Tribology International 55 (2012) 59–67

Contents lists available at SciVerse ScienceDirect

Tribology International

0301-67

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

The ice friction of polymeric substrates

Christos Stamboulides, Peter Englezos, Savvas G. Hatzikiriakos n

Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z3

a r t i c l e i n f o

Article history:

Received 19 January 2012

Received in revised form

22 April 2012

Accepted 1 May 2012Available online 5 June 2012

Keywords:

Sliding friction

Friction measurement

Frictional heating

Surface wetting

9X/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.triboint.2012.05.001

esponding author.

ail address: savvas.hatzi@ubc.ca (S.G. Hatzikir

a b s t r a c t

A fixture has been designed and utilized in conjunction with a conventional rotational rheometer to

measure and study the mechanisms of friction of polymeric surfaces (ultra-high molecular weight

polyethylene, polytetrafluoroethylene and polymethylmethacrylate) on ice. Ice friction experiments

were performed at various sliding velocities (from 0.0079 to 1.96 m/s), temperatures (from �25 1C

to �1.5 1C), and various degrees of surface roughness. It was found that the magnitude of the sliding

velocity and temperature play important roles in ice friction. Friction coefficient decreases with

increasing temperature at high sliding velocities (from 0.79 to 1.96 m/s) due to surface lubrication.

However, at lower sliding velocities (from 0.0079 to 0.079 m/s) the friction coefficient initially

decreases and then increases due to the modification of the real contact area on the ice surface. The

ice friction coefficient was found to be nearly independent of relatively small differences in surface

roughness. Finally, the ice friction coefficient of three polymer substrates exhibiting very different static

contact angles, from hydrophilic to hydrophobic, was found to scale inversely with their water static

contact angle.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Interest in snow and ice friction comes from the need tounderstand and control phenomena of practical importance suchas glacier and avalanche movement, traction of automobile tires,and snow and ice sports [1–6]. Sports are now an indispensablepart of our daily lives and the world economy and there is astrong industrial interest that promotes research and develop-ment to advance technology. One area of interest is to control andminimize friction. Sliding over snow is more complicated thansliding over ice because of the large variety of possible snowconditions and due to the fact that snow surfaces exhibit manycomplexities. It becomes very difficult to reproduce and/orharvest snow of the same condition and properties and toperform a reliable experimental study on snow friction [6].Therefore, this study is limited to the investigation of themechanisms of friction of polymeric surfaces on ice, althoughrelevant literature of snow friction is discussed below.

The sliding process of a solid surface on ice may induce a phasechange on the water (from solid to liquid) mainly by frictionalheating; that is dissipation of mechanical into thermal energy[7–12]. It has been shown by many researchers [8–32] that thecoefficient of friction (COF) of materials sliding over ice (andsnow) should follow the following trend as represented in Fig. 1

ll rights reserved.

iakos).

and on the right-hand side of ‘‘point A’’. Colbeck [9] consideredthe friction phenomena and expressed the total coefficient offriction (COF) as a function of the frictional contributions due tothe solid deformation (dry friction), melt-water film (wet friction),and capillary drag effects with all three contributions to occursimultaneously in many cases. As the sliding velocity increases,frictional heating increases and thus the thickness of the meltwater film increases. It has generally been accepted that the COFdecreases with increase of sliding velocity in the dry-frictionregime (regime I after ‘‘point A’’) and the initial dry friction isfollowed by mixed dry and wet friction, where the frictionalresistance reaches a minimum (regime II in Fig. 1) [22,33]. Afterthis minimum point it starts increasing again (regime III in Fig. 1).These three frictional regimes are referred to as dry-to-boundary,mixed, and hydrodynamic friction regimes respectively. In the dry-

to-boundary friction, a lubricating layer exists; however, thethickness is much less than the height of the asperities of thetwo surfaces in contact. The very thin lubricating layer betweenthe two surfaces in contact is either a few nanometers thick[34,35] or even has smaller dimensions comparable to molecularlevels [19,33,36,37]. In the mixed friction (or mixed lubrication)

regime the thickness of the lubricating film is still less than thecharacteristic roughness of the surfaces but it keeps increasing,which decreases the solid-to-solid contact, thus enhancing sur-face lubrication. At the same time, a wetting lubricant induces thebuild-up of capillary water bridges between the asperities. In thisregime, the load of the slider is partly supported by the surfaceasperities and partly by the lubricating layer. The resistance from

A

Coe

ffici

ent o

f fric

tion

Sliding velocity

Dry / BoundaryFriction

Mixed friction /Mixed Lubrication

Hydrodynamic friction /Capillary drag

Regime I Regime II Regime III

Substantialmelt-watercreation

A

Fig. 1. Sketch of the effect of sliding velocity on the ice coefficient of friction

(COF).

C. Stamboulides et al. / Tribology International 55 (2012) 59–6760

solid-to-solid contact decreases, however, resistance increasesdue to capillary drag. The contributions of these two resistingmechanisms lead to a minimum value of the frictional force,which is the characteristic of this regime [10,13,17,33]. In thehydrodynamic regime (regime III in Fig. 1), the thickness of themelt-water film is greater than the average height of the surfaceasperities; it overlays the surface and becomes capable of carryingthe applied load and the phenomenon that dominates is thecapillary drag and viscous shearing. There is no more solid-to-solid contact and the ‘‘breakage’’ and shearing of the adhesivebonds between the two surfaces in contact does not contribute tothe resisting force.

