High-temperature analysis of the thermal degradation of silicon-based materials. II: Ternary Si–C–O, Si–N–O, and Si–C–N compounds
Philippe Rocabois, Christian Chatillon, Claude Bernard
The thermal degradation under vacuum, ie the vaporisation behaviour of ternary Si – N – O, Si – C – O, and Si – C – N, is analysed by means of thermodynamic concepts of indifferent states (ie azeotropic or congruent vaporisation) on the basis of previous Knudsen-cell mass spectrometric studies. Partial pressures were measured that indicated hindered vaporisation for any main gaseous species existing at equilibrium over these compounds. These hindered vaporisations are similar to those occurring for binary silicon compounds, and moreover new reactions such as those occurring with free carbon are also hindered in such a manner that these reactions seem not to contribute significantly to the thermal degradation. Changes of compositions with time as a result of vaporisation matter loss are analysed, first from a thermodynamic or quasi-equilibrium point of view, and second taking into account hindered vaporisations, for chemical systems in the same triphasic compositions such as fibres, black-glass, or matrix composites. The present analysis shows that hindered vaporisations do not modify the general composition changes in the Si – C – O and Si – C – N systems, but drastically modify those in the Si – N – O system. Indeed, in the latter, the diphasic partial congruent vaporisation lines are different from those at equilibrium.