Showing papers on "Tribology published in 2000"

Thin films of fullerene-like MoS2 nanoparticles with ultra-low friction and wear

Abstract: The tribological properties of solid lubricants such as graphite and the metal dichalcogenides MX2 (where M is molybdenum or tungsten and X is sulphur or selenium) are of technological interest for reducing wear in circumstances where liquid lubricants are impractical, such as in space technology, ultra-high vacuum or automotive transport. These materials are characterized by weak interatomic interactions (van der Waals forces) between their layered structures, allowing easy, low-strength shearing. Although these materials exhibit excellent friction and wear resistance and extended lifetime in vacuum, their tribological properties remain poor in the presence of humidity or oxygen, thereby limiting their technological applications in the Earth's atmosphere. But using MX2 in the form of isolated inorganic fullerene-like hollow nanoparticles similar to carbon fullerenes and nanotubes can improve its performance. Here we show that thin films of hollow MoS2 nanoparticles, deposited by a localized high-pressure arc discharge method, exhibit ultra-low friction (an order of magnitude lower than for sputtered MoS2 thin films) and wear in nitrogen and 45% humidity. We attribute this 'dry' behaviour in humid environments to the presence of curved S-Mo-S planes that prevent oxidation and preserve the layered structure.

Synthesis of diamondlike carbon films with superlow friction and wear properties

Abstract: In this study, the authors introduce a new diamondlike carbon (DLC) film providing a friction coefficient of 0.001 and wear rates of 10{sup {minus}9} to 10{sup {minus}10} mm{sup 3}/N.m in inert-gas environments (e.g., dry nitrogen and argon). The film was grown on steel and sapphire substrates in a plasma enhanced chemical vapor deposition system that uses using a hydrogen-rich plasma. Employing a combination of surface and structure analytical techniques, they explored the structural chemistry of the resultant DLC films and correlated these findings with the friction and wear mechanisms of the films. The results of tribological tests under a 10-N load (creating initial peak Hertz pressures of 1 and 2.2 GPa on steel and sapphire test pairs, respectively) and at 0.2 to 0.5 m/s sliding velocities indicated that a close correlation exists between the friction and wear coefficients of DLC films and the source gas chemistry. Specifically, films grown in source gases with higher hydrogen-to-carbon ratios had the lowest fiction coefficients and the highest wear resistance. The lowest friction coefficient (0.001) was achieved with a film on sapphire substrates produced in a gas discharge plasma consisting of 25% methane and 75% hydrogen.

A crystal-chemical approach to lubrication by solid oxides

Abstract: This paper introduces a new approach to the selection, classification, and mechanistic understanding of lubricious oxides that are used to combat friction and wear at elevated temperatures. Specifically, it describes a crystal-chemical model that enables one to predict the shear rheology or lubricity of an oxide or oxide mixture at elevated temperatures. This model can be used to formulate new alloy compositions or composite oxide structures that can provide low friction at high temperatures. In the case of composite oxides, the model allows one to estimate the solubility limits, chemical reactivity, compound forming tendencies, as well as the lowering of the melting point of one oxide when a second oxide is present. From a tribological standpoint, a prior knowledge of these details is important because they are strongly related to the extent of adhesive interactions, shear rheology, and hence to lubricity of oxides. In light of certain crystal-chemical considerations, general guidelines are provided for the selection of those oxides that can provide low friction at high temperatures. The major goal of this paper is to establish model relationships between relevant crystal-chemical and tribological properties of oxides that can be used as lubricants at high temperatures. Such a model may help eliminate guesswork in high-temperature lubrication and provide a new means to address the difficult lubrication problems experienced at high temperatures.

Smoothest bearings: interlayer sliding in multiwalled carbon nanotubes

Abstract: The well-defined geometry and extreme structural anisotropy of a multiwalled carbon nanotube can bring qualitatively new features to its nanometer-scale tribology. Efficient cancellation of registration-dependent interactions in incommensurate tubes (and also, surprisingly, certain axial commensurate tubes) can induce extremely small and nonextensive shear strengths.

