Kamo, L., Kleyman, A., Bryzik, W., and Schwarz, E., "Recent Development of Tribological Coatings for High Temperature Engines," SAE Technical Paper 950979, 1995, doi:10.4271/950979.
Lubrication of advanced high temperature engines has been one of the greatest obstacles in the development of the Adiabatic engine. Liquid lubricants which gave lubricating properties as well as heat removal function can no longer carry out this duty when piston ring top ring reversal temperatures approach 540°C. Solid lubricants offer some hope. Since solid lubricants cannot perform the heat removal function, its coefficient of friction must be very low, at least <0.10, in order to prevent heat build up and subsequent destruction to the piston rings and cylinder liners. The Hybrid Piston concept developed in the U.S. Army Advanced Tribology program offers some hope, since the top solid lubricant ring slides over the bottom hydrodynamic lubricant film section during each stroke.This paper presents the progress made with the solid lubricant top ring in the Hybrid Piston. Four materials have shown promise in the laboratory to fullfil its mission. The characteristic pairs are: Detonation Gun (D-Gun) sprayed and densified chrome oxide versus densified plasma sprayed chrome oxide. Densified plasma sprayed chrome oxide versus densified plasma sprayed Tribaloy T-800. Densified plasma sprayed iron oxide versus densified plasma sprayed Tribaloy T-800. Densified Slurry iron oxide versus densified hardened M-2 steel. Densification or thermal sealing the plasma spray layer, a process developed on the project, improves the coating's tribological properties. These coating materials are awaiting to be tested in the single cylinder engine.