Mechanical and tribological properties of phenolic resin-based friction composites filled with several inorganic fillers

The calcined petroleum coke (CPC), talcum powder (TP) and hexagonal boron nitride (h-BN) were used as the friction modifiers to improve the mechanical and tribological properties of phenolic resin-based friction composites (the resin matrix was coded as PHE). Thus the composites filled with the inorganic particulates of laminar structures were prepared by compression molding. The hardness and bending strength of the friction composites were measured. The tribological properties of the composites sliding against cast iron were evaluated using a pin-on-disc test rig. The morphologies of the worn surfaces of the composites and the transfer films on the counterpart cast iron disc were analyzed by means of scanning electron microscopy, and the elemental plane distributions on the transfer films were analyzed using energy-dispersive X-ray analysis (EDXA). It was found that the friction composites of different compositions showed different friction and wear behaviors, which was highly dependent on the volume fractions of the friction modifiers in the composites. Namely, the inclusion of CPC, h-BN, and TP at a volume fraction of 10% helped to greatly increase the bending strength and wear resistance of the composites, and in these cases the coefficients of friction for the composites were ranged within 0.43–0.47. In particular, the PHE-based composite with 10% h-BN had excellent friction stability at various testing conditions and showed the best wear resistance above 125 °C, which was attributed to the formation of a compact friction film (third-body-layer) on the rubbing surface of the composite and of a durable transfer film on the rubbing surface of the counterpart cast iron. The PHE-based composite with 10% CPC showed the best wear resistance below 125 °C, which was ascribed to the same reasons mentioned above. The different actions of various friction modifiers in terms of their effects on the friction and wear behavior of the phenolic resin-based friction composites could be related to their different bonding strengths with the resin matrix and their different abilities to form friction films (third-body-layer) on the surfaces of the composites and transfer films on the counterpart cast iron surface as well.     

The tribological properties of bronze–SiC–graphite composites under sea water condition

Bronze–SiC–nickel coated graphite composites were fabricated by powder metallurgy technique (P/M). The tribological properties of composites sliding against AISI321 stainless steel pin were studied under sea water condition. The graphite is an effective solid lubricant in sea water environment. The SiC improved the hardness and tribological properties of composites. The friction coefficient of bronze–SiC–graphite composites increased with the increase of SiC. However, the specific wear rate of bronze–SiC–graphite composites decreased with increasing SiC. Bronze-2 wt% SiC-11.7 wt% nickel coated graphite composite showed the best tribological properties due to the synergistic effects of reinforcements.     

Novel polytetrafluoroethylene (PTFE) composites with newly developed Tribaloy alloy additive for sliding bearings

A group of novel polytetrafluoroethylene (PTFE)-based composite materials are developed for sliding bearing applications. The reinforcements include the newly developed T-401 Tribaloy alloy, which possesses better ductility compared to conventional Tribaloy alloys, spherical bronze particles, chopped carbon fibers and milled graphite. The specimens are fabricated with the compression moulding technique under different preforming and sintering cycles. The mechanical and tribological properties as well as corrosion resistance of the new composites are investigated. It is demonstrated that these properties are influenced by the type of fillers and the content level of fillers. The wear resistances of all the developed PTFE composites are much higher than that of pure PTFE with very low coefficients of friction. Among the developed composites, the mixture of 40% PTFE + 15% T-401 + 45% bronze exhibits the best combination of properties.     

Characterization and tribological investigation of sol–gel ceramic films on Ti–6Al–4V

SiO₂, TiO₂, and hydroxyapatite (HA) thin films with good biocompatibility were grown on Ti–6Al–4V (coded as TC4) substrate by sol–gel and dip-coating processes from specially formulated sols, followed by annealing at 500 °C The chemical states of some typical elements in the target films were detected by means of X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM) are applied to characterize the original unworn films. The tribological properties of thin films sliding against an AISI52100 steel ball were evaluated on a reciprocating friction and wear tester. As the result, the target films composed of nano-particles ranging from 30 nm to 100 nm around were obtained. All the sol–gel ceramic films are superior in resisting wear compared with the TC4 substrate. Among all, HA film shows the best resistance while SiO₂ film shows the worst wear resistance both under higher (3 N) and lower load (1 N). TiO₂ shows good wear resistance under lower load (1 N). SEM observation of the morphologies of worn surfaces indicates that the wear of TC4 is characteristic of abrasive wear. Differently, abrasion, plastic deformation and micro-fracture dominate the wear of ceramic films. The superior friction reduction and wear resistance of HA film is greatly due to the slight plastic deformation of the film. It is supposed that the deformation of the HA film is closely related to the special arrangement of the nano-particles and microstructure. HA film is recommended for clinical application from the point of wear resistance view.     

