Lubrication of Ceramics in Ring/Cylinder Applications

In support of efforts to apply ceramics to advanced heat engines, a study was performed of the wear mechanisms of ceramics at the ring/cylinder interface. A laboratory apparatus was constructed to reproduce most of the conditions of an actual engine, but used easily prepared ring and cylinder specimens to facilitate their fabrication. Plasma-sprayed coatings of Cr₂O₃ and hypersonic flame-sprayed coatings of cobalt-bonded WC performed particularly well as ring coatings. Similar performance was obtained with these coatings operating against SiC, Si₃N₄, SiC whisker-reinforced Al₂O₃, and Cr₂O₃ coatings. The study demonstrated the critical need for lubrication and evaluated the performance of two available lubricants.

SIGNIFICANT EFFICIENCY IMPROVEMENTS have been predicted resulting from the practical application of low-heat-rejection engines (1,2). With associated predictions of top-ring-reversal temperatures exceeding 300 C and possibly reaching 650 C (3,4), these engines require solutions to wear problems for such sliding components as rings, cylinders, pistons, and valve mechanisms. Liquid lubricants, the traditional proven approach for controlling wear of metals used in conventional internal combustion engines, are not yet available for continuous service at the predicted temperatures to be encountered in low-heat-rejection engines. Therefore, ceramics as a class of materials are of interest to address the wear problems. High hardnesses, corrosion resistance, strength at elevated temperatures, and high elastic moduli are attractive properties not generally available in metals, which could be expected to compensate at least partially for the lack of availability of liquid lubricants. All-ceramic and part-ceramic engines have been assembled and evaluated on a limited basis (5,6). Basic studies of the friction and wear of ceramics in sliding contact have also been made, mostly using pin-on-disk apparatuses (7-9). However, few studies have been made to study the wear mechanisms of ceramics using a controlled laboratory apparatus to reproduce the important sliding parameters of actual engines without the problems associated with operating fired engines. The study described in this paper addressed the wear mechanisms of ceramics applied to the reciprocating sliding contact of the ring/cylinder interface in low-heat-rejection engines. The performance of two lubricants in reducing friction and wear at the interface was also evaluated. The approach was to use easily prepared ceramic specimens in a laboratory apparatus that reproduced the important parameters of actual engines. Models of She wear process were developed based on experimental data.