Friction between a sliding surface and ice is influenced byseveral parameters including the type of ice, ambient conditions(moisture and temperature), sliding base properties, normal loadand sliding velocity [7,10,11,29,37]. Bowden and Hughes [7]performed friction tests on sliding surfaces on snow and ice andconcluded that if the sliding speed is low, the kinetic friction isvery similar to the static friction and as the sliding speedincreases there is a significant drop in friction due to the waterfilm formed locally. On the other hand, Spring [29] as well asBaurle et al. [30] reported that the COF increases with increasingspeed and at temperatures close to the melting point of ice; thiswas attributed to the increase of the lubricating layer between thetwo surfaces in contact. Oksanen and Keinonen [2] have shownthat the kinetic coefficient of friction decreases with increasingspeed at cold snow/ice temperature (�15 1C) and increases withincreasing speed at relatively warm temperatures (�1 1C). Thisimplies that increasing friction melting may be beneficial tosliding at low temperatures (lower than �5 1C) but not at higherones (4�5 1C). It is safe to conclude that for the same snow/icetype at constant temperature and constant normal stress, frictionis strongly dependent on velocity [28,30]. A minimum COFvalue of about 0.02 for polyethylene sliding on ice was foundwhen the temperature is around �3 1C [12]. Further temperatureincrease to 0 1C, resulted into an increase of the COF. In summary,at low temperatures, below �3 1C, an increase in temperatureleads to a decrease of the COF and on the other hand fortemperatures above �3 1C, an increase in temperature increaseleads to a higher COF values. This was attributed to the increasedamount of melt water that in turn leads to larger real contactareas and capillary bridges [29].

While several papers have been published on ice friction incontact with metallic, ice and polymeric surfaces, a systematicstudy that covers a wide range of sliding speeds and temperaturedoes not exist. Moreover, a study for ice friction with surfaces of

various levels of hydrophobicity/hydrophilicity and levels ofroughness is needed to address the effects of surface energy.The main focus of this present work was first to design a reliableexperimental fixture for studying the friction of various polymericsurfaces on ice in order to study the various mechanisms under-lining the friction phenomena. The effects of sliding friction,temperature, surface roughness and type of polymeric material(of various hydrophobic levels) on ice friction are also examinedand discussed in detail. The experimental findings are discussedin context of previously published results in the literature and itappears that new insights on the mechanisms of ice friction aregained.

The main hypothesis in this current work is that the COFincreases until a point (‘‘point A’’ in Fig. 1) where the amount ofwater produced on the interface due to frictional heating issignificant enough to induce surface lubrication. Hence, theremust always be a ‘‘dry region’’ at the initiation of ice frictionexperiments or a region where the amount of water produced dueto frictional heating is not enough to fully ‘‘lubricate’’ the slidingsurfaces and therefore reduce friction. This ‘‘dry region’’ has beeninvestigated by different research groups that studied the fric-tional behavior of different materials on ice under dry condi-tions [38–40]. The main observation is that friction increaseswith increase of sliding velocity at ‘‘cold and dry’’ conditions[5,23,41–44]. Furthermore, another interesting observation is thatthere is an initial increase in the friction force due to the increasein real contact area during sliding, and this does not necessarilytake place in ‘‘cold and dry’’ conditions [28]. In other words,initially the motion of the slider causes the ‘‘shearing’’ of theice asperities until they become of uniform height. Furthermore,at this transition ‘‘point A’’ in Fig. 1, the slider velocity reachesits critical point for the onset of lubricated friction [42–44]. Priorto point A, there is no enough heat being produced from friction,or the temperature does not allow for the formation of melt water[28].

2. Experimental

2.1. Equipment

Different experimental set-ups have been utilized by variousresearchers [7–9,12,14,23,25,32] to determine the friction coeffi-cient of various surfaces sliding over ice. They are either set-upson-the-field (mountain slopes or ice rinks) [5,9,13,23], or set-upsin laboratory environments such as pin-on-disk, four-ball, paral-lel-plate (translating or rotational) and block-on-ring [8,12,21,25,27,28,31,32]. Obviously, laboratory set-ups offer better controlof the parameters influencing friction, namely sliding velocity,normal load, temperature and moisture.

In this work, a standard conventional rotational rheometer(Physica MCR 501) was used and conveniently converted into atribometer by designing a new fixture, essentially a modifiedparallel-plate geometry in rotational mode (Fig. 2a). This modifiedgeometry is enclosed in a temperature controlled chamber thatcan reproduce temperatures as low as �150 1C through the use ofa liquid nitrogen evaporator unit. The equipment’s torque resolu-tion is 0.001 mNm, the normal force resolution is 0.002 N and thetemperature control is 70.2 1C. The rotating upper plate from theparallel plate setup is replaced by a ring slider sample that isattached to a sample holder, and the stationary lower plate isreplaced by the ice dish (Fig. 2a). To account for non-perfectparallelism between the two surfaces, the ice dish holder ismounted via a miniature ball coupling and a compressionspring to the rigid lower shaft as shown in Fig. 2b. All resultsreported here were obtained with a fixed normal force of 3 N.

ω (t)

ice

Sliding basesampleR

Fig. 2. (a) Schematic of the parallel plate geometry used for the friction measurements. (b) A picture of the dish holder that instantaneously keeps parallel the two surfaces

under sliding (lower plate).

