The growth and characterization of MoS thin films grown by pulsed laser evaporation is investigated. TOF anafysis of the ions evaporated from an MoS2 target indicates that PLE results primarily in the evaporation of atomic Mo and S species; MoxSy clusters were also detected, but were present at a significantly Iower intensity. TOF velocity analysis indicates an effective plasma temperature of 1500K. Stoichiometric MoS2 films were grown at substrate temperatures between room temperature and 500ºC under the above laser conditions. XPS data is used to develop a Wagner chemical state plot. Analysis of the films by Raman spectroscopy and glancing angle x-ray diffraction indicates the films to be crystalline, hexagonal MoS2, with a tendency for basal plane orientation parallel to the substrate.

Tungsten disulfide (WS2)–zinc oxide (ZnO) composite is a candidate material that exhibits adaptive lubricant behavior. Adaptive lubricants undergo chemical changes with changing environment to provide lubrication in extreme environments. In the current study, the tribological characteristics of WS2–nanocrystalline ZnO films have been investigated from ambient to 500°C. The composite films were powder burnished on inconel substrates. Using a ball-on-flat tribometer, friction tests were conducted on WS2–ZnO nanocomposite films containing 50% by weight of the oxide. For comparison, measurements were made on pure WS2 films burnished under identical conditions. The room temperature tests were performed in dry nitrogen, while the elevated temperature tests were run in air. Wear scars and transfer films on the counterface were analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. Results showed that the nanocrystalline ZnO additive resulted in significant reduction in friction coefficient of WS2 both at 300°C as well as at room temperature. Third-body analyses from 300°C tests revealed that tribooxidation is less prevalent in nanocomposite films. At 500°C, the friction coefficient of pure WS2 films increased to 0.50 within the first 2000 cycles, whereas the nanocomposite films lasted the entire duration of 10,000 cycles with steady state friction coefficient of 0.22. Raman spectroscopy identified the formation of zinc tungstate (ZnWO4) during the 500°C tribotests, confirming the adaptive lubricant concept in WS2–ZnO nanocomposites.

As a result of tribo-induced oxidation, tungsten disulfide (WS2) loses its lubricating behavior in humid environments. The purpose of this study is to explore the role of a tailored metal-matrix composite (MMC) substrate in imparting oxidation resistance to WS2 films in sliding contact. The substrate is an aluminum MMC disk reinforced with 20 vol.% silicon carbide (SiC) particles. The MMC disk was metallographically polished and etched to create SiC particle protrusions. The films were grown on the MMC substrates using a pulsed laser. Friction and wear tests were performed in dry nitrogen and in humid air with 90% relative humidity. The counterface was a 440C steel ball. The wear scars and third-body transfer films were characterized by scanning electron microscopy and Raman spectroscopy. In dry nitrogen, the friction coefficient of WS2 films on MMC substrates was low (0.035–0.050), indicating that the carbide protrusions did not adversely affect the lubricating behavior. In humid air, the friction coefficient of WS2 films on polished steel substrates increased to 0.4 during the first 1000 cycles of sliding, whereas the ones on MMC substrates lasted for the entire duration of 50,000-cycle tests, with friction coefficients ranging from 0.15 to 0.22. The counterface wear, or scratching of the steel ball by SiC, was practically absent. The role of carbide protrusions in controlling chemically assisted crack propagation is discussed. The implications of this work as a model study for the design of thin film composite coatings is highlighted.

The amorphous structure of MoS2 lms prepared by pulsed laser deposition (PLD) has been evaluated with the use of Raman and X-ray photoelectron spectroscopy (XPS). The initial study of the room-temperature deposited films indicated a featureless Raman spectrum. On closer examination, however, four weak reproducible bands were observed. There has been some confusion in the literature as to the nature of this spectrum—whether it represents an amorphous MoS3 structure or a mixture of MoS2 and sulfur. Our interpretation of the Raman and XPS data indicates that the laser-deposited films represent a mixture of small domains of MoS2 and amorphous sulfur.

