High Temperature Tribological Coatings for Advanced Military Diesel Engines

Experimental results focused towards developing tribological surface coatings coupled with liquid lubricant boundary layer effects, for advanced high temperature military diesel engine applications are presented. The primary focus of this work is in the area of advanced, low heat rejection (LHR) high output diesel engines, where high temperature boundary lubrication between the piston ring and the cylinder liner wall surface is critical for successful engine operation. The target temperature focused upon in our research is an operating top ring reversal (TRR) temperature of approximately 538°C. The technology advancement used for this application involves treating porous iron oxide/titanium oxide (Fe₂O₃/TiO₂) and molybdenum (Mo) based composite thermal sprayed coatings with chemical binders to improve coating strength, integrity, and tribological properties. This process dramatically decreases open porosity to form an almost monolithic appearing coating at the surface¹. The effectiveness of this “densification” process through the coating thickness was studied and some characterization is presented. The densification process improves the plasma spray layer's mechanical properties, reduces the friction coefficient of the coating layer, and dramatically improves coating wear resistance. Good results were obtained using densified plasma spray Iron Titanate for cylinder liner coating vs. plasma sprayed Molybdenum compound for piston ring. Small bore single cylinder LHR engine test demonstrated the feasibility of this tribological pair for possible future applications. Inherent properties of these coatings prove to reduce production and manufacturing costs thereby allowing introduction into the commercial marketplace.