Ionic Liquids as Lubricants of Titanium–Steel Contact. Part 2: Friction, Wear and Surface Interactions at High Temperature

The tribological behaviour and surface interactions of titanium sliding against AISI 52100 steel have been studied at 200 and 300 °C in the presence of two commercial imidazolium room temperature ionic liquid (ILs): 1-octyl-3-methylimidazolium tetrafluoroborate (L108) and 1-hexyl-3-methylimidazolium hexafluorophosphate (LP106). L108 presents the higher thermal stability but gives higher friction coefficients and wear rates than LP106, with long running-in periods and high friction values, both at 200 and 300 °C. Friction and wear rates for LP106 are lower and decrease as the temperature increases from 25 to 200 °C. At 200 °C, LP106 shows a constant friction coefficient, without running-in, produces a mild wear on titanium and no surface damage on steel. LP106 fails at 300 °C, close to its degradation temperature, due to tribochemical decomposition through partial dissociation of the hexafluorophosphate anion, with formation of a phosphorus-rich layer on the steel ball, while the titanium wear track surface is heterogeneous, showing regions with the presence of fluoride and others with the presence of phosphate. When the steel ball is substituted for a ruby sphere under the same conditions at 300 °C, a low friction coefficient and mild wear is observed, due to the higher stability of the LP106 lubricant at the ruby–titanium interface. The friction coefficients, wear mechanisms and surface interactions have been studied by means of friction-distance records, SEM, EDX and XPS.     

Friction and Wear Properties of MoS<Subscript>2</Subscript>-Overcoated Laser Surface-Textured Silver-Containing Nickel-Based Alloy at Elevated Temperatures

The solid lubricant that is coated on a flat surface is easily removed during friction. Surface texture dimples, which act as reservoirs of solid lubricant, can prolong the wear life of solid lubricant films. We textured silver-containing nickel-based alloys by a pulse laser and filled the micro-dimples with molybdenum disulfide powders. The tribological properties of the alloys were tested by rubbing against alloyed steel on a ring-on-disk tribometer at temperatures ranging from room temperature to 600°C . After laser surface texturing, the friction coefficients of the silver-containing nickel-based alloy smeared with molybdenum disulfide powders were reduced at temperatures ranging from room temperature to 400°C. With increasing dimple density, the wear life of the MoS₂ film increased while the wear rate of the nickel-based alloy decreased. The wear life of the textured surface with a dimple density of 11.2% exceeded 10,000 m at room temperature. We conclude that molybdenum disulfide and its oxides stored in the micro-dimples play a role in lubrication at room temperature and high temperatures, respectively.     

Influence of temperature on tribological behaviour of ionic liquids as lubricants and lubricant additives

Ionic liquids are low‐melting‐point salts composed entirely of ions, and many of them are liquids at room temperature. In recent years, studies have indicated that they might be good candidates for lubricants, either in neat or additive form. In this work, a sulfate‐based ionic liquid with a pyrollidinium cation was studied as neat lubricant and as additive for glycerol in lubrication of steel–steel contacts. Glycerol was chosen as the base oil because of its high polarity, which allows full miscibility with polar ionic liquids. Tests were performed on an oscillating friction and wear tribometer. The coefficients of friction and wear were measured. The tests were run at room temperature, 50 °C, 100 °C and 150 °C. By using profilometry, optical microscopy, scanning electron microscope and atomic force microscopy (AFM) analyses, it was shown that the ionic liquid plays an important role in the friction and wear reduction, as well as in the smoothening of the worn surface. Copyright © 2013 John Wiley & Sons, Ltd.     

Sliding wear response of zinc based alloy as affected by suspended solid lubricant particles in oil lubricant

Abstract

Wear behaviour of a zinc based alloy has been studied in partially lubricated condition. The test environment comprised a mixture of oil plus graphite/talc particles. The composition of the lubricant mixture was varied by changing the concentration of the solid lubricant particles suspended in the oil lubricant. Wear response of the alloy was noted to improve in terms of decreased wear rate, frictional heating and friction coefficient initially with the increasing concentration of the solid lubricant particles suspended in the oil lubricant. A critical content of the solid lubricant led to the best wear performance of the samples. This was followed by a reversal in the trend at concentrations of the solid lubricant particles in the lubricant mixture that were greater than the critical one. Wear behaviour of the alloy has been substantiated through the characteristics of wear surfaces, subsurface regions and debris particles.     

