Crystallization of polymer-derived silicon carbonitride at 1873 K under nitrogen overpressure

Abstract

The chemical stability of an amorphous silicon carbonitride ceramic, having the composition given by 0.57SiC:0.43Si3N4:0.49C is studied as a function of nitrogen overpressure at 1873K. The ceramic suffers a weight loss at pn2 < 3.5 bar, does not show a weight change from 3.5 - 11 bar, and gains weight above 11 bar. Interestingly, the structure of the ceramic changes with pressure: it is crystalline from 1-6 bar, amorphous at » 10 bar, and then again crystalline at higher pressures. The weight loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, silicon carbide and silicon nitride are set equal to those of the crystalline forms, implying that the material crystallizes prior to decomposition. The amorphous-to-crystalline transition that occurs near 10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen on the surface of the sample is proposed as the likely mechanism. The supersaturation required to nucleate a-silicon nitride crystals is calculated to be 30 kJ/mole.