Tribology of newly developed EAL versus

water in hydropower turbine bearings

Nayan Dhakal

Mechanical Engineering, master's level (120 credits)

2019

Luleå University of Technology

Department of Engineering Sciences and Mathematics

i

Acknowledgment

The work presented in this thesis is carried out at the Division of Machine Elements at the Luleå

University of Technology, Sweden as a part of the Erasmus Mundus Joint European Master in Tribology

of Surfaces and Interfaces – TRIBOS programme. I would like to thank Education, Audiovisual &

Culture Executive Agency (EACEA) and Erasmus+ programme of the European Union for funding this

programme. I would also like to express my appreciation to the TRIBOS consortium for providing me

with this opportunity.

I would like to express my sincerest gratitude to my supervisors, Professor Nazanin Emami and Associate

Professor Yijun Shi for their valuable suggestions, motivations, and encouragement. Finally, I am always

grateful to my colleagues, staff and laboratory technicians at the TRIBOLAB, Luleå University of

Technology, Sweden for their assistance during this project.

Nayan Dhakal

Luleå, August 2019

ii

Abstract

The increasing demand for more readily bio-degradable, renewable and non-toxic environmentally

adaptive lubricants with a lower degree of bioaccumulation has escalated the investigation of water-based

lubricants as the potential alternatives for conventional mineral-based oils. Water is an excellent

environmentally adapted lubricant; however, it is a low viscous fluid and holds downsides of having

corrosive nature and extremely low pressure-viscosity coefficient leading to insufficient load-carrying

capacity. These shortcomings make pure water a poor choice for operations involving high load and low

speed, for instance, within hydropower applications. Therefore, the choice of appropriate modifiers and

additives is crucial to improve the viscosity, friction-reducing performance and anti-wear properties of

water as a base lubricant. Appropriate selection and combination of bearing materials also significantly

improve the tribological performance of the lubricants. In the presented work, tribological behavior of

polyvinylpyrrolidone (PVP) and poly(sodium 4-styrenesulfonate) (PSS) thickening agents with water as

a base fluid in the lubrication of pure UHMWPE and SCF reinforced UHMWPE bearing materials has

been investigated individually under reciprocating tribometer. The results showed that both PVP and

PSS are excellent viscosity modifiers. PVP exhibited excellent friction-reducing and anti-wear

performance, while PSS revealed increased wear rates with an insignificant reduction of friction

coefficients. This study aims to explore the potential for using newly developed water-based lubricants

in the replacement of traditional mineral-based lubricants for hydropower turbine bearings.

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iv

Table of Contents

Acknowledgment ........................................................................................................................... i

Abstract ........................................................................................................................................ ii

List of Figures .............................................................................................................................. vi

List of Tables ............................................................................................................................... vi

1 Introduction .......................................................................................................................... 1

1.1 Hydropower ....................................................................................................................... 1

1.2 Hydrodynamic Sliding Bearings .......................................................................................... 1

1.3 Wear Mechanisms .............................................................................................................. 3

1.3.1 Abrasive Wear ................................................................................................................ 3

1.3.2 Adhesive Wear ............................................................................................................... 3

1.4 Environmentally Adaptive Lubricants (EALs) ...................................................................... 4

1.5 Water Lubrication .............................................................................................................. 5

1.6 Materials ............................................................................................................................. 7

Polyvinylpyrrolidones (PVP) ........................................................................................... 7

Poly(sodium 4-styrenesulfonate) (PSS) ............................................................................ 8

UHMWPE ..................................................................................................................... 9

Short Carbon Fiber (SCF) ............................................................................................... 9

1.7 Research Gap ................................................................................................................... 10

1.8 Research Objectives ......................................................................................................... 11

2 Experimental Methodology ................................................................................................... 12

2.1 Preparation of modified water-based lubricants ................................................................. 12

2.2 Viscosity measurements .................................................................................................... 12

2.3 Measurement of tribological properties ............................................................................. 13

2.4 Scanning Electron Microscope (SEM) ............................................................................... 15

2.5 Wettability ....................................................................................................................... 16

3 Results and Discussion .......................................................................................................... 17

3.1 Enhancement of Viscosity ................................................................................................. 17

3.2 Tribological Characterization ............................................................................................ 19

3.2.1 Friction and wear performance ...................................................................................... 19

3.2.2 SEM Analysis ................................................................................................................ 27

3.3 Contact angle measurements ............................................................................................. 31

4 Conclusion .......................................................................................................................... 32

5 Recommendations ............................................................................................................... 33

References .................................................................................................................................. 34

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vi

List of Figures

Figure 1 Tilting pad thrust bearing (Kingsbury Bearing Inc.) .............................................................................. 2 Figure 2 Dynamic viscosity of water versus temperature ..................................................................................... 5 Figure 3 Chemical Structure of PVP ................................................................................................................. 7 Figure 4 Brookfield viscosity of PVP at various % w/w concentrations ............................................................... 8 Figure 5 Chemical Structure of PSS .................................................................................................................. 8 Figure 6 Experimental workflow ..................................................................................................................... 12 Figure 7 Bohlin Rheometer & schematic diagram of Cylinder on Cup ............................................................. 13 Figure 8 Cameron Plint TE77 Tribometer ...................................................................................................... 13 Figure 9 Schematic diagram of test configuration & wear scar on SS countersurface .......................................... 14 Figure 10 JEOL JSM-IT300 SEM ................................................................................................................... 15 Figure 11 Attension Theta optical tensiometer ................................................................................................. 16 Figure 12 Enhancement of viscosity with different concentrations of PVP and PSS ........................................... 17 Figure 13 Variation of viscosity with time for aqueous solutions of PVP and PSS ............................................. 17 Figure 14 Variation of the viscosity for aqueous solutions of a) PVP and b) PSS ................................................ 18 Figure 15 Dependence of friction coefficient of UHMWPE on the concentration of PVP ................................ 19 Figure 16 Variation of friction coefficient of UHMWPE on friction time for 20% PVP .................................... 20 Figure 17 Dependence of specific wear rates of UHMWPE pins on the concentration of PVP.......................... 20 Figure 18 Dependence of friction coefficient of UHMWPE on the concentration of PSS ................................. 22 Figure 19 Dependence of specific wear rates of UHMWPE pins on the concentration of PSS........................... 22 Figure 20 Variation of friction coefficient of UHMWPE on friction time for 20% PSS ..................................... 23 Figure 21 Dependence of friction coefficient of SCF reinforced UHMWPE on the concentration of PVP ........ 24 Figure 22 Dependence of specific wear rates of SCF reinforced UHMWPE on the concentration of PVP ......... 25 Figure 23 Dependence of friction coefficient of SCF reinforced UHMWPE on the concentration of PSS ......... 25 Figure 24 Dependence of specific wear rates of SCF reinforced UHMWPE on the concentration of PSS .......... 26 Figure 25 Worn UHMWPE pin surfaces (20% PVP, 20 MPa, 2 hours) ............................................................ 27 Figure 26 Worn UHMWPE pin surfaces (20% PVP, 20 MPa, 20 hours) .......................................................... 27 Figure 27 Worn UHMWPE pin surfaces (20% PSS, 20 MPa, 2 hours) ............................................................. 28 Figure 28 Worn UHMWPE pin surfaces (20% PSS, 20 MPa, 20 hours) ........................................................... 28 Figure 29 Worn SCF reinforced UHMWPE pin surfaces (20% PVP, 20 MPa, 2 hours) .................................... 29 Figure 30 Worn SCF reinforced UHMWPE pin surfaces (20% PSS, 20 MPa, 2 hours) ..................................... 29 Figure 31 SS counter surface plate after sliding lubricated with A) 20% PVP and B) 20% PSS ........................... 30 Figure 32 Dependence of contact angles on the concentration of PVP ............................................................. 31

List of Tables

Table 1 Properties of PVP40 ............................................................................................................................. 7 Table 2 Properties of PSS .................................................................................................................................. 8 Table 3 Characteristics of UHMWPE ............................................................................................................... 9 Table 4 Experimental conditions for Cameron Plint TE77 ............................................................................... 14 Table 5 Test sequence 1 PSS (20 MPa contact pressure) ................................................................................... 21

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1

1 Introduction

1.1 Hydropower

Energy generation from the water with hydropower plants is the preferred form of renewable energy.

Swedish Energy Agency (2019) stated that over 40% (64.6 TWh) of total electricity produced in 2017

(160.2 TWh) was from the hydroelectric power plants in Sweden, while 96% of Norway’s electricity is

from hydropower [1]. Increasing demand and positive perspective towards renewable energy are

continuously attracting researchers to find the aspects which will increase the efficiency of hydropower

generation and reduce energy loss during production as well as supply.

Hydropower turbines frequently operate at low speed and extremely high load conditions. In such conditions, friction behavior due to contact between the opposing surfaces and lubrication plays a vital role in the improvement of efficiency. Friction between mating surfaces results in the generation of localized heat and stress leading to a rise in temperature and generation of wear particles, ultimately causing early material damage and unexpected plant failure. In addition, during peak electricity demand, for instance, during the evenings or when electric heaters are used during the sudden temperature drop, power plants are being started and stopped frequently to meet these power demands. These rapid changes and extra load placed on the power generation system increases the mechanical strain [2]. All these factors result in increased maintenance costs and reduced operating efficiency of the whole system.

Conventional mineral oils have been used as the lubricating fluid for turbine bearings for long. However,

the environmental effects of mineral-based oils due to the spillage, leakage, or even after the disposal

have raised concerns about the safety of the earth, plants, and animal life. The demand for more readily

bio-degradable, renewable, non-toxic environmentally adaptive lubricants has increased the investigation

and search of potential alternatives for mineral-based oils. Improving the performance of hydropower

from the tribological perspective can be concerned with friction improvement and lubrication

performance. It has been established that the reduction in friction can be achieved by improving the

lubrication performance. Increasing the load-carrying capacity of the bearings within the existing

dimensional constraints, the influence of rapid changes in load and speed, reduced wear with an increased

life of lubricants using environmentally adaptive alternatives can be the prime focal points leading to the

reduced energy loss and increased efficiency.