The objective of this study is to develop improved procedures for characterizing amorphous carbon films. Raman spectroscopy is used to characterize amorphous carbon thin films grown by pulsed laser deposition at temperatures between 293 and 873 K. The amount of bond-angle disorder is shown to decrease with increasing substrate temperature. However, a shift of the Raman D peak to higher wave numbers is not observed to coincide with the presumed decrease in sp3 bonding as the deposition temperature increases. The graphitic domain size is shown to initially decrease, pass through a minimum, and then increase as temperature increases. Mass densities, measured independently by x-ray specular reflectometry, are seen to decrease from a maximum of 2.4 g/cm−3 as deposition temperature increases. The trend in the observed density measurements correlates well with the Raman spectroscopy data. The importance of x-ray specular reflectometry as part of a strategy to completely characterize amorphous carbon films is discussed in terms of these data.

ZnO-WS2 is a candidate high temperature solid film lubricant for aerospace applications that exhibits adaptive lubricant behavior. In the as-deposited state, room temperature (RT) pulsed laser deposited (PLD) ZnO-WS2 films are amorphous, but when wear-tested, the crystalline phases WS2, WO3 and ZnWO4 are produced. Of these, WS2 is a lubricant phase at low temperatures (≤ ∼ 450°C) while ZnWO4, which is formed by reaction of the film with air, becomes lubricious above 600°C. If this material is to be used at elevated temperatures, the characterization of the microstructural and chemical changes that occur when these films are heated in air is extremely important to the understanding of the dynamics of this system. As-deposited films and films heated in air at increasing temperatures to 800°C were examined by transmission electron microscopy (TEM), Raman spectroscopy, and scanning electron microscopy (SEM). Cross-sectional TEM (XTEM) of the as-deposited RT-PLD ZnO-WS2 films showed that they were fully dense and amorphous. A periodic structure was seen that was due to density variations and was attributed to a smaller angular distribution of W in the plume compared to the other elements. At 500°C, an approximately 37 nm-thick film of WO3 and ZnWO4 formed at the surface. At 600°C, a 150–200 nm-thick mixed oxide layer of ZnWO4 and WO3 formed at the surface with the WS2 phase forming below it. The volume fraction of WS2 decreased with increasing depth from the surface. Above 600°C, surface roughening of the film was seen as well as significant grain growth of the WO3 and ZnWO4 phases. The ZnO phase was not detected in any of the films heated in air. The dynamics of the nucleation of these lubricant phases are advantageous with respect to applications: the high temperature lubricant phase, ZnWO4, is available at the surface while the low temperature phase, WS2, remains intact to provide lubrication when the temperature is lowered.

The fact that dopants improve the friction and weal properties of sputtered MoS2 films is well known. However, the role of dopants in the mechanisms governing friction and wear are not well understood. The purpose of this work is to gain a fundamental understanding of their role by co-depositing a number of materials, i.e., Ni, Fe, Au, and Sb2O3, with MoS2 and evaluating their effects on film chemistry, crystallinity, microstructure, and tribology. Friction and wear measurements were collected using ball-on-flat and dual-rub shoe tribometers. Other physical and chemical properties were obtained using SEM, XPS, XRD and Raman spectroscopy. Crystalline MoS2 was seen in all of the films. In Sb2O3-doped films, an amorphous phase was also observed. The presence of dopants caused film densification and affected crystallite size. They had little effect on the overall crystallite orientation. In addition, dopants caused a reduction in the mean and variance of the friction coefficient and an increase in wear life. The correlation between dopants, film properties, and tribology is discussed in detail.

