Long-term stability of nonoxide ceramics in an oxidative environment at 1500 °C

In the present study nonoxide ceramic materials based on silicon nitride were evaluated with respect to their long-term oxidation stability at temperatures of 1500 °C. Various Si3N4 ceramics with different additive systems were examined after long term oxidation tests (several thousand hours) at 1500 °C in ambient air. Special focus was placed on the processes occurring in the materials during oxidation. In the case of Si3N4 materials with sintering additives these processes resulted in alteration of the bulk microstructure which consequently led to degradation of the mechanical properties of the ceramics. The damage mechanisms were discussed on the basis of a comprehensive chemical and microstructural analysis of the materials after the oxidation tests. The diffusion of oxygen into the material and its further reaction in the bulk of the material were found to be the most critical factors during long term oxidation treatment at elevated temperatures. For that reason the long-term bulk stability should be considered as a key requirement for the confident application of potential ceramic materials at temperatures of more than 1400 °C. Finally some possibilities to hinder the processes that occur during long-term service at elevated temperatures are provided.