1.2 Hydrodynamic Sliding Bearings

Hydrodynamic sliding bearings are the type of bearings where two conformal surfaces slide relative to

one another with the distributed load across the interface, for instance, journal bearings, thrust bearings,

and shaft bushings. The choice of lubricant in such bearings can be a key to reducing the friction between

contacts and the formation of a stable lubricant film. Sliding bearings are supposed to have a long service

life and operate for their lifespan if the employed lubrication method generates fluid film thickness

enough to separate the contacting surfaces. This, however, becomes challenging when high electricity

demand is placed on the system and such bearings become susceptible to damage during start-up.

During the normal running phase, the bearing face is separated from the shafts by a hydrodynamically

generated lubricant film. At start-up, due to the inevitable high load and low speed, the lubricant film

breaks down, and the surfaces come in contact. Figure 1 [3] shows the tilting pad thrust bearing with a

converging gap. During steady-state operating conditions, lubricant flows in this converging wedge due

to the relative motion of contacting surfaces and the available volume is reduced. This tilting pad helps

to the pressure build-up in the lubricating film. The resulting pressure distribution by such pressurized

film provides maximum load-carrying capacity, as it is operating under the hydrodynamic lubrication

domain.

2

On the contrary, during transient state conditions, the system is dragged under the boundary/mixed

lubrication domain due to high load and low speed, whilst the load is primarily transferred through the

surface asperities in contact. This lubrication regime is associated with high friction and wear conditions

relative to the hydrodynamic lubrication. Thereby, sliding bearings are susceptible to damage due to the

rapid changes in operating parameters like load and speed during the start-up [2]. Thus, the appropriate

employment of lubricants can overcome such threats by reduction of friction with the formation of an

adequate film. Simultaneously, the choice of appropriate materials for the turbine bearings can also be

crucial for eliminating such damages as well.

Figure 1 Tilting pad thrust bearing (Kingsbury Bearing Inc.)

The most common choice of turbine bearing materials in hydropower applications for a long time was

Babbitt or white metal, a soft metal alloy coating along with tin, lead, copper, aluminum and bronze

base alloys. However, weaknesses such as thermal ratcheting, prone to corrosion, poor fatigue strength,

poor thermal and elastic properties had negative impacts over the efficiency of the system. The need to

improve the bearing performance by overcoming the drawbacks of babbitt faced bearing materials led to

the research on polymer composite materials [2]. Over the past 2 decades, polymer composite bearings

have been tested and used to be a suitable replacement for babbitted bearings improving the effectiveness

and overall machine efficiency.

The study for polymer-based materials, for instance, ultra-high molecular weight polyethylene

(UHMWPE) as hydropower bearing materials, has provided a significant amount of tribological

improvements in friction reduction and increased wear resistance [4]. Earlier, polyetheretherketone

(PEEK) and polyphenylene sulfide (PPS) in water-lubricated sliding contacts have been reported to yield

lower friction and wear behavior, attributed to the superior wear resistance of PPS to its chemical reaction

with steel countersurface [5]. Some polymers, however, such as polytetrafluoroethylene (PTFE) in water-

lubricated sliding contacts are known to decrease the wear-resistant properties of the polymer [6]. PTFE

alone has also been associated with high wear rates despite its low friction properties [7] in dry and wet

sliding conditions. The use of fillers and reinforcements are well known to increase the wear-resistant

behavior of PTFE [8-10]. In addition, it was concluded that the use of PTFE/glass-fiber composite faced

pad in hydro turbine bearings is capable of thicker lubricant film formation along with the reduction of

contact temperature leading to the increased load-carrying capacity [11].

Recently, high-performance bulk polymers were reviewed by Nunez (2019) [12] and demonstrated that

different blends of polymers, including PTFE and PEEK, show excellent friction and wear reduction

compared to unfilled polymers. Jean-Fulcrand A. et al. (2019) [13] observed that certain blends of

polymers, PEEK-PBI (polyetheretherketone-polybenzimidazole) in contact with steel ball can drastically

reduce the friction even at an elevated temperature of 280°C. Among the commercially available bearing

materials, Thordon Thorplas, homogeneous thermoplastic polymer bearing materials capable of

operating under high pressure of over 45 MPa and up to 90 MPa, are widely investigated for material

characterization [14] and applications in hydro turbines due to their low friction and wear rates under

wet conditions [15, 16].

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1.3 Wear Mechanisms

Wear can be defined as a progressive loss of material due to the damage of a solid surface resulting from

the relative motion with another contacting surface. It can also sometimes be the degradation of the

surface material without any loss of material whilst changing the surface topography leading to damaging

of the surface [17]. Within hydropower applications, wear of the bearing surfaces and shaft increases the

clearance in between, which has adverse effects on bearing performance and efficiency of the overall

system.

Wear may be due to the various mechanical and chemical effects involving plastic deformation,

ploughing action, micro-cutting, cracking, fracture, chemical interaction, delamination and such. Wear

of polymers can be defined by mechanisms like abrasive wear, adhesive wear, fatigue wear, fatigue-

related delamination wear, and chemical wear. The discussion on polymer wear mechanisms in this

section will be constrained within abrasive and adhesive wear.

1.3.1 Abrasive Wear

Abrasive wear is caused by the abrasion action of harder material over softer surface interacting in physical

contact. Softer material wears out by the penetration of relatively hard asperities generating wear particles.

Primary modes of abrasive wear are cutting and ploughing. Sharp particle or hard asperity cuts the softer

surface and material removal occurs as wear debris, leading to the formation of wear tracks. Such micro-

chipping/cutting results in severe wear but at a shallow depth. Ploughing action, however, does not

remove the material from the surface but the material is shifted to the sides of wear tracks. Brittle materials

experience micro-fracture due to the merging of a few smaller cracks. Relatively ductile material, on the

other hand, is abraded by a blunt asperity and results in fatigue due to ploughing. Wear particles, in this

case, are not created immediately, but later get peeled off as the worn surface is repeatedly loaded and

unloaded and consequently, leads to the delamination wear.

The wear between two surfaces in contact is called two-body abrasion, where the particles are firmly

clamped and there is continuity of movement. It causes deep scratches and a high wear rate. The abrasion

caused by hard loose particles trapped in between two mating surfaces is termed as three-body abrasion.

Such particles are free to roll, slide and slip between the rubbing surfaces since they are not held rigidly.

Wear rate is lower in three body abrasion and generates shallow scratches.

In theory, lubricated contacts experience normal operation and minimal wear, however, in practice,

contact surfaces under lubrication also experience surface damage and degradation if hard particle/grit

gets ingested due to contamination and/or from wear debris. Such wear debris and contaminants trapped

in between the mating surfaces act as abrasive particles which leads to enhanced damage and accelerated

wear of surfaces resulting in premature failure of the bearings [18]. If abrasive particles stay suspended in

between the surfaces, it acts as three-body abrasion, while, it will behave as two-body abrasion if the

particles get wedged in-between with any of the surfaces [19].

1.3.2 Adhesive Wear

Adhesive wear occurs when two surfaces are pressed together and are joined by the action of cold welding

at the contact interface. The bonding between the asperities of surfaces in contact results in junction

growth. Due to the high load and continued motion, asperities get deformed and adhere to counter

surface forming adhesion bonds, ultimately leading to the rupture of micro-junctions. Hardening of the

adhered particles due to repeated load cycles may result in abrasion as well. That is why abrasion and

adhesion often occur simultaneously.

4

On the other hand, the transfer of material by the rupture of bonded polymer asperities onto the counter

bodies forms a transfer film. The formation of polymer transfer film is governed by interfacial energy

between polymer-metal surfaces (adhesive work) and polymer surface energy (cohesive work). After the

polymer transfer film is generated, the polymer surface is rubbing against the same polymer film

preventing the contact between polymer-metal. The lower surface energy of polymer-polymer than

polymer-metal also aids in the reduction of friction and wear. It is believed that the formation of transfer

films results in steady-state wear from transient wear phenomena [20] and thus, the ability to transfer film

formation of a polymer composite is often attributed to its wear-resistant behavior.

1.4 Environmentally Adaptive Lubricants (EALs)

The adverse effects of mineral-based lubricating oils on the environment for the extended duration of

usage have raised the necessity of environmentally adaptive lubricants (EALs) to reduce the possible

contamination of mineral-based oils on the human, plants, aquatic life, and earth. Most EALs are

chemically produced from simple hydrocarbons such as ethane or propene. Polyalphaolefins (PAO),

vegetable oils, synthetic esters, alkylated aromatics, polyglycols, and phosphate esters are several base

fluids to produce EALs. An EAL is not, however, completely safe for plant and animal life, but has a less

detrimental impact on the environment than mineral oil. On a similar note, EALs are formulated to

exhibit superior performance with the goal of increased lubricant life and improved efficiency, which

ultimately will reduce the negative impacts on the environment. Higher bio-degradability, low toxicity,

renewability and low degree of bioaccumulation are the major advantages. EAL lubricants compared to

typical mineral-based oil provide extended stability limits, increased load-carrying capacity, reduced start-

up friction and ultimately, reduced power losses.

Along with the increased concern for the environmental safety, there have been various studies on

vegetable oils as a potential base fluid for lubricants because of their excellent lubricity, bio-degradability,

viscosity-temperature characteristics, and low volatility. The strong intermolecular interactions of

vegetable oils can form durable lubricant film as well as reduction of friction coefficients and wear while

compared to the most mineral base oil lubricants, however, this strong intermolecular interaction results

in poor flow properties at low temperatures. In addition, vegetable oils have poor corrosion protection,

narrow viscosity range, low thermal stability and oxidation stability, which limit vegetable oils as a

potential EAL for the replacement of mineral oils [21-23]. Scholars have established that the anti-wear

properties, friction behavior and other lubricating characteristics of vegetable-based oils can be improved

by the addition of additives [24, 25].

