Introduction

It has been clearly established that significant improvements in heat engine operating efficiencies can be realised with the use of uncooled ceramic components at temperatures above those attainable with superalloys. A large number of investigations have focused on introducing monolithic silicon nitride (SN) and silicon carbide (SC) components into gas turbine engines. However, due to the hard and brittle nature of these ceramics, high surface stresses in contact regions cannot redistribute as in metals. This can result in localized stresses which may exceed the baseline strength, thus damaging the surface of the component and reducing its strength. This susceptibility to contact stress damage has lead to projections that ceramic heat engine components under sliding contact may fail unpredictably and prematurely. Subsequent research helped define the nature of the contact stresses and the parameters influencing them (1,2).

It has been suggested that substantial reductions in surface damage and strength loss under such conditions can be attained by the application of a thin ceramic coating. Exploratory studies using plasma sprayed oxide coatings (3) demonstrated improvements in the contact stress damage resistance of both SN and SC ceramics. The adherence of these coatings, however, was inadequate under long term heat engine application. The use of an electron beam physical vapor deposition (EB-PVD) process has been reported to produce adherent coatings of yittria (V2O3) stabilized zirconia (Z1O2) on such ceramic substrates (4). However, these coatings were observed to lose adherence and spall under static air oxidation and contact stress. It has therefore become evident that the stringent conditions imposed by heat engine environments mandates that coatings be adherent, capable of withstanding thermal cycling, and oxidation resistant at extremely high temperatures. This paper discusses the conceptual design of a coating configuration for three specific substrates, namely monolithic silicon carbide (SC), monolithic silicon nitride (SN), and reaction bonded silicon nitride (RBSN), that may satisfy these objectives.

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