Mullite Based Oxidation Protection for Silicon Carbide-Carbon/Carbon Composites in Air at Temperatures up to 1900K

For an industrial Si-SiC coated C/C material (reference material) the temperature dependence of the isothermal linear mass loss rate is interpreted in the temperature range 500°C < T < 1700°C. The Arrhenius plot of the thermogravimetrically determined oxidation rate shows four typical regimes, which can be individually modelled. Only in the temperature range 1050°C < T < 1550°C is the oxidation rate close to or even lower than the limit for long-term application. In a second step a mullite coating was deposited by Pulsed Laser Deposition (PLD). As-ablated green layers do not yet show IR peaks typical for mullite. After a short oxidation treatment (15 min at 1400°C) the formation of mullite in the coating is completed as was confirmed by IR spectroscopy and XRD investigations. PLD mullite coatings with a thickness of 2.5 µm on preoxidised samples improved the isothermal oxidation behaviour significantly. Because of Si02 formation at the mullite-SiC interface all samples exhibited a mass increase on oxidation. The inward diffusion of oxygen across the outer mullite-containing layer controlled the kinetics of the reaction as was deduced from 18O diffusivity measurements in PLD mullite layers by secondary neutral mass spectrometry (SNMS). The calculated oxidation rates resulting from the diffusion parameters in SiO2 and mullite are close to the thermogravimetric data. For oxidation durations of three days only amorphous SiO2 is formed at the mulliteSiC interface. In a further investigation, the material was tested under thermal cycling conditions between room temperature and 1100°C < T < 1600°C. An oxidation protection effect was observed which can be modelled on the basis of the isothermal oxidation rate and the cyclic testing conditions.