Synthetic esters and glycerol can be another alternative form of EAL possessing great potential with

possible improvements to their lubricating characteristics. Synthetic esters can be designed to provide the

appropriate viscosity, volatility, thermal stability, pour point, flash point, biodegradability and even cost

by appropriate selection of organic alcohol and carboxylic acid ingredients. Synthetic fatty acid esters

have superior oxidative, hydrolytic and bio-stability and can be designed to meet the desired viscosity,

volatility and lubricity, while polyol esters are preferred in high-temperature operations where they give

long life and resist varnish and deposit formation. Studies show that the use of synthetic esters within

hydro turbine applications considerably improved friction behavior and wear performances with bronze-

steel contacts in Kaplan turbine as compared to mineral-based turbine oil [11], with tin-bronze contacts

at a high bearing pressure of 15 MPa [26] and with copper-based journal bearings [27].

Glycerol is a polyol compound, obtained as the by-product of the transesterification of triglycerides to

produce bio-diesel from biological products. The high viscosity, abundancy, biodegradability, non-

toxicity and eco-friendly nature of glycerol have rapidly increased the use of glycerol as lubricants in

food packaging industries and pharmaceuticals. The viscosity of pure glycerol was recorded as 890 Pa.s

at room temperature [28], which is almost 20 times greater than that of typical mineral-based oils. Such

high viscosity is not desirable for low-temperature applications as high heat generation occurs due to the

5

extra energy required to shear the thick lubricating film, resulting in lubrication decomposition and

possibly early failure. The works of literature show that the freezing point and film thickness of glycerol

can be reduced by adding an appropriate amount of water in pure glycerol producing a very low

coefficient of friction in boundary and elastohydrodynamic lubrication domain [29, 30]. Shi et al. [30]

studied the effects of addition of water in pure glycerol and observed that the addition of 10% w/w of

water can reduce the viscosity of pure glycerol by one fifth and up to 200 times lower when the water

content was increased to 50% w/w. On the contrary, higher water content (>20% w/w) could not

provide enough viscosity for full separation and increasing friction coefficient was observed at decreasing

slide-to-roll ratio (SRR). Almost 12 times higher wear rate with 50% w/w of water content was noted

as compared to the pure glycerol. This was attributed to the highly corrosive nature of water leading to

increased wear. Similar results were generated by Tortora et al. [31] with the aqueous solutions of

glycerol. It was evident that the friction and wear increased with an increasing amount of water after a

certain level of water content, thus an optimum concentration of water should be established.

Moreover, Zheng et al. (2018) [32] studied ricinoleic acid-based ionic liquid [Cho][ricinoleic] as an

additive for glycerol solution with different concentrations and observed a significant reduction in friction

coefficients and wear volumes with increasing loads at light load conditions (25N-50N), and it was

attributed to the formation of compact fluid films by strong adsorption onto the metal surface due to the

presence of carboxylate group, unsaturation and hydroxyl group. The optimal concentration of IL was

observed to be at 6% w/w. On the contrary, the increasing pattern of wear volumes at the higher load

range greater than 100N is worth to be noted. In recent years, bio-based ionic liquids (ILs) have been

an interest in the exploration of its performance on tribological applications as a green lubricant because

of its biodegradability and non-toxic properties [33-37]. As the conventional ionic liquids are associated

with drawbacks such as; imidazolium-based ILs showing lower oxidative stability at higher temperatures,

the corrosive nature of pyridinium-based ILs, phosphonium-based ILs not being environmentally

adaptive and halogen-containing ILs enhancing corrosion, the focus is shifted towards the bio-based ILs

[38-40]. It has been established that bio-based ILs are capable of improving lubricating properties of fully

green lubricants reducing friction and wear for various surface contacts [32, 34, 35, 41-46].

1.5 Water Lubrication

Water alone has been used for lubrication for a long time because it is non-inflammable, cheap, readily

available, non-toxic, biodegradable and relatively incompressible. Its low viscous nature, on the other

hand, holds disadvantages for applications like hydro turbines where high contact load and low speed are

frequent and thus water is not an ideal choice as a lubricant due to its low load-carrying capacity [47].

Figure 2 Dynamic viscosity of water versus temperature

6

Figure 2 [48] presents the dynamic viscosity of water at different temperatures, with 8.91x10-4 Pa.s at

25°C. This value is taken as the pure water viscosity reference for the comparison with modified water-

based lubricants hereafter. The low pressure-viscosity coefficient of water is a major drawback for its use

within hydropower applications. In addition, water is highly corrosive and possesses low freezing and

boiling points which affects the lubrication mechanism leading to poor lubrication and early failure. The

addition of various additives can overcome these disadvantageous properties of water, but again those

additives are environmentally harmful and toxic, even sometimes are insoluble in water [49].

In an investigation on sodium lauryl sulfate (SLS) and ethoxylated sodium lauryl sulfate (ESLS) additives,

Sulek et al. [50] observed improved lubricating properties of water. Pure water provides poor lubricating

film formation due to its low viscosity. It was claimed that the migrating nature of the amphiphilic

structure of surfactant aqueous solution influences the lubricating property of water. It was seen that the

formation of adsorption layers is capable of film generation at low speed which can be further enhanced

at higher speeds [47]. In another study by Wang et al. (2012) [51], poly(sodium 4-styrenesulphonate)

(PSS) resulted in nearly 60% increment in viscosity with 4% w/w concentration with over 60% reduction

in friction. The inclusion of PSS in the water-glycol hydraulic fluid base also exhibited excellent anti-

wear properties. The author attributed this improvement of tribological performances to the formation

of the chemically stable lubricant film. It was the result of probable iron oxide and iron hydroxide

generation by the interaction of dissolved oxygen and sulfonate anions of the lubricant sample with the

sliding steel surface.

Polymers are concluded as good additives modifying the lubricating properties of water as a base fluid in

various studies [52-54], observed to enhance the lubricating properties of aqueous solutions by the

formation of films in boundary lubrication domain leading to the reduction of friction coefficient [55,

56]. Researchers have established that the proper formulation of water content with other lubricating

base stock, such as ionic liquids, glycerol, and the addition of performance-enhancing additives, have

huge potential as EAL. Amino containing poly (ethylene glycol) [PEG-NH2] and carboxylated graphene

[G-COOH] was used to formulate PEGlated graphene [PEG-G] as nano additives for deionized (DI)

water solution with a varying concentration of PEG-G [57]. The study of friction and wear behavior

gave an optimum concentration of 0.05% w/w PEG-G yielding maximum reduction of wear by 81.23%

and a 39.04% reduction in the coefficient of friction, as compared to pure water solution. The hydrophilic

nature of PEG [58], the formation of fluid adhesive film and carbon protective film, and the dispersion

of PEG-G was the reason behind the observed best performance.

Sulek et al. [59] used polyvinylpyrrolidones (PVP) as green additives to modify the tribological

performances at water-lubricated contacts and observed that the degree of polymerization of PVP has a

huge impact on the viscosity as compared to that with a lower degree of polymerization, even at a lower

concentration of 5%. The coefficient of friction was noted to decrease with an increase in the load.

Almost 30% reduction in wear scar diameters and a great increase in load-bearing capacity was observed

as compared to that for water. Guo et al. [60] later concluded PVP as a promising lubricating additive

after investigating the viscosity, surface tension, wettability, and tribological performance. Inclusion of

PVP additive depicted coefficients of friction trend decreasing significantly at first but subsequently

tapering off. Ti6Al4V ball – UHMWPE disc friction pair showed a significant reduction in friction

coefficients up to 50% with an effective improvement of wear resistance with PVP at concentrations

from 0 to 15% w/w in deionized water as well as bovine serum aqueous solutions for artificial joints.

The ease of using pure water as base fluid leading to the possibility of using modified water as a hydro

turbine bearing lubricant is gaining increased interest in water-based or aqueous lubrication towards the

development of EAL. Numerous researchers are focusing on improving the viscosity and anti-corrosion

property of water by the addition of different thickeners and viscosity modifiers as additives, however,

the complexity of friction characteristics and wear mechanisms of water and aqueous lubricant solutions

is always challenging within the hydro turbine applications.

7

1.6 Materials

This section comprises of the specific materials studied in this work. Two commercially available

polymeric thickening agents for water-based lubrication and polymer bearing materials for hydropower

applications were investigated.

Polyvinylpyrrolidones (PVP)

Polyvinylpyrrolidone (PVP), also termed as Polyvidone or Povidone, is a hygroscopic amorphous

powder consisting of polar amide groups and non-polar ethylene groups which can be obtained from

vinylpyrrolidone [61]. Figure 3 [62] represents the chemical structure of PVP. Initially used as a blood

plasma substitute in World War II, PVP now finds its use as a serum albumin substitute, artificial tears

for irritation and drying of eyes, disinfectants, dispersants, and binders. Polyvinylpyrrolidone is a highly

biodegradable, non-toxic, biocompatible water-soluble polymer additive [63, 64]. It also has a very low

degree of bioaccumulation as it does not decompose due to the action of microorganisms and thus,

provides stability as a lubricating component.

Figure 3 Chemical Structure of PVP

PVP has also been widely used in pharmaceutical, medical devices, food, paints, cosmetic and detergent

industries [65-67]. It also finds its use in multiple technical applications like membranes, hydrogels, hot-

melt adhesives, and glue sticks. The research works on PVP have been somewhat limited to the

experimental study on surface tension, density, viscosity, clinical effects and such applications [68-74]

while the study focusing on the tribological performance of PVP are very little [59, 60, 66].

Owing to its versatile features such as water solubility, toxicological harmlessness, good adhesion and

film formation, PVP is selected with an expectation that its aqueous solution with pure water can be

studied as a potential EAL.

Table 1 [62] lists the typical properties of the PVP40.

Table 1 Properties of PVP40

Form Powder

Linear Formula (C6H9NO)n

Molecular weight average Mw ~40,000 Viscosity 20-30 mPa.s, 20% (25°C, Brookfield)

Melting Point 150°C

Solubility water and lower glycols: soluble

Density 1.2 g/mL at 25 °C

Figure 4 represents the standard measurement of Brookfield viscosity for polyvinylpyrrolidone at different

temperatures, provided by Ashland Polymers [75]. Brookfield viscosity can be referenced for the

validation of viscosity measurements for the aqueous solutions of PVP at different % w/w concentrations,

reported later in this study.

