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The Tribological Behaviors of Self-LubricatingComposites as Filler in Copper FoamKeju Ji a b , Weigen Shan a , Yanqiu Xia b & Zhendong Dai aa Institute of Bio-inspired Structure and Surface Engineering, Academy of Frontier ScienceNanjing University of Aeronautics and Astronautics 29 Yudao Street, Nanjing, 210016, Chinab State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics ChineseAcademy of Sciences, Lanzhou, 730000, China

Version of record first published: 04 Nov 2011.

To cite this article: Keju Ji, Weigen Shan, Yanqiu Xia & Zhendong Dai (2012): The Tribological Behaviors of Self-LubricatingComposites as Filler in Copper Foam, Tribology Transactions, 55:1, 20-31

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Tribology Transactions, 55: 20-31, 2012Copyright C© Society of Tribologists and Lubrication EngineersISSN: 1040-2004 print / 1547-397X onlineDOI: 10.1080/10402004.2011.622069

The Tribological Behaviors of Self-Lubricating Compositesas Filler in Copper Foam

KEJU JI,1,2 WEIGEN SHAN,1 YANQIU XIA,2 and ZHENDONG DAI1

1Institute of Bio-inspired Structure and Surface Engineering,Academy of Frontier Science

Nanjing University of Aeronautics and Astronautics29 Yudao Street

Nanjing 210016 China2State Key Laboratory of Solid Lubrication

Lanzhou Institute of Chemical PhysicsChinese Academy of Sciences

Lanzhou 730000 China

A copper foam–based (CFB) self-lubricating composite

was developed as a friction pair material. The composite

consists of copper foam with three-dimensionally interpene-

trated pores and solid lubricant fillers (polytetrafluoroethylene

[PTFE] and MoS2). The thermal and electrical conductivity,

bending strength, and tribological properties of the new com-

posite were investigated. The friction and wear properties were

investigated on a M-2000 model ring-on-block test rig. An elec-

tric field was imposed between the sample and ring to monitor

the tribochemical reaction and formation of transfer film by

means of contact resistance. Measurement of friction temper-

ature was carried out by means of three thermocouples em-

bedded in the material. The friction coefficients of the CFB

composite decrease slightly, and the wear rate substantially de-

creased overall compared with that of the homologous poly-

mers. The optical and scanning electron micrographs (SEM) of

the frictional surfaces show the worn surface of the CFB com-

posite. The main wear mechanism was three-body abrasion,

caused and promoted by plastic deformation, abrasive wear,

and fatigue spalling. X-ray photoelectron spectroscopy (XPS)

analysis showed that the reactions of MoS2 with Cu accompa-

nied by their respective oxidation reactions are involved in the

identifiable copper area.

KEYWORDS

Copper Foam; Self-Lubricating Composites; Solid Lubri-cants; Thermal Conductivity; Electrical Conductivity; WearMechanisms

Manuscript received July 6, 2011Manuscript accepted September 6, 2011

Review led by David Burris

INTRODUCTION

The increasing demand for good thermal and/or electricalconductivity in friction materials has led to the quest for poly-meric materials with comprehensive properties. Previous workhas shown that various methods are used to improve thermaland/or electrical conductivity of polymer matrices (Balic andBlanchet (1); L. Q. Wang, et al. (2); Ng, et al. (3); He, et al. (4);Mu, et al. (5)). Materials possessing different advantages havefound their way into lubrication-related fields, such as powdermetallurgical materials with solid lubricants as additives, porousembedded self-lubricated bronze bearings, self-lubricating ce-ramic matrix composites, and so on. However, it is complicated tobalance and optimize the mechanical, frictional, thermal, and/orelectrical properties of such materials. For instance, the continu-ity of substrate hard phases is broken up by solid lubricant phases,which spontaneously reduces the material strength and wear re-sistance; low-melting lubricants are partially burned or oxidizedat high sintering temperature, which may adversely affect the lu-bricating performance (Liu (6)); the effective lubrication area isdependent on the compactness of solid lubricants and the porestructure (Pelletiers, et al. (7)); the conductivity of composites issuppressed by the dispersion of conductive in the powder metal-lurgy method; and solid lubricant fillers are difficult to compactevenly when the typically incompressible and micro–pore cermetsinter is filled with soft metal by a vacuum.

Foamed metals have been used in many experimental andcommercial applications as their manufacturing methods haverapidly developed (Kim, et al. (8); Banhart (9)). They are appliedin various thermal applications (heat exchangers, heat sinks, andheat pipes), high-capacity anode materials of lithium-ion batter-ies, electrochemical catalysis, filters, gas-flow-controlling devices,and biomedical implants (Hong and Herling (10); Xu, et al. (11);Li, et al. (12); Sanz, et al. (13); Banhart and Weaire (14); Lefeb-vre, et al. (15)). These applications greatly profit from the id-iosyncratic combinations of conductivity, permeability, low den-sity, specific mechanical properties, and high surface area.

