Surface Engineering Design of Alumina/Molybdenum Fibrous Monolithic Ceramic to Achieve Excellent Lubrication in a High Vacuum Environment

Al₂O₃/Mo fibrous monolithic ceramics are potential candidates for space applications because of their excellent mechanical properties and low density. This study aims at achieving low friction and long life of this material in a high vacuum environment. Three-dimensional composite-lubricating layers were fabricated by considering texture pattern as storage dimples and MoS₂ synthesized via hydrothermal method as lubricant. The tribological properties were studied sliding against Si₃N₄ ceramic and GCr15 bearing steel balls under high vacuum condition. Results showed that the lubricating properties of the Al₂O₃/Mo fibrous monolithic ceramics were improved greatly by the micro-texture and MoS₂ solid lubricant; the friction coefficients were as low as approximately 0.08 and 0.04, respectively, when Si₃N₄ ceramic and GCr15 bearing steel balls acted as the pairing materials. It was also demonstrated that the low friction coefficient can be realized with various normal loads and sliding speeds, indicating the composite-lubricating layers have good adaptation of working conditions. This excellent performance of the material is mainly because of MoS₂ stored in dimples can be easily dragged onto the friction surface to form lubricating and transferring films during the friction process. This work is an extension of studies that were previously published in Tribology Letters journal.     

The Antiwear Action of Zinc Di-n-Butyl Phosphate

The dynamics of MoS₂ particles in a mineral oil dispersion are studied in the same manner as reported in Part I for graphite dispersions. A Hertzian contact consisting of a steel ball in contact with a glass disk is lubricated with MoS² dispersions and observed by optical microscopy at various. slide/roll conditions. In general, the behavior of MoS₂ and graphite are similar. That is, the solids lend to enter the contact and form a film on the contacting surfaces whenever a rolling component of motion is used, but solid particles seldom enter the contact during pure sliding. MoS₂ has more pronounced plastic flow behavior than graphite. However, the polished steel ball is more readily scratched by MoS₂ than by graphite. Under the conditions of these studies, lower friction and wear are observed with pure oil rather than with the dispersions. However, under other conditions (such as different contact geometry or rougher surfaces), the solid-lubricant dispersions might be beneficial.     

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.     

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.     

Embeddability behaviour of some Pb-free engine bearing materials in the presence of abrasive particles in engine oil

One of the tribological requirements on engine bearing material is its ability to safely embed contaminant particles onto its surface and minimise damage to both the bearing and crankshaft surfaces. In this work, a journal bearing test rig that operates under constant load has been employed to investigate the embeddability behaviour of selected multi-layered Pb-free engine bearing materials at three different rotational speeds using engine oil contaminated with SiC particles. Experimental results have shown that third-body abrasive wear is influenced by the lubricant film thickness. There was also difference in embeddability of the different materials. Bismuth-based overlay and MoS₂ containing polyamide-imide-based overlay-coated materials show higher wear compared to tin-based overlay and a polyamide-imide-based composite overlay-coated material. Steel counter surfaces sliding against bismuth-based overlay and MoS₂ containing polyamide-imide-based overlay exhibited higher wear than those sliding against tin-based overlay and polyamide-imide-based composite overlay.     

Fabrication and evaluation of D-gun sprayed WC–Co coating with self-lubricating property

A WC–Co coating with self-lubricating property was deposited by detonation gun (D-gun) process, using a commercial WC–Co powder doped with a MoS₂–Ni powder, under a proper spray condition. It is proved that the MoS₂ composition in the feed powder was kept, which is attributed to the protection of Ni around it, and its content is a little higher in the resulting coating. Evaluation on sliding wear property indicates that the MoS₂ composition plays an important role in lowering both coefficient of friction and wear rate for the resulting coating, which is confirmed by observations on wear track, as well as X-ray photoelectron spectroscope (XPS) results on worn surface. It suggests that the deposition of WC–Co coating with self-lubricating property by D-gun spray is feasible by controlling lubricant powder and spray conditions, which can exhibit higher sliding wear resistance.     