8

Figure 4 Brookfield viscosity of PVP at various % w/w concentrations

Poly(sodium 4-styrenesulfonate) (PSS)

Poly(sodium 4-styrenesulfonate) (PSS) is an anionic polyelectrolyte derived from the polymerization of

polystyrene by the addition of sulfonate functional groups. PSS is extremely low toxic with high water

solubility compared to other homogeneous compounds of sulfonate, as a result, it has been widely used

in wastewater treatment, adhesives, chemical and pharmaceutical industry [76]. Sulfonates are used as

detergent, anti-rust additives and are known as excellent dispersants. There are studies concerned with

synthetic sulfonates as extreme pressure and wear-resistant additives for lubricating oils, however, there

are very little investigations on PSS as water-soluble additives [50, 51]. Figure 5 [77] represents the

chemical structure of the PSS.

Figure 5 Chemical Structure of PSS

Most water-soluble high molecular polymers are found to significantly increase the viscosity of pure

water and PSS are also known for its excellent water solubility and extremely low toxicity. Thus, PSS is

expected to exhibit good tribological performances as a water-based lubricant additive. Table 2 [77] lists

the typical properties of the PSS.

Table 2 Properties of PSS

Form Powder

Linear Formula (C8H7NaO3S)n Molecular weight average Mw ~70,000 Viscosity 15-55 mPa.s, 20% (25°C, Brookfield)

Melting Point 460°C

Solubility water and lower glycols: soluble

Density 0.801 g/mL at 25°C

9

UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) is a polymer-based material used as the bearing

material for the hydropower turbine in this presented work. It is a semi-crystalline thermoplastic polymer

which consists of long chains of ethylene groups. UHMWPE possess high abrasion and wear resistance,

high impact strength, good lubricity, and excellent biocompatibility. UHMWPE is a relatively cheap

general-purpose plastic which shows low friction and good wear resistance with excellent load-bearing

capacity for water-based lubrication system [4]. The detailed characteristics of the UHMWPE are

presented in Table 3 [78].

Table 3 Characteristics of UHMWPE

Particle size 30 [μm] Material Density 0.93 [g/cm3]

Elastic Modulus 0.8 [GPa]

Tensile Strength 23 [MPa]

Flexural Strength 40 [MPa]

Yield Strength 23 [MPa]

Elongation at Break >450 [%]

Hardness 65 [MPa]

Melting point 135 [°C]

Glass Transition Temperature, Tg -160 [°C]

UHMWPE has been extensively used in biomedical applications, such as acetabular cups in hip implants

and bearing surfaces in artificial knee arthroplasty, because of its excellent mechanical and tribological

characteristics [78]. In addition, UHMWPE composites are popular in numerous applications that require

very low friction, durability, wear and chemical resistance in aerospace, biomedical, military applications,

and manufacturing industries.

The use of pure UHMWPE sometimes is associated with wear problems because of its low creep

resistance compared to its metal counterpart. Recent research at Luleå University of Technology (LTU)

[79] shows that the addition of fillers and reinforcements to virgin UHMWPE improves its friction-

reducing and anti-wear performances. It is believed that the reinforcement of virgin UHMWPE with

fillers and nanoparticles can improve the mechanical, thermal and tribological properties of this polymer

[80]. UHMWPE reinforced with short carbon fibers (SCF), nanodiamonds (ND), graphene oxide (GO)

and carbon nanotubes (CNT) are some of the major research interests on polymer matrix composites at

Division of Machine Elements, LTU. This thesis, however, will cover the investigation of pure

UHMWPE along with the SCF reinforced UHMWPE as a means of comparative analysis.

Short Carbon Fiber (SCF)

Short carbon fibers are popular fillers for polymer composites. High surface to volume ratio, outstanding

thermal and mechanical properties and excellent dispersibility in polymer matrices are significant

characteristics of SCFs. The smaller fibers can be consolidated in complex geometries making the

manufacturing process easier and cost-effective than with conventional carbon fibers. Inclusion of SCFs

increased Young’s modulus, compressive yield stress and creep resistance, along with the more stable

coefficients of friction trend and reduced wear as compared to unfilled UHMWPE polymer composites

[80]. Capela et al. (2019) studied the fatigue response of SCF reinforced epoxy resin-based composites

and established that the mechanical properties are proportional with fiber content; stiffness and tensile

strength increased by 52% and 400% respectively for the composites with 17.5% volumetric fraction.

The fatigue life was observed to be influenced primarily by fiber dispersion and porosity [81]. It was

previously established that short carbon fibers can increase the compressive modulus and hardness of

10

UHMWPE along with the significant reduction of friction and wear. The main load between the surfaces

in contact is carried by the exposed SCFs preventing the polymer matrix from severe wear leading to

improved tribological performance of the polymer matrices [82].

The selection of turbine bearing materials was based on commercial applications, availability, and earlier

studies with water-based lubricated contacts. MIPELONTM UHMWPE XM-220 (Mitsui Chemicals,

Japan) with an average size of 30 μm and a molecular weight of 2x106 g/mol was used as the base

polymer. Short carbon fibers (Tenax-A HT M100 100mu) with a bulk density of 1.82 g/cm3, an average

length of 100 μm and 7 μm diameter were used as reinforcements for virgin UHMWPE [79]. SCF at a

concentration of 10% w/w with pure UHMWPE is selected based on the earlier studies. The polymer

specimens used in this work were manufactured using direct compression molding (DCM) technique at

TRIBOLAB, Division of Machine Elements, LTU, Sweden.

1.7 Research Gap

As mentioned earlier, there has been extensive research on environmentally accepted solutions to prevent

the problems associated with environmental safety and lubrication issues. Tribological properties of

polymers for hydro turbine bearings are mostly studied for dry sliding conditions and/or pure water-

lubricated conditions. The prime problem of using water lubrication being the poor lubricity of water,

researchers are focused on improving the tribological properties of water as a base. Friction and wear

behavior of polymer composites for turbine bearings vary widely during dry and water-lubricated

conditions. On a similar note, transfer film formation behavior, hydrodynamic effects, and

adsorption/absorption mechanisms of lubricant molecules on bearing surfaces also have different effects

on the friction pairs under lubricated conditions.

Recently, the search for a potential environmentally adaptive lubricant for hydropower turbine bearings

is widely revolving around ionic liquids (ILs), synthetic esters, glycerol-based lubricants, polyhydric

alcohol-based lubricants, surfactant-water solutions, and such. The studies on improving lubrication of

general-purpose bearing materials for industrial applications have been widely explored from an

environmental perspective, but only a limited number of studies have been undertaken in view of

modified water lubricating the hydropower turbine bearings. While most studies to enhance the lubricity

of water are directed towards changing the type of base fluid, little research has been performed on the

study of the modification of water properties by the addition of thickeners alone.

Earlier studies on polyvinylpyrrolidone [59] and poly(sodium 4-styrenesulfonate) [51] have shown

promising results at low contact pressure conditions, however, its performance during high load and

low-speed conditions, typical to hydropower applications, is not explored till date. Furthermore, the

bearing material, UHMWPE, has been widely investigated for biomedical applications. Very little is

known about the tribological performance of UHMWPE in the presence of modified water for

hydropower applications involving high contact pressure and low-speed conditions. The presented work

is an attempt to further evaluate the effects of PVP and PSS as water modifiers during transient operating

conditions and to understand the friction and wear mechanisms of UHMWPE and SCF reinforced

UHMWPE polymer composites.

11

1.8 Research Objectives

The aim of this work is primarily focused on improving the tribological performance of pure water by

the addition of certain polymeric thickeners/modifiers in the lubrication of hydropower turbine bearings.

The main objectives of the presented thesis can be listed as:

• The search for potential EALs in water-based lubrication for hydropower turbine bearings.

• Investigation of viscosity and tribological performances of modified water lubrication using

polymeric thickeners within hydropower applications.

• Comparative study of friction behavior and wear performances with modified water versus pure

water.

The preparation of new water-based lubricants, experimental analysis methodology and characterization

techniques carried out to achieve these objectives will be discussed in greater detail in the following

sections.

12

2 Experimental Methodology

Figure 6 depicts a simple experimental workflow executed in this thesis.

2.1 Preparation of modified water-based lubricants

Polyvinylpyrrolidone (PVP40, CAS Number 9003-39-8, Sigma-Aldrich Sweden AB) was obtained in

powder form with an average molecular weight Mn ~40,000 and a density of 1.2 g/mL at 25°C.

Poly(sodium 4-styrenesulfonate) (PSS, CAS Number 25704-18-1, Sigma-Aldrich Sweden AB) was

obtained in powder form with average molecular weight Mn ~70,000 and density 0.801 g/mL at 25°C.

PVP and PSS were used without further purification to prepare their aqueous solutions.

Aqueous solutions of both PVP and PSS at different concentrations of 3, 6, 9, 15 and 20% w/w were

prepared at room temperature by mixing with distilled water and shaking manually for 60 seconds. The

solutions were kept for an hour and manually shaken again for 60 seconds duration. The process was

repeated twice, and the lubricant solution was left for at least 24 hours and used for further analysis after

the lubricants were manually shaken thoroughly. Both PVP and PSS easily dissolved in distilled water

and formed fully transparent sample solutions. The appearance of the solutions changed from

colorlessness to light amber to brownish with an increase in concentration. PSS aqueous solutions were

observed to be darker than the PVP solutions. Note that mechanical (magnetic) stirrer was not used for

mixing purposes, as the thickening agents possess excellent solubility with distilled water.

2.2 Viscosity measurements

Viscosity is the measure of the resistance of a lubricating fluid to deformation due to shear. It is also

described as the internal friction of a moving fluid resulting from molecular interaction. A fluid with

higher viscosity has higher internal friction between the molecules which resists the motion. Viscosity

measurement is one of the most reliable and accurate flow behavior characterization techniques that helps

to understand the flow performance towards the development of a new lubricant. The viscosity of a

lubricant is significantly dependent on temperature, meaning viscosity of a lubricating fluid decreases

with increased operating temperature due to frictional heating. Applied pressure also can affect the

viscosity and is subsequently associated with its load-carrying capacity.