Graphite fluoride (CFx) is investigated as an additive for WS2 thin films to reduce its sensitivity to moisture. The films are grown onto hardened 440C stainless steel disks by pulsed laser deposition using the 248 nm line from an excimer laser. Substrate temperature and additive concentration are varied to control film chemistry and crystal structure. The effect of relative humidity (i.e., < 1 to 85% RH) on friction is evaluated. Coatings grown at RT from targets with a low concentration of CFx exhibit ultralow friction (ULF) behavior in dry air (i.e., μ ⩽ 0.01), but friction increases with RH. Mechanisms for the ULF behavior are proposed which suggest that further reductions in friction are possible. Films grown at 300 °C or with higher concentrations of CFx are relatively insensitive to humidity, but have more typical friction coefficients (μ ⩽ 0.04) in dry air.

This research describes the friction behavior of pulsed laser-deposited tungsten disulfide films. A ball-on-flat apparatus, in which a 440C stainless steel ball was held on a rotating dish coated with a WS2 film, was used as the test configuration. Friction measurements were made in dry nitrogen and in laboratory air. Wear surfaces were characterized by scanning electron microscopy and Raman spectroscopy. The friction coefficient of the film in dry nitrogen was 0.04, and in laboratory air it rose to between 0.10 and 0.15. In the dry nitrogen case, friction induced some degree of crystallinity into the otherwise amorphous film, while rubbing in air mostly resulted in oxidation of the film. Transfer films formed in a dry environment were smooth, tenacious and firmly adherent to the steel counterface. By contrast, the films formed in air were patchy and powdery in nature.

Raman scattering is utilized to investigate structural disorder in solid lubricant films. MoS2 films used in tribological studies for aerospace applications may be prepared by pulsed laser deposition (PLD) or by ion sputter deposition. Raman scattering spectra were obtained on a set of standard MoS2 compounds and a set of PLD films. These deposition processes incorporate changes into the crystallinity of the deposited films and the Raman data from these films were studied with a curve fitting program to analyze the defects generated by the PLD process. The curve fitting program shows that MoS2 PLD films can be categorized by peak position, half-band width, integrated area and linear intensity measurements. The Raman data appear to be more sensitive to crystal order in the x−y symmetry plane than to order in the z axis of the crystal.

PbMoO4 is a potential solid lubricant for use at elevated temperatures in oxidizing environments. Pulsed laser deposition (PLD) was utilized to grow thin films of this material because it allows good control over film chemistry and crystallinity. Films were grown at different substrate temperatures in vacuum and in partial pressures of oxygen. The chemistry and crystallinity of the films were evaluated using X-ray photoelectron spectroscopy, Raman spectroscopy, and glancing angle X-ray diffraction. Friction coefficients and wear lives were measured using a ball-on-flat tribometer at room temperature and 700‡C. Films deposited in vacuum, at room temperature and at 300‡C, were oxygen deficient. To adjust chemistry and crystallinity, films were grown in a partial pressure of oxygen (i.e. 6.7×10−1 Pa). Stoichiometric, crystalline films of PbMoO4 were produced when films were grown at 300‡C in this environment. PbMoO4 films were lubricious (Μ = 0.35) and long lived at 700‡C, but at room temperature had high friction and failed quickly. The properties of the films grown at the different conditions are discussed.

Crystalline disorder in thin films plays an important role in determining their properties. Disorder in the crystal structure of MoS2 films prepared by magnetron sputtering and pulsed laser deposition was evaluated with the use of Raman spectroscopy. The peak positions and bandwidths of the first-order Raman bands, in the region 100 to 500 cm−1, were used as a measure of crystalline order. In addition, a low-frequency feature was observed at 223 cm−1 that is not part of the normal first-order spectrum of a fully crystalline specimen. Data presented here demonstrate that this band is characteristic of crystalline disorder, and its intensity depends on the annealing history of the film. This behavior seems to be analogous to the disorder found in graphite thin films.

