High‐Temperature Gaseous Corrosion of Si 3 N 4 in H 2 –H 2 O and Ar–O 2 Environments

Samples of Si 3 N 4 made by chemical vapor deposition (CVD) and hot isostatic pressing (HIP) were exposed for 10 h at 1400°C to H 2 –H 2 O and Ar–O 2 environments. Variations in weight and surface morphology of the CVD‐Si 3 N 4 samples were monitored to determine the corrosion behavior of ‘pure’ Si 3 N 4 as baseline information. Additionally, the room‐temperature flexural strengths of the HIP‐Si 3 N 4 were determined and correlated with observed changes in weight and microstructure. The corrosion behavior of both materials depended on the level of oxidant (H 2 O or O 2 ) in the atmosphere, but the magnitudes of the weight changes observed in the HIP‐Si 3 N 4 were significantly greater. When the P H2O in the H 2 atmospheres was low ( P H2O < × 10 −5 MPa), significant weight loss and surface degradation occurred in both ceramics, accompanied by reductions in strength of the HIP‐Si 3 N 4 . However, in atmospheres with P H2O 1 × 10 −4 MPa, the weight loss was smaller and the strength of the HIP‐Si 3 N 4 was greater than that of the as‐received material. Somewhat different behavior was observed in samples exposed to Ar–O 2 environments. Weight loss of both silicon nitrides increased with increasing P o2 until a level of about 2 × 10 −5 MPa was reached. However, contrary to the results for H 2 –H 2 O exposures, even under these conditions the strength of the HIP‐Si 3 N 4 material was found to have increased above that of the as‐received material.