Silica Scales on Si, SiC and SiSiC: Chemical and Mechanical Characterization After Oxidation in Different Water Vapor Containing Atmospheres

[Automatically extracted and translated] In oxidizing atmospheres Si containing materials can develop dense passivating silica scales. Oxide scales on Si, SiC and silicon infiltrated SiC (SiSiC) were grown at 1300°C in N2 with 30% water vapor (type w1), air with 30% water vapor (type w2) and air with 4% water vapor (type d) for 170 hours. The chemical and mechanical properties of the respective oxide scales were characterized as a function of the various substrate materials. The w1 scales and the w2 scales are relatively thick (10-20 µm and 7-10 µm, respectively) and have smooth surfaces, while the d scales are thinner (1-3 µm) and exhibit multiple cracking. The fracture surfaces of oxidized specimens showed that w1- and w2-scales had a bilayered morphology with a glassy part next to the surface and a crystalline part adjacent to the interface. The bilayered scale morphology was difficult to detect on the d type scales. The surface compositions of the scales showed that the w1- and w2-scales contained about 9 at% K and Al, while d type scales contained less than 2 at% of both. Une scans of taper sections made with SIMS revealed a decrease in Al and content with increasing depth of the scales, in correlation with the bilayered structure. Vickers hardness of H=8 GPA was measured for the scales. The d-type scales on SiSiC and SiC-variants showed chipping at any applied load so the hardness as well as toughness could not be measured. The toughness values are in the order of KR-0.9 MPa-Jm. The toughness and hardness are in fair agreement with literature values of vitreous silica. The good adhesion recognized in the scratch tests with the w1 - and w2-scales makes these layers favorable for protection of the substrate material. In contrast, the oxide scales grown in dry atmosphere exhibit poor mechanical properties.