Tribological behavior of graphite-containing nickel-based composite as function of temperature,load and counterface

Nickel-based graphite-containing composites were prepared by powder metallurgy method. Their mechanical properties at room temperature and friction and wear properties from room temperature to 600 °C were investigated by a pin-on-disk tribometer with alumina, silicon nitride and nickel-based alloy as counterfaces. The effects of graphite addition amount, temperature, load, sliding speed and counterface materials on the tribological properties were discussed. The micro-structure and worn surface morphologies were analyzed by scanning electron microscope (SEM) attached with energy dispersive spectroscopy (EDS). The results show that the composites are mainly consisted of nickel-based solid solution, free graphite and carbide formed during hot pressing. The friction and wear properties of composites are all improved by adding 6–12 wt.% graphite while the anti-bending and tensile strength as well as hardness decrease after adding graphite. The friction coefficients from room temperature to 600 °C decrease with the increase of load, sliding speed while the wear rates increase with the increasing temperature, sliding speed. The lower friction coefficients and wear rates are obtained when the composite rubs against nickel-based alloy containing molybdenum disulfide. Friction coefficients of graphite-containing composites from room temperature to 600 °C are about 0.4 while wear rates are in the magnitude of 10^?5 mm³/(N m). At high temperature, the graphite is not effective in lubrication due to the oxidation and the shield of ‘glaze’ layer formed by compacting back-transferred wear particles. EDS analysis of worn surface shows that the oxides of nickel and molybdenum play the main role of lubrication instead of graphite at the temperature above 400 °C.     

Sliding wear performance of nano-SiO2/short carbon fiber/epoxy hybrid composites

In the present work, epoxy based composites filled with hybrid nano-SiO₂ particles and short pitch based carbon fiber were prepared. Copolymer of styrene and maleic anhydride was grafted onto the nanoparticles prior to the compounding so that the nanoparticles can be covalently connected to the composites’ matrix through the reaction between anhydride and epoxide groups during curing. Consequently, the nano-SiO₂/matrix interfacial interaction was enhanced. By evaluating sliding wear properties of the composites as a function of the components concentrations, positive synergetic effect was found. That is, both wear rate and friction coefficient of the hybrid composites were significantly lower than those of the composites containing individual nano-SiO₂ or short carbon fiber. The composite with 4 wt.% nano-SiO₂ and 6 wt.% carbon fiber offered the greatest improvement of the tribological performance. Compared to the results of hybrid composites reported so far, the above composite is characterized by relatively lower filler content, which would facilitate processing in practice. Increased surface hardness, lubricating effect of the sheet-like wear debris reinforced by nano-SiO₂ and rapidly formed transfer film were believed to be the key issues accounting for the remarkable wear resisting and friction reducing behaviors.     

An investigation of friction and wear performances of bonded molybdenum disulfide solid film lubricants in fretting conditions

The friction and wear performances of bonded MoS₂ solid film lubricants with the counterpart steel ball rubbing were investigated in fretting wear conditions in order to inquire into the load-carrying capacity and wear mechanisms of bonded MoS₂ solid film lubricants under dry friction conditions. Experimental results show that the bonded MoS₂ solid film lubricants have excellent anti-friction and wear-resistance performances within a wide load range between 20 N and 800 N and within a wide oscillatory frequency range between 5 Hz and 30 Hz. It is found through analyses of the transfer films formed in the surface of the counterpart steel ball investigated by SEM, XPS and AES, that the thickness of the transfer film formed is about 38 nm and the oxidation of MoS₂ in the transfer films does not occur during dry friction process. The high load and frequency promote the formation of a compact transfer films. The compact transfer films are believed to be the predominant mechanism giving rise to high load-carrying capacity, and excellent wear-resistance performances of the bonded MoS₂ solid film lubricants.     

