A study on tribological behaviour of transfer films of PTFE/bronze composites

Transfer films of PTFE/bronze composites with 5–30% volume content of bronze were prepared using a RFT friction and wear tester on surface of AISI-1045 steel bar by different sliding time (5–60 min). Tribological properties of these transfer films were studied using a DFPM reciprocating tribometer in a point contacting configuration under normal loads of 0.5, 1.0, 2.0 and 3.0 N. Thickness and surface morphology of the transfer films were investigated. It was found thickness of the transfer films slightly increased along with the increase of bronze content of corresponding composites. Increased sliding time of transfer film preparation is helpful to form transfer film with better ductibility and continuity, but sliding time almost has no effect on tribological properties of the transfer film. Higher bronze content in the composite improved tribological properties of the corresponding transfer film, i.e., reduced friction coefficient and prolonged wear life. All these transfer films are sensitive to load change. Their wear life becomes shorter along with the increase of load. SEM image of the worn surface show fatigue wear and adhesion wear have happened on the transfer film during the friction process. The author believe bronze in the transfer film effectively partaked in shear force applied on the transfer film and its good ductibility helped to improve tribological properties of the transfer films.     

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.     

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 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.     

Tribological performance of silicone composite coatings filled with wax-containing microcapsules

The effect of wax-containing microcapsules incorporated in silicone composite coatings deposited on aluminum (Al) alloy substrates on the tribological performance of the coatings was systematically investigated. The wax-containing microcapsules were prepared via in situ polymerization. The tribological behavior of the composite coatings was evaluated using ball-on-disk tribological test. It was found that the increase in microcapsule concentration in the composite coatings apparently reduced the friction coefficient of the coatings because the lubricant released from the broken microcapsules during the tribological test of the coatings lubricated the rubbing surfaces. The results showed that the silicone composite coatings rubbed by a smaller Cr6 steel ball (3 mm diameter) under a lower normal load (100 mN) produced higher friction coefficients via reduced complication of their underlying strong substrates compared to the same coatings tested against a larger Cr6 steel ball (6 mm diameter) under a higher normal load (1 N).     

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.     

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.     

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.     

Friction and wear behaviors of C/C-SiC composites containing Ti3SiC2

In the present paper, friction and wear behaviors of a carbon fiber reinforced carbon–silicon carbide–titanium silicon carbide (C-SiC–Ti₃SiC₂) hybrid matrix composites fabricated by slurry infiltration and liquid silicon infiltration were studied for potential application as brake materials. The properties were compared with those of C/C-SiC composites. The composites containing Ti₃SiC₂ had not only higher friction stability coefficient but also much higher wear resistance than C/C-SiC composites. At an initial braking speed of 28 m/s under 0.8 MPa pressure, the weight wear rate of the composites containing 5 vol% Ti₃SiC₂ was 5.55 mg/cycle, which was only one-third of C/C-SiC composites. Self-lubricious film-like debris was formed on the composites containing Ti₃SiC₂, leading to the improvement of friction and wear properties. The effect of braking speed and braking pressure on the tribological properties of modified composites were investigated. The average friction coefficient was significantly affected by braking speed and braking pressure, but the wear rate was less affected by braking pressure.     

Effect of rubber component on the performance of brake friction materials

Various composite friction materials containing 40 vol.% organic binder (phenolic resin plus styrene–butadiene–rubber (SBR)) with varying phenolic-resin/SBR ratio were prepared. The content of phenolic resin in each composite was indicated by the resin value (RV) index ranging between 0 and 100%. The composites with RVs greater than 50% form resin-based friction materials in which the primary binder is the phenolic resin. For RVs less than 50%, the composites become the rubber-based materials where the primary binder is the SBR. The analysis of mechanical properties exhibited that the conformability of the composites increases upon incorporation of SBR. The frictional analysis revealed that type of polymeric binder, i.e. resin or rubber, dominates greatly the frictional behavior of the composites. The increment of friction force and higher improvement in the frictional fade and recovery with sliding velocities are the general features of rubber-based friction materials. It was attributed to the inherent properties of rubber on the viscoelastic response at higher sliding velocities and entropic contribution on the mechanical properties at higher temperatures. The wear rate of resin-based materials and its drum temperature is lower than those of rubber-based materials. It was attributed to the strongly adhered multilayer secondary plateaus formed on the surface of resin-based materials.     

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.     

