Effect of electrical current on the tribological behavior of the Cu-WS2-G composites in air and vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.     

The Mechanical Mixed Layer and Its Role in Cu-15Ni-8Sn/Graphite Composites

This article provides an in-depth investigation into the formation of the mechanical mixed layer (MML) and its role in Cu-15Ni-8Sn/graphite composites. Wear tests were conducted at room temperature using a ring–block configuration with an applied load of 50 N and sliding speed of 0.42 m/s. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were performed to analyze the worn surfaces and subsurfaces. Results indicated that high graphite content contributed to the formation of a protective MML. When the MML formed on the tribosurface as the graphite content increased, both the friction coefficient and wear rate greatly decreased. The friction coefficient with a stable value of 0.075 and wear rate of 6.10 × 10^?16 (m³/N· m) were the lowest when an apparent tribolayer appeared at the graphite content of 38 vol%. The characteristics of the MML and its influence on wear mechanisms of the composites are discussed. The MML existing on the worn surface protected the materials from severe adhesion and abrasion and the predominant wear mechanisms changed to delamination, which resulted in the drastic changes in wear resistance and friction coefficient.     

Impact of Viscoelasticity on the Tribological Behavior of PTFE Composites for Valve Seals Application

In this article, we studied and explored the impact of viscoelasticity on the friction and wear behavior of pure polytetrafluoroethylene (PTFE), carbon–graphite PTFE composite, and glass fiber–MoS₂ PTFE composite. Tests were carried out using a specific reciprocating tribometer for valve seal application. The worn surfaces of the PTFE composites and the transfer films formed on the counterface were examined with a scanning electron microscope (SEM). Experimental results revealed that the addition of filler materials was effective in reducing the wear volume in all composites studied. In addition, the friction coefficient and wear resistance showed high sensitivity to the viscoelastic behavior of the PTFE seal. SEM investigation showed that the incorporation of particulate fillers into the PTFE matrix could dramatically reduce and stabilize the transfer films to the counterface, so they largely decreased the wear of the PTFE composites.     

Load bearing capacity of bronze, iron and iron–nickel powder composites containing fullerene-like WS2 nanoparticles

In the past few years, inorganic fullerene-like (e.g. IF) supramolecules of metal dichalcogenide MX₂ (M=Mo,W, etc.; X=S, Se), materials with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. Recent experiments showed that IF possess lubricating properties superior to those of commercially available layered solid lubricant (2H–WS₂ polytype) in a wide range of operating conditions. It was shown before that the impregnation of a small amount of such nanoparticles into porous bronze matrix largely improve the tribological properties of bronze–steel contact pairs. In the present work, the effect of the PV (pressure–velocity) parameter on the transition to seizure for powdered bronze–graphite, iron–graphite and iron–nickel–graphite composites impregnated 2H and IF lubricant has been studied. The tribological tests were performed using a ring–block tester at loads of 150–1200 N and sliding speeds from 0.5 to 1.7 m/s. It was found that impregnation of IF into the pores improves the tribological properties of the powdered composites in comparison to 2H–WS₂ solid lubricant. Furthermore, it was established that the impregnation of oil together with the IF nanoparticles allows to provide very high load bearing capacity of the powdered materials.It is suggested that the transition to seizure occurs when the wear debrises accumulate in the pores on the surface. This process practically blocks the surface pores and limits the supply of the solid lubricant particles to the contact surface. The main advantage of the IF nanoparticles is attributed to: (a) slow release and supply of nanoparticles from the open pores to the surface; (b) sliding/rolling of the IF between the rubbing surfaces; and (c) prevention of the accumulation of the agglomerated wear particles in the pores. The model of third body was used in order to explain the effect of the wear particles, oil and solid lubricant particles on the friction and wear behavior of powdered composites.     