A number of materials have been added to MoS2 to improve its lubricating properties. However, the mechanism underlying this improvement is not fully understood. Perhaps the most widely used and studied additive is Sb2O3. While not a lubricant itself, it acts synergistically with MoS2 to improve friction and wear properties. This paper is directed towards developing a better fundamental understanding of the synergism between Sb2O3 and MoS2 in adhesively bonded films. A commercial preparation was used to form the films and laser Raman spectroscopy was used to analyze them before and after rubbing. It is shown that films layer as a result of tribostress -MoS2 preferentially coats film surfaces and Sb2O3 becomes enriched in the next deeper layer. The mechanism proposed to explain the synergistic behavior is that only the thin layer of MoS2 residing on top is exposed to degradation from the environment. The Sb2O3 layer acts as a thermal and oxygen diffusion barrier to retard oxidation deeper into the film. Sb2O3 also acts as a beneficial support for MoS2 as was demonstrated earlier by Centers (Tribol. Trans., 31/32 (1987) 149). The proposed mechanism suggests that protection is provided against tribo-oxidation but not necessarily thermal oxidation. To validate this concept, wear oxidation is evaluated in situ using a Raman tribotester and the results are compared with thermal oxidation data. Films containing Sb2O3 are indeed more resistant to tribo-oxidation than MoS2 films. It is also shown that the transfer film surface is enriched with MoS2. This indicates that friction is primarily governed by MoS2 rubbing against MoS2 and that Sb2O3 plays a secondary role.

Raman spectroscopy is used to investigate the tribochemistry of thin film solid lubricants. The development of an in situ tribotester (pin-on-disk), to be used in conjunction with a Raman spectrometer, is described in detail and its utility is demonstrated in several MoS2-based thin film systems. The films studied include two commercial films and two films generated in-house by pulsed laser deposition (PLD). The commercial films were MoS2-PbO-graphite and MoS2-Sb2O3. The PLD films were MoS2 and MoS2-PbO. Wear test studies on the commercial films generally produced MoO3 as a reaction product. PLD films were deposited at room temperature and 300 °C. The PLD films formed at room temperature are amorphous and do not exhibit Raman scattering. The 300 °C films contain enough crystalline domains to exhibit Raman scattering. Initial rubbing of the room-temperature PLD films imparts enough crystallinity to support Raman scattering. Wear tests on the PLD films produce reaction products containing MoO3 and PbMoO4. These crystalline oxides have a large Raman scattering cross section, making them easily detectable. These data provide a unique insight into the tribochemical interactions of MoS2-based materials.

The potential of pulsed laser deposition to incorporate additives into MoS2 films was investigated. Composite PbOMoS2 films were grown on stainless steel substrates using the 248 nm radiation from a KrF excimer laser. The PbO:MoS2 ratio and the substrate temperature during deposition were varied to determine the optimum conditions for producing lubricious, long-lasting coatings. X-ray photoelectron spectroscopy was used to evaluate surface chemistry while Raman spectroscopy was used to determine bulk chemistry and crystallinity. Spectroscopic data were correlated to friction and wear measurements collected from a ball-on-flat tribometer. Films deposited at 300 K were composed of amorphous MoS2 and other MoSPbO compounds. Crystalline PbMoO4, MoS2, and MoO3 were produced after the films were annealed in air at 773 K or after tribomechnical stressing. Composite films perform significantly better during tribotesting than films composed entirely of MoS2 or PbO. In addition, composite films demonstrate the properties of “adaptive” lubricants. MoS2 provides lubrication at room temperature; however, when the films are exposed to oxidizing environments at elevated temperature, they adapt by forming PbMoO4. This compound has been noted to posses lubricant properties at high temperature. Thus, there is significant potential for tailoring film compositions so that the components react to produce lubricious wear debris and to provide lubrication over extended temperature ranges.

