Kinetic effect of boron on the thermal stability of Si-(B-)C-N polymer-derived ceramics

The isothermal mass loss of two polymer-derived ceramics with compositions SiC(1 4)N(0 9) and SiC(1 5)N(1 0)B(0 05) were measured as a function of time using thermal gravimetric analysis at various temperatures ranging between 1580 and 1720 degrees C. The process of mass loss, attributed to the reaction Si(3)N(4) + 3C -> 3SiC + 2N(2)(up arrow), takes substantially more time for the boron-containing ceramic compared with the boron-free one. The continuous formation of SiC crystallites as the product of the reaction between Si(3)N(4) and C was revealed through X-ray diffraction (XRD) measurements during the course of the reaction. The kinetics of this reaction was studied using a generalized model for the analysis of chemical reaction kinetics Consequently, the effective activation energies for the Si(3)N(4) degradation were estimated to be 11 6 +/- 0 5 eV and 17.1 +/- 0.7 eV for the Si-C-N and Si-B-C-N ceramics, respectively. Moreover, the results obtained indicate that the dominant mechanisms of the Si(3)N(4) degradation are strongly Influenced by the presence of boron. For the Si-C-N ceramic, the chemical reaction at interfaces of the reactants and the crystallization of SiC as the reaction product are proposed to be the main probable stages controlling the progress of the investigated reaction. However, the local diffusion of C out of BNC(x) turbostratic layers surrounding the Si(3)N(4) nanocrystals and the gas (N(2)) release from the reaction zone are suggested to be the most plausible processes limiting the progress of Si(3)N(4) degradation for the Si-B-C-N ceramic. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.