Composite thermal protection coating on woven silica fiber fabrics

Abstract

A composite coating with inner and outer layers was prepared for the thermal protection of woven silica fiber fabrics. Using a sol mixture of a silica sol and AlF3/SiO2 particles mixed in the stoichiometric molar ratio for mullite, hollow silica spheres and short mullite fibers were added to the inner layer and outer layer, respectively. The phase composition and thermal evolution of the coating, along with the interfacial microstructure between the coating and the matrix, were characterized by means of X-ray diffraction, differential scanning calorimetry/thermogravimetry, scanning electron microscopy, and tensile strength testing. Mullite whiskers grew between 950°C and 1200°C and helped prevent thermal cracking during the drying and densification processes. The hollow silica spheres might play dual roles, weakening the adhesion between the coating and the fibers by reducing their direct contact, but strengthening the joining between the coating and substrate by embedding themselves among the fabrics.