Paul D. Fleischauer

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...

Fundamental aspects of the electronic structure, materials properties and lubrication performance of sputtered MoS2 films☆

Abstract An understanding of the fundamental properties of solid lubricants at the atomic level ( e.g. crystalline structure and chemical bonding to substrates) and their relationship to tribological performance is essential to the development of new lubricants to satisfy the demands of advanced mechanical technologies. Detailed measurements of the physical and chemical properties of the surfaces of lubricant films and their substrates, and of their bulk crystalline and electronic structures can be correlated with their friction, wear life and resistance to reactive degradation. In this paper experimental results concerning the electronic and crystal structures, film adhesion and bulk friction of sputter-deposited MoS 2 films are reviewed and discussed. A model using a molecular orbital description of the electronic energy levels of MoS 2 is presented that can be used to interpret these data. This model also provides a rationale for the differences in lubrication effectiveness between NbSe 2 and MoS 2 , and it provides predictive capabilities for improving the performance of MoS 2 films.

Applications of solid lubricant films in spacecraft

Abstract The use of solid (dry) lubricants, particularly solid lubricant thin films, in space systems, including satellites and launch vehicles, is reviewed. Various types of satellites and their generic mechanical requirements are outlined. Mechanisms that use solid lubricant films are described. The types of solid lubricant films available, including burnished, bonded, and sputter-deposited, are reviewed and their properties assessed relative to application requirements. Future opportunities for insertion of solid lubricants as replacements for liquid or grease lubricants are identified.

Effects of Crystallite Orientation on Environmental Stability and Lubrication Properties of Sputtered MoS2 Thin Films

The endurance lives of rf sputtered MoS2 lubricant films are severely degraded by exposure to high humidity environments prior to wear testing. Surface chemical and electron microscopic analyses show that the degraded films are oxidized to form MoO3 and that the oxidation rate is a function of the orientation of the crystallites composing the films before any run-in or burnishing activity. If the crystallites are arranged with their basal planes parallel to the substrate surface, the films have lower reactivity (good stability) and longer endurance lives than film with randomly oriented crystallites. The results of surface analyses and wear tests are presented to support these observations. Auger electron spectroscopy results indicate that oxidation of the randomly oriented films (designated Type I films) occurs to a depth of at least 10 nm into the bulk of the films when they are exposed to adverse environments. Oxidation of films with coplanar orientations (Type II films), however, occurs only within 1 ...

Crystallinity of RF-Sputtered MoS2 Films.

The crystallinity and morphology of thin, radio-frequency (rf) -sputtered MoS2 films deposited on 440C stainless steel substrates at both ambient (∼70°C) and high temperatures (245°C) were studied by scanning electron microscopy (SEM) and by x-ray diffraction (Read thin-film photography and 0−20 scans). Under SEM the films exhibited a “ridgelike” (or platelike) formation region for thicknesses between 0.18 and 1.0 μm MoS2. X-ray diffraction was shown to give more detailed and accurate information than electron defraction, previously used for elucidating the structure of sputtered lubricant films. Read thin-film x-ray diffraction photographs revealed patterns consistent with the presence of polycrystalline films and strong orientation of the MoS2 crystallites. Correlation of those patterns with 0−20 scans of the films indicated that the basal planes of the MoS2 crystallites [i.e., the (001) planes] were perpendicular to the substrate surface plane, and that various edge planes [i.e., the (h k 0) planes] in the individual crystallites were parallel to the surface plane, in agreement with previous observations of thinner films. Sliding wear caused the crystallites to orient with their basal planes parallel to the surface plane. The crystallite lattices in all films in this study were shown to exhibit compressive stress (∼ 3%–5% with respect to natural molybdenite) in the direction perpendicular to the (h k 0) planes, and the worn films were expanded (i.e., exhibited tensile stress) perpendicular to the (001) plane. In addition, the shapes of the x-ray diffraction peaks were strongly influenced by the presence of oxygen impurities and/or sulfur vacancies in the MoS2 lattice, indicating that x-ray diffraction may provide a simple quality-control test for the production of a film with optimum lubricating properties.

A comparison of oxidation and oxygen substitution in MoS2 solid film lubricants

Abstract Significant advancements in the production of low friction, long wear life, sputter-deposited MoS 2 lubricant coatings have been made in the last decade. The introduction of multi-layered coatings, the establishment of careful controls on doping during DC and magnetron sputter deposition, and the implementation of ion assisted deposition have resulted in lubricants with substantially longer wear lives (up to a factor of ten greater than in the early 1980s) and lower sliding friction coefficients. A major research effort, designed to improve the performance of solid lubricants, involved a number of laboratories during this time period, resulting in these major breakthroughs. However, even with this concentrated effort, the typical investigation involved making an educated guess, based on previous experience, of the deposition conditions, target compositions, or post treatments that might be expected to provide improved performance of resulting coatings. One notable discovery during this time period was that typical MoS 2 films contain large quantities (up to 20 atom %) of oxygen substituted for sulfur in individual crystal lattices. In this paper we will compare the effects of this oxygen substitution with the effects of oxidation which involves a change in the oxidation number of the central molybdenum atoms within the crystals. A discussion of the relationship(s) between chemistry and coating structure and tribological performance will be presented with emphasis on defect chemistry and multiple phase interactions. Speculations on the role of coating chemistry in determining coating performance in applications such as in ball bearings will be presented.

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...

Assessment of the Tribological Requirements of Advanced Spacecraft Mechanisms

Abstract : A survey was conducted of existing technologies for moving mechanical assemblies used in spacecraft applications. The purpose was to identify areas where future requirements for lifetimes in excess of ten years with anticipated speeds, loads, and temperatures might not be satisfied. Some specific mechanisms, such as momentum/reaction wheels, high speed turbines, pointing and tracking mechanisms, despin mechanisms, and gimbal mechanisms, were identified as areas for potential application of existing but unused technologies. Two major problem areas identified involve boundary regime lubrication and lubricant supply (active or passive) for long life. Areas where substantial, near term improvements appear practical include the use of hybrid bearings, new synthetic fluid lubricants, new bearing retainer materials, and properly designed solid film lubricants.

Structural Studies of Sputter-Deposited MoS2 Solid Lubricant Films.

Abstract : The relationship between the morphologies and crystal structures of sputter-deposited MoS2 films and their lubricating capabilities is discussed. In particular, aspects of plastic deformation processes are presented. Scanning electron microscopy (top surface and cross sectional), transmission electron microscopy (lattice imaging and dark field), and x-ray diffraction techniques were used to characterize film structure. The as-deposited morphology, which can be described in terms of zone models, 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 films was found. Applied stress was found to reorient crystallites and to induce crystallization, with the degree of both processes being related to the initial structure of the film. Criteria are presented for selecting the film type and preparation conditions to fit various application. Keywords: Lubricant films; Molybdenum disulfide; Solid lubricants; Tribology. (KT)