NN

average

N

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

mN

m

M

-1

0

0.5

1

1.5

2

2.5

3

3.5

4N

0 10 20 30 40 50 60 70 80s

Time t

Rheoplus

Anton Paar GmbH

NormalLoad (FN)

Torque(FT)

-0.5

Fig. 3. A typical frictional experimental measurement produced by the MCR 501, showing that the excellent control of the normal load and the measured torque M.

C. Stamboulides et al. / Tribology International 55 (2012) 59–67 61

The instrument recorded the torque FT, which was converted intothe coefficient of friction m using the following equation:

m¼ FT

r � FNð1Þ

where r is the average radius of the slider ring and FN is theapplied normal force. Since the operation controls the normalforce and measures the tangential force, the calculation of the COFthrough Eq. (1) is independent of the area of contact.

To limit the variability of experimental results each ice surfacewas used for only two runs, before it was polished again using thecommercial drill press as describe below. At each temperature,runs with different linear speeds were randomized to average outchanges in the slider sample’s surface over time. In the end, atleast 10 runs were carried out for each sliding velocity. Typicalresults from a friction measurement run is plotted in Fig. 3.Results reported in this work were obtained for a standard normalforce of 3 N, and a slider ring having and outer diameter of 25 mmand an inner of 21 mm ðr¼ 23 mmÞ. This translates into an‘‘apparent’’ surface pressure of 21 kPa, which assumes a perfectcontact. However as explained above the COF calculations

through Eq. (1) are independent of the actual area of contact ofthe slider.

2.2. Materials

2.2.1. Polymeric surfaces

Ultra-high molecular weight polyethylene (UHMWPE) is themain polymer used in this study. UHMWPE is a linear polymerwith a high molecular weight with several unique propertiesincluding superior abrasion resistance, impact strength and a lowcoefficient of friction (COF). Compression molding was used inthis study to prepare the testing samples. Additionally, two morepolymers, a hydrophilic polymethylmethacrylate (PMMA) and ahydrophobic polytetrafluoroethylene (PTFE) were studied. Themain goal was to investigate the effect of surface energy (staticcontact angle) of different polymeric substrates on the COF withice. Although the advancing and receding contact angles of thevarious substrates are more relevant in assessing the exactwettability of the various substrates, it is sufficient in this studyto report only the static contact angle in simply classifying thehydrophilic/hydrophobic nature of the three polymeric surfaces

C. Stamboulides et al. / Tribology International 55 (2012) 59–6762

studied. The basic properties of UHMWPE, PMMA, and UHMWPEare listed in Table 1.

2.2.2. Polymer surfaces preparation and characterization

The prepared polymeric surfaces were polished by usingdifferent grit sandpaper. Grits of 420, 600 and 1200 were used.The average roughness and static contact angle of all surfaces wasmeasured prior to any friction experiment to ensure that allsurfaces under study exhibit similar surface properties. A Scan-ning Electron Microscope (Hitachi S-4700 FESEM, Tokyo, Japan)was used for analyzing and evaluating the modification/deforma-tion of sliding base sample surfaces by comparing the surfacesbefore and after friction experiments, and before and after varioussurface treatments. Non-contacting profilometer using light inter-ferometry (WYKO, from Veeko instruments, NY, USA) with com-puterized XY table capable of measuring surface heights fromsub-nm to mm-range was used for the surface characterization(roughness measurements). It has a vertical and spatial resolutionof 0.1 nm and 40 nm respectively and capabilities of measuringheight asperities up to 2 mm.

Contact angle, y (Fig. 4), measurements were conducted byusing the sessile drop technique and they were performed atroom temperature (25 1C). The contact angle of water, y, of allsubstrates with distilled and de-ionized water was measuredusing images obtained with a high resolution camera-NikonD80, Digital SLR Camera. The camera was attached to a SigmaAF-MF zoom lens (105 mm, F2.8 EX DG Macro) and a Kenkoextension tube set for better image magnification and resolution.The FTA32 software from First Ten Angstroms was used toanalyze the images and calculate the static contact angles of thevarious surfaces under study. The volume of the water dropletwas maintained constant for all substrates, 1 mL, in order toprevent possible changes in the droplet due to gravity effects.

Table 1Polymers used in the friction experiments and some physical properties.

UHMWPE PMMA PTFE

Density (g/cm3) at 25 1C 0.93 1.18 2.20

Thermal conductivity (W/m K) 0.40–0.50 0.17–0.25 0.25

Static contact angle (deg.) 90 65 118

Sliding velocity (m.s-1)

0.001 0.01 0.1 1 10

Coe

ffici

ent o

f fric

tion

0.04

0.06

0.08

0.10

0.12

-1.5° C-4° C-7° C-10° C-15° C-25° C

UHMWPE

Fig. 4. Effect of sliding velocity on the coefficient of friction (COF) for UHMWPE

substrates sliding on ice at different temperatures.

2.2.3. Ice surfaces preparation

The preparation of reproducible ice surfaces to obtain consis-tent and reproducible experimental results was one of thechallenges of this work. Small bumps and imperfections on theice surface can lead to a great variability of the experimental data.To obtain similar ice surfaces the following experimental protocolwas followed. Distilled and deionized water was used in thepreparation of ice. To minimize bubble formation in the ice, thewater was boiled to minimize the dissolved gas content. About6 ml of water were poured into dishes of 40 mm diameter andfrozen in a commercial freezer unit. The obtained ice surfaceswere smoothed by using a commercial drill with a smooth high-density polyethylene at a rotational speed of about 1500 rpmat �4 1C.