Au/Cr Sputter Coating for the Protection of Alumina During Sliding at High Temperatures

A sputter-deposited bilayer coating of gold and chromium was investigated as a potential solid lubricant to protect alumina substrates in applications involving sliding at high temperatures. The lubricant was tested in a pin-on-disk tribometer with coated alumina disks sliding against uncoated alumina pins. Three test parameters—temperature, load and sliding velocity—were varied over a wide range in order to determine the performance envelope of the Au/Cr solid lubricant film. The tribo-tests were run in air at temperatures of 25° to 1000°C, under loads of 4.9 to 49.0 N and at sliding velocities from 1 to 15 ms^?1. Posttest analyses included surface profilometry, wear factor determination and SEM/EDS examination of worn surfaces.

Compared to unlubricaled Al₂O₃ sliding, the use of the Au/Cr film reduced friction by 30 to 50 percent and wear by one to two orders of magnitude. Increases in test temperature resulted in lower friction and the Au/Cr film continued to provide low friction, about 0.3, even at 1000°C. Pin wear factors and friction were largely unaffected by increasing loads up to 29.4 N. Sliding velocity had essentially no effect on friction, however, increased velocity reduced coaling life (total sliding distance). Based upon these research results, the Au/Cr film is a promising lubricant for moderately loaded, low-speed applications operating at temperatures as high as 1000°C.     

Rolling Contact Testing of Vapor Phase Lubricants—Part II: System Performance Evaluation

The relative effects of several vapor lubrication parameters on bearing performance were examined using a ball-on-rod tester. Lubricants included in the evaluation were a tertiary-butylphenyl phosphate (TBPP), a 2 cSt polyalphaolefin blended with 15 percent TBPP (PAO+), the TBPP blended with 33 percent tributyl phosphate (TBPP+), a cyclophosphazine (X-1P), a polyphenylether (5P4E), and a perfluoroalkylether (Z). Parameters included in the study were bearing temperature, vapor concentration, and vapor temperature. Additionally, a solid lubricant coating was included to improve the bearing performance under cold-start conditions. The lubricants containing phosphorus demonstrated the best high temperature performance. The TBPP lubricant failed shortly after test at 650°C., while the X-1P lubricant performed satisfactorily over an eight-hour period at 650°C. The TBPP+ lubricant demonstrated the widest temperature range capability, with 600°C operation and a projected pump-ability point of–-45°C. Lubricant concentration was the most significant system parameter affecting bearing friction and wear.     

Tribological Behavior of WC-12Co Air Plasma–Sprayed Coating at Elevated Temperatures

The friction and wear performance of WC-12Co air plasma–sprayed (APS) coating at temperatures of 25–650°C under loads of 8 and 28 N in at atmospheric environment have been studied by a ball-on-disc tribometer. The effect of temperature and load on the tribological behavior of WC-Co coating was investigated. The results show that under a load of 8 N, the wear volume of the coating increases at 250°C due to the coating splat delamination and then it gradually decreases at 350–500°C. The friction could promote the formation of double oxide (CoWO₄), which is beneficial to reduce friction and wear. At higher temperatures, the wear volume increases again due to the removal of oxides. Under a load of 28 N, the wear volume of the coating increases enormously at 250°C due to the serious splat delamination. At 350°C, the load promotes double oxide formation, resulting in an early decrease in the coefficient of friction and a rapid reduction in wear volume. Although the wear volume decreases at 350–500°C, it is 10-fold higher than that under a load of 8 N. Above 500°C, the differences of the wear volumes of coatings under the two loads become less obvious, and similar trends also appear for the coefficients of friction. The synergistic effect between the load and temperature on the friction and wear mechanism of WC-12Co APS coating is discussed.     

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.     

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.     

Tribological Properties of Water Glass Lubricant for Hot Metalworking

Tribological properties of water glass (sodium silicate aqueous solution) with graphite powders, a lubricant for hot metalworking, were studied by means of pin-on-disk type tribotesting at a wide variety of temperature ranges. The lubricant is a water suspension consisting of water glass with graphite powders at room temperature. It becomes solid or a suspension state consisting of solid or liquid glass with the graphite powders owing to evaporation of the water inside. The frictional behavior of the lubricant over 100°C was strongly affected by the state of the glass, which is a factor of temperature, and was categorized into the following three regions in accordance with the state of the glass: solid film lubrication by the powder glass and the graphite (Region I), fluid film lubrication by the viscous liquid glass (Region II), and fluid film lubrication by the low-viscosity liquid glass and solid film lubrication by the graphite (Region III). The lowest friction coefficient was obtained in Region III.     