The objective of the current study is to gain a greater understanding of the atomic structure of carbon films deposited by UV-pulsed-laser ablation. Films deposited onto Si substrates at 293 K, 473 K, 673 K, and 873 K are characterized using X-ray reflectometry and Raman spectroscopy. Film densities are shown to initially decrease from a maximum density of 2.39 g/cm3 as temperature increases. Above 673 K the density remains constant. When compared with Raman spectroscopy data, the reflectometry data are shown to be sensitive to microstructural changes within the films. The possibility of using density measurements as a boundary condition for more quantitative analysis is discussed. Also, Raman spectroscopy indicates that the graphitic domain size initially decreases, passes through a minimum, and then increases as temperature increases. The amount of bond-angle disorder is shown to decrease with increasing substrate temperature. However, an upward shift in frequency of the Raman D peak is not observed with the presumed decrease in sp3 bonding as the deposition temperature increases. These and other experimental results are presented and discussed.

Thin films of titanium carbide (TiC) and boron carbide (B4C) were grown by excimer pulsed laser deposition (PLD) at room temperature (RT) and 300°C. Films were deposited using the output of an excimer laser operating with KrF gas (λ = 248 nm, 15 ns pulse duration) to ablate hot-pressed targets. Film chemistry, morphology, and crystallinity were investigated. Stoichiometric, crystalline TiC films were grown on 440C stainless steel and NaCl substrates at room temperature and at 300°C. The films grown on NaCl were nanocrystalline, cubic TiC, with a grain size ranging between 2 and 10 nm in diameter. Boron carbide films were grown on silicon {100} substrates at room temperature and at 300°C. Film chemistry and stoichiometry duplicated that of the B4C target, which contained B4C and a mixed C-B-O-N binder phase. SEM analysis indicated that the morphology of the films was uniform, non-porous, and fine-grained. The films exhibited good adhesion and wear resistance, based on friction and wear data collected with a ball-on-disc tribometer.

Pulsed laser deposition (PLD) of tribological materials is an emerging technology that offers the possibility to tailor film properties for an application. Early research efforts to deposit MoS2 utilized a frequency doubled Nd:YAG laser, while recent effort is focussed on laser processing of tribological materials using the UV wavelengths available from an excimer laser. PLD provides a mechanism to deposit highly adherent thin films of a variety of tribological materials. The materials of interest include metal dichalcogenide solid lubricants such as MoS2 and wear resistant carbides such as TiC and Cr3C2. Applications of interest include solid lubricants for satellite precision direction mechanisms, wear resistant coatings for turbine engine components, and protective coatings for harsh environments. PLD films often exhibit superior performance, as compared to films deposited by more traditional methods. Improved film performance is due to increased adhesion, full density - low porosity, and optimized crystal structure and morphology. PLD is also being utilized to develop new materials formed by laser processing. Based on the results with lubricious films, PLD offers the possibility to tailor film properties by the appropriate choice of substrate materials, deposition parameters and post deposition treatments. The properties of films deposited by PLD are investigated as a function of: (1) substrate material, (2) laser deposition parameters (wavelength, pulse energy, fluence, and rep rate), (3) duration of post deposition laser annealing treatments, and (4) substrate temperature during deposition.

Pulsed laser ablation (PLA) is an emerging technology that provides a mechanism to deposit lubricious films and to tailor film properties by the appropriate choice of substrate materials, deposition parameters and post deposition treatments. The properties of MoS2 films deposited by PLA are evaluated as a function of: (1) substrate material, (2) duration of post deposition annealing treatments using 248 nm laser radiation and (3) substrate temperature during deposition. The chemistry and crystal structure of the different films are determined using small angle X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS). The tribological properties of the films are then evaluated as a function of their chemistry and crystal structure.

Films of MoS2 have been successfully deposited on 440C stainless steel using an excimer laser. A comparison was made of films ablated with the laser operating at 193 nm and at 248 nm. The effects of substrate temperature were also studied. X-ray Photoelectron Spectroscopy (XPS) measurements indicated that the films were sulphur rich as compared to single crystal MoS2. Laser Raman measurements indicated that annealing was necessary to obtain crystalline films. All films exhibited coefficients of friction in the neighborhood of 0.03 in a dry nitrogen environment. Coefficients of friction in laboratory air were significantly higher.