3. Results and discussion

3.1. The effect of sliding velocity on ice friction

Fig. 4 depicts the ice frictional curves with UHMWPE asfunctions of the sliding velocity at different temperatures from�25 1C to �1.5 1C. Each data point represents the average of atleast ten measurements and the error bars in the graphs representthe corresponding 95% confidence interval. It should also be notedthat all ring polymeric test specimens under study had an averagesurface roughness of about 760 nm with a cross sectional surfacepattern (more details are presented in Sections 3.3 and 3.4). Thecoefficient of friction at �1.5 1C and �4 1C initially increases andreaches a plateau with a tendency to decrease at higher slidingvelocities. This behavior resembles that depicted in Regimes I andII as described previously (Fig. 1). At the lower sliding velocities,there is not enough melt-water production to induce decrease infriction. The melt-water production that is sufficient to inducefriction reduction occurs at velocities greater than 0.079 m/s at�1.5 1C and greater than 0.79 m/s at �4 1C. At lower tempera-tures such a decrease would have been observed at even highersliding velocities not accessible by our experimental apparatus.This decrease of friction at higher sliding speeds is in agreementwith observations reported by Blaum [27], who found that for agiven load, as frictional heating increases, the thickness of themelt-water film increases with velocity which results into adecrease of the coefficient of friction.

The sliding velocity of a slider at ‘‘point A’’ depicted in Fig. 1 isdefined in this work as the critical velocity, uc. This is the velocityat which the friction coefficient reaches its maximum value andthe mechanism of friction shifts from dry to mixed friction. It canbe seen from Fig. 4 that uc (�1.5 1C)E 0.079 m/s and uc

(�4 1C)E0.79 m/s (the exact maximum cannot be easily locatedby the available experimental results). It can also be assumedfrom the available data that uc (�7 1C)E1.96 m/s, the minimumpossible value. Thus uc9�1.5 1Couc9�4 1Couc9�7 1C which is anexpected behavior, that is the lower the temperature the higherthe critical sliding velocity for the onset of the lubricated water-film formation. Unfortunately, the experimental setup in thiswork does not allow for friction measurements at higher velo-cities. It is reasonable to assume that the critical velocities attemperatures lower than �7 1C are greater than the value of1.96 m/s. Table 2 summarizes the critical velocities for all tem-peratures for the occurrence of lubricated friction.

It should be noted at this point that the thermal conductivityof the two materials in contact, namely the ice and the UHMWPEsurfaces is very important in the estimation of the criticalvelocity. It is safe to hypothesize that most of the heat beinggenerated during the sliding process remains on the ice/sliderinterface. The thermal conductivities of ice and polyethylene

Table 2Critical sliding velocities for the onset of lubricated

friction.

Temperature (1C) tc (m/s)

�1.5 E0.079

�4 E0.79

�7 E1.96

�10, �15, �25 41.96

Table 3Estimation of the minimum times (s) required to observe the onset of lubricated

friction (transition ‘‘point A’’ in Fig. 1).

Temperature (1C)Time (s)

0.0079 m/s 0.079 m/s 0.79 m/s 1.96 m/s

�1.5 98,116 633 9 2

�4 1,081,469 8322 53 11

�7 3,106,777 28,264 141 20

�10 6,402,218 61,308 227 27

�15 10,176,042 110,154 418 53

�25 17,755,127 162,471 620 77

Temperature (°C)-30 -25 -20 -15 -10 -5 0

Coe

ffici

ent o

f fric

tion

0.04

0.06

0.08

0.10

0.120.0079m/s0.079m/s0.79m/s1.96m/s

UHMWPE

Fig. 5. Effect of temperature on the COF of UHMWPE substrates sliding on ice at

different sliding velocities.

C. Stamboulides et al. / Tribology International 55 (2012) 59–67 63

slider are 2.3 and 0.42 W/m K, respectively. Therefore, most of theproduced heat is being transported to the ice surface, whichinduces the formation of the melt-water film.

Higgins et al. [19] performed pin-on-disk friction tests ofrubber on ice. They have reported that for the lower velocitiesthere was not enough time required for the ice to melt. Ho et al.[45] used the following equation to calculate the time necessaryfor the ice to melt by a specified heat flux on the surface:

tc ¼2ðDTÞ2kCpr

3q002ð2Þ

where, DT is the temperature rise to reach the melting tempera-ture (0 1C), k is the thermal conductivity of ice (2.3 W/m K), Cp isthe heat capacity of ice, r is the density of ice and q00 is the heatflux at the interface. The heat flux can be calculated with thefollowing equation Higgins et al. [19]:

q00 ¼ umsn ð3Þ

where u is the sliding velocity, m the coefficient of friction and sn

the normal stress at the interface.Eqs. (2) and (3) were used in order to estimate the time for the

ice surface to melt during the ice friction experiments. Theobjective of this exercise is to investigate if the slider’s runningtime of each friction experiment performed (60 s) at the criticalvelocity agrees with the time required to produce a water film asestimated from these equations. In other words, if the hypothesisis correct, the wetting calculated time should be less than 60 s. Itshould be noted here that the average friction coefficient valuesplotted in Fig. 4 are averages of 10 different ice surfaces over 60 sof sliding times. The calculated results are listed in Table 3. Theresults at lower velocities (0.0079 and 0.079 m/s) agree well withthe observations and calculations made by Higgins et al. [19].