Tribological characterization of electroless Ni–10% P coatings at elevated test temperature under dry conditions

An experimental study of wear characteristics of electroless Ni–10% P coating sliding against hard AISI 52100 steel pin is investigated. Experiments are carried out at room and 550°C temperatures. Heat treatment effects on tribological behavior of this coating are studied. The wear surface and the microstructure of the coatings are analyzed using optical microscopy, scanning electron microscopy, energy dispersion analysis X-ray, and microhardness testing equipment. It is observed that the forming of continuous oxide film on contacting surfaces of pin and disk improves wear resistance and decreases friction coefficient of the Ni–10% P coating. The results indicate that the wear resistance of electroless Ni–10% P coating has improved with heat treatment at room temperature wear test, but it reverses in the wear test at 550°C. In addition, specimens without heat treatment have the highest wear resistance and the lowest friction for wear tests at elevated temperatures.     

Theoretical Study on Tribological Mechanism of Solid Lubricating Films in a Sand–Dust Environment

A model based on energy dissipation was developed to describe the tribological behavior of solid lubricant films in sand–dust environment. A relationship between wear rate and coefficient of friction was obtained. Theoretical results kept well consistent with the experimental data in reported publications (Qi et al., Tribol Lett 38:195–205, 2010; Surf Interface Anal 43:836–846, 2011; Wear 271:899–910, 2011). It was pointed out that the absolute value of slope of the simulated straight line is closely related with mechanical properties of solid lubricant films. The results increased our understanding about the individual friction and wear mechanism for solid lubricant films in sand–dust environment.     

Investigation on Tribological Behavior of Advanced High Strength Steels: Influence of Hot Stamping Process Parameters

In order to investigate the friction and wear behavior of high strength steel in hot stamping process, a hot strip drawing tribo-simulator is developed and the friction coefficient, which is an important parameter in the finite element modeling, is measured. The results have shown that the friction coefficient remains almost unchanged until temperature reaches 500 °C. It then increases sharply as temperature is increased from 500 to 600 °C. It has also been shown that the friction coefficient decreases as the drawing speed increases. The change in the dominate wear mechanism as the temperature and the drawing speed increases has been identified from SEM analyses of the worn surface. The dominate wear mechanism is the groove cutting at temperatures between room temperature and 500 °C, which changes to the adhesive wear at temperatures above 500 °C. The main wear mechanism is the adhesive wear at 25 mm/s, which changes to the slight groove cutting at 75 mm/s.     

The influence of TiC,CaF2 and MnS additives on friction and lubrication of sintered high speed steels at elevated temperature

Friction behaviours of sintered high speed steels containing TiC, CaF₂ and MnS additives and lubrication mechanisms of these additives have been investigated at sliding conditions at 600°C. Results shown that these additives strongly affected friction behaviours of the sintered high speed steels. Ceramic carbide TiC, as a bonding agent and enhancement phase, bonded the solid lubricant CaF₂ and MnS surrounding it and supported these solid lubricant particles so that friction process become stable. Fluoride calcium CaF₂ has a better high temperature lubrication properties than MnS, and that the addition of TiC + CaF₂ in the sintered high speed steels achieved excellent friction performances both a stable and a low friction coefficient value. Analysis results by X-ray indicated that the surface film appearing on worn path consists of some molten metal matters and very fine carbide particles. During sliding, the surface film separated contacts and resulted in a lower friction coefficient. Because of the sintered high speed steels usually to be used to manufacture high temperature components, these results are helpful for their engineering use.     

Tribological property of self-lubricating PM304 composite

PM304 composite has been prepared by high-energy ball milling and powder metallurgy. The composition of the PM304 composite is the same as that of PS304, but the microstructure is quite different. The microstructure of PM304 composite was fine and dense, the size of self-lubricating particles in the composite was very small. The tribological properties of PM304 composites against Inconel X-750 were examined in the temperature range from room temperature to 800 °C. The friction coefficient of PM304 was ranged from 0.32 to 0.41. At room temperature, brittle fracture occurred on the worn surface. With the increase of temperature up to 200 °C, a protective layer consisting of fluorides and Ag existed on the worn surface and led to a low wear rate. The wear resistance of the PM304 was superior to that of the PS304 in the temperature range from room temperature to 650 °C. The improvement in wear resistance of the PM304 was discussed in the terms of its microstructural characteristics.     

Solid Film Lubrication at High Temperature

The behavior of several solid film lubricants has been experimentally established as a function of temperature. These films are formed of a suspension of lubricant particles (graphite, molybdenum disulfide) in a thermosetting resin baked onto a hard surface. The test consists of heating such a layer and continually observing the friction coefficient until failure. Sliding conditions of high unit loading and low rubbing velocity were chosen to provide a comparison between the films. They were evaluated as to the highest temperature for which the friction coefficient remained low and subsequently as to the number of load cycles sustained at temperature before an abrupt increase in friction. Results for all films were similar in that failure was caused by a breakdown of the binder rather than the lubricating particles and that the minimum friction coefficient (～ 0.03) was observed just before the film failure at temperatures as high as 1200° F.     

