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MATERIALS RESEARCH

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Infiltration of C/SiC composites with silica sol-gel solutions: Part I. Infiltration by dipping Mario Aparicio and Alicia Dura´n Instituto de Cera´mica y Vidrio (CSIC), E-28500 Arganda del Rey, Madrid, Spain (Received 15 March 1999; accepted 30 August 1999)

Oxidation resistance of ceramic matrix composites (CMC) of SiC reinforced with C fibers (C/SiC) can be improved by filling the residual porosity. The aim of this work was to design and analyze a dipping infiltration process under ambient conditions (1 atm pressure and room temperature) with silica sol-gel solutions prepared from tetraethyl orthosilicate. Different substrates and solutions have been studied. Thermal treatments, i.e., curing or sintering between infiltrations, increase the efficiency of the process since the densification of infiltrated silica opens up the remaining porosity. Increasing viscosity and/or concentration of the solution lead to greater weight gains. Weight gains are higher in the initial stages of the process because larger diameter porosity remains unfilled. As the process advances, the average pore size decreases, and only the lower viscosity solution can enter the residual porosity. I. INTRODUCTION

The lack of light-weight structural materials capable of reliable operation at high temperatures has posed a real challenge to the development of many aerospace systems. One example is structural components for reentry vehicles. Carbon matrix composites reinforced with carbon fibers have attractive mechanical properties and chemical stability at high temperatures, plus they have damagetolerant fracture behavior. However, C/C composites are easily oxidized in air at temperatures higher than 400 °C. The addition of various inhibitors improves the oxidation resistance in air but only up to 800 °C.1,2 The use of protective systems, which are composed of different layers with complementary oxidation properties—like SiC, TiC, HfC, MoSi2, SiO2, SiO2, B2O3, and Y2SiO5—give some protection to C/C composites against oxidation,3–5 but the high reactivity of carbon and the severe thermal shocks that materials suffer in many applications require additional protection systems. An alternate approach is the replacement of the carbon matrix by materials with better oxidation resistance that are also compatible with carbon fibers. SiC shows very good mechanical properties up to 1500 °C and has an acceptable oxidation resistance. A silica coating forms on SiC during high-temperature oxidation, and it acts as a barrier against oxygen diffusion.6–8 SiC reinforced with carbon fibers (C/SiC) is a possible alternative to C/C composites. The mechanical properties (toughness, flexure strength, and thermal shock resistance) and the thermal stability are slightly lower for C/SiC than C/C, but the oxidation resistance is much higher. One problem is 4230

http://journals.cambridge.org

J. Mater. Res., Vol. 14, No. 11, Nov 1999 Downloaded: 15 Apr 2015

the mismatch between SiC matrix and C fibers thermal expansion coeffic