[NZP], NaZr 2 P 3 O 12 -type materials for protection of carbon-carbon composites
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[NZP], NaZr2 P3 O12 -type materials for protection of carbon-carbon composites Dinesh K. Agrawal, Girish Harsh´e, Else Breval, and Rustum Roy Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 28 February 1995; accepted 29 July 1996)
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 Ca0.5 Sr0.5 Zr4 P6 O24 , SrZr4 P6 O24 , and Ba1.175 Zr4 P5.65 Si0.35 O24 . 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.
I. INTRODUCTION A. Application of NZP materials
A variety of carbon-carbon composites have been successfully utilized as integral throat and reentry components in space vehicles. These C-C (carbon-carbon) composites possess unique thermomechanical properties such as low rates of ablation, thermal shock resistance, high strength to weight ratio, and strength-retention at elevated temperatures, which have made them very reliable with high performance capability in extreme thermal environments.1,2 However, these materials suffer from one major limitation: the ease of oxidation of the surface in normal atmospheres at temperatures above 350 ±C.1,3,4 For this reason they need to be protected from oxidizing environments by either using an impermeable coating or a sacrificial protective coating of a suitable material and/or doping of an oxidation inhibitor in the carbon. In this study we have investigated a new class of materials for oxidation protection coatings for C-C composites. One of the major difficulties encountered in such oxidation protection coatings is the thermal expansion mismatch between the C-C composite and the protective layer. This thermal expansion mismatch leads to tensile stresses and coating cracks, and spalls during thermal cycling, and hence lowers the performance of the system.3 Most of the materials used at present for protective coatings, such as SiC, metal, ZrC, etc., have relatively high coefficient of thermal expansion (CTE) and hence have met with little success.1,3,5 Therefore, a material that (a) is thermodynamically compatibl
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