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On the use of a brale indenter to evaluate the cross-sectional morphology of sputter-deposited MoS2 solid lubricant films
Adhesion measurement of brittle coatings on ductile substrates is examined with emphasis on diamond–coated metals. The experimental aspects of determining film adhesion is discussed through measurement of the interface toughness by a brale indentation method.
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 molybdenum disulfide produced by rf, dc, 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: good coating-substrate adhesion, dense, small grain size (low porosity), and high chemical (phase) purity. Dry lubricants, Molybdenum sulfide, Molybdic sulfide, Plasma deposition, Sputtering, Surface properties, Anisotropic platelets, Adhesion, Solid lubricants, Coatings, Tribology.
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 MoS2 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.
The surface composition of hot-pressed silicon carbide (SiC) samples and the growth of lubricating molybdenum disulfide (MoS{sub 2}) films on these surfaces are examined as a function of surface treatment. SiC samples polished in air exhibit significantly less surface silicon oxide (approximately one monolayer) relative to unpolished samples, as determined by x-ray photoelectron spectroscopy. The surface oxide and surface carbon contamination are both reduced by etching with aqueous HF under a nitrogen atmosphere. Annealing the sample to 350{degree}C under vacuum removes still more surface carbon contamination, particularly C-O species. The crystallographic orientation of radio frequency sputter-deposited MoS{sub 2} films on the SiC substrates at 220{degree}C is strongly dependent on the amount of surface contamination. As the contamination concentration decreases, more MoS{sub 2} crystallites grow with basal planes oriented parallel to the sample surface, as detected by x-ray diffraction. These results support an active site model for the growth of MoS{sub 2} films with basal planes oriented perpendicular to the surface and suggest that the active site on SiC is a chemisorbed carbon contaminant.
The effects of substrate pretreatment on substrate surface chemistry and on film-substrate adhesion, and film endurance (wear life) of sputter-deposited molybdenum disulfide (MoS2) 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.
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 vacuum friction performance and temperature insensivity of MoS2 makes it the lubricant of choice for many spacecraft mechanism applications. However, most of these applications historically utilized bonded MoS2 lubricants where control of thickness and film quality were not optimum. This paper reviews the development and problems encountered in applying recently developed sputtered MoS2 films to precision spacecraft gimbal bearings which require low torque signatures and long life. Monolithic metal cosputtered and metal interlayered films were evaluated.The vacuum life of MoS2 film lubricated bearings under gimbaling improved substantially with self-lubricating polytetrafluoroethylene composite retainers. Some bearings exceeded 45 × 106 cycles. Multilayer MoS2 films on even long-lived bearings were found to have fractured early in bearing life from the combined effects of poor film toughness and substrate roughness. Debris from these films produced irregular torque signatures. Increasing the metal interlayer thickness in multilayer films provided greater film ductility and fracture toughness. Bearing torque levels were also reduced.Application of a more fracture-resistant multilayer film to large spacecraft-quality bearings did not completely eliminate the film fracture mode. The scale-up of the sputter deposition system for larger bearings produced some questionable effects on film quality. Large thin-section bearings appeared more torque sensitive to wear debris than the smaller screening-test bearings.
Lubrication properties of rf-sputtered MoS//2 films deposited by 2-target sputtering have been studied and compared with those deposited by conventional sputtering. The endurance life of a film depends on the temperature of the substrate during the sputtering process independent of the sputtering method. This is attributed to the porous growth of a film on the heated substrate, which might be associated with the compositional change of the deposited MoS//2. The improvement of the wear life of a chemically converted film originally proposed by J. E. Brophy and R. W. Ingraham is also discussed. The addition of oxygen to hydrogen sulphide for the conversion process is effective in improving the reproducibility and the durability of films. In contrast to the sputtered MoS//2 films, the compositions of the films made by the chemical process have an excess of sulphur.
TiN films were deposited onto M2 tool steel, type 304 stainless steel and glass substrates using plasma-assisted chemical vapor deposition. TiCl4 and NH3 were the reactive gases used, in the pressure range 26.6–133 Pa. The use of the plasma accelerated the rate of film deposition by about one order of magnitude. Auger electron spectroscopy, X-ray diffraction and electron microscopy studies revealed the growth of two types of film depending on the substrate temperature. At 300°C and above, highly crystalline stoichiometric TiN formed with (200) surface orientation and having a columnar grain structure. These films showed good adherence and hardness. Below 300°C an amorphous TiN-Cl complex formed that exhibited poor mechanical properties. Above the transition temperature the formation of gaseous HCl was thermodynamically favored as a reaction byproduct, while the formation of solid NH4Cl was favored at the lower temperatures. Vickers' microhardness and scratch adhesion measurements were made. At thicknesses greater than 5 μm the films exhibited poorer adhesion because of the build-up of residual stress.