Synthesis of superlow-friction carbon films from highly hydrogenated methane plasmas☆

Abstract: In this study, we investigated the friction and wear performance of diamond-like carbon films (DLC) derived from increasingly hydrogenated methane plasmas. The films were deposited on steel substrates by a plasma-enhanced chemical vapor deposition process at room temperature and the tribological tests were performed in dry nitrogen. Tests results revealed a close correlation between the hydrogen in the source gas plasma and the friction and wear coefficients of the DLC films. Specifically, films grown in plasmas with higher hydrogen-to-carbon ratios had much lower friction coefficients and wear rates than did films derived from source gases with lower hydrogen-to-carbon ratios. The lowest friction coefficient (0.003) was achieved with a film derived from 25% methane, 75% hydrogen, while a relatively high coefficient of 0.015 was found for films derived from pure methane. Similar correlations were observed for wear rates. Films derived from hydrogen-rich plasmas had the least wear, while films derived from pure methane suffered the highest wear. We used a combination of surface analytical methods to characterize the structure and chemistry of the DLC films and worn surfaces.

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

Abstract: 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. Presented at the 55th Annual Meeting Nashville, Tenn...

Tribological properties of plasma sprayed alumina-titania coatings : Role and control of the microstructure

Abstract: Plasma sprayed ceramic coatings (Al 2 O 3 +40% TiO 2 ) were studied using two different kinds of powder: one spray dried and one crushed and blended. The cooling rate during the thermal spray process was also considered, with the use of cryogenic cooling. The friction behavior and the wear resistance of the corresponding coatings were then evaluated in dry sliding conditions in two different configurations: block-on-ring (Amsler) and ball-on-disc. The microstructure of a coating is conditioned by the morphology of the powder and the cooling rate which modify the residual stresses in the coating.

A frictional study of total hip joint replacements.

Abstract: Polymeric wear debris produced by articulation of the femoral head against the ultra-high-molecular-weight polyethylene socket of a total hip replacement has been implicated as the main cause of osteolysis and subsequent failure of these implants. Potential solutions to this problem are to employ hard bearing surface combinations such as metal-on-metal or ceramic-on-ceramic prostheses. The aim of this study was to investigate the difference in lubrication modes and friction of a range of material combinations using synthetic and biological fluids as the lubricants. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing carboxy methyl cellulose (CMC) fluids as the lubricant. Under these conditions the ceramic-on-ceramic joints showed full fluid film lubrication while the metal-on-metal, metal-on-plastic, diamond-like carbon-coated stainless steel (DLC)-on-plastic and ceramic-on-plastic prostheses operated under a mixed lubrication regime. With bovine serum as the lubricant in the all ceramic joints, however, the full fluid film lubrication was inhibited due to adsorbed proteins. In the metal-on-metal joints this adsorbed protein layer acted to reduce the friction while in the ceramic coupling the friction was increased. The use of bovine serum as the lubricant also significantly increased the friction in both the metal-on-plastic and ceramic-on-plastic joints. The friction produced by the DLC-on-plastic joints depended on the quality of the coating. Those joints with a less consistent coating and therefore a higher surface roughness gave significantly higher friction than the smoother, more consistently coated heads.

Mechanical and tribological performance of MoS2 co-sputtered composites

Abstract: Co-sputtered films of MoS2 with Au, Ti, Cr or WSe2, respectively, have been deposited in order to investigate the relationships between dopant and tribological and mechanical behaviour. The alloying constituents were selected based on possible chemical effects, for example substitution in the MoS2 structure, intercalation, edge-bond passivation, and with and without simultaneous oxygen gettering of the sputtering environment. The mechanical properties of the composite films were investigated by pin-on-disk sliding wear testing and linear, oscillating wear testing both in 50% relative humidity air, microscratch and indent adhesion characterisation, and nanoindentation hardness as a function of the dopant element(s) and concentration. All of the dopants investigated exhibited superior tribological and mechanical behaviour compared to the pure MoSx films. In tribological tests, results indicate a significant improvement in the lifetime, mean friction coefficient, and friction ‘noise’ of the films. Tribological performance was found to be strongly related to both the dopant element and dopant concentration. The best dopant for improved performance was found with WSe2 followed by Au, Ti, and Cr.