A higher value of viscosity can maintain better film separation of moving contact surfaces and helps to

reduce wear and fatigue, resulting in the extended service life of the bearings. Low viscous fluids have

minimum resistance to flow and low load-carrying capacity, while higher viscosity results in less leakage

and thus improving the volumetric efficiency.

>24 hours

Thickening agent + Distilled water

(Aqueous Solution)

SEM Analysis

Viscosity Analysis

Wettability

Pin-on-plate

Friction & Wear test

Figure 6 Experimental workflow

13

The dynamic viscosity of a lubricant is determined by directly measuring shear stress and shear rate.

Dynamic viscosities of the aqueous solutions of PVP and PSS at different concentrations were measured

directly by Bohlin Rheometer at 25°C using Cylinder-on-Cup (C25 DIN 53019) setup, Figure 7.

Figure 7 Bohlin Rheometer & schematic diagram of Cylinder on Cup

Initially, viscosities were recorded for every hour during the friction test in order to study if it was

necessary to add an additional amount of water to the lubricant sample to accommodate the loss of water

content by the action of evaporation. In general, the analysis was carried out before and after the sample

lubricants have been used in tribological tests. The amount of lubricant used was ~11 ml for all tests as

recommended for Bohlin C25. Each sample solution was tested at least 5 times within shear rates of 1-

200 [1/s] taking 15 number of data points and averaged.

2.3 Measurement of tribological properties

Cameron Plint TE77 High-Frequency Reciprocating (HFR) Tribometer (Plint & Partners, Berkshire,

England), Figure 8, was used to perform tribological tests using the pin-on-flat configuration lubricated

with newly developed sample lubricants. The counter surface plates were fixed in a bath, while the

polymer pins were held by a reciprocating arm. Pin-on-flat configuration was chosen to ensure the

consistency of applied contact pressure over one test cycle which helps to eliminate the converging gap

and resulting wedge action of typical hydrodynamic sliding bearings.

Figure 8 Cameron Plint TE77 Tribometer

The polymer pin samples were reciprocated in a horizontal plane with a pure sinusoidal motion provided

by a scotch yoke mechanism driven by a variable speed motor. The frictional force was measured

14

continuously by a piezoelectric transducer, and the coefficients of friction were recorded directly by a

data-acquiring software linked with the TE77 tribometer. The coefficient of friction gives the measure

of resistance to motion. A spring balance and beam were used to apply the normal load directly to the

moving specimen. The load varies by approximately 2% of the total load value (~ 5-7 N) over one test

cycle. The friction is highest at the end of the stroke with zero sliding velocity due to boundary

lubrication and lowest at mid-stroke with maximum sliding velocity where mixed lubrication occurs.

The friction coefficients recorded during the beginning of the sliding process were higher than during

the running-in phase. The reported friction data are the average coefficients of friction measured after

reaching the steady-state, excluding the running-in phase. The tribometer was calibrated before starting

the test series and the same calibration setup was used for all the friction tests. The schematic diagram of

the test configuration and wear scar generated on the counter surface is shown in Figure 9.

Figure 9 Schematic diagram of test configuration & wear scar on SS countersurface

Polymer test specimens of bearing materials were used with a size of 4 x 4 x 4 mm3 cubes. Stainless steel

plates, SS2333 grades were used as counter surfaces as a typical shaft material used in hydropower

applications. The plates were laser cut from a rolling sheet and had a dimension of 60 x 30 x 3 mm3

(length x width x height), measured surface roughness Sa of 0.31 ± 0.02 μm and with 205 ± 5 HV100

hardness. Polymer test specimens and counter surface plates were cleaned thoroughly with ethanol prior

to each test in order to remove potential contaminations.

Friction tests were performed in two different test sequences with two different normal loads, 320 N and

160 N. The first test sequence was performed at room temperature under an applied load of 320 N

producing an initial apparent contact pressure of 20 MPa which accounts for the results presented under

high load conditions. The second test sequence, low load condition under 160 N equivalent to 10 MPa

contact pressure, was then performed for comparative analysis with the high load conditions. These

contact pressures were chosen to imitate the maximum apparent contact pressure at highly loaded

condition during start/stop operations of hydropower turbine applications. All tests were carried out at

a constant sliding velocity (20 mm/s) provided by 2 Hz indicated rotor frequency with 5 mm stroke

length. A full description of the experimental parameters can be found in Table 4.

Table 4 Experimental conditions for Cameron Plint TE77

Contact pressure 20 MPa, 10 MPa Load 320 N, 160 N

Indicated rotor frequency 2 Hz

Stroke length 5 mm

Sliding velocity 20 mm/s

Test duration 2 hours *

Total sliding distance 144 m *

Polymer specimen 4 x 4 x 4 mm3

Stainless steel plate, SS2333 grade 0.31 ± 0.02 μm

Temperature RT (23-25°C)

* Except for SEM analysis which was performed on the specimens after 20 hours of pin-on-plate friction test resulting for a total sliding distance of 1440 m

15

These conditions were chosen to accelerate testing of the materials and to increase the interaction of

friction surfaces during sliding to allow high load and low speed at the boundary/mixed lubrication

regime. Selected contact pressures and sliding speed, are typical for hydropower applications in reference

to the Swedish hydropower industry. All tests were performed at room temperature and each friction

test was repeated three times.

Linear potentiometer or LVDT (linear variable differential transformer) sensor attached to the tribometer

was used to continuously measure the vertical displacement experienced by the polymer pin face in

contact with the counter surface to study the wear. The linear displacement in microns gives the measure

of linear dimensional change due to pin wear and is further used to calculate the total wear volume of

the polymer pins lubricated with aqueous solutions of PVP and PSS at different concentrations. The

specific wear rate coefficients were calculated after the recorded linear wear depths were plotted as a

function of applied normal load and total sliding distance using Equation 1 [56]. The measured data

points were selected after the steady-state was achieved, excluding the running-in phase in all tests and a

specific wear rate coefficient was calculated by linear fitting a line to the plotted data points based on

Equation 1.

𝑊𝑒𝑎𝑟 𝑟𝑎𝑡𝑒 =𝑇𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑙𝑜𝑠𝑠

𝐴𝑝𝑝𝑙𝑖𝑒𝑑 𝐿𝑜𝑎𝑑 𝑥 𝑆𝑙𝑖𝑑𝑖𝑛𝑔 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 (𝑚𝑚³)

(𝑁.𝑚) Equation (1)

2.4 Scanning Electron Microscope (SEM)

Scanning Electron Microscope (SEM) works by creating images of the sample surface due to the action

of high-energy electron beams directed on the surface producing various signals. Various types of signals

are generated mainly by the detection of secondary electrons and backscattered electrons. Electrons

emitted by the atoms of the specimen are secondary electrons, while those reflected from the sample

surface by elastic scattering are backscattered electrons. Backscattered electrons emerge from deeper

locations within the surface, thus the resolution of images produced by backscattered electrons is less

than the resolution of images from secondary electrons. SEM utilizes an electron beam with a wavelength

shorter than that of light and therefore detailed information on the structure and wear behavior can be

obtained visualizing the wear scar in the nanometer scale.

The wear mechanisms explained earlier are expected to be more pronounced while the specimens are

analyzed under SEM. Both the polymer pins and stainless-steel counter surface plates were analyzed using

a JEOL JSM-IT300 Environmental Scanning Electron Microscope (JEOL, Peabody, MA, USA), Figure

10.

Figure 10 JEOL JSM-IT300 SEM

16

2.5 Wettability

Wetting can be explained as the phenomena of how a liquid deposited on a surface spread out. The

wetting ability of a liquid to a surface is determined by the contact angle measurements. Attension Theta

optical tensiometer (Biolin Scientific, Sweden) was used for the contact angle measurements, shown in

Figure 11.

Figure 11 Attension Theta optical tensiometer

The contact angle is governed by the interaction and repulsion forces between the liquid and solid

surface. The molecules of liquid tend to interact more with solid molecules than with each other if the

cohesive forces are weaker than adhesive forces. This results in a smaller contact angle between the liquid

and solid surfaces. The contact angle will be larger when cohesive forces are stronger than adhesive forces

leading to the interaction of liquid molecules with each other than with the solid surface. This also helps

to explain the hydrophilicity and hydrophobicity of the polymer surfaces along with the philicity and

phobicity of other liquids [83].

The ability to wetting of newly developed water-based lubricants was analyzed by executing contact

angle measurements on the unmodified polymer surfaces and stainless steel countersurface. Aqueous

solutions of PVP and PSS were deposited on UHMWPE and SCF reinforced UHMWPE polymer

surfaces using the Sessile drop method with a droplet size of 4 µl. The angle made by the lubricant with

the surface was then recorded and compared against distilled water. The measurements for wettability

were repeated eight times to obtain the average and standard deviation error value, as the contact angle

may vary from place to place.

17

3 Results and Discussion

3.1 Enhancement of Viscosity

It can be clearly seen from Figure 12 that both PVP and PSS were observed to effectively increase the

viscosity of water with an increase in concentration [% w/w]. The dynamic viscosity of 0.891 mPa.s for

pure water is taken as the reference [48] and at the concentration of 3%, the viscosities for both PVP and

PSS aqueous solutions are more than two times higher than the value for pure water. The viscosity of a

slightly higher than 2 mPa.s at 3% wt. concentrations for both PVP and PSS have gradually increased to

nearly 20 mPa.s for PVP and 16 mPa.s for PSS at 20% w/w concentration. The viscosity enhancement

is recorded 8 to 10 times higher when the concentration was increased from 3% to 20% w/w. This shows

the excellent ability of the thickeners to viscosity enhancement when used with water as base fluid. The

increased viscosity is expected to increase the load-carrying capacity and lubricating performance of the

modified water.

0 3 6 9 12 15 18 21

0

2

4

6

8

10

12

14

16

18

20

22

Vis

cosity [m

Pa.s

]

Concentration [% w/w]

PVP

PSS

Figure 12 Enhancement of viscosity with different concentrations of PVP and PSS

compared with the reference viscosity of pure water at 25°C

0 2 4 6 81.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

Vis

cosity [m

Pa.s

]

Time [hours]

PVP

PSS

Figure 13 Variation of viscosity with time for aqueous solutions of PVP and PSS

at 3% concentration (Pin-on-plate, 320 N, 20 mm/s, 8 hours)

18

A separate analysis was carried out by taking viscosity measurements of aqueous solutions with both

thickeners at 3% w/w concentration. The measurements were recorded at every two-hour interval of

test duration on the lubricant samples after friction test. The logic behind this process was to observe the

loss of water content by the action of evaporation from the lubricating sample during friction analysis.