Tribological characterization of tungsten nitride coatings deposited by reactive magnetron sputtering

The structure, hardness, friction and wear of tungsten nitrides prepared by d.c. reactive magnetron sputtering were investigated. The coatings were deposited with different nitrogen to argon ratios; the total pressure was kept constant. The tribological tests were performed on a pin-on-disc tribometer in terrestrial atmosphere with 100Cr6 steel, Al₂O₃ and Si₃N₄ balls as sliding counter-bodies. The wear tracks, the ball-wear scars and the wear debris were analysed by scanning electron microscopy in order to characterize the dominant wear mechanisms.The coatings exhibited different phases as a function of the nitrogen content: films with low N content exhibited the α-W phase; β-W phase was dominant for nitrogen contents from 12 to 15 at.% and β-W₂N was observed for nitrogen content higher that 30 at.%. The mechanical and tribological properties of the tungsten nitride coatings were strongly influenced by the structure. The hardness and the Young's modulus values were in the ranges (29–39 GPa) and (300–390 GPa), respectively; the lowest values correspond to the coatings with the highest nitrogen content. Generally, the friction and wear rate of tungsten nitride coatings sliding against ceramic balls increased with nitrogen content reaching a maximum at 12 at.%; further increase of the nitrogen content led to a decrease of the friction and wear. The sliding with the steel balls did not wear the coatings under the selected testing conditions.     

Effects of sliding velocity and normal load on friction and wear characteristics of multi-layered diamond-like carbon (DLC) coating prepared by reactive sputtering

Friction and wear tests were performed to investigate effects of sliding velocity and normal load on tribological characteristics of a multi-layered diamond-like carbon (DLC) coating for machine elements. The DLC coatings which consist of sequentially deposited gradient Cr/CrN, W-doped DLC (a-C:H:W) and DLC (a-C:H) layers were formed on carburized SCM 415 Cr–Mo steel disks using a reactive sputtering system. The tests against AISI 52100 steel balls were performed under various sliding velocities (0.0625, 0.125, 0.25, 0.5, 1 and 2 m/s) and normal loads (6.1, 20.7 and 49.0 N) in ambient air (relative humidity=26±2%, temperature=18±2 °C). Each test was conducted for 20 km sliding distance without lubricating oil. The results show that friction coefficients decrease with the increase in sliding velocity and normal load. Wear rates of both surfaces decrease with the increase in normal load. The increase in sliding velocity leads initially to the increase in wear rates up to the maximum value. Then, they decrease, as the sliding velocity increases above specific value that corresponds to the maximum wear rate. Through surface observation and analysis, it is confirmed that formation of transfer layers and graphitized degree of wear surfaces of DLC coatings mainly affect its tribological characteristics.     

Extremely low coefficient of friction of diamond sliding against Ag thin films on Si(1 1 1) surface under ultrahigh vacuum condition

The tribological properties of a diamond (1 1 1) pin slid on subnanometer thick Ag films, which were deposited on a cleaned Si(1 1 1) substrate, were studied using a pin-on-plate tribometer. The preparation of Ag ultrathin films and frictional experiments were performed in an ultrahigh vacuum (UHV) chamber at a pressure of 10⁻⁸ Pa. The frictional experiments were carried out at a sliding speed of 0.1 mm/s and at a normal load of 250 mN. An extremely low coefficient of friction, less than 0.01, was obtained when the pin was slid on Ag films, whose thicknesses were 1 and 2.6 monolayer (ML) under reciprocal motion. The minimum coefficient of friction was less than 0.004 for Ag 1 ML film. After the extremely low coefficient of friction was obtained, Ag remained on the worn track without any transfer to the diamond pin, as confirmed by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS).The mechanisms of extremely low coefficients of friction are discussed in terms of the chemical bonding force between atoms of the topmost layers, and Ag coverage on the Si surface.     

