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.     

A comparison of boundary wear film formation on steel and a thermal sprayed Co/Cr/Mo coating under sliding conditions

In this paper, the action of the zinc dialkyl dithiophosphate (ZDDP) anti-wear additive has been examined on two different materials (Steel AISI 52100 and a Co/Cr/Mo thermal spray coating) sliding against cast iron in reciprocating mode. Tests have been conducted under lubricated wear conditions at relatively low (20, 50 °C) and elevated (up to 100 °C) bulk oil temperatures. A comparison is made between the friction, wear and chemical nature of the wear film formed under varying temperatures, on two materials, in two lubricants (one free from and one containing ZDDP) and after different test durations. The wear film has been examined by energy dispersive X-ray analysis (EDAX) and X-ray photoelectron spectroscopy (XPS).In this work, it has been shown in this work that the friction coefficient is dependent on the temperature, the lubricant and the nature of the contacting surfaces. In the presence of ZDDP, a wear film, comprising Zn, S and P, forms even at the lowest bulk oil temperature of 20 °C. The nature of the film is dependent on the substrate material and the steel and Co/Cr/Mo coating showed contrasting film characteristics. In this paper, the wear and friction results for each couple in oil containing and free from additives is discussed with reference to the nature of the wear film. A correlation has been made between the wear, friction and chemical analysis measurements.     

Role of smoke from combustion of oil in tribological applications to ceramics

The influence of smoke from combustion of light oil on friction properties was investigated with various ceramic specimens (Al₂O₃, Si₃N₄ and SiC) at a normal load of 58.8 N, a circumferential velocity of the rotating ring of 0.1 m/s. The variation of friction torque with sliding distance was measured with these specimens over a range of testing temperatures from 100°C to 400°C. It was found that a stable sliding condition was characterized by the formation of a smooth carbon film on the surface. The carbon films thus formed were investigated in relation to the change in friction coefficient with film thickness. The result showed that the optimum film thickness for minimizing the friction coefficient is 0.2–0.3 μm, almost irrespective of the kind of ceramic samples. It was deduced that an effective amount of soot as a lubricant is about 10–40 g/m²/h depending on the kinds of ceramic couple.     

Tribological Evaluation of an Al2O3-SiO2 Ceramic Fiber Candidate for High Temperature Sliding Seals

A test program to determine the relative slitting durability of an alumina-silica candidate ceramic fiber for high temperature sliding seal applications is described. Pin-on-disk tests were used to evaluate the potential seal material by sliding a tow or bundle of the candidate ceramic fiber against a superalloy test disk. Friction was measured during the tests and fiber wear, indicated h the extent of fibers broken in the tow or bundle, was measured at the end of each test. Test variables studied included ambient temperature from 25° to 900°C, loads from 1.3 to 21.2 N, and sliding velocities from 0.025 to 0.25 m/sec. In addition, the effects of fiber diameter and elastic modulus on friction and wear were measured. Thin gold films deposited on the superalloy disk surface were evaluated in an effort to reduce friction and wear of the fibers.

In most cases, wear increased with test temperature. Friction ranged from 0.36 at 500°C and low velocity (0.025 miser) to over 1.1 at 900°C and high velocity (0.25 m/sec). The gold films resulted in satisfactory lubrication of the fibers at 25°C. At elevated temperatures diffusion of substrate elements degraded the films. These results indicate that the alumina-silica (Al₂O₃SiO₂) fiber is a good candidate material system for high temperature sliding seal applications. More work is needed to reduce friction.     

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.     