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Tribological Behaviors of Copper Foam–Based Composites 21

Copper foam is one type of foamed metal and is characterizedby a highly porous open-cell structure, large ductility, as wellas extraordinary thermal and electrical conductivity. Burrisand Sawyer (16) introduced aluminum foam to a polymerictribological composite for increased thermal conductivity andpressure–velocity (PV) limit. Their equal models and preciseinvestigation showed that the conductivity of the compositewas strongly dependent on the structure of the filler withinthe composite, and the continuity of the aluminum foam wassubstantially more effective than the particle dispersion of thereference indium at dissipating thermal energy. Qu, et al. (17)studied the wear characteristics of novel graphitic foam materialsand proposed three primary tribological advantages: they canefficiently remove frictional heat; their natural porosity can trapwear debris; and their porosity can serve as a lubricant reservoirfor the contact surface. Y. J. Wang (18) and Y. J. Wang, et al.(19), (20) reported that porous metal ceramic preforms with aninterpenetrated network were infiltrated by Pb-Sn– and Pb-Sn-Ag–based solid lubricants and pointed out that the resultantcomposite has good mechanical and tribological properties. So itis interesting to introduce copper foam to the friction materials,especially those requiring good electric and thermal conductivity.This article represents a new self-lubricating material with aninterpenetrating network and compacting fillers. The compositeswere obtained by filling the open pores of copper foam with solidlubricants such as polytetrafluoroethylene (PTFE) and MoS2

with the help of a vacuum filtration apparatus. During sliding, the

solid lubricants were dispersed to the frictional surface to providelubrication by the differencein the thermal expansion coefficientbetween the solid lubricant phase and the matrix base (Sang, etal. (21)) while the copper foam transfers heat and supports theload. The friction and wear characteristics of the newly formedmaterials and the mechanisms of transfer film formation in theseself-lubricating composites were studied in detail.

EXPERIMENTAL WORK

Materials

Open-cell copper foam was supplied by our laboratory. Twokinds of copper foam used in the current tests had open-celledstructures composed of dodecahedron-like cells, pentagonal orhexagonal faces, and hollow metallic struts, as shown in Fig. 1a.The characteristics and some mechanical parameters of copperfoam are shown in Table 1. The PTFE powder had an averageparticle size 60 µm (Shanghai Electrochemical Plant, Shanghai,China) and the MoS2 flake powder had an average particle size 4µm (Shanghai Colloid-Chemical Plant, Shanghai, China).

Specimen Preparation

Copper foam samples were prepared using a bench saw orwire cutting machine. The samples were pretreated in petroleumether and dilute hydrochloric acid solution to remove oil andoxide on the surface. The copper foam–based (CFB) compositeswere fabricated using vacuum filtration, compression molding,

Fig. 1—(a) Optical micrograph (50×) of two kinds of open-cell copper foam, (b) specimen preparation, and (c) and (d) photos of the CFB composites(P/M25-1 and P/M25-2) showing surface morphology (P/M25: 75% PTFE + 25% MoS2; P/M25-1: #1 copper foam filled with P/M25 component;P/M25-2: #2 copper foam filled with P/M25 component). (color figure available online.)

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22 K. JI ET AL.

TABLE 1—CHARACTERISTIC PARAMETERS OF THE COPPER FOAM

DesignationAverage

Aperture (mm) Porosity (%) Pores per Inch Density (g/cm3) Relative DensityBending

Strength (MPa)Compressive

Strength (MPa)

#1 0.96 93 28 0.60 0.07 1.15 1.56#2 1.58 96 17 0.28 0.03 0.42 0.50

and sintering as follows: Firstly, different ratios of solid lubricantswere mixed in ethanol using a magnetic stirrer. Secondly, themixed solution was filtered and dried with the help of a vacuumfiltration apparatus (Fig. 1b). The apparatus consisted of a filterflask with a fine mesh strainer (300 mesh) or a sand core (averageaperture 15–40 µm) and a vacuum pump that connected the filterflask. The design of the filtration apparatus enabled uniformflow of the suspension and was suitable for samples of largedimension. In the process, a flexible pipe was applied to imbibethe mixture of solid lubricants and splash it into the copper foamsample while the organic solvent was filtered out with the helpof a vacuum pump. The filtering process took about 4 h forsufficient volatilization of ethanol. After the filtering process, thebottom interspaces of copper foam adjacent to the mesh straineror sand core were filled up. Therefore, the filling was actually apowder deposition process in the interspaces from the bottomto top, and this marked the end of the filling process. Then,the specimens were preformed with compression molding at 60MPa for about 5 min to allow the preparation of rectangular andcylindrical samples of sizes 6 mm × 7 mm × 30 mm, 3 mm × 7mm × 30 mm, and Ø12.5 mm × 2 mm at room temperature. Thesame pressure was used to fabricate the polymers without copperfoam. A pressure of 60 MPa was chosen because the compressivestrengths of the initial foam were low (Table 1). Lastly, thesecomposite blocks were sintered at 330◦C for 60 min and 370◦Cfor 60 min in a sintering furnace and then cooled to room temper-ature. In the process of cold molding, the copper foam acting as aporous support was compacted with the solid lubricants, and thecompressibility in the process was about 45% of its original sizecompared with the initial copper foam without fillers up to 90%.