Effects of Surface Pretreatment of Ti-6AI-4V on the Adhesion and Wear Lifetimes of Sputtered MoS2 Coatings

Improving the adhesion and wear endurance lifetimes of the solid lubricant molybdenum disulfide (MoS₂) on titanium (Ti) alloys was studied in this experimental investigation. Ti-6Al-4V alloy specimens were implanted with gas ions or coated with ceramic layers prior to coating with sputtered MoS₂ to investigate the adhesion and wear lifetimes of the MoS₂ coatings. The greatest improvement in scratch adhesion (2.4 times Ti-6Al-4V coated directly with MoS₂) was observed for an MoS₂/diamond-like carbon/Si multilayer coating. Sliding wear tests revealed the greatest lifetime improvement (3.2 ×) was for an MoS₂/TiC dual-layer coating. Increased MoS₂ adhesion was observed for pretreated surfaces with a Vickers microhardness greater than 800 kgf/mm². Increased adhesion of MoS₂ for bond layers with lower elastic moduli (estimated) is suggested. Therefore the ratio hardness/elastic modulus may be a potential figure of merit for surface pretreatment selection.     

Morphological Influence of Molybdenum Disulfide on the Tribological Properties of Rapeseed Oil

The morphological influence of molybdenum disulfide (MoS₂) on the tribological properties of rapeseed oil (RO) was investigated. MoS₂ nano-vesicles improved the tribological properties of RO, especially the wear resistance. However, nano-platelets and micro-platelets weakened the wear resistance because of their bad dispersibility in RO. Positive synergistic lubrication was observed between nano-vesicles and nano-platelets, but not between nano-vesicles and micro-platelets or nano-platelets and micro-platelets. Abrasive plowing is the main wear manner for the steel balls lubricated by RO or ROs with platelet-like MoS₂. However, chemical corrosive wear occurred on the friction surface as lubricated by RO with nano-vesicles. The chemical corrosive wear confirmed the entering of MoS₂ nano-particles to the contact area for lubrication. This lubrication of nano-vesicles was ascribed to their spherical structure and good dispersibility in RO. The nano-vesicles did not influence the stability of RO and assembled beside furrows to alleviate the wear. Thus, potential applications of MoS₂ nano-vesicles in vegetable oils were revealed.     

Tribological properties of γ-Y2Si2O7 ceramic against AISI 52100 steel and Si3N4 ceramic counterparts

Reciprocating ball-on-flat dry sliding friction and wear experiments have been conducted on single-phase γ-Y₂Si₂O₇ ceramic flats in contact with AISI 52100 bearing steel and Si₃N₄ ceramic balls at 5–15 N normal loads in an ambient environment. The kinetic friction coefficients of γ-Y₂Si₂O₇ varied in the range over 0.53–0.63 against AISI 52100 steel and between 0.51–0.56 against Si₃N₄ ceramic. We found that wear occurred predominantly during the running-in period and it almost ceased at the steady friction stage. The wear rates of γ-Y₂Si₂O₇ were in the order of 10^?4 mm³/(N m). Besides, wear debris strongly influenced the friction and wear processes. The strong chemical affinity between γ-Y₂Si₂O₇ and AISI 52100 balls led to a thick transfer layer formed on both contact surfaces of the flat and counterpart ball, which changed the direct sliding between the ball and the flat into a shearing within the transfer layer. For the γ-Y₂Si₂O₇/Si₃N₄ pair, a thin silica hydrate lubricant tribofilm presented above the compressed debris entrapped in the worn track and contact ball surface. This transfer layer and the tribofilm separated the sliding couple from direct contact and contributed to the low friction coefficient and wear rate.     

