Influence of Oxidation on Long Term Reliability of Silicon Nitride

Abstract

The long term reliability of a silicon nitride ceramic at high temperature under oxidative conditions and a mechanical load was studied. The material, hot pressed with yttria, has excellent oxidation resistance at temperatures below 1500°C and forms a protective oxide layer which consists of yttrium-disilicate crystals embedded in a silica layer which is partially crystalline (cristobalite). The structure and the composition of this layer is influenced by the presence of stress.

This material, with an intergranular phase consisting of a crystalline silicon-yttrium-oxynitride with apatite structure, has excellent creep and cavitation resistance in comparison with other silicon nitrides. Steady-state creep rates are only marginally influenced by the testing environment or by pre-oxidation.

In an inert environment such as nitrogen, the material can survive at a stress of 300 MPa for more than 1000 hours without significant cavitation or other damage. Under oxidising conditions, lifetime may be curtailed by deep oxidation pits, which occurred rather randomly on some samples after relatively short oxidation times.

After lifetimes longer than 10⁶ secs in air, a new crack initiation mechanism appears on all surviving samples. A porous layer is formed underneath the surface oxide. Cracks are generated by the interlinkage of these pores under influence of the applied tensile stress. One of these cracks lead to ultimate failure of the sample. The relationship between crack nucleation time and the applied stress could be derived from the observations.