The microstructure, mechanical properties and surface transformations of a syalon ceramic
The potential application of nitride based ceramics in structural applications is dependent on their performance at elevated temperatures. To meet these demands the microstructure of a syalon ceramic has been refined. Improved processing techniques, by the use of high purity powders and "balanced" compositions, have been shown to improve the degree of inter granular crystallisation. The key aspects are the reduction of impurity levels, particularly calcium, and the use of high nitrogen compositions such that full crystallisation is achieved. The achievement of complete crystallisation results in the elimination of subcritical crack growth during fracture and high temperature creep processes dominated by non-cavitational grain boundary diffusional processes.\ud \ud The mechanism for oxidation has been identified by determination of the kinetics, surface and sub-surface reactions. A temperature limit of l300 oC has been identified by the reversion of YAG to a eutectic liquid by reaction with the Si02-rich oxidation layer. In this regime oxidation rates are determined by the YAG reversion and cation out-diffusion to the oxide layer. Oxidation effects have been shown to be responsible for the onset of sub-critical crack growth at temperatures above 1300oC, where crack extension results directly from YAG reversion effects.\ud \ud Enhanced high temperature performance above and beyond l300oC by surface transformation and surface coating was investigated. Surface microstructures based upon BI and Si2N20 have been shown to increase oxidation resistance upto 1375oC but are difficult to form without substantial surface degradation. Si3N4 and SiC coatings deposited by chemical vapour deposition (CVD) were found to have similar effects, increasing oxidation resistance beyond 1300oC.
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