Figure 13 depicts the variation of viscosity with time for the aqueous solutions of PVP and PSS at 3%

concentration under reciprocating tribometer pin-on-plate test at 20 MPa contact pressure up to 8 hours

sliding at 20 mm/s constant speed. It can be noted that 2.15 mPa.s viscosity of 3% PVP aqueous solution

at 0 hours before the friction test changed to 3.53 mPa.s after 8 hours of test duration, which can be

considered as a negligible difference. Similarly, the aqueous solution of PSS at 3% concentration also

showed a trivial difference and thereby the effect of evaporation of water is not taken into consideration

for the friction tests in this study.

Figures 14 presents the overall change in viscosity of the sample solutions lubricating pure UHMWPE

after the test duration of 2 hours, with different concentrations of PVP and PSS, respectively. The rate

of change in viscosity increases with concentration. The negligible change in the recorded data further

makes it evident that the change in viscosity with time does not play a significant role in lubricant flow

behavior over a short test duration of 2 hours. However, it should be noted that the tests carried out for

a longer duration, for instance, more than 24 hours will have a higher rate of viscosity change, as

observed. This increment in the rate of change of viscosity will play important significance over the

friction behavior for both PVP and PSS solutions, and thus appropriate means needs to be employed to

accommodate the loss of water by evaporation during the tribo tests. The preferred method is to use an

IV drip lubricating system to ensure uniform lubrication between the friction pairs.

3.21

11.3

20

3.4

12.7

23

3.34

15.1

23.5

6 15 200

5

10

15

20

25

Vis

cosity [m

Pa.s

]

Concentration [% w/w]

0 hour

2 hour @ 10 MPa

2 hour @ 20 MPa

a) PVP

4.28

9.36

15.4

4.63

10

16.8

4.39

10.7

18.2

6 15 200

5

10

15

20

25

Vis

co

sity [

mP

a.s

]

Concentration [% w/w]

0 hour

2 hour @ 10 MPa

2 hour @ 20 MPa

b) PSS

Figure 14 Variation of the viscosity for aqueous solutions of a) PVP and b) PSS

at different concentrations [% w/w] with pure UHMWPE at 20 mm/s

Furthermore, the change in viscosity is seen to be higher under high contact pressure of 20 MPa

(equivalent to 320 N applied normal load) for both PVP and PSS aqueous solutions with higher

concentrations. This is due to the generation of higher frictional heat with a high load which increases

the operating temperature and thereby increasing the evaporation of base water. A similar and parallel

trend in viscosity change was observed with both PVP and PSS aqueous solutions lubricating both pure

UHMWPE and SCF reinforced UHMWPE bearing materials. This can also be further explained by the

results obtained from friction performances, discussed later.

19

3.2 Tribological Characterization

3.2.1 Friction and wear performance

a) Pure UHMWPE with aqueous solutions of PVP

To start with, the aqueous solutions of PVP at different concentrations of 0%, 3%, 6%, 9%, 15% and 20%

[w/w] were taken in the first test sequence for tribo test under high contact pressure of 20 MPa providing

equivalent normal load of 320 N. The better-performing friction pairs with aqueous PVP at

concentrations of 6%, 15% and 20% w/w were then selected for the second test sequence for further

evaluation with a lower contact pressure of 10 MPa, see Figure 15.

Figure 15 shows the averaged values of coefficients of friction versus concentration plot at different

contact pressures with standard deviation error bars. All tests were repeated 3 times. It can be observed

that the friction coefficient is decreasing with increasing concentration of PVP. A constant sliding speed

of 20 mm/s for 2 hours was used to study the variation of friction coefficients under varying contact

pressures of 20 MPa (high load conditions) and 10 MPa (low load conditions). COF for high load

conditions is significantly higher than at low load conditions. This shows that the change in a single

parameter can result in a considerable difference in variation of friction coefficient, and ultimately with

the performance of the system. While the hydropower shifts from steady-state to transient state, there is

a great alteration with the performance of friction pairs, which can have an adverse effect on the efficiency

of the overall system.

0.156

0.129

0.115

0.132

0.1270.118

0% 3% 6% 9% 15% 20%

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Friction C

oeffic

ient [µ

]

Concentration [% w/w]

a) 20 MPa

0.104

0.090.085

0.081

0% 6% 15% 20%

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Friction C

oeffic

ient [µ

]

Concentration [% w/w]

b) 10 MPa

Figure 15 Dependence of friction coefficient of UHMWPE on the concentration of PVP

(Pin-on-plate tests, 20 mm/s, 2 hours)

Moreover, it can also be observed that the trend of friction reduction with 20 MPa contact pressure is

more fluctuating with increasing concentrations [Figure 15.a], while friction reduction at low load

conditions is gradual but the reduction is not huge. Under high load condition, COF of 0.156 for pure

water (0% w/w) reduced to 0.115 at 6% w/w concentration, which slightly increased and reached 0.118

at 20% w/w, which shows more than 26% reduction from 0% to 20% w/w concentration of PVP. On

the contrary, close inspection of Figure 15.b) shows that the reduction of COF from 0% to 20% w/w is

not immense.

20

Variation of friction coefficient on friction time for pure UHMWPE lubricated with an aqueous solution

of PVP at 20% concentration compared with pure water for both high and low load conditions is

presented in Figure 16. The friction coefficient as a function of time increases in the presence of pure

water, however, the friction curve tends to stabilize after running-in phase during operation in the

presence of PVP solutions. The friction-time curve for pure water is fluctuating and many peaks can be

observed throughout the curve, while the curves with PVP solutions are smoother and stable. The stable

plateau for an aqueous solution of PVP is more prominent with low load conditions as compared to a

higher load in the friction-time curve.

Figure 16 Variation of friction coefficient of UHMWPE on friction time for 20% PVP

(Pin-on-plate tests, 20 mm/s, 2 hours)

7.34

6.26

5.54 5.49

0% 6% 15% 20%

0

2

4

6

8

10

Sp

ecific

wea

r ra

te [1

0-5

mm

3/N

.m]

Concentration [% w/w]

a) 20 MPa

7.29

5.875.73

5.63

0% 6% 15% 20%

0

2

4

6

8

10

Specific

wear

rate

[10

-5 m

m3/N

.m]

Concentration [% w/w]

b) 10 MPa

Figure 17 Dependence of specific wear rates of UHMWPE pins on the concentration of PVP

(Pin-on-plate tests, 20 mm/s, 2 hours)

1000 2000 3000 4000 5000 6000 7000 8000

0.00

0.05

0.10

0.15

0.20

Frictio

n C

oe

ffic

ien

t [µ

]

Friction time [s]

Pure water @20 MPa

Pure water @10 MPa

20% PVP @20 MPa

20% PVP @10 MPa

21

The specific wear rates of the pure UHMWPE measured after friction test in lubrication with aqueous

solutions of PVP at 0%, 6%, 15% and 20% w/w concentrations can be found in Figure 17. The

corresponding specific wear rates for aqueous PVP solutions are analogous with the friction trend. The

wear rate is observed to reduce with the increasing concentration, the lowest wear rate value attained is

at 20% w/w PVP at both high and low load conditions.

The inclusion of PVP molecules in the lubricant sample shows a significant reduction in wear at 6% w/w

concentration as compared to pure water. Further increase in concentration exhibited slightly reduced

wear, however, it should be noted that the rate of reduction is relatively smaller from 6% reaching up to

20% w/w PVP concentration with large standard deviations. Approximately, 15% wear reduction is

observed with 6% w/w PVP solution compared to pure water with 20 MPa contact pressure, while only

about 6% wear reduction is recorded when the concentration is increased from 15% to 20% w/w. An

identical trend can be seen with 10 MPa contact pressure although the reduction of wear is more

prominent with a high load condition at 20 MPa. This suggests that the inclusion of PVP molecules in

pure base water significantly increased the wear-reducing properties of pure water, while a further

increase to higher concentrations of PVP might not facilitate the wear reduction in the same manner.

It is interesting to notice that the overall tendency for both friction and wear behavior with aqueous

solutions of PVP is found to have a drastically decreasing pattern with increasing concentrations. The

rate of reduction is then subsequently reduced and both friction and wear plots almost taper off. The

reason for such behavior is due to the adherence of PVP molecules at the SS counter plate which accounts

for the reduction of friction and successively wear, up to a certain concentration. Theoretically, once the

amount of PVP molecules on the surface reaches saturation, the increased concentration would not aid

in friction reduction or influence the friction and wear properties whatsoever, as there would be much

lesser space available for additional PVP molecules [60].

b) Pure UHMWPE with aqueous solutions of PSS

The aqueous solutions of PSS at different concentrations of 0%, 3%, 6% and 9% [w/w] were taken for

the first test sequence at high load conditions under TE77 pin-on-plate tribometer for the lubrication of

pure UHMWPE and SS counter plate friction pair. The inclusion of 3% w/w PSS concentration revealed

a huge increase in the coefficient of friction, which was not expected.

Table 5 Test sequence 1 PSS (20 MPa contact pressure)

PSS Concentration [w/w] Coefficient of friction [µ]

0% 0.156

3% 0.262

6% 0.239

9% 0.179

The average coefficient of friction of 0.156 with pure water increased to 0.262 at 3% w/w concentration,

only to decrease gradually with further increase in concentration. However, the friction coefficients at

3%, 6%, and 9%, all showed higher values than with pure water, Table 6. To further examine the

probable reason for this rise in COF, other solutions of PSS at 15% and 20% w/w concentrations were

prepared. The second test sequence was carried out with 9%, 15%, and 20% PSS to perform comparative

analysis at both high and low load conditions and the averaged friction coefficients are presented with

standard deviation error.

