Thermal Degradation of Fiber Coatings in Mullite‐Fiber‐Reinforced Mullite Composites

Abstract

The thermal degradation behavior of single‐layer BN and of double‐layer BN/SiC chemically vapor‐deposited fiber coatings in mullite‐fiber‐reinforced mullite composites was investigated by means of transmission electron microscopy after processing and heat treatment of the composites at 1000°, 1200°, and 1300°C for 6 h in air. The single‐layer BN coatings were ˜0.7 mu m thick and consisted of turbostratic BN with (0001) basal planes lying parallel to the surfaces of the fibers plus nanosized areas that had no preferential orientation. This microstructure remained unchanged up to 1000°C; however, distinct coarsening of the randomly oriented BN crystallites occurred in the temperature range of 1000°‐1200°C. The single‐layer BN coatings were stable against oxidation, up to 1200°C. At higher temperatures, degradation of the coatings via oxidation occurred. Double‐layer BN/SiC coating systems consisted of BN that was 0.08 mu m thick and SiC layers that were 0.16 mu m thick and deposited onto the mullite fibers. The turbostratic BN was highly anisotropic and did not undergo any microstructural change, up to 1300°C. The outer SiC layer of the double‐layer coating system improved the oxidation resistance of BN in the 1200°‐1300°C temperature range, despite a partial oxidation of SiC to SiO 2 .