A thermodynamic study of the thermal degradation of Nicalon NL-202 and of Tyram~o Lox-M fibres was proposed recently, with the aims of predicting the temperature limit for their use and understanding the mechanisms responsible for their behaviour under these extreme conditions [1][2][3]. This work was conducted by minimizing the total Gibbs energy of the Si-C-O-H(-Ti) chemical systems and was based on a critically selected thermodynamic database of the participating compounds. The results were compared to recently reported transmission electron microscope (TEM) studies of as-received and heat treated materials. One of the main features in this approach was the differentiation of the environments in the core and at the skin of the fibres by adopting different inert gas dilution of the reactants. This situation allowed consideration in the calculations of the fact that the external part of the fibres is found in contact with a larger free volume compared to the core and, consequently, its degradation is more advanced at a given temperature. The stability limit of both fibres was thus calculated to be 1800 K (1527 Β°C) or lower for operating conditions referring to the core environment. At this temperature the available carbon, i.e. other than the carbon linked to silicon to form SiC, is exhausted, while the oxygen content is highly reduced before vanishing at higher temperatures. The main reaction products were silicon carbide, carbon and silicon monoxides, and, in Lox-M, stoichiometric titanium carbide.
It has already been shown that this behaviour is due to the decomposition of an intergranular disordered solid solution or continuum, which, in the as-received state of these materials, surrounds together with a free carbon network, nanometre sized SiC grains. The composition of the continuum in NL-202 was determined, among others, by Le Coustumer et al. [4] who proposed a formula of SIO1.15C0.85 . During the heat treatments of NL-202 the diameter of the SiC grants increased from 1.8 nm in the asreceived state to 10.4 nm at 1773 K (1500 Β°C) [4]. In the same conditions the SiC grains in Lox-M increased from -2 n m to 60nm at 1673K (1400Β°C) [5]. This phenomenon, associated with the disappearance of the intergranular "cement" phase, led to the deterioration of the mechanical properties of the fibres.