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Sialons and Silicon Nitrides; Microstructural Design and Performance

Published online by Cambridge University Press:  25 February 2011

M.H. Lewis*
Affiliation:
Centre for Advanced Materials Technology, University of Warwick, Coventry, CV4 7AL, U.K.
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Abstract

survey is presented of developments in silicon nitride and sialon ceramic microstructures designed for application in differing temperature regimes.

For low temperature (< 1000°C) application, pressureless-sinterable sialons with moderate (10-15%) intergranular glass, anisotropic β′ grains and high values of MOR and Kc (1 GPa and 6-10MPa| m respectively) are preferred.

Improved hardness and high temperature capability may be achieved by tailoring intergranularp hases for crystallisation and further enhanced by the introduction of mixed α′/β′ Sialon microstructures. Examples are given of microstructural evolution in β′- Y3Al5O12(garnet), β′-Nd3Si3Al3O12N2 and α′/β′/garnet ceramics and a comparison of their mechanical behaviour.

Novel Sialon ceramics containing dispersed transition metal compounds (TiN, TiB2) may be formed by in-situ redox reaction, utilising the α′ Sialon phase as an oxygen receptor. The dispersed phase may enhance hardness and toughness and confer electro-discharge machinability.

Oxidation instability of Sialon compositions dictates the use of diphasic Si3N4/M2Si2O7 microstructures for application above 1300°C. The thermal cycle during pressurised sintering of these non-sialon compositions is critical in avoiding crystallisation of mixed polymorphs of the intergranular disilicate, with consequent microcracking.

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