The unlubricated reciprocating sliding wear of 316 stainless steel in CO2 at 20–600°C

The friction and wear behaviour of 316 stainless steel in CO₂ has been investigated in the load range 8–50 N from 20 to 600°C. Wear transitions occurred at all temperatures but were load-dependent. At and below 300°C, wear transitions only took place at low loads, whereas above 300°C transitions were observed at all loads. The low temperature wear transition, representing an order of magnitude decrease in wear rate, was associated with a change in friction behaviour. The friction force across the specimen was initially widely fluctuating but after a time, which did not necessarily coincide with the wear transition, became much smoother. The smoother sliding is thought to indicate a trend to oxide-oxide contacts. At higher temperatures wear transitions result in a two orders of magnitude reduction in wear. The corresponding friction transition was similar to the low temperature friction change but also included a marked temporary drop in the coefficient of friction.Pits or troughs up to 450 μm deep were seen in wear scars above 400°C. It is proposed that isolated sections of grooves formed during the initial stages of wear become back-filled with loosely adhering oxide particles. These troughs are then further deepened, possibly by abrasive fretting action of the semi-fluid oxide material.     

Performance and Durability of High Temperature Foil Air Bearings for Oil-Free Turbomachinery

The performance and durability of advanced, high temperature foil air bearings are evaluated under a wide range (10 to 50 kPa) of loads at temperatures from 25° to 650 °C. The bearings are made from uncoated nickel based superalloy foils. The foil surface experiences sliding contact with the shaft during initial start/stop operation. To reduce friction and wear, the solid lubricant coating, PS304, is applied to the shaft by plasma spraying. PS304 is a NiCr based Cr₂O₃ coating with silver and barium fluoride/calcium fluoride solid lubricant additions.

The results show that the bearings provide lives well in excess of 30,000 cycles under all of the conditions tested. Several bearings exhibited lives in excess of 100,000 cycles. Wear is a linear function of the bearing load. The excellent performance measured in this study suggests that these bearings and the PS304 coating are well suited for advanced high temperature, oil-free turbomachinery applications.     

Effect of Temperature on the Dry Sliding Friction and Wear of Rice Bran Ceramics against Different Counterpart Materials

In this study, we investigated the effect of temperature on the friction and wear of rice bran (RB) ceramics, a hard porous carbon material made from rice bran, sliding against alumina, stainless steel, and bearing steel balls under dry conditions. Friction tests were performed using a ball-on-disk-type friction tester wherein a ceramic heater was installed in the rotational stage. The surface temperature of the RB ceramic disk specimens was controlled at 20, 100, 150, or 200°C. The normal load was 1.96 N, sliding velocity was 0.1 m/s, and number of cycles was 20,000. The effect of surface temperature on the friction and wear of RB ceramics substantially differed among the ball material types. The friction coefficient for the RB ceramics sliding against an alumina ball decreased with increasing temperature and exhibited an extremely low value (0.045) at 200°C. The friction coefficient in the case of the RB ceramics sliding against a stainless steel ball exhibited a stable value as the temperature was increased to 150°C and slightly decreased as the temperature was increased further, reaching a low value of 0.122 at 200°C. The friction coefficient for the RB ceramics sliding against bearing steel ball drastically increased with increasing temperature, reaching 0.381 at 200°C. The specific wear rate of the RB ceramics increased with increasing temperature; it was lowest when sliding against alumina and highest when sliding against bearing steel. The wear of the alumina ball was the lowest and that of the bearing steel ball was the highest under all investigated temperature conditions. On the basis of these results, we concluded that alumina is a promising counterpart material for RB ceramics sliding at high temperatures (≤200°C).     

The tribological characteristics of TiCN coating at elevated temperatures

The tribological behaviour of TiCN coating prepared by unbalanced magnetron sputtering is studied in this work. The substrates made from austenitic steel were coated by TiCN coatings during one deposition. The measurements were provided by high temperature tribometer (pin-on-disc, CSM Instruments) allowing measuring the dependency of friction coefficient on cycles (sliding distance) up to 500 °C. The evolution of the friction coefficient with the cycles was measured under different conditions, such as temperature or sliding speed and the wear rate of the ball and coating were evaluated. The 100Cr6 balls and the Si₃N₄ ceramic balls were used as counter-parts. The former were used at temperatures up to 200 °C, the latter up to 500 °C. The wear tracks were examined by optical methods and SEM. The surface oxidation at elevated temperatures and profile elements composition of the wear track were also measured.The experiments have shown considerable dependency of TiCN tribological parameters on temperature. Rise in temperature increased both friction coefficient and the wear rate of the coating in case of 100Cr6 balls. The main wear mechanism was a mild wear at temperatures up to 200 °C; fracture and delamination were dominating wear mechanisms at temperatures from 300 to 500 °C.     

