Performance of Ceramics in Ring/Cylinder Applications

In support of the efforts to apply ceramics to advanced heat engines, a study is being performed of the performance of ceramics at the ring/cylinder interface of advanced (low heat rejection) engines. The objective of the study, managed by the Oak Ridge National Laboratory, is to understand the basic mechanisms controlling the wear of ceramics and thereby identify means for applying ceramics effectively. Attempts to operate three different zirconias, silicon carbide, silicon nitride, and plasma-sprayed ceramic coatings without lubrication have not been successful because of excessive friction and high wear rates. Silicon carbide and silicon nitride perform well at ambient temperatures with fully formulated mineral oil lubrication, but are limited to temperatures of 500 F because of the lack of suitable liquid lubricants for higher temperatures. Wear is confined to surface polishing during conditions of successful operation. The zirconias encounter high wear rates at room temperature even in the presence of liquid lubricants because of thermal-shock-induced surface cracking. The low thermal diffusivity of zirconia allows local areas to heat excessively from friction. The periodic heating induces thermal shock cracks and subsequent spalling. The studies are continuing with evaluations of plasma sprayed ceramic coatings with liquid lubricants.