Sliding wear behavior of Fe-Al and Fe-Al/WC coatings prepared by high velocity arc spraying

A comparative study was carried out to investigate the microstructure and tribological behavior of Fe-Al and Fe-Al/WC iron aluminide based coatings against Si₃N₄ under dry sliding at room temperature using a pin-on-disc tribotester. The coatings were prepared by high velocity arc spraying (HVAS) and cored wires. The effect of normal load on friction coefficient and wear rate of the coatings was studied. The microstructure and the worn surfaces of the coatings were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscope (EDS). The results showed that, the main phases in both coatings were iron aluminide (Fe₃Al and FeAl) and α. WC/W₂C particles were embedded in the matrix of the composite coating. With adding WC hard particles, the Fe-Al/WC composite coating exhibited higher wear-resistance than Fe-Al coating. But the friction coefficient of both coatings showed little difference. As the load increased, the friction coefficient decreases slightly due to a rise of friction contact temperature and larger areas of oxide film formation on the worn surface, which act as a solid lubricant. Increasing load causes the maximum shear stress occurring at the deeper position below the surface, thereby aggravating the wear. The coating surface is subjected to alternately tensile stress and compression stress during sliding, and the predominant wear mechanism of the coatings appears to be delamination.     

Effect of Temperature on Tribological Properties and Wear Mechanisms of NiAl Matrix Self-Lubricating Composites Containing Graphene Nanoplatelets

Studies have been carried out to explore the friction and wear behaviors of NiAl matrix self-lubricating composites containing graphene nanoplatelets (NG) against an Si₃N₄ ball from 100 to 600°C with a normal load of 10 N and a constant speed of 0.2 m/s. The results show that NG exhibits excellent tribological performance from 100 to 400°C compared to NiAl-based alloys. A possible explanation for this is that graphene nanoplatelets (GNPs) contribute to the formation of a friction layer, which could be beneficial to the low friction coefficient and lower wear rate of NG. As the temperature increases up to 500°C, the beneficial effect of GNPs on the tribological performance of NG becomes invalid due to the oxidation of GNPs, resulting in severe adhesive wear and degradation of the friction layer on the worn surface of NG. GNPs could hold great potential applications as an effective solid lubricant to promote the formation of a friction layer and prevent severe sliding wear below 400°C.     

Dry Sliding Friction and Wear Characteristics of Cu-Sn Alloy Containing Molybdenum Disulfide

The wear and sliding friction response of a hybrid copper metal matrix composite reinforced with 10 wt% of tin (Sn) and soft solid lubricant (1, 5, and 7 wt% of MoS₂) fabricated by a powder metallurgy route was investigated. The influence of the percentages of reinforcement, load, sliding speed, and sliding distance on both the wear and friction coefficient were studied. The wear test with an experimental plan of six loads (5–30 N) and five sliding speeds (0.5–2.5 m/s) was conducted on a pin-on-disc machine to record loss in mass due to wear for two total sliding distances of 1,000 and 2,000 m. The results showed that the specific wear rate of the composites increased at room temperature with sliding distance and decreased with load. The wear resistance of the hybrid composite containing 7 wt% MoS₂ was superior to that of the other composites. It was also observed that the specific wear rates of the composites decreased with the addition of MoS₂. The 7 wt% MoS₂ composites exhibited a very low coefficient of friction of 0.35. The hardness of the composite increased as the weight percentage of MoS₂ increased. The wear and friction coefficient were mainly influenced by both the percentage of reinforcement and the load applied. Wear morphology was also studied using scanning electron microscopy and energy-dispersive X-ray analysis.     

Low temperature investigations on frictional behaviour and wear resistance of solid lubricant coatings

The tribological characteristics of polymer-based solid lubricant coatings under frictional stressing in vacuum at 293, 120 or 77 K were studied. Vacuum-friction apparatus with cryogenic pumps and low-temperature tribometer designed at SR&DB of ILTP&E was used for sliding tests. It was found that the coefficient of friction is somewhat higher at low than at room temperatures. The effect of temperature decrease on the wear life of solid lubricant coatings is ambiguous and determined by the direction of changes in physical and mechanical characteristics of a solid lubricant coating under cooling and by the rate of the process of a binder tribodestruction.