Tribological Properties of Organic Functionalized ZrB<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Epoxy Composites

The tribological behaviors of epoxy composites filled with organic functionalized ZrB₂–Al₂O₃ were environmentally investigated and compared with those with as-received fillers under both dry and oil sliding conditions in this work. The worn surfaces and the transfer films on the counterparts were characterized by scanning electronic microscope (SEM), and the frictional temperature rising was investigated by infrared thermometer. The results demonstrated that the coefficient of friction (CoF), the wear rate, as well as the frictional temperature rise of the epoxy composites were all decreased due to the introduction of ZrB₂–Al₂O₃ fillers. And with the increase in filler content, similar variation tendencies of CoF and wear rate of epoxy composites were observed under the different sliding conditions. Besides, the organic functionalization of ZrB₂–Al₂O₃ fillers, which made the epoxy composites exhibit lower CoF and wear rate than those with as-received fillers, lowered the frictional temperature as well. In comparison, the epoxy composites filled with 5 vol% modified fillers presented better tribological properties, suggesting a stronger interfacial bonding between modified fillers and epoxy matrix. The dominant wear mechanisms of filled composites under dry and oil sliding conditions could be inferred as the combination of adhesive wear and abrasive wear and the fatigue wear, respectively, on the basis of SEM images of worn surfaces.     

石墨含量对铜基石墨自润滑复合材料摩擦过程中石墨润滑膜的影响

为研究石墨含量对铜基石墨自润滑复合材料摩擦过程中形成石墨润滑膜的影响,采用粉末冶金法制备了不同石墨含量的铜基石墨自润滑复合材料,测试了复合材料的力学性能,用自制环-块摩擦试验机测试评估了材料的耐磨性能,用光学显微镜实时原位观察了摩擦表面组织形貌的变化,用扫描电镜对磨痕进行观察和分析,通过能谱仪成分扫描分析接触面石墨润滑膜的覆盖率。结果表明:随着复合材料中石墨含量的增加,材料的力学性能逐渐降低,石墨润滑膜的覆盖率先升高后降低,磨损量先减小后增大;当石墨体积分数为14%时,石墨润滑膜的覆盖率最高,磨损量最小,耐磨性能最好。     

Microscratch behavior of copper–graphite composites

Microscratch tests were carried out on Cu–graphite composites with graphite content of 0–30 vol% and normal loads of 0.5–2 N. Scratch grooves generated by the plastic deformation of surfaces were characterized for detailed friction and wear mechanisms investigation. The influence of normal load and graphite content on friction coefficient was also studied. It is found that the dominant wear mechanism transits from ploughing to micro-cutting with increasing the normal loads. A friction model for knowing the contribution of ploughing and adhesion components to friction is presented. This friction model is useful in understanding the friction mechanism of composites during scratching.     

The friction behavior of Ni-, SiO2- and mica sodium silicate-based solid lubrication composites

The friction behavior of Ni⁻-, SiO⁻₂- and mica sodium silicate-based lubricant composites, which included BN, WS₂ and graphite as lubricants, were examined. A ring-on-disk apparatus, in which a solid lubricant composite disk was held against a rotating stainless ring, was used as the test configuration. The tests were run with a load from 62 to 250 N in temperatures from 20 to 800°C in the laboratory environment. The wear surface was characterized by scanning electron microscope and X-ray photo spectroscopy. The major findings were that both mica sodium silicate- and SiO⁻₂-based composites failed at above 500°C due to severe wear and surface damage; in contrast, Ni⁻-based composite showed a stable friction coefficient and low wear from 20 to 800°C.     

Tribological performance of microwave sintered copper-TiC-graphite hybrid composites

Copper matrix composites are finding many applications due to their inherent properties such as good electrical and thermal conductivity and corrosion resistance. New series of copper-TiC (5-15 vol%)-graphite (5-10 vol%) hybrid composites are fabricated through a novel microwave processing technique. Pin-on-disc is used to evaluate their tribological properties under testing parameters of normal loads 12-48 N and sliding speed of 1.25-2.51 m/s. The formation of mixed smooth layer with higher graphite hybrid composites improves the wear resistance and reduces the friction coefficient. Morphology of worn out surfaces and wear debris were analyzed to understand the wear mechanisms.     

