High performance machines such as gas turbine engines demand efficient solid lubricants at high temperature and in vacuum. The current conventional solid lubricants need to be further improved. This research evaluates carbon based solid lubricants using a high vacuum, high temperature pin-on-disc tribometer. The objectives of this research were to develop an understanding of the tribological properties of solid lubricant coatings under extreme operating conditions, and to determine whether using a carbon based solid lubricant would be acceptable for use in those conditions. Experimentally, two solid lubricant coatings on tungsten carbide substrate were tested against two different materials. The coatings were carbon based and molybdenum disulfide based. The other materials were 440C stainless steel and tungsten carbide. The temperature, pressure, and relative humidity are independent variables. The results showed that the carbon based coating increases friction and wears out quickly due to high temperature, high vacuum, and low humidity. Abrasive wear is the dominating mechanism. At elevated temperatures and in dry environment, the carbon based coating underwent significant oxidation and phase transformation. This research is beneficial for future design and development of solid lubricants for aerospace applications, as well as other industries requiring lubricants that must operate in extreme conditions. This thesis includes five chapters. Chapter I is an introduction to tribology and to the materials being used in this research. Chapter II describes the motivation and objectives behind this research. Chapter III discusses the experimental procedure and further explains the materials used. Chapter IV presents and discusses the results obtained. Chapter V discusses the major conclusions obtained from the results and offers some future work that may be conducted concerning this research.

Solid lubricants possessing low friction coefficients, low wear rates, and long wear lives are vital for significantly increasing the life span of instruments undergoing extreme frictional wear due to harsh environments. Solid lubricants currently used in high temperatures or excess humidity, such as MoS2, WS2, graphite, or noble metals, are unable to sustain superior frictional qualities over extended amounts of time or in changing environments. To ameliorate these limiting properties, a composite solid lubricant is produced to enable the favorable frictional properties of one lubricant to overcome the lacking frictional properties of the other. This composite uses the combined materials to produce a solid lubricant that can sustain a low friction coefficient and wear rate for a longer amount of time than each individual material. MoS2 electrodeposited on to carbon nanotubes (CNTs) has a lower friction coefficient in humid (~0.16), non-humid (~0.05) and non-humid/humid cycled (~0.075-0.2) environments than either bare MoS2 or bare CNTs. Similarly, silver deposited on CNTs, by electrodeposition, electroless deposition, and sputter coating, performs better in room temperature, high temperature (500°C), and room temperature/high temperature cycling environments than either of its individual materials. Using the techniques used to produce these solid lubricant composite coatings with appealing frictional properties will provide a variety of tribological applications involving high temperature and/or high humidity environments with necessary solutions and further facilitate the improvement of solid lubricants used in other extreme environments.

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Thesis (M.S.)--University of North Texas, Aug., 2009. / Title from title page display. Includes bibliographical references.

Thesis (Ph. D.)--University of North Texas, Dec., 2008. / Title from title page display. Includes bibliographical references.

Ceramics have a wide variety of applications due to their unique properties; however, the low fracture toughness leads the formation and propagation of unpredictable cracks, and reduces their reliability. To solve this problem, self-healing adaptive oxides were developed. The aim of the work is to gain new insights into self-healing mechanisms of ceramics and their application. Binary oxide systems were investigated that are at least partially healed through the extrinsic or intrinsic addition of silver or silver oxide to form ternary oxides (e.g., Nb2O5 + Ag → AgNbO3). Sintered pellets and coatings were tested. For self-healing TBCs, model systems that were studied include YSZ-Al2O3-SiC, YSZ-Al2O3-TiC, YSZ-Al2O3-Nb2O5, and YSZ-Al2O3-Ta2O5. Laser cladded samples and sintered pellets were produced to test. The healing process occurs due to the formation of oxidation products and glassy phases depending on the self-healing mechanism. X-ray diffraction was used to explore phase evolution, chemical compositions, and structural properties of these samples. SEM equipped with EDS was used to investigate the chemical and morphological properties for the cross-sectional area. Pin-on-disc test was applied to test tribology performance for Nb2O5-Ag2O system, and infiltration test was applied to test CMAS-resistance for TBCs at elevated temperature. The improvements in the performance of these materials were demonstrated.

This research describes the process of manufacturing and machining of wire and arc additive manufactured (WAAM) thin wall structures on integrated and non¬integrated WAAM systems. The overall aim of this thesis is to obtain a better understanding of deposition and machining of WAAM wall parts through an integrated system. This research includes the study of the comparison of deposition of WAAM wall structures on different WAAM platforms, namely an Integrated SAM Edgetek grinding machine, an ABB robot and a Friction Stir Welding (FSW) machine. The result shows that WAAM is a robustly transferable technique that can be implemented across a variety of different platforms typically available in industry. For WAAM deposition, a rise in output repeatedly involves high welding travel speed that usually leads to an undesired humping effect. As part of the objectives of this thesis was to study the travel speed limit for humping. The findings from this research show that the travel speed limit falls within a certain region at which humping starts to occur. One of the objectives of this thesis was to study the effect of lubricants during sequential and non-sequential machining/deposition of the WAAM parts. Conventional fluid lubricants and solid lubricants were used. In addition, the effect of cleaning of deposited wall samples with acetone was also studied. A systematic study shows that a significant amount of solid lubricant contamination can be found in the deposited material. Furthermore, the results indicate that even cleaning of the wire and arc additive manufactured surfaces with acetone prior to the weld deposition can affect the microstructure of the deposited material.

