THERMAL-DEGRADATION OF FIBER COATINGS IN MULLITE-FIBER-REINFORCED MULLITE COMPOSITES

The thermal degradation behavior of single-layer BN and of double-laye r BN/SiC chemically vapor-deposited fiber coatings in mullite-fiber-re inforced mullite composites was investigated by means of transmission electron microscopy after processing and heat treatment of the composi tes at 1000 degrees, 1200 degrees, and 1300 degrees C for 6 h in air, The single-layer BN coatings were similar to 0.7 mu m thick and consis ted of turbostratic BN with (0001) basal planes lying parallel to the surfaces of the fibers plus nanosized areas that had no preferential o rientation. This microstructure remained unchanged up to 1000 degrees C; however, distinct coarsening of the randomly oriented BN crystallit es occurred in the temperature range of 1000 degrees-1200 degrees C. T he single-lager BN coatings were stable against oxidation, up to 1200 degrees C. At higher temperatures, degradation of the coatings via oxi dation occurred, Double-layer BN/SiC coating systems consisted of BN t hat was 0.08 pm thick and SiC layers that were 0.16 mu m thick and dep osited onto the mullite fibers, The turbostratic BN was highly anisotr opic and did not undergo any microstructural change, up to 1300 degree s C, The outer SiC layer of the double-layer coating system improved t he oxidation resistance of BN in the 1200 degrees-1300 degrees C tempe rature range, despite a partial oxidation of SiC to SiO2.