The results listed in Table 3 suggest that the frictional behaviorfor the sliding velocity of 1.96 m/s at temperatures equal orhigher than �15 1C should be in the mixed lubrication zone. Thisis because the calculated wetting times are lower than the actualrunning experimental time of 60 s. The results also suggest thatthe COF would drop with increase of sliding velocities greaterthan 1.96 m/s at �7, �10, and �15 1C, and that the value of1.96 m/s is possibly close to the wetting velocity. Unfortunately,and due to equipment limitations higher sliding velocities couldnot be achieved in order to determine the exact wetting velocityat these lower temperatures. However, the agreement is excellent

for the �4 1C curve; the calculated wetting velocity and theexperimental value are both at 0.79 m/s. For the case of the curvethat corresponds to �1.5 1C, the calculated wetting velocity isabout 0.79 m/s and the experimental observation is somewhatlower, at a value between 0.079 and 0.79 m/s (the exact locationof the maximum is difficult to be located).

3.2. The effect of temperature on ice friction

Fig. 5 depicts the friction curves plotted in Fig. 4, this time as afunction of temperature at different sliding velocities. Again, eachdata point represents the average of at least ten measurementsand the error bars in the graphs represent the corresponding 95%confidence interval. The frictional behavior for the lower slidingvelocities of 0.0079 m/s and 0.079 m/s and for temperatures equalor higher than �10 1C resembles the one depicted in Regime I(Fig. 1) before the transitional ‘‘point A’’. For the higher velocitiesof 0.79 m/s and 1.96 m/s, the behavior resembles that after ‘‘pointA’’ where there is a drop in the COF. This implies that there is aminimum value for the COF at the lower sliding velocities.Theoretically, the higher the sliding velocity is, the more frictionalheat should be generated at the slider/ice interface, and thereforemore melt-water should be produced. Thus, the mixed lubricationzone should be reached faster for the higher sliding velocitiesthan the lower ones. However, this is not the case presented here.Based on the interpretations of experimental results in Fig. 5, itcan be concluded that at the lower sliding velocities there is notenough melt-water production to induce the transition towardsthe mixed lubrication zone.

Several research groups have studied the effect of ice tem-perature on the sliding coefficient of friction [8,9,12,14,23,25,32].Buhl et al. [12] found that there is a minimum value for thefriction coefficient at around �3 1C similar to what has beenobserved in the present work. The minimum was attributed tosurface lubrication. On the other hand, Albracht et al. [17] andFiorio et al. [41] mentioned that for temperatures lower or equalto �7 1C the dominant phenomenon during the sliding process onice is the plastic deformation on ice and the consequence of this isincreased friction. Glenne [8] has shown that the COF increasedwith increasing temperature and that was due to the fact that theice surface became ‘‘softer’’ and therefore it was easier for theslider to ‘‘rub-in’’ the ice surface. The asperities on the slider’ssurface were essentially penetrating the ice surface. Therefore, thelonger the sliding time on the ice surface, the more the asperitiespenetrate the ice. Based on these reports, it is safe to conclude

C. Stamboulides et al. / Tribology International 55 (2012) 59–6764

that the observed minimum at the lower sliding velocities is dueto the local deformations occurring on the ice surface and not dueto the surface lubrication.

3.3. The surface topography on ice friction

The effect of surface roughness on COF has also been studied.Three different grit sandpapers as a final finishing were used toprepare the testing specimens. These are 420, 600 and 1200 grits,which resulted in surfaces exhibiting average roughness of 1300,1000 and 760 nm respectively. Contact angle measurements thatwere performed on the substrates with different average rough-ness showed no significant change on the static water contactangle values. The average roughness and contact angle data aresummarized in Table 4.

Table 4The effect of preparation method, on the average roughness and the static contact

angle of UHMWPE substrates.

Preparation methodAverage roughness,Ra (nm)

Static contact angle (deg.)

400 grit 1300 8475

600 grit 1000 8474

1200 grit 760 8273

0.02

0.04

0.06

0.08

0.10

1300 nm1000 nm760 nm

0.001 0.01

Coe

ffici

ent o

f fric

tion

0.02

0.04

0.06

0.08

0.10

T = -7°C

Sliding velocity (m.s-1)

Sliding v

0.001 0.01 0.1 1 10

Coe

ffici

ent o

f fric

tion

0.02

0.04

0.06

0.08

0.101300 nm1000 nm760 nm

T = -1.5°C

Fig. 6. Effect of sliding velocity on the COF for UHMWPE substrates

Fig. 6 plots the experimental data for all the roughnessesstudied as a function of sliding velocity at three differenttemperatures. The COF exhibits the same trend for all threesubstrates of different roughness. A deviation from this trend isobserved only at the lowest sliding velocity and this is becausethe friction regime is in the dry region where the increasedroughness results into more asperity shearing and thereforehigher COF values. There is more penetration of the slider’sasperities on the ice surface at warmer temperatures possiblydue to the fact that the ice surface is softer. During the slidingmotion of solid materials on ice, the slider/ice interface alwaysfails plastically because of the high compressive stresses aroundthe regions of contact [46]. Petrenko [34] found that there is verysoft but elastic layer on the ice. Rist [47] reported that at highrates and/or low temperatures ice frictional sliding appears to bethe result of elastically deforming asperities that undergo shearfailure. Overall, the COF is nearly independent of roughness withthe exception of results at the higher temperatures of �1.5 1C and�4 1C and lower sliding velocities. Based on the results shownpreviously (Fig. 4) where it was seen that the frictional behaviorfor those two temperatures lies at the borders of the mixedlubrication regime. It has been shown that the COF increases inthe mixed friction regime as roughness increases [39]. This canalso be observed from Fig. 6. It should be mentioned that theeffect of the melt-water film thickness depends on the structure