Zinc oxythiomolybdate (ZnMoO2S2) powder as a solid lubricant for high-temperature rolling contact applications

Conventional liquid lubricants being used in today's gas turbine engines will not be able to operate effectively in the hostile bearing environments expected in future turbine engines. The expected high operating temperatures (500–800°C) mandate new and innovative lubrication schemes to achieve success. Recent studies have demonstrated that a new class of solid lubricants, the complex chalcogenides or metal ocythiomolybdates, have good potential for high temperature lubrication. This paper describes the friction, wear and rolling contact endurance of three high-temperature bearing materials using a zinc oxythiomolybdate (ZnMoO₂S₂) powder lubricant. Rolling contact tests were conducted using VIM-VAR M50, micromelt T15 tool steels and silicon nitride (Si₃N₄) at temperatures ranging from 23°C to 649°C, using a modified ball-on-rod type rolling-contact fatigue tester. Significant improvements in friction, endurance and wear were observed at all test temperatures, and with all three materials evaluated, when ZnMoO₂S₂ was used as a lubricant. Overall, silicon nitride exhibited the best frictional and antiwear performance. The lubricant powder exhibited the best tribological performance with T15 and M50 specimens between 177°C and 316°C. Energy Dispersive X-Ray Analysis (EDAX) of wear tracks showed the presence of iron (Fe) on the Si₃N₄ specimens as well as the presence of zinc (Zn) on both the T15 and the M50 specimens.     

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.     

Friction and Wear Characteristics of BaCr2O4 Ceramics at Elevated Temperatures in Sliding Against Sintered Alumina Ball

In this article, friction and wear characteristics of BaCr₂O₄ ceramics have been investigated using a high-temperature friction and wear tester from room temperature to 800?°C in dry sliding against sintered alumina ball. At room temperature, the friction coefficient and wear rate of BaCr₂O₄ ceramics are quite high. BaCr₂O₄ ceramics exhibit low friction coefficients and small wear rates with temperature increasing up to 400?C600?°C. The oxidation reaction of BaCr₂O₄ during high-temperature wear tests is responsible for the tribological properties. The oxidized product of BaCr₂O₄ is BaCrO₄, which forms a smooth self-lubricating film on the worn surface to effectively reduce friction and wear. However, at 800?°C, severe oxidation reduces the relative density of sintered BaCr₂O₄ ceramics, and further expedites the materials removal process.     

Wear and Friction of TiAl Matrix Self-Lubricating Composites against Si3N4 in Air at Room and Elevated Temperatures

More durable, low-friction bearing materials over a wide temperature range are needed for turbine components and other high-temperature bearing applications. The current study reported the tribological properties of TiAl matrix self-lubricating composites (TMC) containing MoS₂ (a low-temperature lubricant, below 500°C), hBN (a medium-temperature lubricant, below 600°C), and Ti₃SiC₂ (a high-temperature lubricant, above 600°C) designated as MhT against an Si₃N₄ counterface at temperatures ranging from 25 to 800°C in air. The load was 10 N and the sliding speed was 0.2 m/s for all tests. Tribological studies indicated that TMC containing MhT showed a lower friction coefficient and wear rate in comparison to TiAl-based alloy at all test temperatures, which was attributed to the excellent synergetic lubricating effect of MoS₂, hBN, and Ti₃SiC₂. TMC containing 5 wt% MhT exhibited the best tribological properties over a wide temperature range.     