Tribological Properties and Wear Mechanisms of NiCr–Al2O3–SrSO4–Ag Self-Lubricating Composites at Elevated Temperatures

NiCr–Al₂O₃–SrSO₄–Ag self-lubricating composites were prepared by powder metallurgy method and the tribological properties of composites were evaluated by a ball-on-disk tribometer against alumina ball at wide temperature range from the room temperature to 1,000 °C in air. The linear coefficient of thermal expansion was evaluated for investigation of thermal stability of composites. The tribo-chemical reaction films formed on the rubbing surfaces and their effects on the tribological properties of composites at different temperatures were addressed according to the surface characterization by SEM, XRD, and XPS. The results show that the NiCr–Al₂O₃ composite with addition of 10 wt% SrSO₄ and 10 wt% Ag exhibits satisfying friction and wear properties over the entire temperature range from room temperature to 1,000 °C. The composition of the tribo-layers on the worn surfaces of the composites is varied at different temperatures. The synergistic lubricating effect of SrAl₄O₇, Ag, and NiCr₂O₄ lubricating films formed on worn surfaces were identified to reduce the friction coefficient and wear rate from room temperature to 800 °C. Meanwhile, at 1,000 °C, the SrCrO₄ and NiAl₂O₄ was formed on the worn surfaces during sliding process, combining with the NiCr₂O₄, Al₂O₃, Cr₂O₃, Ag, and Ag₂O, which play an important role in the formation of a continuous lubricating film on the sliding surface.     

Tribological Behavior of Composites Based on ZA-27 Alloy Reinforced with Graphite Particles

The objective of this investigation is to assess the influence of graphite reinforcement on tribological behavior of ZA-27 alloy. The composite with 2 wt% of graphite particles was produced by the compocasting procedure. Tribological properties of unreinforced alloy and composite were studied, using block-on-disk tribometer, under dry and lubricated sliding conditions at different specific loads and sliding speeds. The worn surfaces of the samples were examined by the scanning electron microscopy (SEM). The obtained results revealed that ZA-27/graphite composite specimens exhibited significantly lower wear rate and coefficient of friction than the matrix alloy specimens in all the combinations of applied loads (F _n ) and sliding speeds (v) in dry and lubricated tests. The positive tribological effects of graphite reinforcement of ZA-27 in dry sliding tests were provided by the tribo-induced graphite film on the contact surface of composite. In test conditions, characterized by the small graphite content and modest sliding speeds and applied loads, nonuniform tribo-induced graphite films were formed leading to the increase of the friction coefficient and wear rate, with increase of the sliding speed and applied load. In conditions of lubricated sliding, the very fine graphite particles formed in the contact interface mix with the lubricating oil forming the emulsion with improved tribological characteristics. Smeared graphite decreased the negative influence of F _n on tribological response of composites, what is manifested by the mild regime of the boundary lubrication, as well as by realization of the mixed lubrication at lower values of the v/F _n ratio, with respect to the matrix alloy.     

石墨铝基自润滑材料的制备及性能表征

以石墨为固体润滑剂,铝为基体材料,添加硅、铜、铁等元素作为强化成分,添加Cr3C2、粉煤灰来提高自润滑材料的耐磨性,通过液态铸造法制备铝基自润滑材料。借助MM-200磨损试验机研究不同石墨含量以及粉煤灰的添加对铝基复合材料摩擦磨损性能的影响;在Olympus金相显微镜下观察材料显微组织;用S-3000N扫描电子显微镜对材料拉伸断口和磨损表面进行观察。结果表明:随着石墨含量的增加,摩擦因数逐渐降低,磨损率逐渐升高,抗拉强度、硬度都逐渐降低;粉煤灰的加入使得初生硅得到细化,自润滑材料的强度上升,摩擦因数得到进一步的改善。     