Various MoS2-, WS2- and C-Based Micro- and Nanoparticles in Boundary Lubrication

Various solid lubricant particles have been experimentally evaluated as possible additives to oils. However, information in terms of a direct comparison of their tribological properties is still missing. In this study, we have compared the tribological properties of seven different solid lubricant micro- and nanoparticles as additives in polyalphaolefin (PAO) oil: MoS₂ nanotubes, MoS₂ platelets (2 and 10 μm), WS₂ nanotubes, WS₂ fullerene-like nanoparticles, graphite platelets (20 μm) and multi-walled carbon nanotubes. The experiments were performed in the boundary lubrication regime under a contact pressure of 1 GPa (Hertz, max) using a ball-on-disc tribotester. In general, the particles significantly decreased the friction and wear compared to the base PAO oil. We found that it was the material of the particles that largely determined their tribological performance. The effect of the size of the particles was much less important, and the morphology (shape) of the particles had little or no influence. We have also investigated the effect of ultrasonication during suspension preparation on particle damage and found that the solid lubricant particles were not notably affected, except the MoS₂ and WS₂ nanotubes, which became somewhat shorter.     

Formation of Low-Friction Particle/Polymer Composite Tribofilms by Tribopolymerization

Inorganic fullerene-like (IF) solid lubricant particles based on MoS₂ or WS₂ have recently gained attention in various tribological applications, for instance incorporated in coatings, porous materials, greases and dispersed in oils. However, their effect in oil can be limited often due to inadequate penetration into the contact zone. Meanwhile, tribopolymerization of monomer additives in oil have proven to be efficient to reduce wear, but without significantly reducing the friction. This investigation combines these two lubrication techniques in order to form particle/polymer composite tribofilms, aimed to give low friction and wear in high-pressure sliding contacts. A cyclic amine, caprolactam, was used as the monomer together with IF as well as normal 2H WS₂ particles. Zn-DTP was used as a reference antiwear additive and poly-alfa-olefin (PAO) was used as the base oil. Reciprocal ball-on-flat sliding test results found that monomer plus particles reduced the friction by 10–50%, depending on sliding speed and type of particles, compared with particles or monomer alone. And the scattering between different tests was also considerably lower. The wear rate was also substantially reduced to a level similar to that of Zn-DTP. The particle/polymer composite tribofilms were generally much larger than the actual contact area and its thickness varied from below a few nanometres in the contact centre to several micrometers in the outer parts. Consequently, the reduced friction is believed to come from two effects: one being a trapping of particles in the contact zone, which reduces the boundary friction level; and the other a shift in lubrication regime towards EHD-lubrication due to reduced contact pressures accompanied with the formation of the composite tribofilm.     

Multi-environmental lubrication performance and lubrication mechanism of MoS2/Sb2O3/C composite films

MoS₂–Sb₂O₃–C composite films exhibit adaptive behavior, where surface chemistry changes with environment to maintain the good friction and wear characteristics. In previous work on nanocomposite coatings grown by PVD, this type of material was called a “chameleon” coating. Coatings used in this report were applied by burnishing mixed powders of MoS₂, Sb₂O₃ and graphite. The solid lubricant MoS₂ and graphite were selected to lubricate over a wide and complementary range including vacuum, dry air and humid air. Sb₂O₃ was used as a dopant because it acts synergistically with MoS₂, improving friction and wear properties. The MoS₂–Sb₂O₃–C composite films showed lower friction and longer wear life than either single component MoS₂ or C film in humid air. Very or even super low friction and long wear-life were observed in dry nitrogen and vacuum. The excellent tribological performance was verified and repeated in cycles between humid air and dry nitrogen. The formation of tribo-films at rubbing contacts was studied to identify the lubricating chemistry and microstructure, which varied with environmental conditions. Micro-Raman spectroscopy and Auger electron spectroscopy (AES) were used to determine surface chemistry, while scanning electron microscopy and transmission electron microscopy were used for microstructural analysis. The tribological improvement and lubrication mechanism of MoS₂–Sb₂O₃–C composite films were caused by enrichment of the active lubricant at the contact surface, alignment of the crystal orientation of the lubricant grains, and enrichment of the non lubricant materials below the surface. Sb₂O₃, which is not lubricious, was covered by the active lubricants (MoS₂ – dry, C – humid air). Clearly, the dynamics of friction during environmental cycling cleaned some Sb₂O₃ particles of one lubricant and coated it with the active lubricant for the specific environment. Mechanisms of lubrication and the role of the different materials will be discussed.     