Tribology of Diamond, Diamond-like Carbon and Related Films

Abstract: Diamond, diamond-like carbon (DLC), and other related materials (i.e., carbon nitride and cubic boron nitride [CBN]) are some of the hardest materials known and offer several other outstanding properties, such as high mechanical strength, chemical inertness, and very attractive friction and wear properties, that make them good prospects for a wide range of tribological applications, including rolling and sliding bearings, machining, mechanical seals, biomedical implants, microelectromechanical systems (MEMS), etc. The dry sliding friction and wear coefficients of these materials are among the lowest recorded to date (Brookes and Brookes, 1991; Feng and Field, 1991; Field, 1992; Miyoshi, 1995; Erdemir, 2001a,b). In fact, if they were inexpensive and readily available, they would undoubtedly be the materials of choice for a wide range of applications. Besides their exceptional mechanical and tribological properties, most of these superhard materials offer broad optical transparency, high refractive index, wide bandgap, low or negative electron affinity, transparency to light from deep UV through visible to far infrared, excellent thermal conductivity, and extremely low thermal expansion. Briefly, these exceptional qualities make diamond, DLC, and other related materials ideal for numerous industrial applications in addition to tribology. Ali Erdemir Argonne National Laboratory

Tribological behaviour and microscopic wear mechanisms of UHMWPE sliding against thermal oxidation-treated Ti6Al4V

Abstract: Tribological behaviour of ultra-high molecular weight polyethylene (UHMWPE) pins sliding against thermal oxidation (TO)-treated Ti6Al4V alloy discs with different levels of average surface roughness was investigated under water lubrication conditions. When rubbing against a smooth counterface (RaB 0.030‐0.035 mm), UHMWPE was found to be worn predominantly via a micro-fatigue mechanism. To advance the scientific understanding of the microscopic wear mechanisms of UHMWPE, a technique involving permanganic etching coupled with high resolution SEM analyses of wear surfaces and cross-sections was adopted to yield new insight into the micro-fatigue mechanism. It was found that stress-induced preferential orientation of the crystalline lamellae in the UHMWPE led to the origin of ripples containing micro-cracks at their valleys. The cyclic loading promoted lateral propagation and inter-connection of these micro-cracks, thus giving rise to eventual spallation of the surface material as wear debris. Based on the experimental results, a micro-fatigue wear mode is proposed. © 2000 Elsevier Science S.A. All rights reserved.

Nano-scratch behavior of a bulk Zr-10Al-5Ti-17.9Cu-14.6Ni amorphous alloy

Abstract: The tribological behavior of a Zr-10Al-5Ti-17.9Cu-14.6Ni (at.%) bulk amorphous alloy, in both the as-cast and annealed states, was investigated using nano-scratch tests, including ramping load scratch and multiple sliding wear techniques. The crystallization sequence of the alloy was also characterized. Mechanical properties, such as Young's modulus, hardness, friction coefficient, and tribological wear were measured. These properties were found to vary with microstructure. In general, an increase in annealing temperature results in an increase in hardness, which in turn produces a decrease in friction coefficient but an increase in wear resistance. Samples having a structure consisting of supercooled liquid matrix with dispersed nanocrystalline particles exhibit the best wear performance. (c) 2000 Materials Research Society.

Hard lubricating coatings for cutting and forming tools and mechanical components

Abstract: Two new coatings based on graphite and MoS2 have been developed. They combine low friction with high hardness, high load capacity and exceptionally low wear. Both coatings act as solid lubricants, providing protection for both the coated surface and any opposing uncoated surface. The coatings are finding application in improving the general performance of cutting and forming tools and also make possible high-speed machining. The graphite-based coatings have exceptional wear properties under water or oil and results from wear tests under a wide range of conditions are given. A number of practical applications are given, including the protection of artificial hip joints. The advantages offered by the use of such coatings for many mechanical components are demonstrated.