In this work, five compositions were filled into the preparedcopper foam and homologous PTFE-based polymers withoutcopper foam were studied for comparison. The compositionaldetails are listed in Table 2 and the densities of select specimensare shown in Table 3.

TABLE 2—COMPOSITIONS (VOL%) OF PTFE-BASED AND CFBCOMPOSITES

Description of FillingLubricants Description of CFB Composites

PTFE: 100% PTFEP/M5: 95% PTFE + 5%MoS2

PTFE-1: #1 copper foam filled withpure PTFE

P/M15: 85% PTFE + 15%MoS2

P/M25-1: #1 copper foam filled withcomponent P/M5

P/M25: 75% PTFE + 25%MoS2

P/M25-2: #2 copper foam filled withcomponent P/M5, etc.

P/M35: 65% PTFE + 35%MoS2

Figures 1c and 1d show the surface morphologies of the CFBcomposites (P/M25-1 and P/M25-2) before wear testing. In thephotos we can see that the interspaces of the copper foam are ef-fectively filled with solid lubricants, and the hollow metallic strutsare widely distributed throughout the body, which is expected tobe helpful in storing lubricants in interfacial friction.

The cured samples were used for mechanical, thermal andelectrical conductivity, and tribological capability tests.

EXPERIMENTAL DETAILS

Mechanical Testing

A three-point bending test of the selected samples was per-formed on a universal material tester (DY35, Adamel Corpora-tion, Paris, France) at a load-applying velocity of 1 mm/min. Thedimensions of the cured rectangular samples were 3 mm × 7 mm× 30 mm, and tests were performed at room temperature. Themaximum loads were obtained and the bending strength (MPa)was calculated using the following formula:

σ = 3FL2BH2

[1]

where F is the maximum load (N), L is the distance between thesupports (mm), B is the width of the specimen (mm), and H is theheight (mm).

The bending modulus (GPa) was determined as:

E = FL3

4BH3d[2]

where F is the maximum load, L is the distance between the sup-ports, B is the width of the specimen, H is the height of the spec-imen, and d is the deflection (mm) corresponding to the load F.

Electrical Resistivity Testing

An equivalent substitution was used to calculate the electricalresistivity of the CFB composite (Fig. 2a).

Definition. The mass of the substitute (m0) is equal to thatof the CFB composite; m1 refers to the mass of the upper sheetcopper, the same mass as the copper foam; m2 refers to the massof the lower polymer; A1 and A2 refer to the copper density andpolymer density, respectively; R1 and R2 refer to the copper andpolymer resistances, respectively; and ρ1 and ρ2 refer to the elec-trical resistivities of copper and polymer, respectively.

TABLE 3—DENSITIES OF SELECT SPECIMENS

Specimen PTFE MoS2 P/M25 P/M25-1 P/M25-2

Density (g/cm3) 2.16 4.80 2.69 3.04 2.83

P/M25: 75% PTFE + 25% MoS2; P/M25-1: #1 copper foam filled withP/M25 component; P/M25-2: #2 copper foam filled with P/M25 com-ponent.

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Tribological Behaviors of Copper Foam–Based Composites 23

Fig. 2—(a) Schematic diagram of the equivalent substitution of the CFBcomposite and (b) a key sample preparation step for the DCKelvin bridge test. (color figure available online.)

The electrical resistivity ρ was calculated from the definition:

ρ = RSL

= ρ1ρ2 [m1A2 + (m0 − m1) A1]ρ1 (m0 − m1) A1 + ρ2m1A2

[3]

Then a DC Kelvin bridge test was applied to confirm the va-lidity of the proposed model. The specimens were evenly coatedon both lateral faces and the potential contact areas with the con-ductive silver paste in order to increase the contact area in thetesting process (Fig. 2b).

