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Oxynitride glasses exist as grain boundary phases in Si3N4 ceramics. This paper provides an overview of oxynitride glasses outlining effects of composition on properties. A review of the effects of grain boundary glass chemistry on fracture resistance of silicon nitride is given. A knowledge of overall additive compositions and their quantities in Si3N4 combined with measured properties of bulk glasses allows residual stresses in the interfacial glasses to be calculated. Increase in Y:Al ratio leads to higher thermal expansion mismatch and higher residual stresses in intergranular glasses. Values are in good agreement with those obtained using micromechanical finite element analysis.