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.     

Theoretical surface temperatures generated from sliding contact of pure metallic elements

Surface temperatures and thermal effects produced in tribological processes are important not only in influencing possible mechanisms of friction, wear, and lubricant film failure but also in initiating protective film-formation. As part of a continuing combined theoretical and experimental study of surface temperatures generated by friction, the fundamental Green's function approach has been applied to a number of pure metallic elements to compare and discuss their predicted behavior in A-on-A sliding contact. Assuming a single area of real contact, calculated ratios of surface temperature rise to coefficient of friction plotted against area of contact, velocity and load on a logarithmic scale are presented and summarized for several pure metallic elements in the first transition series of the Periodic Table (e.g., Ti, V, Cr, Mn, Fe, Co, Ni) as well as members in connecting groups, e.g., Cr, Mo, and W in Group VIa and Cu, Ag, and Au in Group Ib. These include metals which are tribologically difficult to machine and use (e.g., Ti), common elements in bearing steels (e.g., Fe, Cr), and metals useful in reducing friction or wear when applied as thin surface coatings (e.g., Ag, Au). The results of this comparison are interesting and surprising. They may add to our understanding of why some metals are very “difficult” in a tribological sense while others provide benefits in controlling friction and/or wear.     

Tribological Properties of Hard Carbon Films on Zirconia Ceramics

In this study, the authors investigated the tribological properties of hard diamondlike carbon (DLC) films on magnesia-partially stabilized zirconia (MgO-PSZ) substrates over a wide range of bads, speeds, temperatures, and counterface materials. The films were 2 μm thick and produced by ion-beam deposition at room temperature. Tribological tests were conducted on a ball-on-disk machine with MgO-PSZ balls, in open air of 30 to 50% relative humidity under contact loads of 1 to 50 N, at sliding velocities of 0.1 to 6 m/s, and at temperatures of 400°C. Al₂O₃ and Si₃N₄ balls were also rubbed against the DLC-coaled MgO-PSZ disks, primarily to assess their friction and wear performance and to compare it with that of MgO-PSZ balls. A series of long-duration lifetime tests was run at speeds of 1, 2, and 6 m/s under a 5 N load to assess the durability of these DLC films. Results showed that the friction coefficients of MgO-PSZ balls sliding against MgO-PSZ disks were 0.5-0.8, and the average specific wear rates of MgO-PSZ balls ranged from 1 × 10^?5 to 5 × 10^?4 mm³/N·m, depending on sliding velocity, contact load, and ambient temperature. The friction coefficients of MgO-PSZ balls sliding against the DLC-coaled MgO-PSZ disks ranged from 0.03 to 0.1. The average specific wear rates of MgO-PSZ, balls were reduced by three to four orders of magnitude when rubbed against the DLC-coaled disks. These DLC films could last 1.5 to 4 million cycles, depending on sliding velocity. Scanning electron microscopy and micro-laser Raman spectroscopy were used to elucidate the microstructural and chemical nature of the DLC films and worn surfaces.     

Tribological characteristics of low-pressure plasma-sprayed A12O3 coating from room temperature to 800 °C

The tribological characteristics of low-pressure plasma-sprayed (LPPS) Al₂O₃ coating sliding against alumina ball have been investigated from room temperature to 800 °C. These friction and wear data have been compared quantitatively with those of bulk sintered alumina to obtain a better understanding of wear mechanisms at elevated temperatures. The friction and wear of Al₂O₃ coating show a strong dependence on temperature, changing from a mild to a severe wear regime with the increase of temperature. The coefficient of friction at room temperature is approximately 0.17 to 0.42, depending on applied load. The tribochemical reaction between the coating surface and water vapor in the environment and the presence of the hydroxide film on the Al₂O₃ coating reduce the friction and wear at room temperature as contrasted to those of bulk sintered alumina. At intermediate temperatures, from 400 to 600 °C, the friction and wear behavior of Al₂O₃ coating depends on the inter-granular fracture and pull-out of Al₂O₃ grains. At above 700 °C, formation and deformation of fine grain layer, and abrasive wear in the form of removal of fine alumina grains further facilitate the friction and wear process of Al₂O₃ coating.     

