Preparation of the mullite coating precursor solution
All the materials used for mullite precursor synthesis were of the AR grade. The mullite coating precursor solution was prepared by mixing a mullite precursor sol with mullite powder, along with other ingredients. The processing steps for synthesizing mullite precursor sol are shown in Fig. 1. Tetraethoxysilane (TEOS) was mixed with aluminum nitrate nonahydrate (ANN) and aluminum acetate (2:5 molar ratio of ANN to aluminum acetate) in pure ethanol medium in the stoichiometric ratio of mullite. Acetic acid was added (0.1–0.2 mol/L) to promote gelation. After 6 h of refluxing at 353 K, a milky, viscous monophasic sol was obtained. The mullite precursor sol and 3/2-mullite powder (70–75 wt.% of the Fig. 2. Surface XRD spectra of the as-prepared mullite coating on SiC-C/C. total mixture) were mixed by high energy planetary ball-milling at 300 rpm, with a crack-sealing agent (polyvinylpyrrolidon, PVP, average molecular weight of 1.3×106 g/mol, about 0.1–0.25 wt.%) and oxalic acid (3–5 wt.%) as drying control chemical additive (DCCA). Ball milling was performed for 4 h after the addition of all the ingredient and, at the end of the ball-milling process, acetone was added to reach an optimized solution viscosity. The solid concentration in the optimized mullite coating precursor solution was about 20 wt%.The mullite powder was prepared by a sol–gel process and ANN
and TEOS were used as the starting materials. In short, the source chemicals were dissolved in absolute ethanol in the molar ratio of 3:1 of ANN: TEOS (3/2-mullite) after which acetic acid was added as a catalyst and chelating agent. Subsequently, the mixture was heated in a water bath under reflux at 353K for 12 h to form a gel
which was then oven dried at 393K for 12 h and further fired at