Tribological behaviors of Ti–6Al–4V modified by chemical methods

In order to improve the tribological properties of titanium-based implants, sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂) solutions, sol–gel hydroxyapatite (HA) film, thermal treatment and combined methods of NaOH solution/HA film, H₂O₂ solution/HA film are used to modify the surfaces of Ti–6Al–4V (coded TC4). The chemical states of some typical elements in the modified surfaces were detected by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of modified surfaces sliding against an AISI52100 steel ball were evaluated on a reciprocating friction and wear tester. As the results, complex surfaces with varied components are obtained. All the methods are effective in improving the wear resistance of Ti–6Al–4V in different degrees. Among all, the surface modified by the combined method of NaOH solution/HA film gives the best tribological performances. The friction coefficient is also greatly reduced by the modification of NaOH solution. The order of the wear resistance under 3 N is as following: Ti–NaOH–HA>Ti–NaOH>Ti–HA>Ti–H₂O₂–HA>Ti–H₂O₂ >Ti–500; under 1 N is Ti–HA, Ti–NaOH–HA>Ti–NaOH. For Ti–H₂O₂, a very low friction coefficient and long wear life over 2000 passes is obtained under 1 N. SEM observation of the morphologies of worn surfaces indicates that the wear of TC4 is characteristic of abrasive wear. Differently, abrasion, plastic deformation and micro–crack dominate the wear of Ti–HA; slight abrasive wear dominate the wear mechanism of Ti–NaOH and microfracture and abrasive wear for Ti–NaOH–HA and Ti–H₂O₂–HA, while the sample modified by thermal treatment is characterized by sever fracture. The superior friction reduction and wear resistance of HA films are greatly attributed to the slight plastic deformation of the film. NaOH solution is superior in improving the wear resistance and decreasing the friction coefficient under relative higher load (3 N) and H₂O₂ is helpful to reduce friction and wear under relatively lower load (1 N). Combined method of Ti–NaOH–HA is suggested to improve the wear resistance of Ti–6Al–4V for medial applications under fretting situations.     

Effect of surface laser texture on friction properties of nickel-based composite

Laser surface texturing (LST) was performed on the nickel-based composites by a Nd:YAG pulsed laser and the regular-arranged dimples with diameter of 150 μm were fabricated on their surfaces. The textured surfaces were smeared with molybdenum disulfide powder. The tribological properties of the textured and filled composites were investigated by carrying out sliding wear tests against an alumina ball as a counterface using a high temperature ball-on-disk tribometer. The tests were conducted at a sliding speed of 0.4 m/s and at normal loads ranging from 20–100 N and from room temperature to 600 °C. The friction coefficient of nickel-based composite textured and smeared with molybdenum disulfide was found to reduce from 0.18 to 0.1 at the temperature range from 200 to 400 °C. The texture with a dimple density of 7.1% was observed to prolong wear life of MoS2 film by more than four times in comparison to the texture with other dimple densities. The lubricious oxide particles stored in the dimples reduce friction coefficient at elevated temperatures and compensate for the extra lubricant owing to the degradation of MoS₂ caused by its oxidation at high temperatures.     

Evolution of wear,roughness, and friction of Alloy 600 superalloy surfaces in water-submersed sliding

Self-mated wear and friction of Alloy 600 superalloy was studied in a water-submersed ring-on-rod configuration, loading the side of a 6.35 mm diameter rod across the flat surface of a rotating annular ring of 100 mm outer diameter and 70 mm inner diameter producing two sliding contacts along the ring. Tests were conducted at sliding speeds of 0.178 and 0.330 m/s for sliding distances of 100 m. Normal loads of 51 and 204 N were applied, and initial R_a surface roughnesses of the rings along the sliding direction were either smooth (～0.2 μm) or rough (～7.5 μm). Increased initial ring roughness caused a ～20-fold increase in rod wear at the lighter load, whereas at the heavier load increased initial roughness only caused a ～4-fold increase in wear. At lower initial ring roughness the 4-fold decrease in normal load caused a large (one order-of-magnitude) decrease in rod wear, whereas for rings of higher initial roughness the 4-fold decrease in normal load caused only minor (2-fold or less) decreases in rod wear. Wear during this 100 m sliding distance only experienced a minor effect from the 1.8-fold change in sliding speed, as did friction. In all cases friction coefficient rapidly settled into the range 0.6–0.7, except in the cases of lower load on rings of lower initial roughness where friction coefficient remained above 1 for most of this sliding duration. At this lower load the initial ～0.2 μm rod roughnesses increased to nearly 0.8 μm by the 100 m sliding distance, whereas at the higher load this same sliding distance resulted in roughnesses returning near to the initial 0.2 μm. It was hypothesized more highly loaded cases also went through initial roughening prior to smoothening back to 0.2 μm roughness within the 100 m sliding distance, and given additional sliding the more lightly loaded cases would also experience subsequent smoothening. Increasing sliding distance to 400 m, roughnesses indicated a smoothening back to 0.2 μm level during those lightly loaded tests, with friction coefficient correspondingly dropping from 1 into the 0.6–0.7 range observed in all other cases. Extended sliding to 400 m at light loading against rings of lower initial roughness also allowed a rod wear rate which increased with increased sliding distance to be observed, approaching the same rate observed against initially rough rings within the 100 m sliding distance.     

