Oxidation weight gain and strength degradation of Si3N4 with various additives [oxides of magnesium, yttrium and zirconium]

A systematic study has been made of the oxidation behavior and strength degradation of Si3N4 bodies with MgO, ZrO2, or Y2O3 used as densification aids. The weight change and strength degradation show definite dependence on the kind of additive. White porous layers are formed on bodies with MgO additions, with both the weight gain and the thickness of the layers increasing with MgO content. Additions of ZrO2 or Y2O3 result in the formation of continuous glassy layers which do not change with the amount of additive. Strength degradation, which is greatest (up to 60%) at higher additive levels, is shown to be related to the formation of pits on the specimen surface. Possible mechanisms of pit formation are examined.