Oxynitride Glass Systems and Subsequent Glass-Ceramic Heat Treatments
Silicon nitride based ceramics contain oxynitride glass phases at the grain boundaries which can impair subsequent high temperature properties. Studies of the effect of nitrogen on properties of these glasses show that nitrogen increases viscosity, hardness and glass transition temperature. Heat treatment of the ceramics to crystallise these glasses is well established but further improvements are possible if glass-ceramic processes using two-stage heat treatments are introduced. This paper reports on the optimisation of glass-ceramic heat treatments in various oxynitride systems (notably RE-M-Si-O-N systems where (RE = Nd, Sm, M=Mg, Al) and the types of crystalline phases produced. One of the major crystalline phases is apatite, MRE4Si3O12N, which exists as a range of solid solution. The use of the glass-ceramic heat treatment results in improved hardness and this is used to optimise the heat treatment processes.
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