Influence of graphite content on the dry sliding and oil impregnated sliding wear behavior of Al 2024–graphite composites produced by in situ powder metallurgy method

The influence of graphite content on the dry sliding and oil impregnated sliding wear characteristics of sintered aluminum 2024 alloy–graphite (Al/Gr) composite materials has been assessed using a pin-on-disc wear test. The composites with 5–20 wt.% flake graphite particles were processed by in situ powder metallurgy technique. For comparison, compacts of the base alloy were made under the same consolidation processing applied for Al/Gr composites. The hardness of the sintered materials was measured using Brinell hardness tester and their bending strength was measured by three-point bending tests. Scanning electron microscopy (SEM) was used to analyze the debris, wear surfaces and fracture surfaces of samples. It was found that an increase in graphite content reduced the coefficient of friction for both dry and oil impregnated sliding, but this effect was more pronounced in dry sliding. Hardness and fracture toughness of composites decreased with increasing graphite content. In dry sliding, a marked transition from mild to severe wear was identified for the base alloy and composites. The transition load increased with graphite content due to the increased amount of released graphite detected on the wear surfaces. The wear rates for both dry and oil impregnated sliding were dependent upon graphite content in the alloy. In both cases, Al/Gr composites containing 5 wt.% graphite exhibited superior wear properties over the base alloy, whereas at higher graphite addition levels a complete reversal in the wear behavior was observed. The wear rate of the oil impregnated Al/Gr composites containing 10 wt.% or more graphite particles were higher than that of the base alloy. These observations were rationalized in terms of the graphite content in the Al/Gr composites which resulted in the variations of the mechanical properties together with formation and retention of the solid lubricating film on the dry and/or oil impregnated sliding surfaces.     

Mechanical and tribological properties of carbon fabric composites filled with several nano-particulates

The carbon fabric composites filled with the particulates of nano-SiO₂, nano-TiO₂, and nano-CaCO₃, respectively, were prepared by dip-coating of the carbon fabric in a phenolic resin containing the particulates to be incorporated and the successive curing. The friction and wear behaviors of the resulting carbon fabric composites sliding against AISI-1045 steel in a pin-on-disc configuration were evaluated on a Xuanwu-III high temperature friction and wear tester. The tensile strength and adhesion strength of the filled carbon fabric composites were determined on a DY35 universal materials test machine. The morphologies of the worn surfaces of the unfilled and filled carbon fabric composites and the transfer films on the counterpart steel pins were analyzed by means of scanning electron microscopy, and the elemental plane distributions on the transfer films were analyzed with an energy dispersive X-ray analyzer (EDAX). It was found that the nano-particles as the fillers contributed to significantly improve the mechanical properties and wear-resistance of the carbon fabric. Nano-CaCO₃ as the filler was the most effective in increasing the wear-resistance, while nano-SiO₂ was the most effective in increasing the friction-reducing ability and mechanical properties. This was because the nano-particulates as the fillers contributed to enhance the bonding strength between the carbon fabric and the adhesive resin. Moreover, the friction and wear properties of the carbon fabric composites were closely dependent on the characteristics of the transfer films formed on the counterpart steel pin surfaces and on the environmental temperature as well. Namely, the differences in the wear-resistance of various filled carbon fabric composites were related to the differences of their transfer films on the counterpart steel pin surface. The wear rates of the composites at elevated temperature above 180 °C were much larger than that below 180 °C, which was attributed to the degradation and decomposition of the adhesive resin at excessively elevated temperature.     

Tribological properties of surface-modified Pd nanoparticles for electrical contacts

A fully comprehensive study of the tribological behavior of palladium nanoparticles (Pd NPs) capped by tetrabutylammonium chains using a ball-on-disk tribometer under different conditions of applied load, concentration, tribometer motion, linear speed and nature of the counterface is revised. A low concentration of NPs (2 wt%) in tetrabutylammonium acetate was found sufficient to improve the tribological properties due to the formation of a protective transfer film (TF) comprised of metallic Pd. The increase of the applied load (up to 20 N, 1.82 GPa of contact pressure) confirmed the excellent extreme-pressure behavior avoiding the counterfaces from severe wear. After a running-in period whose duration depends on the operating conditions, the TF build-up allows to maintain a low contact electrical resistance through the contact (<0.1 kΩ) during the entire test. When the Pd NPs are used with ceramic counterfaces, the nanoparticles increase the load-bearing capabilities and performance of the base without forming TF, likely by mixed or boundary lubrication and healing effects. Finally, the Pd NPs are demonstrated to be useful as a thin solid lubricant film in reciprocating motion yielding a comparable tribological behavior. Hence, the presented surface Pd NPs can be very helpful to extend life of sliding components due to their high strength resistance providing a gateway to electrical conduction as well.     

