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(a) Typical actual diesel truck engine experience.

(b) Surface modified by Ceraprep process.

(a) Typical actual diesel truck engine experience.

(b) Surface modified by Ceraprep process.

operating conditions was a factor of 100 lower, which could be directly attributed to the improved wear resistance offered by this ceramic coating.

The study also showed that a lubricant will be needed to maintain wear rates in the acceptable range unless improvements in self-lubricating ceramics are forthcoming. Since there are no liquid lubricants currently available for operation over the temperature from ambient to 650 C, this currently remains as a critical need in the overall engine development efforts. The testing at 260 C using a polyalphaolefin demonstrated the important role of the lubricant even though its viscosity was below the normally considered usable range and it was in the process of rapid decomposition. The measured friction coefficients of 0.03 to 0.08 were in the range expected for hydrodynamic lubrication. Apparently the decomposition products were playing a role in maintaining a useful separation of the sliding members. Spectrographic surface analyses on worn specimens run at room temperature also showed the presence of an organic film that was helping to control wear by separating the sliding surfaces. Based on these successful results, the lubricant development efforts need to address additive packages as well as basic thermal stability to enhance further the protective films remaining at the sliding interfaces as the lubricants decompose.

The formation of hot spots and associated thermal shock cracking with zirconia demonstrated that low thermal conductivity is inherently incompatible with satisfactory ring/cylinder sliding performance. The use of structural ceramics having low thermal conductivities would therefore require surface coatings having higher conductivities or reliable lubricating coatings to maintain low friction coefficients over the entire operating range.

The wear mechanism of ceramics operating with low wear coefficients was found to be a very fine polishing mode. The resulting wear debris would be expected to have sub-micron sizes, which would be tolerable in the lubricant system of the overall engine. More severe wear mechanisms, which would release larger wear particles, would be unacceptable from the standpoint of both the longevity of the ring and the potential secondary damage to the rest of the engine. The acceptable wear mechanisms observed and the wear coefficients in the range of interest provide encouragement for the ultimate successful application of ceramics to the advanced engines.

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