Tribological Characteristics of Calophyllum inophyllum–Based TMP (Trimethylolpropane) Ester as Energy-Saving and Biodegradable Lubricant

The purpose of this research is an experimental study of Calophyllum inophyllum (CI)-based trymethylolpropane (TMP) ester as an energy-saving and biodegradable lubricant and compare it with commercial lubricant and paraffin mineral oil using a four-ball tribometer. CI-based TMP ester is a renewable lubricant that is nonedible, biodegradable, and nontoxic and has net zero greenhouse gases. The TMP ester was produced from CI oil, which has high lubricity properties such as higher density, higher viscosity at both 40°C and 100°C and higher viscosity index (VI). Experiments were conducted during 3,600 s with constant load of 40 kg and constant sliding speed of 1,200 rpm at temperatures of 50, 60, 70, 80, 90, and 100°C for all three types of lubricant. The results show that CI TMP ester had the lowest coefficient of friction (COF) as well as lower consumption of energy at all test temperatures, but the worn surface roughness average (R_a) and wear scar diameter were higher compared to paraffin mineral oil and commercial lubricant. Before 80°C, CI TMP ester actually has a higher flash temperature parameter (FTP) than paraffin mineral oil and as the temperature increases, the FTP of TMP ester decreases. The worn surfaces of the stationary balls were analyzed by scanning electron microscopy (SEM) and results show that CI TMP ester has the highest wear compared to paraffin mineral oil and lowest wear compared to commercial lubricant. However, CI TMP ester is environmentally desired, competitive to commercial lubricant, and its use should be encouraged.     

A Tribological Study of Multiply-Alkylated Cyclopentanes and Perfluoropolyether Lubricants for Application to Si-MEMS Devices

Lubrication of Micro-Electro-Mechanical Systems (MEMS) is a major constraint in MEMS applications, restricting the designs and practical usages of such devices. Possible lubricants and methods have been investigated in this paper, comparing perfluoropolyether (PFPE) lubricant with multiply-alkylated cyclopentanes (MACs). The effectiveness of both the lubricants in reducing friction and enhancing the wear life was investigated in a new method of MEMS lubrication known as Localised-Lubrication or “Loc-Lub.” Friction and wear tests were conducted in a flat-on-flat test geometry under a normal load of 50 g and a sliding velocity of 5 mm s^?1 in reciprocation, with Si as the substrate. Further tests were conducted at higher loads, to compare wear durability between lubricants and methods. It was found that MACs have a propensity to remain cohesive during the tests due to higher surface tension and provide better friction and wear properties when tested under reciprocating sliding conditions, as a complete film is present between the two surfaces. The results show that MAC lubricant is more effective in extending the wear life and reducing friction under the tested conditions compared to PFPE.     

Study of the tribological performance of zirconia-based materials in pair with VT-9 titanium alloy

The paper presents a study of the possibility of using composite nanostructured ceramic materials and crystals based on zirconia as bearing materials for dry operation at temperatures of up to 150°C. The results of experimental study of the dry friction of the materials in pair with alpha-titanium alloy VT-9 are presented. It is shown that at a sliding velocity of 0.004 m/s, contact pressure of 3.3 MPa, and temperature of 150°C, their antifriction properties do not differ significantly. ZrO₂ crystals have a slight superiority in wear resistance because of their higher microhardness. The wear resistances of the alpha-alloy in pair with the ceramics and the crystal are practically the same. A power pattern of the dependence of the wear rate on the pressure for the ZrO₂ crystal and the VT-9 (α) alloy is confirmed.