Load-adaptive crystalline–amorphous nanocomposites

Advances in laser-assisted deposition have enabled the production of hard composites consisting of nanocrystalline and amorphous materials. Deposition conditions were selected to produce super-tough coatings, where controlled formation of dislocations, nanocracks and microcracks was permitted as stresses exceeded the elastic limit. This produced a self-adjustment in the composite deformation from hard elastic to quasiplastic, depending on the applied stress, which provided coating compliance and eliminated catastrophic failure typical of hard and brittle materials. The load-adaptive concept was used to design super-tough coatings consisting of nanocrystalline (10–50 nm) TiC grains embedded in an amorphous carbon matrix (about 30 vol%). They were deposited at near room temperature on steel surfaces and studied using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, Raman spectroscopy, nanoindentation and scratch tests. Design concepts were verified using composition–structure–property investigations in the TiC–amorphous carbon (a-C) system. A fourfold increase in the toughness of hard (32 GPa) TiC–a-C composites was achieved in comparison with nanocrystalline single-phase TiC.     

Synthesis and characterization of tungsten disulphide films grown by pulsed-laser deposition

The synthesis and characterization of tungsten disulphide (WS₂) films grown on 440C stainless steel substrates using the 248 nm line from a KrF excimer laser are reported. Film properties could be adjusted by controlling substrate temperature and by laser or thermal anneals. X-ray photoelectron spectroscopy, glancing angle XRD, Raman spectroscopy and high-resolution scanning electron microscopy were used to evaluate film chemistry, crystallinity and morphology. Films grown at room temperature were amorphous, near stoichiometric, and had a multiplicity of chemical states. Local order and bonding were improved most dramatically through post-deposition laser anneals. Crystallite size could be increased by raising the substrate temperature during deposition and, to a lesser degree, by post-deposition thermal anneals. Local disorder was observed within the larger crystallites compared to those that were laser annealed. Crystallinity was induced in amorphous films by mechanical rubbing at room temperature under conditions where frictional heating was negligible. The degree of control over film properties provided by PLD demonstrates its value for growing/designing tribological coatings.     

Multilayered YSZ–Ag–Mo/TiN adaptive tribological nanocomposite coatings

Adaptive nanocomposite coatings provide low friction in broad ranges of environmental conditions through the formation of lubricious surfaces resulting from interactions with the ambient atmosphere. Designing adaptive coatings to withstand wear through repeated thermal cycles is particularly difficult since most demonstrate irreversible changes in surface composition and morphology. This permanent change in the condition of the surface limits the utility of adaptive coatings in applications where thermal cycling is expected. Moreover, some lubrication mechanisms occur over the entire coating surface in addition to the area experiencing wear, which results in a significant waste of lubricant. In an effort to increase the wear lifetime and move toward thermal cycling capabilities of solid adaptive lubricants, a multilayer coating architecture incorporating two layers of adaptive YSZ–Ag–Mo nanocomposite lubricant separated by a TiN diffusion barrier was produced. The multilayer coating lasted over five times longer than a monolithic adaptive coating of identical composition and total thickness for dry sliding tests at 500 °C in air. Analysis of the structure and composition of the films after heating suggests directed, lateral diffusion of lubricant beneath the diffusion barrier toward the wear scar as a mechanism for the increased wear life of the multilayer coating.     

MEMS lubricants based on bound and mobile phases of hydrocarbon compounds: film deposition and performance evaluation

The concept of a thin layer of mobile hydrocarbon-based lubricant providing protection by replenishment to a surface already protected by a chemically bound material has been explored for the first time, for application to silicon-based microelectromechanical (MEMS) systems. Several bound/mobile pairs of lubricants were evaluated to study the effects of bound phase end group and mobile phase chemical functionality on wettability and performance. The bound species studied were derived from 1-decanol and 1,7-heptanediol. Mobile phases investigated were, a pentaerythritol ester, a multiply alkylated cyclopentane, Pennzane, and a polysilane developed for spacecraft application. Hydrocarbon lubricant performance was evaluated using electrostatic motors run in dry air, and was compared with that provided by Fomblin Zdol under identical conditions. Motors with hydrocarbon coatings showed substantial improvement in performance over uncoated motors, and for some bound/mobile pairs, was equal to Zdol within experimental error. We believe that for silicon-based devices, hydrocarbon coatings will be preferable at least for some applications, as the degradation observed due to aging of a fluorinated lubricant in direct contact with a silicon surface is absent.     

Tribological properties of pulsed laser deposited Mo-S-Te composite films at moderate high temperatures

Tribological investigations of Mo-S-Te composite films were conducted on films grown at room temperature by pulsed laser deposition. The chemistry and microstructure of the films were characterized by X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy, and micro Raman spectroscopy. The films showed a granular morphology and a preferred basal plane growth of 2H-MoS₂ parallel to the substrate after annealing at high temperatures. The friction coefficients of the films were 0.05 at 300 °C and 0.10 at 450 °C for more than 10,000 cycles in air. Smeared hexagonal MoS₂ lubricant films were observed inside wear tracks while the tribochemical formation of wear debris occurred both inside and outside the wear tracks. The Te additives for increasing the film durability were proposed to slow oxidation of the lubricants at elevated temperatures by thermally-induced tellurium migration to the surface and the subsequent formation of the Te diffusion barrier. This mechanism could be significantly effective in high-temperature tribotests because of the increased tellurium mobility at high temperatures.     