Compact non-columnar coatings of MoSx with a thickness of up to 25 μm were deposited by d.c. as well as r.f. magnetron sputtering in argon at low pressure (about 1 mTorr) onto polished AISI 440C steel discs, Si(100) wafers, glass and polished polycrystalline Al2O3. They showed superior properties to the well-known MoSx coatings deposited at gas pressures about one order of magnitude higher.
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 related compounds are suggested. Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987
Methods for the determination of the density of sputtered films of MoS2 are discussed and the problems connected with the determination of the film thickness are elucidated. The density determined for different coating parameters is given and the results are compared with the data of natural MoS2. The deviation from the bulk value is explained by morphological properties of the films. In addition, the influence of sputtering conditions on the structure of the films is mentioned and the results are correlated with these data.
The present study presents an experimental evidence for amorphization of rf sputtered MoS x films by ion bombardment. Even at low doses (3×10<sup>1</sup><sup>5</sup> ions/cm<sup>2</sup>) of 400 keV argon ions a complete amorphization was confirmed by x‐ray diffraction analysis and transmission electron microscopy. As a result of the ion bombardment the film density increased 100% to almost the bulk value for MoS 2 . The friction coefficient for ion beam amorphized MoS x was measured to be 0.04 in agreement with the values reported for crystalline films but disagreeing considerably with the friction coefficient of 0.4 previously reported for amorphous films.
Comparative measurements of adhesion of single-layer and multilayer hard coatings on cemented carbide substrates are made using scratch and indentation test methods. It is shown that the critical load Lc in scratch adhesion is influenced by the hardness of the substrate and extraneously by the surface condition of the indenter and its coefficient of friction relative to the coating material. The average stress calculated from the width of the scratch channel is found to be a more meaningful adhesion parameter than Lc. In the indentation technique, the approximate load for lateral crack initiation and the slope obtained from the indentation load-lateral crack length function are two useful adhesion parameters. The latter is demonstrated to be more discriminating in several test cases. It is also found that increased residual stress in physically vapor deposited TiN coating reduces both measured scratch and indentation adhesion parameters.
From the texture and growth patterns of sputtered MoS2 films deposited onto substrates, three regions can be distinguised: (1) a ridge formation region, (2) an equiaxed transition zone and (3) a columnar-fiber-like structure. The lubricating properties of sputtered MoS2 films can be visually identified with respect to optical changes before and after rubbing. The orientation of the surface microcrystallites is identified, and the change in optical properties is explained. In sliding contact the sputtered film tends to break up at the base of the columnar region. Effective lubrication occurs with the film remaining on the substrate. This film is 0.18–0.22 microm thick.
As the lubricating properties of MoS2 are due to the sliding of lamellae, the crystallographic orientation and structural properties of these lamellae are important for the tribological behaviour. Thus an analysis of the growth morphology of sputtered MoS2 films was carried out. It is shown that morphological properties can be influenced by the parameters of the deposition process; in particular, it is possible to prepare coatings in which the lamellae are oriented parallel to each other.
Thin sputter-deposited MoS2 films in the thickness range 2000–6000 Å have shown excellent lubricating properties when sputtering parameters and substrate conditions are properly selected and precisely controlled. The lubricating properties of sputtered MoS2 films are strongly influenced by their crystalline-amorphous structure, morphology and composition. The coefficient of friction can range from 0.04 which is effective lubrication to 0.4 which reflects an absence of lubricating properties. Visual screening and slight wiping of the as-sputtered MoS2 film can determine the integrity of the film. An acceptable film displays a black sooty surface appearance whereas an unacceptable film has a highly reflective gray surface and the film is hard and brittle.
MoS2 films have been prepared by RF sputtering in a planar magnetron system. Their chemical composition was analysed by energy dispersive X-ray spectroscopy. The morphology and growth characteristics were examined by scanning and transmission electron microscopy and by electron diffraction. Correlation between process conditions, film stoichiometry and morphology has been pointed out by a systematic variation of the experimental sputter parameters with emphasis on a stable substrate temperature during deposition. It has been shown that by a precise control of the substrate temperature specific morphologies can be obtained which are independent of film thickness. In particular, at high deposition temperature the films exhibit a lamellar-type microstructure with the 002 planes of the MoS2 crystallites perpendicular to the substrate. The crystallite size strongly decreases for deposition at lower temperature. The increase of power density was found to give higher sulphur concentration in the films whereas substrate biasing results in a pronounced excess of the metallic element.