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Abstract: Silver tantalate was investigated as a potential lubricious material for moving assemblies in high temperature tribological applications. Three different approaches were explored for the creation of such materials on Inconel substrates: (1) powders produced using a solid state which were burnished on the surface; (2) monolithic silver tantalate thin films deposited by magnetron sputtering; and, (3) an adaptive tantalum nitride/silver nanocomposite sputter-deposited coating that forms a lubricious silver tantalate oxide on its surface when operated at elevated temperatures. Dry sliding wear tests of the coatings against Si3N4 counterfaces revealed friction coefficients in the 0.06–0.15 range at T ~750°C. Reduced friction coefficients were found in nanocomposite materials that contained primarily a AgTaO3 phase with a small amount of segregated Ag phase, as suggested by structural characterization using X-ray diffraction. Furthermore, cross-sectional transmission electron microscopy techniques determined that the reduced coefficient of friction at T ~750°C was primarily the result of the formation of a lubricious AgTaO3 phase that reconstructs during the wear process into a mechanically mixed layer of AgTaO3, Ta2O5, and Ag nanoparticles. [Copyright &y& Elsevier]

Abstract: Nanocomposite films that consist of niobium nitride with silver nanoinclusions were created using unbalanced magnetron sputtering to investigate their potential as adaptive, friction reducing coatings. The coatings were tribotested against a Si3N4 counterface in the 25 to 1000°C temperature range. The coatings displayed coefficients of friction in the 0.15 to 0.30 range at T >700°C. Post-wear testing structural and chemical characterization revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. In situ Raman spectroscopy measurements were taken during heating and wear testing at 750°C to identify the evolution of phases in the coatings surface and in the wear track. The analysis of the in situ Raman spectroscopy data revealed the various stages of formation of these binary metal oxides. The coatings were subsequently doped with MoS2 to investigate the effect of the introduction of a low temperature lubricant. The addition of MoS2 did not appreciably reduce the room temperature coefficient of friction, likely due to the miscibility of this compound with the transition metal nitride. However, the coefficient of friction was significantly reduced at high temperatures because of the synergistic lubricious effect of silver niobates and molybdates. [Copyright &y& Elsevier]

Abstract: A two-phase nanocomposite coating that consists of inclusions of silver in a vanadium nitride matrix (VN/Ag) was investigated as a potential adaptive coating with a reduced friction coefficient from 25 to 1000°C. This nanocomposite structure was selected based on the premise that silver and silver vanadate phases would form on the surface of these coatings, reducing their friction coefficient in the (i) room to mid-range and (ii) mid-range to high temperatures, respectively. Silver and vanadium were expected to react with oxygen at high temperatures and create a lubricious silver vanadate film on the coating. The VN/Ag coatings were deposited using unbalanced magnetron sputtering and their elemental composition was evaluated using X-ray photoelectron spectroscopy. The tribological properties of the materials against Si3N4 balls were investigated at different temperatures. The lowest friction coefficients recorded for samples with identical compositions were 0.35, 0.30, 0.10 and 0.20 at 25, 350, 700 and 1000°C, respectively. Post-wear testing Raman spectroscopy and X-ray diffraction (XRD) measurements revealed the formation of silver vanadate compounds on the surface of these coatings. In addition, real time Raman spectroscopy and high temperature XRD revealed that silver vanadate, vanadium oxide and elemental silver formed on the surface of these coatings upon heating to 1000°C. Upon cooling, silver and vanadium oxide were found to combine at about 400°C, leading predominantly to the formation of silver vanadate phases on the surface of these materials. [ABSTRACT FROM AUTHOR]

Double oxide phases combining a transition metal and a noble metal have recently become a subject of investigation as solid lubricant materials for high-temperature tribological applications. Here, we study the changes in chemistry and crystal structure of silver molybdate (Ag2Mo2O7) and silver tungstate (Ag2WO4) thin films and powders from 25 to 600°C. We show, using ab initio molecular dynamics, high-temperature X-ray diffraction, high-temperature Raman spectroscopy and differential scanning calorimetry in combination with sliding tests, that the layered atomic structure of silver molybdate facilitates sliding, resulting in a low coefficient of friction (<0.2) from 300 to 500°C. [Copyright &y& Elsevier]