Tribological behavior of metal matrix composites

The wear and friction behavior of continuous graphite fiber reinforced metal matrix composites was investigated. Composite materials were tested against 4620 steel at 54 m s^?1 at room temperature in air without lubricant. The graphite fibers studied included rayon-, pitch- and polyacrilonitrile (PAN)-based fibers. Both high modulus and high strength PAN-based fibers were examined. The fibers were incorporated into copper- and silver-based alloys by means of a liquid metal infiltration technique. The results of this study indicate that the type of graphite fiber in the composite is the most significant factor in the wear and friction behavior of metal matrix composites. In some high modulus fiber tin-bronze composites the fiber fraction influences the wear rate but not the coefficient of friction. Neither the matrix alloy nor the composite tensile strength per se correlate with the friction and wear properties; however, there are specific trends for the various matrix alloys.     

Friction and wear properties of bronze–graphite composite under water lubrication

Bronze–graphite composite was prepared using powder metallurgy. The friction and wear behaviors of the resulting composites in dry- and water-lubricated sliding against a stainless steel were comparatively investigated on an MM-200 friction and wear tester in a ring-on-block contact configuration. The wear mechanisms of the bronze–graphite composite were discussed based on examination of the worn surface morphologies of both the composite block and the stainless steel ring by means of scanning electron microscopy equipped with an energy dispersion spectrometry and on determination of some typical elements on the worn surfaces by means of X-ray photoelectron spectroscopy. It was found that the friction coefficient was higher under water lubrication than that under dry sliding and it showed margined change with increasing load under the both sliding conditions. A considerably decreased wear rate of the bronze–graphite composite was registered under water-lubricated sliding than under dry sliding, though it rose significantly at a relatively higher load. This was attributed to the hindered transfer of the composite onto the counterpart steel surface under water-lubricated sliding and the cooling effect of the water as a lubricant, while its stronger transfer onto the steel surface accounted for its higher wear rate under dry sliding. Thus, the bronze–graphite composite with much better wear-resistance under water-lubricated sliding than under dry sliding against the stainless steel could be a potential candidate as the tribo-material in aqueous environment.     

Investigation into sliding wear performance of zinc-based alloy reinforced with SiC particles in dry and lubricated conditions

The objective of the present investigation was to assess the influence of SiC particle dispersion in the alloy matrix, applied load, and the presence of oil and oil plus graphite lubricants on the wear behaviour of a zinc-based alloy. Sliding wear performance of the zinc-based alloy and its composite containing SiC particles has been investigated in dry and lubricated conditions. Base oil or mixtures of the base oil with different percentages of graphite were used for creating the lubricated conditions. Results show a large improvement in wear resistance of the zinc-based alloy after reinforcement with SiC particles. The lubrication improved the wear resistance and friction behaviour of both the reinforced and base alloys. It was also observed that there exists an optimum concentration of graphite particles in the lubricant mixture that leads to the best wear performance. The composite experienced higher frictional heating and friction coefficient than the matrix alloy in all the cases except oil lubricated conditions; a mixed trend was noticed in the latter case. The wear rate and frictional heating increased with load while friction coefficient was affected in an opposite manner. Test duration influenced the frictional heating and friction coefficient of the samples in a mixed manner.Examination of worn surfaces revealed a change of predominating wear mechanisms from severe ploughing and/or abrasive wear for base alloy to delamination wear for the reinforced material under dry sliding conditions. The presence of the lubricant increased the contribution of adhesive wear component while reducing the severity of abrasion. This was attributed to the generation of more stable lubricant films on the contacting surfaces. Cross-sections of worn surfaces indicated substantial wear-induced plastic deformation, thereby suggesting adhesive wear to be a predominant wear mechanism in this study. The debris particles revealed deformed flakes and machining chips signifying the involvement of adhesion and abrasion modes of wear respectively.     