Adaptive coatings are an important development in tribology. These coatings widen the range at which solid lubricants are useful in various environments. In this paper, coatings founded on molybdenum nitride are studied, with a focus on thermal cycling. These coatings were fabricated by unbalanced magnetron sputtering and characterized with techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, energy dispersive x-ray spectroscopy (EDS), and pin-on-disk tribometer. The results of two sets of coatings are reported. The first set of coatings is a nanocomposite of Mo2N/MoS2/Me (Me = Ag, Au, Cu). The second is a complex multi-layer system of Mo2N/Ag and a diffusion barrier of TiN which has been etched, then filled and coated with a layer of MoS2. After heating, these compounds produced silver molybdates. The Mo2N/MoS2/Ag nanocomposite shows promise with a 0.02 coefficient of friction at room temperature, while the multi-layer system eventually equilibrated at approximately 0.6. At high temperatures, again the nanocomposite was better, producing a 0.25 frictional coefficient compared to a 0.3 from the multilayer system. These results provide insight into what is needed to achieve thermal cycling.

Friction and wear mitigation is typically accomplished by introducing a shear accommodating layer (e.g., a thin film of liquid) between surfaces in sliding and/or rolling contacts. When the operating conditions are beyond the liquid realm, attention turns to solid coatings. Solid lubricants have been widely used in governmental and industrial applications for mitigation of wear and friction (tribological properties). Conventional examples of solid lubricants are MoS2, WS2, h-BN, and graphite; however, these and some others mostly perform best only for a limited range of operating conditions, e.g. ambient air versus dry nitrogen and room temperature versus high temperatures. Conversely, lubricious oxides have been studied lately as good potential candidates for solid lubricants because they are thermodynamically stable and environmentally robust. Oxide surfaces are generally inert and typically do not form strong adhesive bonds like metals/alloys in tribological contacts. Typical of these oxides is ZnO. The interest in ZnO is due to its potential for utility in a variety of applications. To this end, nanolaminates of ZnO, Al2O3, ZrO2 thin films have been deposited at varying sequences and thicknesses on silicon substrates and high temperature (M50) bearing steels by atomic layer deposition (ALD). The top lubricious, nanocrystalline ZnO layer was structurally-engineered to achieve low surface energy {0002}-orientated grain that provided low sliding friction coefficients (0.2 to 0.3), wear factors (range of 10-7 to 10-8 mm3/Nm) and good rolling contact fatigue resistance. The Al2O3 was intentionally made amorphous to achieve the {0002} preferred orientation while {101}-orientated tetragonal ZrO2 acted as a high toughness/load bearing layer. It was determined that the ZnO defective structure (oxygen sub-stoichiometric with growth stacking faults) aided in shear accommodation by re-orientating the nanocrystalline grains where they realigned to create new friction-reducing surfaces. Specifically, high resolution transmission electron microscopy (HRTEM) inside the wear surfaces revealed in an increase in both partial dislocation and basal stacking fault densities through intrafilm shear/slip of partial dislocations on the (0002) planes via a dislocation glide mechanism. This shear accommodation mode mitigated friction and prevented brittle fracture classically observed in higher friction microcrystalline and single crystal ZnO that has potential broad implications to other defective nanocrystalline ceramics. Overall, this work has demonstrated that environmentally-robust, lubricious ALD nanolaminates of ZnO/Al2O3/ZrO2 are good candidates for providing low friction and wear interfaces in moving mechanical assembles, such as fully assembled rolling element bearings and microelectromechanical systems (MEMS) that require thin (~10-200 nm), uniform and conformal films.

Mechanical components operating at elevated temperatures can be identified in the aerospace industry as well as in metal forming. Hot stamping is a metal forming technique utilized for manufacturing high strength lightweight components for the automotive industry. Three types of Ni-based hardfacings doped with solid lubricants were manufactured using laser cladding. An additional hardfacing was also manufactured using the Ni-based alloy only as a reference. Solid lubricants added were Ag+WS2, WS2 and Cu+MoS2. These hardfacings were manufactured and tribologically evaluated for potential application in hot stamping tooling. Direct diode laser was used for the cladding process. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used for characterization of the microstructure after deposition. Tribological testing was done using a reciprocating pin-on-disk tribometer under dry sliding conditions at temperatures in the range between RT and 600°C. Bearing steel ball and a flat pin made of the same material were used as counterbodies. Wear volume loss was measured using a 3D optical interferometer. For identification of the wear mechanisms SEM/EDS techniqueswere used. Formation of a lubricious CrxSy phase was detected after deposition, additionally encapsulation of silver particles by CrxSy was also observed. Reduction of friction coefficient was observed for every self-lubricating hardfacing compared to the reference. Lowest friction coefficient (0.23-0.3) was observed when the testing temperature was set to 400°C using a pin-on-flat configuration. Lowest wear rate was observed at 400°C, with the coating containing Ag and WS2 having a specific wear rate of 3.04 · 10−5mm3/Nm. The lubricity of the self-lubricating hardfacings was attributed to the CrxSy phase observed on the worn surface. At low to moderate temperatures abrasive wear is the main wear mechanism. At the highest testing temperature, oxidative wear was dominant.