Wave propagation in an elastic/viscoplastic hollow sphere

An analysis of the dynamic problem of a hollow sphere is presented. The medium is assumed elastic/viscoplastic, satisfying Mises condition, isotropic hardening and viscoplastic incompressibility. The cavity is subjected to a radially applied and continuously maintained impact load. The analysis for viscoplastic waves is based on the method of characteristics and a generalized form of Malvern's theory for strain-rate dependent materials. A bilinear shear stress-shear strain curve is considered and calculations are carried out on the IBM 7094 computer. The paper examines the influence of the important parameters governing spherical wave propagation on the response of the sphere; these parameters include the level of strain hardening, the viscosity coefficient, Poisson's ratio and the geometry of the sphere. It is shown that the degree of strain hardening has a significant effect on the amplitude of the oscillations with the period unaffected. The viscosity coefficient becomes more influential as the wall thickness of the sphere increases.     

Microstructure and lubrication mechanism of multilayered MoS2/Sb2O3 thin films

Multilayered MoS₂/Sb₂O₃ thin films were prepared by pulsed laser deposition on steel substrates. A rotary multi-target holder was used to switch the laser targets for alternative growth of MoS₂ and Sb₂O₃ layers providing nanometers thickness. The tribological properties of the films were measured in dry and wet environments and the wear scars were observed using a scanning electron microscope. The multilayer films showed a much longer wear life than pure MoS₂ films in wet air tribotests. Focused ion beam and transmission electron microscopies were used to investigate the cross-sectional microstructures of wear scars. Lubricious MoS₂/Sb₂O₃ tribofilms were built up on wear scar surfaces, and produced low friction. Micro-cracks occurred along the interface between the tribofilm and the neighboring/topmost Sb₂O₃ underlayer, where the Sb₂O₃ layer effectively inhibited the crack propagation perpendicular to the interface. The orientation of MoS₂ crystals in as-deposited films was mostly random and friction-induced stress oriented the MoS₂ basal planes parallel to the surface. The reorientation was confined to the topmost MoS₂ layer and was not observed below the first intact Sb₂O₃ layer.     

Frictional Characteristics of Quasicrystals at High Temperatures

AlCuFe quasicrystal coatings were deposited by unbalanced magnetron sputtering for study of their friction and wear properties at high temperatures. It has been shown that growth and annealing conditions can be controlled to produce icosahedral quasicrystal or the approximant cubic phase. The comparison of friction and wear properties between quasicrystal and an approximant with nearly the same stoichiometry permits assessment of the unique quasicrystalline structure for tribological applications. The goals of this study are to determine how crystal structure influences tribological properties and to study the general friction and wear behavior of quasicrystals. Tribological properties were evaluated using a pin-on-disk tribometer and crystal structure was characterized using an X-ray diffractometer. The tests specimens were 10 m thick films deposited on a 1 diameter steel disk and a 1/4 diameter steel ball. Data was collected over a range of temperatures from room to 600°C. Friction coefficients and wear rates of quasicrystals and approximants were nearly identical for room temperature tests. The wear process generated Al, Cu, Fe, and oxide debris on the side of the track, but overall wear was mild. The friction coefficient of the icosahedral phase was 25% lower than the cubic phase at 150 thru 450°C. Generally, only moderate differences in the friction and wear properties were observed between the icosahedral quasicrystal phase and the cubic phase.     

Tribological behavior and graphitization of carbon nanotubes grown on 440C stainless steel

Vertically aligned carbon nanotube (CNT) arrays were directly grown onto 440C stainless steel substrates by plasma-enhanced chemical vapor deposition. Tribological properties of both short and long CNTs samples were studied under normal loads of 10 g, 25 g and 100 g. The CNTs had a steady-state friction coefficient of about 0.2 in humid air. In dry nitrogen, a friction of 0.2 was measured under a load of 10 g while high friction was measured at 25 g and 100 g loads. No significant variation of tribological behavior was measured between the short and long CNTs samples. SEM observations showed that rubbing caused the CNTs to align or lay down along the wear scar. They formed aggregates and were compressed by rubbing, which resulted in layer-structured graphite formations. SEM observation of the wear scars revealed loss of CNT structures accompanied by the appearance of dark areas. Micro Raman spectroscopic studies demonstrated that the dark areas were graphitized CNTs. Shear stress aligned the basal planes of the small graphene sheets in the CNT layers to the low friction orientation and eventually caused formation of more ordered graphite. The tribological formation of interfacial carbon layers increased with increasing stress from higher loads.     

Sulfate based coatings for use as high temperature lubricants

Films of CaSO₄, BaSO₄, and SrSO₄ were grown with the pulsed laser deposition technique. The high temperature lubricious properties of these films were evaluated and it was found that all three sulfates exhibited a low coefficient of friction (μ ≈ 0.15) at 600°C. X-ray diffraction studies showed the presence of carbonate crystal structures along with the sulfate crystal structures after testing. The carbonate crystal structure is composed of alternating planes of alkali earth atoms and carbonate ions. The layered structure of the carbonates may be important to the lubricating mechanism as the layered structure of the dichalcogenides contributes to their tribological properties. Various observations of the wear region suggest that the starting surface finish of the metal coupon plays a significant role in the tribological mechanism of these materials. Additional composite films containing CaSO₄ were grown with the plasma spray technique. The high temperature friction properties of these films were measured and compared to those of similar films containing CaF₂/BaF₂. Although both types of films exhibited low coefficients of friction at 600°C, analysis of the wear regions did not show the presence of CaSO₄ and CaF₂/BaF₂, respectively. Ca and/or Ba were present, though, suggesting that a new compound is formed in the wear track and that the sulfate and fluoride phases may not be providing the low friction in these composites. This revised version was published online in July 2006 with corrections to the Cover Date.