Friction and Wear Testing

The friction and wear behaviors of the specimens were studiedon an M-2000 friction and wear tester (Xuanhua testing machinemanufacturing Co., Ltd., Zhangjiakon, China). The frictional pairconsisted of an AISI 52100 steel rotating ring and the block sam-ple. The block-on-ring configuration is shown in Fig. 3. An elec-tric field was imposed between the sample and ring to monitorthe tribochemical reaction by means of contact resistance. Thefriction and wear tests were conducted selectively at a load of

100–700 N at velocities of 0.424 and 0.856 m/s. The testing du-ration ranged from 1 to 60 min. Before testing, the surfaces ofspecimen and counterpart ring were polished with No. 1000 SiCabrasive paper to a surface roughness of 0.08–0.15 µm. The ambi-ent temperature was about 20◦C and the relative humidity was 20± 5%. The friction force was measured with a torque shaft, andthe friction coefficient was calculated and recorded by a computerlinked with a sensor. Then the average over the whole test wastaken as the ultimate value of the friction coefficient. The wearrate of the specimen was determined by measuring the lost vol-ume of the sample after the friction process. The wear scar widthwas measured by a vernier caliper and three repeated trials wereconducted.

The wear rate of each specimen was obtained from the follow-ing formula:

W = VLN

[4]

where L is the sliding velocity multiplied by the time of the fric-tion process (s), is the distance of the friction (m), N is the load(N), and V is the wear volume loss (mm3) and has the relation-ship:

V = B

[πR2

180arc sin

(b

2R

)− b

2

√R2 − b2

4

][5]

where B is the width of the block specimen (7 mm), R is the outerradius of the steel ring (20 mm), and b is the width of the wearscar (mm; H. L. Wang, et al. (22)).

The worn surface morphologies of the specimens and thetransfer films formed on the counterpart steel rings were ob-served by scanning electron microscopy (SEM, JSM-5600LV,Jeol, Tokyo, Japan) and a digital microscope (VHX-600E,Keyence, Osaka, Japan).

Thermal Conductivity and Friction Heat Testing

The efficiency of the heat exchange and dissipation capacityof each composite was measured by the thermal diffusivity usinga laser flash apparatus (LFA 447 NanoFlash, Netzsch, Germany).

Fig. 3—Schematic diagram of the M-2000 friction and wear tester and calculation of wear rate. (color figure available online.)

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24 K. JI ET AL.

Fig. 4—Schematic diagram of LFA 447 NanoFlash. (color figure availableonline.)

A simplified schematic of the apparatus is shown in Fig. 4. Xenonflashlamps fire a pulse at the lower surface of the sample and aninfrared detector measures the temperature increase on the topsurface of sample. The thermal diffusivity (α) of the sample isthen calculated by the software. Specific heat (CP) is measured bycomparing the actual temperature rise of the sample to the tem-perature rise of a reference sample of known specific heat. Thethermal diffusivity and specific heat can be measured simultane-ously. The software uses these values and the bulk density (ρ) tocalculate thermal conductivity (λ) from the LaPlace equation:

λ = α × CP × ρ [6]

Alam and Maruyama (23) have investigated the thermal conduc-tivity of epoxy infiltration of carbon foam based on the similarprinciple and determined that the proposed method is feasibleand practical. The CFB specimens of size Ø12.5 mm × 2 mm were

made using the vacuum filter, compression molding, and sinter-ing technique as mentioned above. Two specimens were testedper immersion time, and each experimental point was obtainedas an average reading of three laser shots per temperature andper specimen.

As a complement, the measurements of friction temperatureswere carried out using thermocouples embedded in the material(Fig. 3). Several researchers have investigated the distribution oftemperature in the friction pairs (Burris and Sawyer (16); Pil-javskij (24); Abdel-Aal (25), (26)). The frictional heat is gener-ated not only on the surface of the sliding pair but in the bulkof the rubbing polymer material as a result of internal friction.The highest temperature occurred inside the polymer material,at some distance from the friction surface. Wieleba (27) foundthat the maximum temperature could occur inside the steel orpolymeric element 1.5 mm from the surface. In this experimentthree 3-mm-deep, 0.32-mm–inner diameter holes were drilled ataxial locations of 1.8, 3, and 4.5 mm as measured from the surface.These holes were used for thermocouple placement for tempera-ture measurement. The effect of copper foam in transmitting anddissipating heat beneath the contact is discussed.

RESULTS AND DISCUSSION

Mechanical Testing

Figure 5 shows the bending strength and modulus of selectspecimens. On the one hand, the addition of MoS2 to the PTFEmatrix resulted in an increase in the values of bending strengthand modulus; on the other hand, the copper foam acting as the in-terconnected metal skeletons embedded in polymer showed thesame tendency. The addition of small-sized MoS2 can improvethe compactness of PTFE. From the samples being loaded to fail-ure it emerged that the ductility of PTFE-based composites de-creased with an increase in filler content, which is consistent withthe findings of previous research (Bijwe, et al. (28); Yan, et al.(29)).