Wear and frictional behaviour of MoS2 pellets and steel lubricated with MoS2 powder

The wear and frictional behaviour of compressed pellets of molybdenum disulphide (MoS₂) were measured when the pellets were slid against a 0.45% C steel disk in air. The wear of the MoS₂ pellets decreased with increasing sliding velocity and increasing contact pressure. These factors are indicated by a parameter μPV which corresponds to the frictional heat. A thin deformed surface layer on the sliding surface of an MoS₂ pellet was different from the MoS₂ substrate.The wear of a bearing steel slider against mild steel was reduced by an MoS₂ film in air and in vacuum. The specific wear rate of the slider and the coefficient of friction were influenced by the degree of vacuum.     

A tribotesting method for evaluating cemented carbide tribo-performance

The abrasive wear properties of materials in sliding contact with solid mineral particles during the comminution process have been studied. The equipment used can simulate the tribo-conditions inside coal pulverisers. Using this experimental apparatus, a number of different cemented tungsten carbides have been tested and classified according to their resistance to abrasive wear in rubbing contact with particulate coal. The paper shows that the wear results can be used to estimate the resistance of a material to brittle fragmentation and chipping of the edges during tribo-contact with solid particles. An equation is presented which enables calculation of the fracture resistance factor, K_WR, based on results from carefully controlled repeated abrasion tests.     

Effect of MoS2 and WS2 Nanotubes on Nanofriction and Wear Reduction in Dry and Liquid Environments

Nano-objects in dry and liquid conditions have shown reductions in friction and wear on the macroscale. Studies in low viscosity liquids with nanoparticles and nanotubes made of lubricating materials such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) are limited. In this research, MoS₂ and WS₂ nanotubes with spherical gold (Au) nano-objects as a control are studied on the nanoscale under dry and low viscosity liquid environments for their effect on friction and wear reduction. Atomic forces microscopy (AFM) experiments on the nanoscale are performed in single-nano-object contact with an AFM tip, where nano-objects are laterally manipulated and multiple nano-object contact with a tip attached to a glass sphere sliding over several nano-objects. Wear tests were performed on the nanoscale by means of AFM as well as on the macroscale using a ball-on-flat tribometer to relate friction and wear reduction on both scales. Results indicate that nano-objects such as MoS₂ and WS₂ nanotubes contribute to friction and wear reduction due to the reduced contact area and the possible rolling and sliding on the nanoscale. On the macroscale, reductions in friction and wear occur due to possible exfoliation of outer layers in addition to other mechanisms just mentioned.     

Evaluation of Hot Pressed Silicon Nitride as a Rolling Bearing Material

A series of tests has been conducted to evaluate the suitability of silicon nitride as a bearing material for rolling contact applications. The ability of silicon nitride to be lubricated by some conventional lubricants was found to be satisfactory. This was determined by wettability studies, lubricant film thickness and traction coefficient measurements on the optical EHD rig and friction coefficient measurements by the pin-on-disk method. The abrasive wear coefficient, measured on a lopping machine using 600 grit SiC abrasive, was found to be high compared to other ceramics. It was also dependent on the composition of the silicon nitride. Comparative rolling contact fatigue tests on steel and silicon nitride flat washers were conducted using steel rollers and balls. A high wear rate leading to grooving in the rolling track on silicon nitride was observed. The spalling resistance of silicon nitride was found to be higher than that of steel under the test conditions used. Surface interactions in the Si₃N₄-M50 steel contacts, detrimental to the life of the steel rolling elements, were recognized. Attempts were made to reduce the severity of these interactions and prolong the life of bearings containing ceramic elements.     

