Boria Fluxing of SiC in Ceramic Matrix Composite Aeropropulsion Applications
Authors: McFarland Bohuslava , Opila Elizabeth 
Published: February 2020

Ceramic matrix composites (CMC’s) are being sought for high temperature applications in extreme environments. The composites are comprised of silicon carbide (SiC) fibers coated with a boron nitride (BN) layer embedded in a SiC matrix. In high temperature oxidizing conditions silicon carbide and boron nitride can react with oxygen, forming silica and boria oxidation products, respectively. The boria and silica can react to form a borosilicate glass over the SiC fiber. In the presence of water vapor the boria and silica can also react with water to form HxByOz and SixOy(OH)z gaseous derivatives, respectively. The composites can eventually degrade due to excessive borosilicate glass formation. These reactions can result in the eventual depletion of the BN interphase and in rapid recession of the SiC fibers, eventually resulting in CMC degradation due to excessive borosilicate glass formation. The effect of the boria concentration in the borosilicate glass on SiC oxidation kinetics is currently unknown since the thermally grown borosilicate glass composition is not currently understood.
In this effort, model borosilicate glass coatings of defined boria concentration were synthesized and applied onto SiC substrates. These glass coatings of known boria composition were synthesized by sol-gel method using silicate and borate sol-gel precursors, which were then converted to glass by heat treatment. Glass compositions ranged from 0 – 30 weight percent boria with the remainder silica. The coatings were applied onto CVD-SiC and SiC fibers by dip-coat method prior to heat treatment. The synthesis parameters were optimized to achieve adherent glass coatings on the substrates with thicknesses less than 1 µm. Free-standing glasses were made by casting molds to investigate boria volatility in isolation from SiC oxidation.
Prior to high temperature oxidation and volatilization experiments, several techniques were used to characterize the sol-derived glasses. The sol-gel synthesis parameters for maximum boria retention and reproducibility were optimized by quantifying glass composition, up to 28 weight percent, by ICP-OES. Thermogravimetric weight change measurements as a function of time, temperature, and gas environment was essential in the optimization of the gel-to-glass conversion in different gas environments, corroborated by FTIR analysis, which confirmed that Si-O-B bridges exist in the glass structure. The carbon retention from trapped organics common to sol-gel derived glasses was quantified by combustion technique. Raman analysis confirmed its presence as free carbon. Further confirmation of the amorphous glass structure lacking crystalline by-products from the synthesis was done by XRD. Imaging of the glass coating surface coverage was accomplished using SEM, during which 2-3 measurements of the coating thickness on 20-30 individual coated fiber cross-sections were recorded.
Following glass coating application, high temperature oxidation experiments were conducted on bare and coated CVD-SiC and also on three bare and coated fiber grades: Sylramic SiC with in-situ grown BN, desized Sylramic SiC without BN, and desized Hi-Nicalon fiber tows. The SiC substrates were exposed to three high temperatures: 1200, 1300, and 1400°C in two different gas environments: dry and wet oxygen. Comparison of bare and glass coated CVD-SiC oxidation demonstrated that the sol-gel derived glass coatings are good models for thermally grown oxides. Initial results showed that the oxidation rate on silica coated CVD-SiC decreased slightly, but repeat experiments are being conducted to determine the uncertainty in the results. The initial fiber diameters were measured by SEM on 20-25 individual bare fibers along with 2-3 measurements of glass coatings on 20-25 coated fibers prior to oxidation experiments. Quantification of fiber diameter recession and oxide growth post oxidation experiments of bare and glass coated fibers by SEM measurement analysis demonstrated in both cases that recession was significant and measurable.
Additionally, free-standing sol-gel derived borosilicate glasses of defined boria concentration were exposed to these oxidation environments to measure boria volatility from the glass. Bulk boria concentration quantified by ICP-OES showed that the boria concentration remained unchanged. On the other hand, preliminary results from XPS depth profiling of the exposed glasses showed a boron concentration gradient up to a sputter depth of ~ 50 nm, whereas oxygen and silicon concentrations remained unchanged for the entire sputter depth of ~ 300 nm. Moreover, the boron surface depletion was greater in wet oxygen than in dry oxygen. These results indicate that boria volatility is a surface reaction.
Preliminary results demonstrate that the sol-gel derived glass coatings are good models for thermally grown oxides of silica and borosilicate compositions. They also demonstrate that boria is volatile from the glass surface in these oxidation environments. Ensuing results from SiC oxidation of varying boria glass coating compositions will be presented.