OXIDATION-RESISTANT CARBON-CARBON COMPOSITES UP TO 1700-DEGREES-C

Ceramic coatings have been developed for the protection of carbon-carbon composites against oxidation at temperatures up to 1700 degrees C. Coatings based on phosphorous oxides derived from POCl3 were quite effective up to 900 degrees C, the weight loss up to 600 degrees C with 60 minutes' isothermal hearing time decreasing from 75% to 2.6%. The coatings based on silicon (SiC) were effective up to a temperature of 1700 degrees C, provided that the cracks that developed in the SiC coatings due to mismatch of thermal expansion of SiC and carbon-carbon composite were filled with a sealant based on boron. In this case the weight loss up to 1000 degrees C decreased from 100% to 1.%, and at 1700 degrees C for 3 minutes it decreased from 30% to 4.8%. At temperatures less than 1200 degrees C, the microstructure of the SiC coating showed variation along the surface as well as along the different surfaces of the sample. The composition of this coating consisted of both SiC and Si. SiC deposited at temperatures of 1400 degrees C and 1500 degrees C showed a uniform microstructure. The microstructure of SiC at 1400 degrees C was of globule type, the globules joining to form a columnar grain with domed top. This microstructure changed to a hill and valley type on increasing the time of deposition from 20 minutes to 40 minutes, whereas the microstructure of SiC deposited at 1500 degrees C consisted of platelets of irregular shape and size, which changed on increasing the time of deposition of 40 minutes to that of a dense layer at the bottom and an upper layer with a globular structure.