[NZP], NaZr₂P₃O₁₂-type materials for protection of carbon-carbon composites

Carbon-carbon composites, if not suitably protected, suffer from the problem of oxidation of the surface in normal atmospheres at temperatures above 350°C. For this reason they need to be protected from oxidizing environments by either using an impermeable coating, or using a sacrificial protective coating of a suitable material, and/or doping of an oxidation inhibitor in the carbon. In this study we have used a new family of materials with tailorable thermal expansion characteristics, namely, the [NZP] family as the materials for developing a suitable coating material for C-C composites. The candidates selected for matching thermal expansion with that of carbon are Ca_0.5Sr_0.5Zr₄P₆O₂₄, SrZr₄P₆O₂₄, and Ba_1.175Zr₄P_5.65Si_0.35O₂₄. They can be sintered in inert atmosphere without decomposition of the phases, and can be hot-pressed in inert atmosphere with C-C composites at 1250°C without decomposition or chemical interaction. They are stable in the presence of carbon up to 1200°C for at least a period of 4 h. They also do not show any weight loss after exposure to various temperatures up to 1200°C for 4 h.