Figure 18 depicts the dependence of coefficients of friction on the concentrations of PSS at 20 MPa and

10 MPa, keeping other operating parameters constant. A parallel trend of friction coefficient can be

clearly seen at both high and low load conditions. Aqueous PSS solutions at 9% concentration showed a

higher friction coefficient, which was seen decreasing with further increase in concentrations with 20%

w/w yielding the lowest friction coefficient in the test sequence. This was true for both high and low

load conditions.

22

0.156

0.179

0.152

0.128

0% 9% 15% 20%

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20F

riction C

oeffic

ient [µ

]

Concentration [% w/w]

a) 20 MPa

0.104

0.152

0.103

0.087

0% 9% 15% 20%

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Friction C

oeffic

ient [µ

]Concentration [% w/w]

b) 10 MPa

Figure 18 Dependence of friction coefficient of UHMWPE on the concentration of PSS

(Pin-on-plate tests, 20 mm/s, 2 hours)

7.34

11.9

8.188.41

0% 9% 15% 20%

0

2

4

6

8

10

12

14

Spe

cific

wea

r ra

te [1

0-5

mm

3/N

.m]

Concentration [% w/w]

a) 20 MPa

7.29

8.83

6.15.28

0% 9% 15% 20%

0

2

4

6

8

10

12

14

Specific

wear

rate

[10

-5 m

m3/N

.m]

Concentration [% w/w]

b) 10 MPa

Figure 19 Dependence of specific wear rates of UHMWPE pins on the concentration of PSS

(Pin-on-plate tests, 20 mm/s, 2 hours)

The specific wear rates of the pure UHMWPE with aqueous solutions of PSS at 0%, 9%, 15%, and 20%

w/w concentrations can be found in Figure 19. The wear rates showed an increasing pattern with the

inclusion of PSS up to 9% w/w concentration, identical to the friction behavior. Under high load

conditions, UHMWPE pins experienced over 50% increased wear rates as compared to pure water when

the PSS concentration was 9%. The wear rates decreased with further increment of PSS concentration

but remained higher than with pure water. These results suggest that the inclusion of PSS with pure

water as a base does not necessarily improve the tribological performances. In a study performed by

Wang et al. [51] on PSS as an additive for water-glycol hydraulic fluid as base, improved friction and

wear performances were reported, however, PSS with pure water as base studied in this work resulted

in higher wear rates than with pure water with high contact pressure of 20 MPa.

23

This shows that although the friction coefficient reduced at higher PSS concentrations, wear resistance

did not achieve any improvement. PSS concentration at 20% under 20 MPa contact pressure showed

the lowest friction coefficient among all, however, the pin wear was enhanced with the presence of PSS

molecules in the lubricant sample. On the other hand, friction pair under low load conditions exhibited

lower wear rates at 15% and 20% w/w concentrations of PSS. The friction coefficients, as well as specific

wear rates, can be observed to be lowest with 20% w/w PSS concentration, with significant wear

reduction by over 25% as compared to pure water, 0.087 COF corresponding to specific wear rate of

5.28e-05 mm3/Nm under 10 MPa contact pressure.

Figure 20 depicts the variation of friction coefficient as a function of friction time for pure UHMWPE

lubricated with aqueous solutions of PSS at 20% concentration compared with pure water for both high

and low load conditions. The friction curves achieved stabilization after a short time in the presence of

PSS solutions. Friction coefficients corresponding to the time for PSS solutions show a trend alike to

that with PVP solutions, as seen from Figures 16 and 20.

Figure 20 Variation of friction coefficient of UHMWPE on friction time for 20% PSS

(Pin-on-plate tests, 20 mm/s, 2 hours)

All the lubricant samples at different concentrations of PVP and PSS generated similar friction-time

curves with a tendency to steadily decline during the running-in phase and stabilizing over the test time.

Pure water exhibited an increasing friction-time curve with fluctuating peaks whilst modified water

solutions generated smoother and stable curves. This indicates that the base water is not capable of

forming a stable boundary lubrication film. The limited friction reduction from higher load to lower

load with pure water can be attributed to the friction-reducing properties of the pure UHMWPE bearing

material being more significant under low load conditions owing to its good mechanical and impact

strength, highly wear-resistant and self-lubricating behavior.

The trend of the friction-time curve was identical for lower concentrations of PSS as well, however, the

average COF values were higher than with pure water. It can be further noted from Figure 19 that the

wear rates are also higher with most of the PSS sample solutions. This suggests that even though the

aqueous solutions of PSS could form a stable lubrication film as compared to pure water, the wear

mechanisms might get enhanced due to the adsorption of PSS molecules to the friction surfaces and thus

lead to increased friction and consequently wear rates.

1000 2000 3000 4000 5000 6000 7000 8000

0.00

0.05

0.10

0.15

0.20

Friction C

oeffic

ient [µ

]

Friction time [s]

Pure water @20 MPa

Pure water @10 MPa

20% PSS @20 MPa

20% PSS @10 MPa

24

c) SCF reinforced UHMWPE with aqueous solutions of PVP

The second phase of this study was carried out with the short carbon fiber reinforced UHMWPE. 10%

SCF reinforced UHMWPE was chosen as the bearing material in contact with the SS2333 grade stainless

steel counter plate during the tribo test. All other operating parameters were kept constant, with two

load conditions corresponding to 20 MPa and 10 MPa contact pressures.

Figure 21 presents the plot of coefficients of friction acquired with the SCF reinforced UHMWPE versus

concentrations of PVP. The aqueous solutions of PVP showed reducing friction coefficients between

the SCF reinforced UHMWPE pin and SS countersurface friction pair under a contact pressure of 20

MPa. The observed average coefficients of friction exhibit the trend within the region of standard

deviation. The reduction is gradual and slow, COF of 0.116 with pure water reaching 0.105 at 20%

w/w PVP concentration. The lower load conditions, on the other hand, showed slightly increased

friction at first which further decreased on the increment of PVP concentrations. PVP solutions at 15%

w/w concentration under lower load conditions showed a reduction in friction by 25% as compared to

pure water.

0.116

0.1130.11 0.105

0% 6% 15% 20%0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Friction C

oeffic

ient [µ

]

Concentration [% w/w]

a) 20 MPa

0.1 0.106

0.075

0.092

0% 6% 15% 20%0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14F

riction C

oeffic

ient [µ

]

Concentration [% w/w]

b) 10 MPa

Figure 21 Dependence of friction coefficient of SCF reinforced UHMWPE on the concentration of PVP

(Pin-on-plate tests, 20 mm/s, 2 hours)

SCF reinforced UHMWPE also showed reduced specific wear rates in the presence of PVP molecules,

analogous to that of pure UHMWPE. The specific wear rates of SCF reinforced UHMWPE bearing

materials with increasing PVP concentrations can be found in Figure 22. In comparison with pure water,

20% w/w PVP concentration showed an almost 25% reduction in wear when loaded with 20 MPa

contact pressure. The lower load conditions at 10 MPa experienced lowest wear rates with PVP solutions

at 15% w/w concentration, with a reduction up to 46% as compared with pure water.

It can be observed from friction and wear performances with 10% SCF reinforced UHMWPE bearing

materials that the optimum concentration of PVP for high load applications is at 20% w/w, while that

for lower loads, is at 15% w/w concentration. When the load is increasing, the inclusion amount of PVP

should also be further increased. Furthermore, it is also evident from Figures 15 and 17 compared with

Figures 21 and 22, that SCF reinforced UHMWPE exhibited considerably lower friction coefficients

and improved wear resistance than the virgin UHMWPE. This clearly shows that the inclusion of SCFs

with pure UHMWPE significantly improves the tribological properties of bearing materials, as expected.

25

6.79

6.22

6.72

5.12

0% 6% 15% 20%

0

2

4

6

8

Sp

ecific

we

ar

rate

[1

0-5

mm

3/N

.m]

Concentration [% w/w]

a) 20 MPa

5.44

4.48

2.92

4.26

0% 6% 15% 20%0

2

4

6

8

Sp

ecific

we

ar

rate

[1

0-5

mm

3/N

.m]

Concentration [% w/w]

b) 10 MPa

Figure 22 Dependence of specific wear rates of SCF reinforced UHMWPE on the concentration of PVP

(Pin-on-plate tests, 20 mm/s, 2 hours)

d) SCF reinforced UHMWPE with aqueous solutions of PSS

The aqueous solutions of PSS exhibited a decreasing coefficient of friction trend with the inclusion of

PSS. SCF reinforced UHMWPE pins experienced the lowest friction with aqueous solutions of PSS at

15% w/w concentrations during both high and low load conditions. The average coefficient of friction

of 0.116 for pure water reached 0.084 under 20 MPa contact pressure, see Figure 23.a), and COF 0.10

reduced to 0.057 under 10 MPa, Figure 23.b). When the PSS concentration was further increased to

20%, the solution resulted in friction higher than with pure water.

0.116

0.094

0.084

0.136

0% 9% 15% 20%0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Frictio

n C

oe

ffic

ien

t [µ

]

Concentration [% w/w]

a) 20 MPa

0.1

0.0650.057

0.121

0% 9% 15% 20%0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Frictio

n C

oe

ffic

ien

t [µ

]

Concentration [% w/w]

b) 10 MPa

Figure 23 Dependence of friction coefficient of SCF reinforced UHMWPE on the concentration of PSS

(Pin-on-plate tests, 20 mm/s, 2 hours)

26

The specific wear rates obtained for SCF reinforced UHMWPE pins showed fluctuating patterns under

both higher and lower contact pressures, see Figure 24. The wear rates first increased up to 9% w/w PSS

concentration, which slightly reduced at 15% concentration, and subsequently increasing again with

concentration reaching 20%. This phenomenon stands true for both high and low load conditions,

exhibiting higher wear rates of SCF reinforced UHMWPE in the presence of PSS molecules with pure

water. On a similar note to friction behavior, lowest wear rates obtained were with 15% w/w

concentration among the PSS aqueous solutions studied, however, exhibited poor wear resistant behavior

as compared to the pure water lubrication. In addition, the friction-time curves with SCF reinforced

UHMWPE also exhibited patterns alike pure UHMWPE lubricated with both PVP and PSS aqueous

solutions, analogous to Figures 16 and 20.

