Novel Thermal Spray Lubricious Oxide Coatings

The extreme operating conditions experienced by many moving mechanical components in aerospace and automotive engines require the uses of high-performance materials and lubricants. Current oil-based lubricants degrade rapidly under such conditions and become useless, especially at high temperatures. As a result, many moving components of engines that are run at high temperatures to improve their performance and efficiency tend to wear out rapidly and to require frequent replacement. Under such conditions, these lubricants with much higher temperature durability are urgently needed. It appears that the manufacturing of oil or liquid-based lubricants is not possible; while the development and implementation of long-lasting solid lubricants can be considered as a viable option. Hence there is a need for finding an enabling technology as a solution for such complex issues in aerospace turbine and automotive bearing applications. One solution to extend engine life is with the implementation of a low friction, high temperature stable, and low wear coatings to the component surface that can perform under extreme conditions. Solid lubricant coating offers a solution for diverse applications involving extreme and difficult running conditions. These coatings make bearings slide more easily and help engine parts last longer and thereby enabling improved energy efficiency and longer durability. The coatings effectively reduce friction, have high hardness and wear resistance, and strongly bond to the bearing steel, providing long operating endurance. A crystal-chemical approach has been proposed recently to classify lubricious oxides on the basis of lubrication performance and operational limits. The principle of the crystal-chemical approach is based essentially on the ionic potential of an oxide and is defined as = Z/r, where Z is the cationic charge and r is the radius of the cation. Erdemir [1] proposed that using this principle, one can establish model relationship(s) between the quantum-chemical characteristics and the lubricity of oxides at high temperatures. Specifically, it is possible to establish a correlation between the ionic potential or the cationic field strength of an oxide and its shear rheology, and hence its lubricity. Table I below shows some of the examples of oxides combinations and their ionic potentials, which will be investigated during the Phase I STTR program. Commercial Applications and Other Benefits Development of solid lubricant protective coating systems which show improved performance at elevated temperatures, will result in improving the current tooling life and efficiency thereby improving overall productivity. The crystal-chemical approach can be used to predict the extent of adhesive interactions between two or more oxides at a sliding interface; hence, it can be used to predict frictional performance. Based on this model, certain complex oxides and oxide-fluorides (i.e., ZnO/SnO/SrF2, NiO/BaTiO3, MgO/ZnO/CaF2, and NiO/SrF2) show enhanced lubricity at elevated temperatures. The dry lubricant ceramic coatings are of interest to our strategic partner Capstone for bearing components applications. Also, Federal Mogul and General Motors has expressed their interest in evaluating these coatings for the piston ring, pins, crack shafts, dies and molds etc.. Being ceramic in nature these coatings can be finish ground and sealed per customer specification. Hence it is proposed to evaluate various combinations of mixed oxide coatings for their room temperature and elevated temperature wear resistance and friction coefficient during the phase I and phase II programs. During phase III, when fully developed, optimized and implemented in engines, such oxide coatings can prevent friction, wear, and oxidation related degradations and increase efficiency, durability and environmental compatibility of these engines.