1300 nm1000 nm760 nm

0.001 0.01 0.1 1 10

1300 nm1000 nm760 nm

0.1 1 10

1300 nm1000 nm760 nm

Sliding velocity (m.s-1)

elocity (m.s-1)

Coe

ffici

ent o

f fric

tion

0.02

0.04

0.06

0.08

0.101300 nm1000 nm760 nm

T = -4°C

of different roughnesses at (a) �1.5 1C, (b) �4 1C, and (c) 7 1C.

Fig. 7. Surface patterns created and the corresponding microscopic images of the two different patterns: (a) schematic of the concentric groove pattern, (b) SEM image of

the slider with concentric groove, (c) schematic of the cross-sectional groove pattern and (d) SEM image of the slide with the cross-sectional groove.

Sliding velocity (m.s-1)0.001 0.01 0.1 1 10

Coe

ffici

ent o

f fric

tion

0.00

0.02

0.04

0.06

0.08

0.10

0.12A - cross-sectionalB - cross-sectionalA - concentricB - concentric

T = -1.5°C

Fig. 8. The effect of sliding velocity on the ice COF of two (A and B) commercial ski

base samples having a cross-sectional and concentric surface patters sliding on ice

at �1.5 1C.

C. Stamboulides et al. / Tribology International 55 (2012) 59–67 65

roughness of the sliding surfaces. If the surfaces are smoothenedat the nano-level, thicknesses in the nano-level might affect COF.If the surfaces are smoothened at the micro-level, thicknesses inthe micro-level might affect the COF [43].

3.4. The effect of surface patterning on ice friction

As mentioned previously the slider in the present study is aring having and an outside diameter of 25 mm and an insidediameter of 21 mm. The average thickness of this ring is 2 mm.Due to the fact that the geometry of the surface is circular it wasdecided to create two distinctly different surface patterns throughpolishing. The two structures are the concentric one where thecreated grooves are parallel to the rotational motion of the ringand the cross-sectional one, where the grooves are more ran-domly distributed over the ring surface. All samples (four in total)for both patterns had an average roughness value of 760 nm.A schematic of the two surface patterns and their microscopic(magnification of �150) images are shown in Fig. 7.

Frictional experiments were conducted at �1.5 1C at severalsliding velocities and the results are depicted in Fig. 8. Sliderswith the cross-sectional pattern exhibited a very similar frictionalbehavior with that of UHMWPE (same surface pattern) that waspresented in Fig. 4. More asperity shearing takes place for the caseof cross-sectional pattern, which results into more resistance atthe low sliding velocities of 0.0079 m/s and 0.079 m/s. At thesame time more water is being produced at the interface whichresults into a decrease of COF at the higher velocities. On theother hand, for the sliders with the concentric surface structure,the COF exhibits a different behavior and increases with increas-ing sliding velocity. The asperity shearing is less at the low slidingvelocities, and therefore the COF is lower than that of the cross-sectional, and as the velocity increases the produced melt-wateris trapped in the concentric grooves, which adds more resistanceto the sliding motion due to capillary drag.

3.5. The effect of temperature on the ice friction of polymers

exhibiting different wettabilities

Finally, the effect of surface static contact angle on thecoefficient of friction (COF) has also been studied in this workby means of using three different polymeric materials of differentwettability (see Section 3.2 for more details). The COF for thethree polymeric substrates are plotted in Figs. 9a and b as afunction of temperature for two different sliding velocities. Theyall exhibit a linear decrease. Evan et al. [16] have also reported alinear dependence of the coefficient of friction on the air tem-perature similar to that depicted in Figs. 9a and b. They have alsoperformed friction measurements of different surfaces (polymericand metallic) on ice. These results also agree with those plotted inFig. 5.

The COF values of the PTFE substrate are lower than those ofthe UHMPWE ones. PMMA shows a similar behavior with one

main difference. At the smaller velocities the effect of the liquidwater film becomes more dominant due to the hydrophilicity ofPMMA. This causes a significant increase to COF at temperaturesclose to 0 1C. It can clearly be seen that at the higher temperatureswhere the creation of the melt water layer is more likely to occurthe more hydrophobic substrate exhibits a lower coefficient offriction. This verifies findings in literature, such as the COF valuesreported by Buhl et al. [12], Colbeck [28], Spring [29] and Baurleet al. [30] as well as gives confidence in the work performed inthis study.