Fig. 5—(a) Bending strength and (b) modulus of specific specimens: pure PTFE, P/M25, P/M25-1, and P/M25-2 (P/M25: 75% PTFE + 25% MoS2; P/M25-1:#1 copper foam filled with P/M25 component; P/M25-2: #2 copper foam filled with P/M25 component).

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Tribological Behaviors of Copper Foam–Based Composites 25

TABLE 4—ELECTRICAL RESISTIVITY OF SELECT SPECIMENS

Specimen P/M25-1 P/M25-2 P/M25 Copper MoS2 PTFE

Electrical resistivity (�·m) 1.27 × 10−7 1.85 × 10−7 1.92 × 102 1.75 × 10−8 8.51 ≥1 × 1015

20◦C; P/M25: 75% PTFE + 25% MoS2; P/M25-1: #1 copper foam filled with P/M25 component; P/M25-2: #2 copper foam filled with P/M25component.

Electrical Resistivity Analysis

The electrical resistivities of P/M25 series specimens(P/M25–1, P/M25–2, and P/M25) and the basic raw materi-als are listed in Table 4. The theoretical analysis results wereproven by the DC Kelvin bridge test. The electrical resistivitiesof the CFB composites were dramatically reduced by the cop-per foam, especially the P/M25-1, compared with the polymerP/M25, MoS2, and PTFE. The low electrical resistivity of thespecimen was attributed to the conductive networks formed bythe interconnected metal skeleton construction, and the smallerthe average aperture of the copper foam, the lower the electricalresistance of the composite.

Friction and Wear Behavior

Figure 6 shows the friction coefficient and wear rate of theCFB composites and PTFE-based polymers as a function of MoS2

content at a velocity of 0.424 m/s and a load of 500 N. It is seenthat the variation in friction coefficient of the CFB compositeswas not fully in agreement with that of the homologous polymersas the MoS2 content changed. At 500 N, the friction coefficientof the CFB composites was lower than that of the homologouspolymers except when the MoS2 content was 15–25%; compositeswith #1 copper foam had higher friction coefficients than thosewith #2 copper foam except when the MoS2 content was greaterthan 30% (Fig. 6a). The wear rates of the CFB composites de-creased compared with those of the homologous polymers with-out copper foam (Fig. 6b). The filler MoS2 contributed to a de-crease in the wear rates even at low content, and the specific wearrates of the CFB composites were approximately in the followingorder: composites #1 CFB < #2 CFB when the MoS2 content wasbelow 15%; as the MoS2 content exceeded 15%, they presentedthe opposite tendency. In addition, the friction and wear proper-

ties of pure PTFE were improved significantly only by the copperfoam. By comprehensive comparison, the optimum MoS2 contentwas 25%.

The influence of the MoS2 content on the trend of friction co-efficients is complicated. For the polymer without copper foam,the PTFE-based composite had a lower friction coefficient whenthe MoS2 content was 15% because a uniform and smooth trans-fer film on the counterface can be formed at this content. For theCFB composite, the formation of transfer film was also affectedby the deformation, adhesion, and chemical reaction of the cop-per debris on the frictional interface. The abrasive wear of thedebris usually enhances the friction coefficient. However, the lo-cal high temperature and high stress produced by the copper strutand debris facilitated a firm bond of the transfer film to the ringsurface, which in turn led to a reduction in the friction coefficientat some MoS2 contents.

Variations in the friction coefficient and wear rates with differ-ent velocities for selected specimens at 500 N are shown in Fig. 7.It can be seen that the friction coefficients of the P/M25-1, P/M25-2, and P/M25 at the higher velocity of 0.856 m/s were lower thanthose at the lower velocity of 0.424 m/s, whereas the wear rateswere greater at 0.856 m/s than at 0.424 m/s. The antiwear prop-erties of the CFB composites were better than that of P/M25 atboth velocities. P/M25-1 had better performance than P/M25-2 atthe higher velocity of 0.856 m/s, contrary to the tendency at 0.424m/s.

PTFE and its composites exhibited viscoelastic properties. Ifcyclic loading was applied, the strain hysteresis (a phase lag) oc-curred for a dissipation of mechanical energy (Wieleba (27)).When the velocity of the ring became higher, the cyclic strainof the polymer was observed to lag behind the stress caused bythe loading force. This led to the development of cavities and

Fig. 6—(a) Friction coefficient and (b) wear rate of the CFB composites and the PTFE-based polymers as a function of MoS2 content at a velocity of 0.424m/s and load of 500 N (P/M: PTFE + x% MoS2; P/M-1: #1 copper foam filled with component PTFE + x% MoS2; P/M-2: #2 copper foam filled withcomponent PTFE + x% MoS2). (color figure available online.)