6.79

10.8

6.43

8.81

0% 9% 15% 20%0

2

4

6

8

10

12

Sp

ecific

we

ar

rate

[1

0-5

mm

3/N

.m]

Concentration [% w/w]

a) 20 MPa

5.446.22

5.84

7.28

0% 9% 15% 20%0

2

4

6

8

10

12

Spe

cific

wea

r ra

te [10

-5 m

m3/N

.m]

Concentration [% w/w]

b) 10 MPa

Figure 24 Dependence of specific wear rates of SCF reinforced UHMWPE on the concentration of PSS

(Pin-on-plate tests, 20 mm/s, 2 hours)

27

3.2.2 SEM Analysis

Polymer pins and SS countersurface were analyzed under a scanning electron microscope (SEM) after

the friction and wear tests. SEM images aided to understand the friction pair interaction and probable

wear mechanisms. All the tests were carried out on the polymer pins under high load conditions of 20

MPa contact pressures and 20 mm/s constant sliding speed. The red-colored double arrowed line shows

the direction of polymer pin sliding over the SS counter surface.

Figure 25 presents the images generated with SEM on the UHMWPE pins lubricated with an aqueous

solution of PVP at 20% w/w concentration after testing for 2 hours. The worn pin surface shows wear

tracks scratched along the sliding direction. As the pin was repetitively loaded and unloaded with a high

contact pressure for 2 hours, the surface also experienced deformation, as seen in Figure 25.B).

Figure 25 Worn UHMWPE pin surfaces (20% PVP, 20 MPa, 2 hours)

A) Wear tracks, Scale 100 µm B) Deformation, Scale 20 µm

Figure 26 Worn UHMWPE pin surfaces (20% PVP, 20 MPa, 20 hours)

A) Scale 100 µm B) Worn-out flakes, Scale 50 µm

Separate wear tests for a longer test duration of 20 hours were carried out for better representation of

wear behavior and compare with 2 hours, keeping all the operating parameters constant. Figure 26 shows

the images obtained from pure UHMWPE pins tested for 20 hours. On the contrary to pins loaded for

A B

A B

28

2 hours, the pin surface loaded for 20 hours shows less visible wear tracks, as noted from Figure 26.A),

due to continuous loading leading to smoothening of the surface. The worn-out flakes are more

prominent with 20 hours wear-tested pin surfaces, Figure 26.B). This is the result of micro-cutting of

the polymer surface causing fragmentation, suggesting to abrasive wear being the dominant wear

mechanism.

The pure UHMWPE pins lubricated with 20% w/w PSS for 2 hours is presented in Figure 27. The

wear tracks are not visible, owing to the lower friction coefficients. However, the fragments of wear

debris on the polymer surface can be observed from SEM images, see Figure 27.B), which could result

in excessive wear.

Figure 27 Worn UHMWPE pin surfaces (20% PSS, 20 MPa, 2 hours)

A) Scale 50 µm B) Scale 100 µm

Figure 28 Worn UHMWPE pin surfaces (20% PSS, 20 MPa, 20 hours)

A) Scale 100 µm B) Scale 10 µm

Worn UHMWPE pin surfaces with 20% w/w PSS tested for 20 hours, on the other hand, shows deep

grooved wear tracks and the attachment of wear debris on the surface is more pronounced, Figure 28.

The chunk of worn flakes gets adhered to the friction pairs due to repetitive loading. The adsorption of

PSS molecules on the surface increases the friction and indirectly can increase the wear. The increased

friction and wear rates observed in the presence of PSS can be attributed to the enhanced wear, where

the further loss of material occurs by peeling from the mating surface. The inclusion of PSS molecules

A

A

B

B

29

on the base water results in higher wear, as the wear is primarily governed by the adhesion mechanism

in the long run.

SEM images of the SCF reinforced UHMWPE pin surfaces lubricated under 320 N load, 20 mm/s

sliding speed and 2 hours test duration with PVP and PSS aqueous solutions were analyzed. Figure 29

depicts the images of pin surface lubricated with 20% w/w PVP. The degrading carbon fibers due to

continuous loading is distinguishable. The fibers are starting to wear out, but the polymer surface is

comparatively less affected than the pure UHMWPE pin surfaces.

Figure 29 Worn SCF reinforced UHMWPE pin surfaces (20% PVP, 20 MPa, 2 hours)

A) Scale 100 µm B) Scale 50 µm

Figure 30 Worn SCF reinforced UHMWPE pin surfaces (20% PSS, 20 MPa, 2 hours)

A) Scale 100 µm B) Scale 50 µm

As the major load is carried by the exposed short carbon fibers, they will wear out first due to continuous

rubbing before the polymer matrix experiences wear. The wear particles adhered to the pin surface in

the presence of PSS with water can be identified while looking at Figure 30. This further supports the

lower friction and wear results obtained earlier on SCF reinforced UHMWPE bearing materials

lubricated with PVP aqueous solutions as compared to the PSS solutions.

A

A

B

B

30

The material transfer on the counter surface could not be seen while analyzing the SS counter surface

after tribological tests, Figure 31. The transfer film formation is relatively difficult to achieve in the

presence of water-based lubricants. The interaction of the UHMWPE polymer pin on the SS

countersurface lubricated with the PVP aqueous solution (Figure 31.A) showed lesser surface damage as

compared to the PSS solution (Figure 31.B). The information obtained after the analysis of SEM images

is in line with the friction and wear results acquired from the pin-on-plate tests.

Figure 31 SS counter surface plate after sliding lubricated with A) 20% PVP and B) 20% PSS

(20 MPa, 20 mm/s, 20 hours, Scale 100 µm)

B A

31

3.3 Contact angle measurements

From the tribological characterization of newly developed modified water lubricants, sample lubricants

with only polyvinylpyrrolidone (PVP) exhibited satisfying friction and wear results which were further

confirmed by the SEM analysis, as seen from the previous sections. To further validate the newly

developed modified water-based lubricant sample, a surface analysis could provide some supporting

information. To investigate the effect of wettability on friction and wear behavior, the wetting ability of

the lubricant sample with PVP was evaluated by the contact angle measurements. Figure 32 shows the

contact angles made by the lubricant samples at different concentrations on the unmodified surface of

pure UHMWPE, SCF reinforced UHMWPE bearing materials, and SS countersurface.

0% 3% 6% 9% 15% 20%50

55

60

65

70

75

80

85

90

95

100

105

Co

nta

ct ang

le [degre

e]

PVP Concentration [% w/w]

SS

UHMWPE

10%SCF+UHMWPE

Figure 32 Dependence of contact angles on the concentration of PVP

Pure base water showed more hydrophobic nature with SS countersurface than with the polymer

surfaces. The inclusion of PVP with base water, however, changed the scenario with larger contact angles

on polymer surfaces than on the SS countersurface. Overall, lower wetting angles on all surfaces were

observed with PVP lubricants relative to distilled water. In addition, the differences in contact angles

made by the lubricant on the pure UHMWPE and SCF reinforced UHMWPE are small. The increment

on PVP concentration increases the phobic nature of the sample lubricants, and thereby reduced wetting

ability. This can be correlated with the results from tribological tests where improved friction and wear

behavior were recorded with the increasing concentrations of PVP molecules on the base water for virgin

UHMWPE as well as SCF reinforced UHMWPE. Reduced wetting ability of the lubricant with the

increasing concentrations of PVP corresponds to the stronger cohesive forces between the lubricant

molecules with each other than with the bearing surfaces suggesting to the ability to lubricant film

formation.

32

4 Conclusion

The presented work is a tribological study of newly developed environmentally adaptive lubricants

(EALs) for hydropower turbine applications with polymer bearing materials. Two thickening agents,

poly(sodium 4-styrenesulfonate) (PSS) and polyvinylpyrrolidone (PVP), with pure water as a base fluid

were investigated. The following conclusions can be drawn after investigating the generated results.

• It is clear from the results that both PVP and PSS effectively increase the viscosity of pure water

used as a lubricant base and was observed to easily dissolve in water to form transparent solutions.

Thus, both thickeners can be used as an effective viscosity modifier.

• PVP exhibited excellent friction-reducing and anti-wear performances versus pure water.

Approximately, 25% reduction on both friction coefficients and specific wear rates for pure

UHMWPE bearing specimen was observed.

• SCF (10%) reinforced UHMWPE showed a significant improvement in friction reduction and

exhibited excellent wear resistance, over 25% wear reduction under high load conditions, and

over 45% wear reduction under lower load conditions with PVP aqueous solutions.

• PSS showed a slightly reduced friction coefficient but increased wear rates of polymer pins by

10% to over 50% at high contact pressure. SEM analysis on worn pin surfaces further revealed

induced adhesion wear in the presence of PSS molecules. Wear enhancement, as observed from

the presented study, makes PSS a poor choice as a thickener for pure water under high load

conditions.

• PSS at the lower load of 10 MPa, however, exhibited a significant reduction in friction and wear

rates at 15% concentration and higher. Thus, PSS can be further studied as a potential additive

for low load applications.

• PVP is recommended as a suitable candidate due to its excellent viscosity enhancing, friction and

wear-reducing properties for further investigation at both higher and lower contact pressure

conditions towards the development of ‘green lubricant’.

33

5 Recommendations

In reference to the concluding remarks made in the previous section, it is important to state that the

presented work still has some limitations. Some recommendations can be made to further improve the

work and strengthen the conclusions presented in this study.

• The friction tests carried out were for 2 hours, further investigation with longer test durations

can be performed to study the aging and life expectancy of lubricants.

• Further analysis of the surface activity of lubricant with PVP molecules will shed more light on

the surface properties of the lubricant.

• An extensive study on UHMWPE with additional fillers and nano reinforcements as bearing

materials lubricated with aqueous PVP can be performed.

• The tests were carried out at laboratory conditions, study with real-life conditions prone to

contamination is recommended to evaluate the performance of lubricants in extreme conditions

and to achieve more understanding over the lubricant performance.

• Poly(sodium 4-styrenesulfonate) (PSS) showed a significant reduction in friction and wear at

lower load conditions, a detailed study on PSS, specifically for low load applications is suggested

to further evaluate the influence of PSS on water-based lubrication under low load conditions.

34

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