Reinhold Bauer

Chemical and Structural Effects on the Lubrication Properties of Sputtered MoS2 Films

Lubricating films of MoS2 have been prepared by sputter deposition onto steel substrates maintained at different temperatures. The surface chemical compositions and bulk structural properties of the films before and after rubbing for various periods of time in dry nitrogen gas were determined by x-ray photoelectron spectroscopy and X-ray diffraction, respectively. Film thickness, density, and morphology measurements were also made. Differences in these properties for the films prepared under different conditions provide some insight into the mechanism(s) of lubrication and failure of the films. On the basis of the reported results, it is proposed that lubrication by MoS2 films occurs through a process of intercrystallite slip and that there is an optimum crystallite size that provides maximum wear life under any particular set of application (use) conditions. Methods for improving film performance based on particle/crystallite size and lattice spacing arguments and on the electronic properties of MoS2 and...

Structural and tribological studies of MoS2 solid lubricant films having tailored metal-multilayer nanostructures

Abstract Molybdenum disulfide (MoS 2 ) solid lubricant films were prepared by r.f. magnetron sputtering on 440C steel, 52100 steel, and silicon substrates. This study concentrated on films that were multilayer coatings of MoS 2 with either nickel or Au-(20%)Pd metal interlayers. Multilayer thicknesses ranged from 0.2 nm to 1.0 nm while the multilayer periodic spacing ranged from 3 to 10 nm. Scanning electron microscopy and X-ray diffraction revealed that the multilayer films had dense microstructures that, in some cases, exhibited significant orientation of their basal planes parallel to the substrate. Film endurance was assessed in sliding contact using thrust washer tests and in rolling contact using thrust bearing tests. Some film microstructures exhibited excellent endurance. Brale indentation indicated that the metal layers can improve film fracture toughness. Friction in air and ultrahigh vacuum (UHV) was investigated using a UHV-compatible test apparatus. Friction coefficients between 0.05 and 0.08 were measured in UHV.

Tribological performance and deformation of sputter-deposited MoS2 solid lubricant films during sliding wear and indentation contact☆

Abstract Microstructural aspects of the plastic deformation of sputter-deposited MoS2 solid lubricant films on 440C bearing steel are presented. The films were produced in three different laboratories using r.f., d.c. and r.f. magnetron sources. Two types of contact were investigated: (1) sliding wear contact in a thrust-washer apparatus used to evaluate film tribological performance and (2) brale indentation contact used to investigated cross-sectional morphology and adhesion. Scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to characterize the films. The as-deposited morphology was found to influence the initial nature of the wear debris and the load-bearing capabilities of the films. In many cases a highly deformed region confined to the surface of the films was found. Applied stress reoriented crystallites and induced crystallization, the degree of both processes being related to the initial structure and the percentage of wear lifetime of the film.

The Influence of Surface Chemistry on MoS2 Transfer Film Formation

A variety of MoS2 films having different initial surface compositions and different degrees of reaction in oxidizing atmospheres have been prepared by sputter deposition onto 440C steel substrates. The compositions of these films, including their degree of oxidation and their molybdenum-sulfur stoichiometry, have been determined by x-ray photoelectron and Auger electron spectroscopy immediately before the films were subjected to sliding wear tests. These tests, which used, an inhouse, “ring-on-disk” apparatus operating in an inert atmosphere, involved rubbing coated specimens against initially uncoated, polished 440C steel coutersurfaces. Films that had approximately 30 to 40 percent of their surface layers oxidized to MoO3 gave the best wear results. In addition, the presence of elemental sulfur in the film surface layers appears to enhance wear life. These data are interpreted to mean that much better adhesion between the transfer film and he initially bare steel surface is obtained through chemical bon...

Chemical Analysis of Hydrocarbon Grease from Spin Bearing Tests

The rotor bearing running torque levels of certain spin bearing units were found to increase significantly during extended life testing. Samples of the greases from these tests were analyzed to determine if lubricant degradation was linked with the increased torque levels. By means of chromatographic and spectroscopic techniques, we were able to determine that oxidation of some of the greases had occurred and that the oxidative degradation was most easily observed in the soap-thickener portion of the grease. A correlation between the disappearance of the fatty acid salts and the appearance of degradation products was observed. Polymerization of the greases war observed in all samples. We also determined that the grease degradation was symptom of adverse mechanical condition in the bearings and not the primary cause of mechanical problems. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference in Washington, D.C., October 5–7, 1982

Oil Exchange Between Ball Bearings and Cotton-Phenolic Ball-Bearing Retainers.