The Role of Cerium in the Resistance of an MoS2-Containing Composite Brush Plating Layer to Humid Atmosphere

MoS₂ is an excellent solid lubricant widely used for reducing friction. However, moisture is very harmful to its solid lubrication property because MoS₂ is easily oxidized to form Mo⁶⁺ and S⁶⁺ in a humid atmosphere. In order to improve its oxidation resistance, a study on the role of a rare earth element Ce in the resistance of a Ni-Cu-P/Mo₂ brush plating layer to the humid atmosphere was carried out. It was found that cerium can effectively stabilize the solid lubrication property of MoS₂ due to its preferential adsorption on the surface of MoS₂ particles, the adsorbed layer serving as a barrier to oxidation. This study shows that the rare earth element Ce can be deposited from a water plating solution.     

Tribological behaviour of polyalphaolefins: Wear and rolling‐contact fatigue tests

Polyalphaolefin (PAO) fluids have become widely accepted as high‐performance lubricants and functional fluids due to certain inherent, and highly desirable, characteristics. One of these characteristics is their low toxicity, which, combined with excellent viscometrics and lubricity, have made low‐viscosity PAO fluids an important component in lubricant formulations. Typical data found in product specifications for lubricants are the kinematic viscosity and the viscosity index. These values do not give enough information with which to choose the optimum lubricant for a lubricated contact. In mechanical systems, rolling, sliding, and rolling/sliding contacts occur, and lubricants have to work optimally under these operating conditions. In this study the rolling‐contact fatigue lives (L₅₀ and L₁₀) of PAOs of different viscosities were experimentally determined. The tests were carried out using a four‐ball machine. Wear tests were also carried out using another four‐ball tester in order to measure the wear‐scar diameter and the flash temperature parameter. The lubricants were characterised by infrared spectroscopy, and the pitting of the balls was observed using scanning electron microscopy.     

Microstructural Characterization of Wear Particles Formed During Tribological Stressing of TiC and Ti(C,N) Coatings

An attempt was undertaken to obtain a better understanding of the tribological properties of two wear-resistant coatings on tool steel by structural and microchemical analysis of wear particles using a transmission electron microscope. Coatings were deposited by physical vapor deposition and plasma-assisted chemical vapor deposition techniques and tribological properties were derived from reciprocating sliding tests of the coatings against alumina balls. Three types of wear particles were identified by electron diffraction and energy dispersive X-ray spectroscopy: nanocrystalline rutile (TiO₂), nanocrystalline graphite and microcrystalline graphite. Low coefficients of friction, of the order of 0.2, were attributed to the formation of solid lubricant films of sub-stoichiometric TiO_2-x Magnéli phases and/or graphite.     

Friction Properties of Carbon Nano-Onions from Experiment and Computer Simulations

The carbon nano-onion can be considered as a new kind of interesting lubricating nanoparticle. Used as lubricant additives, carbon nano-onions lead to a strong reduction of both friction and wear, even at low temperature. To better elucidate the mechanisms by which these processes occur, coupled experimental and computational investigations are carried out. In addition, it is found that lubricious iron oxide nanoparticles are generated in the core of the steel contact through mechanisms that are not yet known. The molecular dynamics simulations of carbon onions placed between sliding diamond-like carbon surfaces at high contact pressure indicate that the lubrication mechanism of the onions is based on a coupled process of rolling and sliding inside the contact area. We conclude that most of carbon onions seem to remain intact under friction processes and do not generate graphitic planes, which is in contrast to the previously determined behavior of MoS₂ fullerenes that are mainly exfoliated inside the contact area and liberate lubricating lamellar sheets of h-MoS₂.