Oxidation and Hot Corrosion Type I of Alternative Bond Coats for SiC/SiC Ceramic Matrix Composites

Abstract

With the goal of increasing the combustion temperature and decreasing the weight of todays aero engines, the application of SiC/SiC ceramic matrix composites is widely investigated and first parts are in service. Besides the many advantages of SiC/SiC ceramic matrix composites, some challenges are encountered and have to be overcome to ensure a reliable operation. One of these challenges is the protection of the ceramic matrix composites from oxidation and volatilization. The oxidation of silicon carbide results in the formation of silicon dioxide, which is generally considered protective. The combustion atmosphere, however, contains water vapor and other contaminations. The water vapor results in the formation of volatile silicon hydroxide, while contaminations, like CMAS or Na2SO4, can result in spallation or an increase in oxidation rates. Also, the fiber matrix interface coatings are more prone to oxidation and need to be protected to avoid loss in mechanical properties. Therefore, a coatings system, consisting of an outer environmental barrier coating and an inner bond coat, is applied to SiC/SiC ceramic matrix composite when used in turbine engines. Within this study alternative bond coats are developed mainly based on aluminum oxide formers, which are in itself less prone to volatilization at the designated temperature of 1200°C. The coating process is realized via chemical vapor deposition, which can coat complex parts cost effectively and without the need for a line-of-sight. The coatings provide a reservoir of the oxide forming species and therefore the oxide scale is able to heal upon crack formation or spallation. One of the developed coatings is based on an oriented aluminum nitride layer, which is in contrast to bulk AlN resistant to high temperature oxidation in humid air. The influence of the orientation and the chlorine content, which is present due to the coating procedure, on the increased oxidation resistance is investigated. Cyclic exposures are done to investigated spallation and cracking behavior due to mismatches in thermal expansion. Further, the resistance to hot corrosion type I is compared to other materials used or investigated for the hottest sections of turbine engines. The microstructure of the coatings was examined before and after the exposures using X-ray diffraction, scanning electron microscopy and electron beam microanalysis.