Nitridation study of reaction-bonded silicon nitride in situ by high temperature X-ray diffraction

Abstract The reaction-bonded silicon nitride (RBSN) nitriding process has been studied using a high temperature X-ray diffractometer (HT-XRD) under isothermal conditions in the temperature interval 1300–1400 °C. With HT-XRD, the nitridation reaction and phases formed could be monitored almost instantaneously at temperature. The experimentally observed kinetics of the nitriding reaction were found to be in fair agreement with a theoretical model which predicts that the nitriding reaction occurs predominantly by Knudsen diffusion of nitrogen molecules through channels in a layer of growing Si 3 N 4 . However, no single rate law is likely to describe the whole nitridation process. Observation of the microstructure after nitridation indicates that the process occurs partly by reaction of Si vapour with nitrogen gas but that inward diffusion of nitrogen into particles also contributes significantly to the overall nitridation. The phase analysis showed that α-Si 3 N 4 formation predominates over β-Si 3 N 4 formation but the proportion of β-Si 3 N 4 increases as nitridation continues.