4. Conclusions

The microscopic friction of polymeric surfaces on ice wasinvestigated with emphasis on the friction of ultra-high molecularweight polyethylene which is the polymer that is used as therunning base in competitive and recreational skis and snowboards.A new fixture was designed for the tribological measurements usedin conjunction with a conventional rotational rheometer. Thedependence of the ice coefficient of friction on parameters suchas sliding velocity, temperature, surface roughness, surface patternand wettability was studied in detail. The experiments wereconducted at sliding velocities ranging from 0.0079 m/s to1.96 m/s and at temperatures from �25 1C to �1.5 1C.

A new friction regime was identified at small sliding velocitieswhere the conditions facilitate ‘‘dry friction’’ and where friction isstrictly dependent on the asperity interaction between the slidingsurfaces. The results have illustrated that there is a significantdifference between the overall behavior of the coefficient offriction at low and high sliding velocities due to the increasing

Temperature (°C)-30 -25 -20 -15 -10 -5 0

Coe

ffici

ent o

f fric

tion

Coe

ffici

ent o

f fric

tion

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

UHMWPEPMMAPTFE

v = 1.96 m/s

Temperature (°C)-30 -25 -20 -15 -10 -5 0

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

UHMWPEPMMAPTFE

v = 0.79 m/s

Fig. 9. Effect of temperature on the ice COF of the three polymeric surfaces under study at (a) 0.79 m/s and (b) 1.96 m/s.

C. Stamboulides et al. / Tribology International 55 (2012) 59–6766

importance of shear heating. This was shown by trends of thecoefficient of friction curves with respect to sliding velocity atdifferent temperatures.

The coefficient of friction was found to depend on bothtemperature and sliding velocity. Friction coefficient decreaseswith increasing temperature at high sliding velocities due tosurface lubrication. On the other hand, at lower sliding velocitiesthe friction coefficient decreases and then increases. This behaviorwas attributed to the modification of the real contact area on theice surface.

Surface roughness was also found to play an important role onthe coefficient of friction mainly in the ‘‘dry friction’’ regime. Itseffect works synergistically with velocity and temperature. Inaddition it was found that the more hydrophobic samples exhibitlower coefficient of friction and when the friction curves are inthe transition between the lubrication and mixed regime, thedrop in the friction coefficient values is more rapid with increaseof hydrophobicity or decrease of hydrophilicity.

Acknowledgments

Financial support from Own The Podium (OTP) of the CanadianOlympic Committee (COC) is gratefully acknowledged.

References

[1] Shimbo M. Friction on snow of ski soles unwaxed and waxed.Scientific studyof skiing in Japan. Tokyo: The Society of Ski Science; 1971 p. 100–13.

[2] Oksanen P, Keinonen J. The mechanism of friction of ice. Wear 1982;78:315–24.

[3] Nachbauer W, Schrocksnadel P, Lackinger B. Effects of snow and air condi-tions on ski friction. In: Mote CD, Johnson JrRJ, Wolfhart H, Schaff PS, editors.Proceedings of skiing trauma and safety, 10th vol. ASTM STP 1266. AmericanSociety for Testing and Materials; 1996. p. 178–85.

[4] Moldestad DA. Some aspects of ski base sliding friction and ski base structure.PhD thesis. Norway: Norwegian University of Science and Technology; 1999.p. 137.

[5] Nikki K, Haizuka S, Tomizawa I, Kaneko K, Saito S, Shirahata K, et al.Measurement of friction coefficient for ski-sliding at low speeds. In: MullerE, Bachacrach D, Klika R, Lindinger S, Schwameder H, editors. Science andskiing III, part four: snow. Oxford: Meyer and Meyer Sports; 2005. p. 423–30.

[6] Kuzmin L. Investigation of the most essential factors influencing ski glide.Licentiate thesis. Sweden: Lulea University of Technology; 2006.

[7] Bowden FP, Hughes TP. The mechanism of sliding on ice and snow. Proceed-ings of the Royal Society of London, Series A, Mathematical and PhysicalSciences 1939;172(949):280–98.

[8] Glenne B. Sliding friction and boundary lubrication of snow. Journal ofTribology 1987;109:614–7.

[9] Colbeck SC. A review of the processes that control snow friction. CRRELmonograph. Hanover (NH): US Army Cold Regions Research and EngineeringLaboratory; 1992 [92-2].

[10] Calabrese SJ, Buxton R, March G. Frictional characteristics of materials slidingagainst ice. Journal of the American Society of Lubrication Engineers1980;36(5):283–9.

[11] Akkok M, Ettles CMM, Calabrese SJ. Parameters affecting the kinetic frictionof ice. Journal of Tribology 1987;109:552–61.

[12] Buhl D, Fauve M, Rhyner H. The kinetic friction of polyethylen on snow: theinfluence of the snow temperature and the load. Cold Regions Science andTechnology 2001;33:133–40.

[13] Bowden FP. Friction on snow and ice. Proceedings of the Royal Society ofLondon, Series A, Mathematical and Physical Sciences 1953;217(1131):462–78.

[14] Lehtovaara A. Friction between plastics and snow. Tribiologio 1985;4(3):14–26.

[15] Bowden FP, Tabor D. The friction and lubrication of solids. third ed. NewYork: Oxford University Press; 2001.

[16] Evans DCB, Nye JF, Cheeseman KJ. The kinetic friction of ice. Proceedings ofthe Royal Society of London, Series A, Mathematical and Physical Sciences1976;347(1651):493–512.

[17] Albracht F, Reichel S, Winkler V, Kern H. On the influences of friction on ice.Materialwiss Werkstofftech 2004;35(10–11):620–5.

