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This paper is a review of ceramic tribological characteristics and application of silicon-based ceramics as engine valves and seat inserts. This review has shown that wear of ceramics is not an intrinsic material property or a constant, rather it depends on the operating conditions as well as on microstructure, environment, measurement techniques and material properties. The review also found that the dominant ceramic wear mechanism is a fracture/crack controlled process. There have been many models developed to describe the wear of ceramics, however, the complexity of the wear and the existence of wear transitions suggest that satisfactory results can be obtained for each wear model/equation only for the operating conditions for which it was derived. The application of silicon-based ceramics for engine valves and seat inserts is also reviewed and field engine test results are presented. The advantages and disadvantages of ceramics for engine valves and seat inserts are discussed.

In this investigation, worn valve seats and inserts have been examined to obtain a fundamental understanding of the primary wear mechanisms. Seats and inserts from a wide variety of engines were studied using optical and electron microscopy. The recessed region on seats and inserts displayed a common feature which can be described as a series of ridges and valleys formed circumferentially around the seat axis. These ridges and valleys were found to be due to either a relatively severe gouging process or to a milder deformation and wearing process. These processes were related to assembly and product geometry factors which caused misalignment between the seats and inserts.

The durability requirements of the diesel engine exhaust valve have been satisfied through the use of several iron, nickel, and cobalt base alloys. There have been cases where iron base austenitic alloys were found deficient in one or more physical or mechanical properties, and they were replaced with a nickel or cobalt base super alloy, often with a wide margin of design safety. Strategic alloying elements such as cobalt, tungsten, and nickel have escalated in price at a significant rate, thus increasing the cost of the super alloys. An improved austenitic alloy has been developed to provide adequate performance while retaining the cost effectiveness of iron base systems. During this development program, the alloy was designed to minimize its susceptibility to manufacturing problems which have been associated with prior valve materials.

A significant contribution to the development of high performance, high specific output automotive engines with extended durability comes from improvements in valve gear efficiency. This paper attempts to review the factors which contribute to valve gear friction and wear and influence its reliable long term function. The roles of the type of valve gear, component design factors, materials, and surface finish in determining the effective stresses and subsequent wear modes under dynamic conditions are discussed. Environmental factors, such as lubrication, corrosion, and temperature, which influence valve gear distress are also outlined. Future trends in seeking new materials and processing for designing lighter, lower friction, extended durability, and higher performance valve gear are suggested. Methods of evaluating new materials, and designs for predicting their wear performance and durability, are also presented.