Wear and friction behavior of sand cast brake rotor made of A359-20 vol% SiC particle composites sliding against automobile friction material

The effect of load range of 30-100 N and speed range of 3-12 m/s on the wear and friction behavior of sand cast brake rotor made of A359-20 vol% SiC particle composites sliding against automobile friction material was investigated. Dry sliding frictional and wear behavior were investigated in a pin-on-disc type apparatus. Automobile friction material was used as pin, while the A359-20 vol% SiC particle composites formed the rotating disc. For comparison, the wear and friction behavior of commercially used cast iron brake rotor were studied. The results showed that the wear rate of the composite disc decreased with increasing the applied load from 30 to 50 N and increased with increasing the load from 50 to100 N. However, the wear rate of the composite disc decreased with increasing the sliding speed at all levels of load applied in the present work. For all sliding speeds, the friction coefficient of the composite disc decreased with applied load. The worn surfaces as well as wear debris were studied using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analyzer and X-ray diffraction (XRD) technique. At load of 50 N and speed range of 3-12 m/s, the worn surface of the composite disc showed a dark adherent layer, which mostly consisted of constituents of the friction material. This layer acted as a protective coating and lubricant, resulting in an improvement in the wear resistance of the composite.     

Effect of Matrix Alloying of Fe on Friction and Wear Properties of Cu-Based Brake Pad Materials

Abstract

Argon-gas-atomized Cu–Fe prealloyed powder was used to prepare Cu-based composites, and the effect of matrix alloying of Fe on microstructure and friction and wear properties of the brake pad material was systematically investigated on an MS3000 friction and wear tester. The results indicate that matrix alloying of Fe induces the precipitation of a uniformly distributed iron-rich phase at the interior of grains, the segregation of the iron-rich phase along SiO₂–matrix interface, and the formation of pearlite in the vicinity of the graphite–matrix interface, which favors the strengthening of copper matrix, improves interfacial bonding, and protects the third body. The prealloyed sample exhibits relatively high friction coefficient and enhanced friction stability, as well as reduced wear loss when the braking speed is lower than 200?km/h. At higher braking speed (>200?km/h), the breakage of the iron-rich phase leads to an unstable friction coefficient and high wear loss.     

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.     

Effect of the Synergetic Action on Tribological Characteristics of Ni-Based Composites Containing Multiple-Lubricants

Ni-based self-lubricating composites with multiple-lubricants addition were prepared by a powder metallurgy technique, and the effect of multiple-lubricants on tribological properties was investigated from room temperature to 700?°C. The synergetic effects of graphite, MoS₂, and metallic silver lubricants on the tribological characteristics of composites were analyzed. XRD analysis showed that new Cr _x S _y and Mo₂C phase were formed in the composites containing graphite, MoS₂ and metallic Ag lubricants during the sintering process. The average friction coefficients (0.69?C0.22) and wear rates (11.90?C0.09?×?10^?5?mm³?N^?1?m^?1) were obtained when rubbing against Inconel 718 alloy from room temperature to 700?°C due to synergetic lubricating action of multiple-lubricants. A smooth lubricating was gradually generated on the worn surface, and the improving of tribological properties was attributed to the formation of lubricious glaze film on the worn surface and their partially transferred to the counterface. The graphite played the main role of lubrication at room temperature, while molybdate phase and graphite were responsible for low friction coefficients and wear rates at mid/high temperatures. The synergetic lubricating effect of molybdate (produced in the rubbing process at high temperatures) iron oxide (transfer from disk material to the pin) and remaining graphite multiple-lubricants play an important lubricating role during friction tests at a wide temperature range.     

The effect of lubricant additives on friction and wear studies of polyphenylene sulphide

Polyphenylene sulphide is a polymer with good thermal stability and high crystallinity. This paper summarizes the results of friction and wear studies of polyphenylene sulphide and its composites made with conventional solid lubricants to ascertain the suitability of the material as a matrix for solid lubricant additives. The polymer itself has a high coefficient of friction. Wear rate increases with load and speed. Addition of solid lubricant additives helps in improving the friction and wear of the polymer. Composites with MoS₂-Sb₂O₃ and PTFE gave better results than composites made by the addition of graphite and MoS₂ graphite. Wear rate of these composites increased with load and speed; but load and speed had little effect on friction.