[18] Ducret S, Zahouani H, Midol A, Lanteri P, Mathia TG. Friction and abrasivewear of UHWMPE sliding on ice. Wear 2005;258:26–31.

[19] Higgins DD, Marmo BA, Jeffree CE, Koutsos V, Blackford JR. Morphology of icewear from rubber-ice friction tests and its dependence on temperature andsliding velocity. Wear 2008;265:634–44.

[20] Kuroiwa D. The kinetic friction on snow and ice. Journal of Glaciology1977;19(81):141–52.

[21] Colbeck SC, Perovich DK. Temperature effects of black versus white poly-ethylene bases for snow skis. Cold Regions Science and Technology 2004;39:33–8.

[22] Fauve M, Buhl D, Rhymer H, Schneebeli M, Ammann W. Influence of snowand weather characteristics on the gliding properties of skis. In: Muller E,Bachacrach D, Klika R, Lindinger S, Schwameder H, editors. Science and skiingIII, part four: snow. Oxford: Meyer and Meyer Sports; 2005. p. 401–10.

[23] Marmo BA, Blackford JR, Jeffree CE. Ice friction, wear features and theirdependence on sliding velocity and temperature. Journal of Glaciology2005;51(174):391–8.

[24] Strausky H, Krenn JR, Leitner A, Aussenegg FR. Sliding plastics on ice:fluorescence spectroscopic studies on interfacial water layers in the mmthickness regime. Applied Physics B—Lasers and Optics 1998;66(5):599–602.

[25] Baurle L. Sliding friction of polyethylene on snow and ice. Diss. nr. 16517.ETH; 2006.

[26] Baurle L, Th.U. Kaempfer, Szabo D, Spencer ND. Sliding friction of polyethy-lene on ice: contact area and modeling. Cold Regions Science and Technology2007;47(3):276–89.

[27] Blaum PJ. Friction science and technology: from concepts to applications.second ed. Boca Raton: STLE and CRC Press (Taylor and Francis Group); 2008.

[28] Colbeck SC. A review of the friction of snow skis. Journal of Sports Sciences1994;12:285–95.

[29] Spring E. A method for testing the gliding quality of skis. Tribologia1988;7(1):9–14.

[30] Baurle L, Szabo D, Fauve M, Rhyner H, Spencer ND. Sliding friction ofpolyethylene on ice: tribometer measurements. Tribology Letters 2006;24(1):77–84.

C. Stamboulides et al. / Tribology International 55 (2012) 59–67 67

[31] Casassa G, Narata H, Maeno N. Shear cell experiments of snow and icefriction. Journal of Applied Physics 1991;69(6):3745–56.

[32] Colbeck SC. The kinetic friction of snow. Journal of Glaciology1988;34(116):78–86.

[33] Bhushan B. Introduction to tribology. New York: John Wiley and Sons; 2002.[34] Petrenko VF. Study of the surface of ice, ice/solid and ice/liquid interfaces

with scanning force microscopy. Journal of Physical Chemistry B1997;101(32):6276–81.

[35] Bluhm H, Inoue T, Salmeron M. Friction of ice using lateral force microscopy.Physical Review B 2000;61(11):7760–5.

[36] Makkonen L. Surface melting of ice. Journal of Physical Chemistry: B1997;101(32):6196–200.

[37] Bowden FP. Introduction to the discussion: the mechanism of friction.Proceedings of the Royal Society of London, Series A, Mathematical andPhysical Sciences 1952;212(1111):440–9.

[38] Unal H, Mimaroglu A, Kadioglu U, Ekiz H. Sliding friction and wear behaviourof polytetrafluoroethylene and its composites under dry conditions. Materi-als and Design 2004;25:239–45.

[39] Bongaerts JHH, Fourtouni K, Stokes JR. Soft-tribology: lubrication in acompliant PDMS–PDMS contact. Tribology International 2007;40(10–12):1531–42.

[40] Burris DL. Investigation of the tribological behaviour of polytetrafluoroethy-lene at cryogenic temperatures. Tribology Transactions 2008;51:92–100.

[41] Fiorio B, Meyssonier J, Boulon M. Experimental study of the friction ofice over concrete under simplified ice–structure interaction conditions.Canadian Journal of Civil Engineering 2002;29(3):347–59.

[42] Maeno N, Arakawa M. Adhesion shear theory of ice friction at lowsliding velocities combined with ice sintering. Journal of Applied Physics2004;95(1):134–9.

[43] Liu S, Luo J, Li G, Zhang C, Lu X. Effect of surface physicochemical propertieson the lubricating properties of water film. Applied Surface Science2008;254:7137–42.

[44] Kennedy FE, Schulson EM, Jones DE. The friction of ice on ice at low slidingvelocities. Philosophical Magazine. A: Physics of Condensed Matter, Defectsand Mechanical Properties 2000;80(5):1093–110.

[45] Ho C-D, Yeh H-M, Wang W-P. Thermal characteristics of ice under constantheat flux and melt removal. Heat Transfer Engineering 2002;23(5):36–44.

[46] Raraty LE, Tabor D. The adhesion and strength properties of ice. Proceedingsof the Royal Society of London. Series A, Mathematical and Physical Sciences1958;245(1241):184–201.

[47] Rist MA. High-stress ice fracture and friction. Journal of Physical Chemistry B1997;101(32):6263–6.