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26 K. JI ET AL.

Fig. 7—(a) Values of the friction coefficient and (b) wear rate ofselect specimens at velocities of 0.424 and 0.856 m/s and load of 500 N (P/M25: 75% PTFE+ 25% MoS2; P/M25-1: #1 copper foam filled with P/M25 component; P/M25-2: #2 copper foam filled with P/M25 component).

the high-speed sliding of the rubbing pair before the strainedmatched stress. The area and depth of contact on the frictionalinterface decreased correspondingly due to the small cavities andthe friction coefficients at the higher velocity decreased in turn.

Figures 8a and 8b show some typical variations in the fric-tion coefficient of select specimens at various loads and velocitiesduring the sliding time. Figure 8a illustrates the continuous vari-ations in friction coefficient of specimens P/M25, P/M25-1, andP/M25-2 with sliding time at 200 N. To be specific, the frictioncoefficients were very similar in the steady wear stage, whereas

that of the CFB composite fluctuated to various degrees in therunning-in stage. Figure 8b shows the variations in friction coef-ficient with sliding time affected by copper foam at velocities of0.424 and 0.586 m/s and load of 500 N. It can be seen that thefluctuations in friction coefficient of the specimens of P/M25 se-ries were analogous in the sliding time. Figure 9c shows the vari-ations in wear rates of specimens P/M25, P/M25-1, and P/M25-2with sliding duration at a velocity of 0.424 m/s and load of 200N. The curves of the wear rates show sharp changes in the be-ginning and flat subsequent areas, and the results show that the

Fig. 8—Variations in friction coefficient, wear rate, and contact resistance of specimens with sliding duration at (a) a velocity of 0.424 m/s and load of200 N; (b) velocities of 0.424 and 0.586 m/s and load of 500 N; (c) a velocity of 0.424 m/s and load of 200 N; and (d) velocities of 0.424 and 0.856m/s and loads of 200 and 500 N (P/M25: 75% PTFE + 25% MoS2; P/M25-1: #1 copper foam filled with P/M25 component; P/M25-2: #2 copper foamfilled with P/M25 component). (color figure available online.)

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Tribological Behaviors of Copper Foam–Based Composites 27

Fig. 9—Optical micrographs (50×) of worn surfaces of composites (left column) and steel rings (right column) of (a) and (b) P/M25-1; (c) and (d) P/M25-2;(e) and (f) P/M25 at 0.424 m/s and load of 500 N (the sliding direction is shown by arrows; P/M25: 75% PTFE + 25% MoS2; P/M25-1: #1 copperfoam filled with P/M25 component; P/M25-2: #2 copper foam filled with P/M25 component). (color figure available online.)

character of the wear rate is mostly consistent with that of thefriction coefficient.

Specifically, the local direct contact between the copper andthe steel ring results in a higher friction coefficient at the begin-ning of the running-in stage. Then, the PTFE matrix with a highexpansion coefficient quickly and ceaselessly intervenes on thefrictional interface due to the plastic deformation and creep. Thedeformation and fracture of platelets of lubricants form the trans-fer layer, which decreases the friction coefficient rapidly. The for-mation process of the transfer layer corresponds to the high wearrate. As the temperature on the frictional interface increases, theformation and stripping of the transfer layer reaches a new dy-namic equilibrium at the end of the breaking-in stage, and thenthe wear enters the steady stage.

Figure 8d shows the variation in DC contact resistances of theP/M25 series specimens under different conditions. It can be seenthat the tendency of contact resistance is consistent with that ofwear rate with sliding duration because the contact resistance isinversely related to the contact area. At some local convex pointsand/or facets, called contact spots, the transfer layer contacts eachother and forms a conductive network within the contact zoneof the steel ring and the CFB composite. The contact spots con-sist of dielectric/conducting film and wear debris. The lubricating

film below a critical value can be conductive due to the tunnelingeffect (Myshkina and Konchits (30)). therefore, the conductivitycan be attributed to the local conducting film and/or conductingwear debris such as copper debris. A further study on the valuesof contact resistance indicated that the resistance of the transferlayer was higher at the wear stage, whereas it was relatively lowerbefore and after the run. The larger applied load corresponded tothe smaller contact resistance due to the increased contact area.The higher sliding velocity slightly increased the contact resis-tance. From the point of view of viscoelasticity, the strain hys-teresis (a phase lag) decreased the area and depth of contact onthe frictional interface at the higher sliding velocity. Although thehigher frictional temperature caused more severe plastic defor-mation, the small cavities due to the strain hysteresis cannot beneglected. The decreased contact area led to a slightly larger con-tact resistance. This is also the cause for the cavities shown inFig. 10.

