Boria Effects on the High Temperature Oxidation of Silicon Carbide

Ceramic matrix composite (CMC) components were introduced into civilian aircraft engines in 2016. CMCs are comprised of silicon carbide (SiC) fibers coated with a boron nitride layer embedded in a SiC matrix. SiC and boron nitride will react with oxygen upon exposure to the engine’s high temperature oxidizing environment, forming silica and boria oxidation products, respectively, which together form a borosilicate glass. In the presence of water vapor the boria and silica can also react with water to form B-O-H and Si-O-H gaseous species. These reactions can result in the depletion of the boron nitride interphase and excessive borosilicate glass formation leading to rapid recession of the SiC fibers and CMC degradation. The thermally grown borosilicate glass compositions are presently unknown, as well as the time, temperature and gas environment that result in excessive borosilicate glass formation. Two approaches were taken in this research project to understand the role of boria concentration and exposure conditions that lead to accelerated SiC oxidation kinetics so that the mechanistic understanding and prediction of CMC degradation rates can be elucidated.
In the first approach, the microstructure, composition, bonding and phase of thermally grown borosilicate glasses were investigated through the oxidation of two different types of boron-containing SiC materials. The role of a boron nitride layer on the oxidation behavior of SiC fibers was investigated. The boria concentrations in the oxides were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Higher boria concentrations led to initial rapid oxidation rates of SiC fibers that were 3 – 10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with silica due to boria volatilization, thereby limiting the observation of boria effects on SiC fiber oxidation kinetics. Reaction-bonded SiC (RB-SiC) ceramics were investigated next to understand the role of boron (B) concentration on the RB-SiC oxidation behavior. Varying concentrations of B were processed into RB-SiC coupons, comprised of SiC particles embedded in a silicon melt-infiltrated matrix. High temperature oxidation studies were conducted in ambient and in elevated oxygen pressure environments. The increased B concentration and elevated oxygen pressure increased the RB-SiC oxidation kinetics. Oxides from B-containing coupons were up to four times thicker than oxides from B-free coupons. Subsurface oxidation was also observed in oxidized B-containing coupons.
In the second approach, glass coatings with well-defined B concentrations were synthesized by a sol-gel method and applied onto SiC substrates to simulate a thermally grown borosilicate glass. Three B concentration ranges were investigated: 0, 24 – 55, and > 95 mol %, balance silica. Coating B concentrations > 95 mol% resulted in fluxing of the SiC substrate, which accelerated the SiC oxidation kinetics by up to 800 times faster than observed for pure SiC. Boria effects on SiC oxidation were not observed with application of borosilicate glass coatings due to a combination of 1) silica incorporation in the boria glass network, which slowed oxygen transport; and 2) boria volatility, which led to reduced B concentrations in the oxide. Sol-gel derived borosilicate glass coatings differed in composition and bonding from thermally grown borosilicate glasses, which limited this approach to directly model the thermally grown borosilicate glass. However, this approach allowed for investigating the interactions of boria with SiC and with the thermally grown silica glass that cannot be investigated by thermally grown borosilicate glasses alone.
The findings in this study demonstrated that high B concentrations flux the SiC, leading to accelerated SiC oxidation kinetics. Oxygen transport in B-rich borosilicate glass is several times faster than in silica by comparison, but is limited when boria volatilization occurs. Thus, the borosilicate glass at CMC free surfaces will likely be protective, but high concentrations of boria formation within the CMC will lead to significant SiC degradation.