Abstract: Adaptive tribological coatings were recently developed as a new class of smart materials that were designed to adjust their surface chemical composition and structure as a function of changes in the working environment to minimize friction coefficient and wear between contact surfaces. This paper provides an overview of the current research developments in this field, including: (1) Chameleon nanocomposite coatings which are produced by depositing a multi-phase structure whereby some of the phases provide mechanical strength and others are lubricious; (2) Micro- and nano-textured coatings which consist of hard nitride films with highly ordered micropores and nanopores that are subsequently filled with solid lubricants using various techniques such as lithography, reactive ion etching, laser texturing, pulsed air arc treatment, and ceramic beads as placeholders for sputter deposition; and, (3) Carbon and nitride nanotubes that are filled electrochemically with solid lubricants. The frictional and wear properties of the above three classes of newly developed adaptive structures, tested in various controlled environmental conditions (temperature, humidity), will be discussed in detail. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Adaptive nanocomposite Mo2N/MoS2/Ag coatings were deposited on Inconel and silicon substrates by magnetron sputtering with individual targets of Mo, MoS2 and Ag. The tetragonal β-Mo2N structure in addition to Ag and MoS2 phases were detected using X-ray diffraction. The elemental composition of the coatings was investigated using Auger electron spectroscopy. The tribological properties of the coatings were studied at room temperature (RT), 350, and 600 °C against Si3N4 balls. The lowest friction coefficients that were obtained were 0.4, 0.3, and 0.1 at RT, 350 °C, and 600 °C, respectively. The average friction coefficient was maintained at 0.1 for more than 300,000 cycles at 600 °C due to the formation of lubricious silver molybdate phases at the contact surfaces. Three types of silver molybdate phases were detected by both X-ray diffraction and micro-Raman spectroscopy in the wear tracks, namely, Ag2Mo4O13, Ag2Mo2O7 and Ag2MoO4 depending on the Mo and Ag contents in the coatings. The superior performance of all three compounds is due to their layered structure with weaker Ag–O bridging bonds. These relatively weak bonds may shear or even break easily at high temperatures to account for the observed friction reduction. [Copyright &y& Elsevier]

Abstract: The production of surface micro-patterns in steel substrates for solid lubricant (SL) reservoirs was previously shown to provide improved wear and frictional properties in sliding contact. We report on the fabrication of micrometer-sized dimple patterns of different sizes on TiAlCN hard coatings produced by cathodic arc vacuum evaporation. These dimples were produced by reactive ion etching in a mixed Ar/CF4 plasma. An overlayer of MoS2 or Mo/MoS2/Ag SLs was deposited on the micro-textured surfaces using unbalanced magnetron sputtering. The tribological properties of the coatings were investigated against Si3N4 in dry sliding at 25 °C and in air at 570 °C. The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy (OM), optical profilometry (OP), energy dispersive x-ray spectroscopy (EDX), and micro-Raman spectroscopy. The frictional and wear properties of textured and flat surfaces were compared. A significant decrease in friction and wear coefficients was achieved at 25 °C and 570 °C for the textured films because the dimples acted as reservoirs that provided a new supply of SLs to the contact surfaces. The chemistry and phase analysis of the coating wear tracks showed temperature adaptive behavior with the lubrication being provided primarily by MoS2 at 25 °C and by silver and silver molybdate compounds at high temperature. [Copyright &y& Elsevier]

Abstract: Aircraft propulsion applications require low-friction and wear resistant surfaces that operate under high contact loads in severe environments. Recent research on supertough and low friction nanocomposite coatings produced with hybrid plasma deposition processes was demonstrated to have a high potential for such demanding applications. However, industrially scalable hybrid plasma technologies are needed for their commercial realization. The Large area Filtered Arc Deposition (LAFAD) process provides atomically smooth coatings at high deposition rates over large surface areas. The LAFAD technology allows functionally graded, multilayer, super-lattice and nanocomposite architectures of multi-elemental coatings via electro-magnetic mixing of two plasma flows composed of different metal ion vapors. Further advancement can be realized through a combinatorial process using a hybrid filtered arc-magnetron deposition system. In the present study, multilayer and nanostructured TiCrCN/TiCr+TiBC composite cermet coatings were deposited by the hybrid filtered arc-magnetron process. Filtered plasma streams from arc evaporated Ti and Cr targets, and two unbalanced magnetron sputtered B4C targets, were directed to the substrates in the presence of reactive gases. A multiphase nanocomposite coating architecture was designed to provide the optimal combination of corrosion and wear resistance of advanced steels (Pyrowear 675) used in aerospace bearing and gear applications. Coatings were characterized using SEM/EDS, XPS and RBS for morphology and chemistry, XRD and TEM for structural analyses, wafer curvature and nanoindentation for stress and mechanical properties, and Rockwell and scratch indentions for adhesion. Coating properties were evaluated for a variety of coating architectures. Thermodynamic modeling was used for estimation of phase composition of the top TiBC coating segment. Correlations between coating chemistry, structure and mechanical properties are discussed. [Copyright &y& Elsevier]