Thermal Conductivity and Friction Heat Analysis

Figure 11a shows a comparison of the thermal conductiv-ity of the CFB specimens (P/M25-1 and P/M25-1) and PTFEcomposites (P/M25 and pure PTFE) at 25 and 100◦C. It can be

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Fig. 10—Schematic representation of the wear mode of the CFB composite during sliding duration: (a) wear mode and the composition of wear debris(oxides included) on the contact surface and (b) the friction process simulation (thickness exaggerated). (color figure available online.)

observed that the thermal conductivity was greatly improved dueto the copper foam. The thermal conductivity of P/M25 was al-most three times that of the pure PTFE due to the addition ofMoS2; the composite P/M25-1 had the best thermal conductivityperformance, about 10 times that of the P/M25. In addition, thethermal conductivity of the specimens was slightly lower at 100◦C

than that at 25◦C with the exception of pure PTFE. The behaviorindicates that the CFB composites were effective in transmittingand dissipating heat.

Figures 11b–11d show the temperature of P/M25 series spec-imens at three temperature measuring points and variations oftemperature with sliding duration at 1.8 mm at given velocities

Fig. 11—(a) Values of the thermal conductivity of specimens at 25 and 100◦C, (b) temperature of specimens at 1.8 mm, (c) temperature at three measuringpoints, and (d) variations in temperature with sliding duration at 1.8 mm at given velocities and loads (P/M25: 75% PTFE + 25% MoS2; P/M25-1:#1 copper foam filled with P/M25 component; P/M25-2: #2 copper foam filled with P/M25 component).

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Tribological Behaviors of Copper Foam–Based Composites 29

Fig. 12—SEM micrographs of the worn surfaces of composite P/M25-1 at 0.424 m/s and load of 500 N: (a) transitional area, (b) hollow metallic strut, and(c) polymer area; and (d) the polymer composite P/M25 (the sliding direction is shown by arrows; P/M25: 75% PTFE + 25% MoS2; P/M25-1: #1copper foam filled with P/M25 component).

and loads. The temperature of the CFB composites approximatedthat of the one without copper foam, and the P/M25-2 specimenhad a slightly lower friction temperature than P/M25 when themeasurement was close to the frictional surfaces. The tempera-ture decreased more slightly due to the copper struts when themeasurement was far from the interface (Fig. 11c). The varia-tions in temperature with sliding duration at 1.8 mm show thatthe temperature heads toward stability. A balance between heatgeneration and dissipation can be seen in the friction process dueto the good thermal conductivity along with the sliding (Fig. 11d).

Wear Mechanism Analysis

Figures 6–8 show that the CFB composites exhibited higherwear resistance in the sliding time despite their slightly higherfriction coefficients at some loads. This behavior can be attributedto the presence of MoS2 and the copper foam. The MoS2 in boththe CFB composites and the polymers without copper foam actedas an effective barrier to prevent large-scale fragmentation ofPTFE. Researchers have investigated the role of MoS2 as an ef-fective filler material (Savan, et al. (31)). They considered that thereinforcement of MoS2 greatly reduced the ultimate elongation tofracture in wear performance. Optical micrographs in Fig. 9 showthe worn surfaces of the P/M25 series composites and counterpartsteel rings. The transfer film formed on the counterface and thedeformation and adhesion of the malleable copper on the surfaceof the material after sliding are shown clearly.

Figure 12 shows the SEM micrographs of the worn surfacesof the P/M25-1 composite and the P/M25 polymer composite at0.424 m/s and a load of 500 N. The worn surface of the CFB

composite can be divided into three areas with the visible copperstruts: a transitional area, the metallic strut, and a polymer area(Figs. 12a–12c). It is seen that the wear scar on the metallic strutis smoother and shows fewer apparently plucked and ploughedmarks than the polymer area. The worn surfaces on the polymerareas of the two composites are not much different with the ex-ception of those due to hard particle erosion and plastic furrowdeformation (Figs. 12c and 12d).

Material flow can be seen clearly along the sliding directionthrough the contacting surface layer (Fig. 12). The flow led by theprogressive inelastic deformation tends to partially or fully coverup the existing porosity. The size and shape of wear debris as aprecursor of the flow were dominated by the ratcheting effect ofthe steel rings to polymer, which had significant effects on the for-mation of continuous transfer layers. The friction heat, compres-sive stress, and tensile stress originate and extend the polymerflow. At these local regions of intense contact high temperaturesand stresses are created and then induce the polymer flow and thechemical and mechanical bonding, and these in turn suppress theratcheting effect and smoothen the transfer film.

