Oxidation Behavior of Y<SUB>2</SUB>SiO<SUB>5</SUB>/SiC Coatings in Low Pressures of Oxygen

The oxidation resistance of a Y2SiO5/SiC coating was evaluated by oxidation tests under thermal cycles between room temperature and 1973 K in pO2=440 Pa, which simulates the re-entry of the HOPE-X (H-II Orbiting Plane-Experimental) space shuttle. The Y2SiO5 layer was produced by atmospheric plasma spraying on a CVD-SiC precoated graphite. Addition of the Y2SiO5 layer to the SiC coating improved oxidation resistance of the coating remarkably at the initial stage up to 7 thermal cycles and a spallation of the Y2SiO5 layer accelerated the oxidation of the SiC layer. An in situ observation of the behavior of opening and closing of cracks in the Y2SiO5 layer, and EPMA analysis on the cross section of the coating revealed that the cracks close at high temperature and hardly accelerate oxidation of the SiC layer. High temperature mass spectrometric analysis of the oxidation behavior in pO2=3.2 Pa revealed that suppression of pO2 at the Y2SiO5/SiC interface by the Y2SiO5 layer prevents active oxidation of the SiC layer and causes an interfacial reaction between Y2SiO5 and SiC.