Effect of electrical current on tribological behavior of copper-impregnated metallized carbon against a Cu–Cr–Zr alloy

A block-on-slip ring-type wear tester was used to investigate the tribological behavior of copper-impregnated metallized carbon against a Cu–Cr–Zr alloy under 2 to 6 N applied load and 0 to 20 A electrical current. The sliding speed was maintained at 25 km/h. The wear loss of copper-impregnated metallized carbon increased with greater electrical current. Under a certain applied load, the wear loss with electrical current was minimized. The tribo-layer had an apparent effect on the friction coefficient. The wear mechanisms were complex, consisting of adhesive wear, abrasive wear and arc erosion.     

Tribological behaviors of hierarchical porous PEEK composites with mesoporous titanium oxide whisker

Hierarchical porous PEEK self-lubricating composites were prepared by mold-leaching and vacuum melting process under high temperature. The tribological behaviors were investigated for the porous PEEK composite and the porous composite after incorporating micro-porogen (NaCl) and mesoporous titanium oxide whiskers. If only micro-porogen was incorporated, the lowest steady state specific wear rate was observed for PEEK composites filled with 30% NaCl. Based on this porous PEEK composite, the effects of mesoporous titanium oxide whiskers and non-perforated titanium oxide whiskers on the friction and wear properties of PEEK composites were studied. Results showed that nano-micro porous PEEK composites with 30 wt% micro-porogen and 5 wt% mesoporous titanium oxide whiskers reached the lowest friction coefficient and specific wear rate, which were recorded as 0.0194 and 2.135×10^–16 m³/Nm under the load of 200 N. Compared with 15 wt% carbon fiber-reinforced PEEK composite which is widely used in industry, the wear resistance of the designed hierarchical porous PEEK composite increased by 41 times, showing outstanding wear resistance.     

Influence of particle content and porosity on the wear behaviour of cast in situ Al(Mn)–Al2O3(MnO2) composite

Aluminium-based tribological materials may reduce the weight of components, leading to significant fuel economy. The aim of the present study is to investigate the wear and friction in cast in situ Al(Mn)–Al₂O₃(MnO₂) composites synthesized by dispersing MnO₂ particles in molten aluminium, which get reduced to form Al₂O₃ particles. Wear tests have been conducted at four normal loads of 9.8, 19.6, 29.4 and 39.2 N and at a constant sliding speed of 1.05 m/s using a pin-on-disc wear testing machine, under dry sliding conditions. The results of the investigation indicate that the cumulative volume loss and wear rate of in situ composites are significantly lower than those observed in either the commercial aluminium or Al–Mn base alloy, under similar loading and sliding conditions. The influences of both reinforcing particle and porosity contents on the tribological behaviour of in situ composites were evaluated. It has been found that at a given particle content, the wear rate increases with increasing porosity content due to its combined effect on real area of contact and subsurface cracking. The wear rate of in situ composites with relatively lower porosity decreases with increasing particle content, but does not appear to change significantly or even increases a little with increasing particle content when the composites have relatively higher porosity. In view of large fluctuations in coefficient of friction during sliding, no effect of particle or porosity contents on the coefficient of friction could be determined unambiguously for different in situ composites.     

WC–ZrO2 composites: processing and unlubricated tribological properties

In the present work, we report the processing and properties of WC–6 wt.% ZrO₂ composites, densified using the pressureless sintering route. The densification of the WC–ZrO₂ composites was carried out in the temperature range of 1500–1700 °C with varying time (1–3 h) in vacuum. The experimental results indicate that significantly high hardness of 22–23 GPa and moderate fracture toughness of ∼5 MPa m^1/2 can be obtained with 2 mol% Y-stabilized ZrO₂ sinter-additive, sintered at 1600 °C for 3 h. Furthermore, the friction and wear behavior of optimized WC–ZrO₂ composite is investigated on a fretting mode I wear tester. The tribological results reveal that a moderate coefficient of friction in the range from 0.15 to 0.5 can be achieved with the optimised composite. An important observation is that a transition in friction and wear with load is noted. The dominant mechanisms of material removal appear to be tribochemical wear and spalling of tribolayer.