Thermochemical modeling and chemical vapor deposition of two-phase borides in the Hf-Si-B-Cl-H system

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Virginia Tech


Advanced, very high temperature materials that are ablation-resistant in oxidizing atmospheres are needed to withstand the severe environments found in rocket engine and aerospace vehicle applications. Boride-based coatings that form a protective layer during oxidation have been found to withstand these extreme conditions. Chemical vapor deposition has been chosen as a viable processing technique for producing these coatings. It is expected that two-phase boride coatings will have enhanced oxidation resistance over the single-phase materials by forming an oxide/glass layer to seal the base material from direct attack. Thermochemical modeling of CVD reactions was done to predict the processing conditions for the deposition of two-phase HfB₂-SiB₆ or HfB₂-SiB₄ coatings. The SOLGASMIX-PV computer program was used for these calculations, which were based on ZrCl₄, SiCl₄ BCl₃, and H₂ reactant gases. An array of temperatures (1100, 1300, and 1500K), total pressures (0.02, 0.1, and 1 atm.) and excess hydrogen concentrations (10:1, 25:1, and 50:1 H:(Zr + Si + B + Cl)) were investigated. These calculations show that two-phase HfB₂-SiB₆ and HfB₂-SiB₃ coatings are possible over a wide range of processing conditions, suggesting that a wide variety of two-phase compositions with a diverse population of microstructures can be deposited. This points to the possibility of optimizing oxidation resistance of these coatings by varying the processing conditions. A hot-wall CVD reactor was designed and constructed specifically for deposition reactions in this system. It was found to be necessary to separately deliver SiCl₄ and BCI₃ to the hot zone to prevent reactions between them and to eliminate interference with the in-situ hafnium chlorination reaction. SiB₄, rather than SiB₆, was the preferred deposition product in the Si-B-CI-H system. HfB₂ was the only compound found to deposit in the Hf-B-Cl-H system. Both borides coatings exhibited several different surface morphologies. The separate delivery of BCI₃ and SiCl₄, while necessary to prevent their gas phase reaction, caused reactant concentration gradients at the substrate surface due to poor mixing. This resulted in a variation of microstructural features across the substrate surface (both domed and faceted morphologies were deposited at the same temperatures), suggesting that reactant supersaturation is more important in determining surface morphology than temperature, pressure, and hydrogen concentration at the conditions studied. The first successful deposition of the two-phase HfB₂-SiB₄ coating was accomplished. Analysis of these two-phase coatings again reveals a broad range of microstructural characteristics, and the compositional gradients across the substrate surface also suggest the need for better gas mixing.