Experiments have been performed that determine for the first time the transfer of oil between cotton-phenolic ball-bearing retainers and operating ball bearings. A full retainer exchanges oil with the metal park of the bearing, probably by diffusional mixing. There is no net delivery of oil from the retrainer to the metal parts of the bearing. A partially filled retainer (such as one that has been incompletely impregnated) absorbs oil from the bearing even during operation, this drying the bearing. A fully-impregnated retainer does not deliver any significant amount of additional oil to the metal parts of a poorly lubricated bearing. The retainer will not prevent lubricant degradation and premature bearing failure under the conditions of these experiments. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Lahaina, Hawaii, October 16–20, 1994

Laser surface processing of molybdenum disulfide thin films

Abstract We have investigated pulsed laser processing of textured MoS2 thin films using excimer radiation at 248 nm wavelength. The films show little change in their crystallographic orientation even after melting and resolidification as determined by X-ray diffraction. The morphology of the films both on the surface and in cross-section is changed somewhat, particularly in the case of melted films, although the planarity of the films is not. Laser processing at high fluences removes material from the surface but does not change the stoichiometry of the remaining film. Measurements of mass loss both by weight loss and by Rutherford backscattering show that the density of the films is unchanged by the processing. The absence of density change is confirmed by the strength of the X-ray diffraction signal. Processed and unprocessed films show no significant difference in wear life. Small spot X-ray photoelectron and Raman spectroscopies of the films indicate that a portion of the surface decomposes, but that MoO3 incorporation in the film is decreased. The stability of the films under energetic laser processing raises the possibility of using this technique to incorporate other materials into the films or to increase substrate-film adhesion.

Paper V (i) Effects of microstructure and adhesion on performance of sputter-deposited MoS2 solid lubricant coatings

Selection of a lubricant for a given application (use) is determined by a number of factors. Analogous to fluid lubricants, where proper base stock, additive package, and formulation are selected for the application, choice of dry lubricants requires consideration of the appropriate deposition procedure, film composition, film structure, and substrate surface preparation procedure. The relationship between film physical and mechanical properties of MoS 2 produced by rf, de, and rf magnetron sputtering is reviewed. We propose that when the requirements of contact geometry, pressures (stresses), and operating life are considered, optimization of three essential properties of the coatings will provide the ultimate in performance: 1) good coating-substrate adhesion, 2) dense, small grain size (low porosity), and 3) high chemical (phase) purity.

Torque Characteristics of Solid-Lubricated Precision Bearings During Oscillatory Motion

MoS2 sputter-coated as well as uncoaled angular-contact ball bearings were tested with various cage materials in a low-speed regime. Various self-lubricating polymers either “neat” or with additives and metallic cages were tested. The emphasis of this paper is to report on significant torque increases, i.e., torque bumps observed during non-ball overlap, oscillatory motion for some race/cage combinations. In some race/cage combinations, torque bumps can be minimized by proper run-in. No attempt was made to run the bearings to failure, although certain race/cage combinations did in fact fail before the end of test. Presented at the 48th Annual Meeting in Calgary, Alberta, Canada May 17–20, 1993

The effect of hydrochloric acid pretreatments on 440C steel surface composition and the adhesion and endurance of sputter-deposited MoS2 solid lubricant films

Abstract The effects of substrate pretreatment on substrate surface chemistry and on film-substrate adhesion, and film endurance (wear life) of sputter-deposited molybdenum disulfide (MoS 2 ) on 440C bearing steel were investigated. Specifically, 20% hydrochloric acid-ethanol mixtures were used to etch the steel surface prior to deposition or X-ray photoelectron spectroscopy (XPS). Acid etching inhibited fractures at the film-substrate interface generated by brale indentation testing. XPS indicated that the acid etching removed an iron-rich oxide surface layer, exposing a chromium-rich oxide underlayer on the steel matrix. Acid etching did not significantly affect thrust washer sliding wear life.