Figure 13 shows the X-ray photoelectron spectroscopy (XPS)spectra on the specific region of the transfer film of the P/M25-1composite where the copper strut is identifiable to determinethe role of copper foam in formation of the composite transferfilm. According to the standard XPS spectrum, the Mo3d peaksat 229.2 and 232.6 eV are assigned to MoS2 and MoOx (MoO2

and MoO3 and/or Mo4O11), which are supported by the weakMo3d peak at 235.8 eV and the main peak in the S2p spectrumat 162.2 eV (Figs. 13a and 13b; Zhang, et al. (32)). The main

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Fig. 13—XPS spectra of the transfer film on the copper-colored area of the P/M25-1 composite: (a) Mo3d, (b) S2p, (c) Cu2p; and (d) F1s at a velocity of0.424 m/s and load of 500 N (P/M25: 75% PTFE + 25% MoS2; P/M25-1: #1 copper foam filled with P/M25 component).

peak in the Cu2p spectrum at 935.6 eV corresponds to CuSO4

supported by the S2p peak at 169.5 eV, and the weak Cu2ppeak at 953.8 eV corresponds to Cu2O, CuO, CuS, and/orCu2S supported partly by the S2p peak at 169.5 eV (Fig. 13c).The worn PTFE has almost the same F1 peaks as the pureunworn one, which indicates that no apparent tribochemicalreactions occurred for the PTFE in sliding against the steel ring(Fig. 13d).

The above XPS results reveal that during the sliding processthe oxidation of MoS2 and Cu and the reactions between them toform new compounds were the dominant chemical processes thatoccurred. The elements Mo, S, and F on the identifiable copperarea verified the presence of the composite transfer film, and thesevere mechanical stress and associated frictional heat promotedthe reactions and enhanced the adhesion of the transfer film tothe steel surface.

Figure 10 shows a schematic representation of the wear modeof the CFB composite during sliding. The abrasion mechanismwas three-body abrasion caused and promoted by plastic de-formation, abrasive wear, and fatigue spalling. The wear debrison the contact surface included copper debris (oxides included),MoS2 debris (oxides included), and PTFE debris. These wear de-bris in the transfer layer maintain a balanced state of accumu-lation and extrusion during friction. Some debris squeezed outby the rotating ring piled up on the outlet is shown on the left

side of Fig. 10a, and some debris peeled off by the sample piledup at the entrance is shown on the right side of Fig. 10a. In theinitial period of wear the lubricants quickly moved into the in-terface, which is in accordance with the variation in contact re-sistance. This indicates a strong possibility of abrasive wear andfurrowing due to the abrasive nature of the copper debris com-ing from the copper foam strut at some local direct contact be-tween the copper and the steel ring. The copper debris serves asabrasive particle and conductive particle. As the friction contin-ued to the steady wear stage, the heat increased in local regionsand the process of the formation and break of transfer layers wasbalanced, and plastic deformation and fatigue wear became dom-inant at the frictional interface. The copper foam was effective informing an interconnecting bond with the filler materials, whichcan interrupt large-scale fragmentation of the polymer, and wasalso high-efficiency in heat conduction. The transfer of copperdebris also improved the adhesion of the film, and the higher thesliding velocity and applied load, the more apparent this trendbecame.

CONCLUSIONS

Two kinds of copper foam samples of different sizes werefilled with solid lubricants such as PTFE and MoS2 with the useof a vacuum filtration apparatus. On the basis of the results pre-sented the following conclusions can be formulated:

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Tribological Behaviors of Copper Foam–Based Composites 31

1. The CFB composites showed a slight improvement in bend-ing properties compared with those without copper foam andexhibited outstanding thermal and electrical conductivity dueto the interconnected metal skeletons embedded in the PTFE-based polymers.

2. The wear rates of CFB composites substantially decreasedcompared with those of the homologous polymers withoutcopper foam, and the friction coefficients affected by the cop-per foam varied with load and velocity. MoS2 as the reinforce-ment lubricant at 25% content was found to achieve optimumtribological properties.

3. An investigation of the contact resistances indicated that thefrictional interface layer had a higher resistance in the wearstage, when the contact resistance was relatively lower beforeand after the run.

4. The main wear mechanism of the CFB composites was three-body abrasion, caused and promoted by plastic deformation,abrasive wear, and fatigue spalling. XPS analysis results ofthe identifiable copper area in the transfer film showed thatoxidation of MoS2 and Cu and the reactions between themto form new compounds were the dominant chemical pro-cesses that occurred. The wear debris on the contact surfaceincluded copper debris (oxides included), MoS2 debris (oxidesincluded), and PTFE debris.

ACKNOWLEDGEMENT

This work was supported by the National Natural ScienceFoundation of China (Grant Nos. 90916021 and 50823008) andthe “Hundred Talents Program” of the Chinese Academy of Sci-ences and the State Key Laboratory of Solid Lubrication.

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