Tribological properties of NiCr–Al2O3 cermet-based composites with addition of multiple-lubricants at elevated temperatures

The tribological properties of NiCr-40 wt% Al₂O₃ (NC40A) cermet-based composites containing SrSO₄ and other lubricant (graphite, MoS₂ and Ag) against alumina ball were evaluated to identify their self-lubrication mechanisms from room temperature to 800 °C. The composites demonstrated distinct improvements in effectively reducing friction and wear, as compared to NC40A cermet. The best results were observed for NC40A–10SrSO₄–10Ag composite, which exhibited satisfactory reproducibility of friction coefficient over a wide temperature range (200–800 °C) through high temperature cyclic friction tests due to the formation of synergistic lubricating films SrAl₄O₇, NiCr₂O₄ and Ag on the contact surface.     

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.     

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.     

Fretting wear behaviors of nanometer Al2O3 and SiO2 reinforced PEEK composites

The influence of diameter and content of Al₂O₃ particles on the tribological behaviors under fretting wear mode was investigated. The surface of PEEK composite and steel ball were examined by SEM and EDS, to identify the topography of wear scar and analyze the distribution of chemical elements in the friction counterparts, respectively. It can be found that the filling of Al₂O₃ powder improves the fretting wear resistance of PEEK composite. With the increase of Al₂O₃ diameter, the area of wear scar on specimen increases first and decreases afterward. However, the wear of composites increases monotonically with increasing Al₂O₃ content. Although the filling of 10 wt.% and 200 nm PTFE powder in PEEK makes the lowest wear of all specimens, no synergistic effect was found when Al₂O₃ and PTFE were filled into PEEK composite together. For the friction pair of PEEK composite and steel ball, abrasive wear and adhesive wear dominate the fretting wear mechanism during fretting. Thermal effect plays a very important role during fretting; thus the property of temperature resistance for polymer material would affect the wear degree on the surface of wear scar.     

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.     

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 PEEK components with compositionally graded PEEK/PTFE surfaces

PEEK is a high strength engineering thermoplastic that suffers from a high friction coefficient and a friction induced wear mode. Past studies with 10 μm PEEK and PTFE powders resulted in composite solid lubricant that (at the optimal composition) had a wear rate of k = 2 × 10⁻⁹ mm³/Nm with a friction coefficient of μ = 0.12. A compositional grading of PEEK and PTFE is implemented in this study to create a bulk composite with the functional requirements of component strength, stiffness and wear resistance while providing solid lubrication at the sliding interface. The tribological performances of three functionally graded PEEK components were evaluated on linear reciprocating, rotating pin-on-disk and thrust washer tribometers. Wear rates comparable to samples of the bulk solid lubricant and comparable or improved frictional performance were achieved by compositionally grading the near surface region of PEEK components.     

A comparison of the reciprocating sliding wear behaviour of steel based metal matrix composites processed from self-propagating high-temperature synthesised Fe–TiC and Fe–TiB2 masteralloys

Steel matrix particulate composites were processed by direct addition of various powders to molten medium carbon steel. Fe–TiC and Fe–TiB₂ powders were produced using a self-propagating high-temperature synthesis (SHS) reaction and consisted of a dispersion of fine TiC (5–10 μm) and TiB₂ particles (2–5 μm), respectively in an iron binder.Addition of the Fe–TiC powder to the steel resulted in the formation of a metal matrix composite containing a homogeneous dispersion of TiC particles. However, addition of the Fe–TiB₂ powder resulted in the formation of a parasitic Fe₂B phase and TiC within the steel microstructure. In response to this an SHS masteralloy composed of Fe–(50% TiB₂+50%Ti) was manufactured which, when added to steel, prevented the formation of Fe₂B and resulted in a composite containing a mixture of TiB₂ and TiC particles.Dry reciprocating sliding wear behaviour of the three composite materials and their unreinforced counterpart was investigated at room temperature against a white cast iron counterface. Relative wear behaviour of the materials varied as a function of load. In all cases, the composite manufactured by addition of Fe–TiB₂ (yielding Fe₂B and TiC phases in the steel) exhibited wear rates greater than three times that of the unreinforced alloy. However, improvements in wear resistance over the base steel of up to two and a half times were observed with the other composites where the desired TiC and/or TiB₂ phases were retained in the steel. Scanning electron microscopy has been used to interpret wear behaviour in relation to both the as-cast microstructures of the composites and the wear scar microstructures observed.     

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.