Michael R. Hilton

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.

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.

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.

MICROSTRUCTURE OF COSPUTTER-DEPOSITED METAL- AND OXIDE-MOS2 SOLID LUBRICANT THIN FILMS

The effect of cosputtering small amounts of Ni (3%, 9%) and SbOx (20%) on the final microstructure of MoS2 lubricant thin films has been studied using a combination of scanning and transmission electron microscopy imaging, and electron and x-ray diffraction techniques. The early-growth, near-interface microstructure of both MoS 2 and 3% Ni‐MoS2 cosputtered films is revealed to be a mixture of (002) basal and elongated, large-size (100) and (110) edge islands. Cosputtering with 9% Ni induces a dramatic change in the microstructure, i.e., primarily basal domains with very small isolated regions of edge islands, while cosputtering with 20% SbOx produces films having no long-range order. The results are compared with and are consistent with previously published x-ray absorption fine structure data. The impact of film morphology on tribological performance is discussed.

Oxygen substitution in sputter-deposited MoS2 films studied by extended X-ray absorption fine structure, X-ray photoelectron spectroscopy and X-ray diffraction

Abstract The morphology and crystallite structure/orientation of sputter-deposited MoS 2 solid lubricant films have been shown to be strongly affected by oxygen-containing species in the sputtering chamber. We have studied the effect of oxygen incorporation within these films using extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Films produced at The Aerospace Corporation, the National Centre of Tribology (U.K.), Hohman Plating and Manufacturing, and NASA Lewis Research Center were studied to determine common characteristics of different MoS 2 films produced by sputter deposition. In agreement with previous results, XPS of the films indicated a bulk anion:cation ratio ( i.e. (S+O):Mo) of about 2, and XRD revealed that the films consisted completely of MoS 2 -like phases that were compressed in the ( h k 0) directions as compared with pure MoS 2 . This “edge plane compression” appeared to correlate with oxygen content. EXAFS confirmed the absence of MoO 2 in the films and indicated that the films actually consist of two similar MoS 2 -like phases. Correlation of the EXAFS results with those obtained from XPS and XRD indicated that the two phases are MoS 2 and an MoS 2- x O x substitutional solution. The solid solution may be described as MoS 2 with oxygen atoms that have substituted for sulfur atoms in the MoS 2 crystal lattice. Relative peak height changes in the EXAFS radial distribution curves indicated that increasing amounts of oxygen cause a reduction in the size of crystallites in the films. The presence of the MoS 2- x O x phase may explain the superior tribological performance of MoS 2 films in some applications, which has been shown to correlate with oxygen impurities in the films.

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.

Nanostructure of Au-20% Pd layers in MoS2 multilayer solid lubricant films

Abstract High-resolution electron microscopy imaging and electron and X-ray diffraction techniques have been used to characterize the structure of low and high flux Au-20% Pd layers in Au-20% Pd/MoS 2 multilayer solid lubricant thin films. Images clearly reveal different morphologies for the metal layers in the two flux regimes, which can be correlated to the variation in fracture resistance reported by an earlier indentation study. In the lower metal flux regime, three-dimensional islands with single-crystal and multiply twinned structures are seen, while quasi-continuous, polycrystalline regions are seen in the higher flux case.

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)

Ultrahigh vacuum high resolution transmission electron microscopy of sputter-deposited MoS2 thin films

Abstract High resolution electron microscopy has been used to characterize the structure of sputter-deposited MoS 2 coatings under both conventional and ultrahigh vacuum (UHV) conditions. As deposited, the films have a mixture of short-range ordered basal-plane and edge-plane oriented grains near the film substrate interface; structural changes were characterized in a UHV transmission electron microscope as a function of two processing variables: temperature and Au deposition. Annealing in an oxygen environment was also carried out to assess chemical stability. During thermal annealing in UHV and in oxygen, substantial long-range ordering of the basal islands followed by grain growth was observed. Inhomogeneous oxidation resulting in the formation of MoO 3 in the initial stages followed by grain growth, yielding the final morphology of a mixture of MoO 3 crystallites of 5–50 nm size was seen on annealing in an oxidizing atmosphere. Au nucleation and growth on both thermally annealed and as-deposited films were seen to follow the Volmer-Weber mode, i.e. three-dimensional islands; these islands were also seen to be highly textured. Also, in comparison with carbon and SiO substrates, Au demonstrated higher stability on MoS 2 to electron beam fluxes, suggesting higher bonding strengths to the substrate. These experiments demonstrated the paramount need for UHV conditions during both deposition and characterization to avoid uncertain contamination artifacts.

EXAFS of sputter-deposited MoS2 films

Abstract The composition and structure of sputter-deposited MoS2 solid lubricant films are difficult to analyze by diffraction techniques because the films typically exhibit poor crystallinity. The extended X-ray absorption fine structure (EXAFS) technique is ideal for analyzing these properties because it probes short-range (local) order parameters such as interatomic bond lengths. EXAFS data for the Mo K X-ray absorption edge were analyzed, including least-squares fitting, to investigate films produced under a number of conditions, as well as those produced with varied amounts of oxygen incorporated in the films. The results were compared with X-ray photoelectron spectroscopy and X-ray diffraction data of the same films. Analysis showed that the films, which contained 9–40 at.% oxygen, consisted predominantly of a MoS2-xOx phase, with x continuously variable. The MoS2−xOx phase exhibited a MoS2-like structure, where oxygen atoms had substituted for sulfur atoms in the MoS2 crystal lattice. In addition, smaller quantities of pure MoS2 (10–;25 times lower) were detected in the films. Increasing oxygen content correlated with increasing values of x in the MoS2−xOx phase, as well as with decreasing relative amounts of the pure MoS2 phase. The presence of the MoS2−xOx phase may explain the decrease in friction with increasing oxygen content that has been observed for MoS2 films. In addition, the poor order (high defect level) in this phase may explain the density enhancement caused by crystallite size reduction; higher densities can be correlated with beneficial wear behavior of the films.