Preparation And Structure Of HfC Ceramic Coating By Chemical Vapor Deposition

HfC is one of the most promising anti-ablation coating materials for C/C composites due to its excellent physical and chemical properties. In this work, HfC ceramic coatings were prepared on the surface of C/C composites by low pressure chemical vapor deposition (LPCVD) with HfCl4-CH4-H2-Ar system. The effects of deposition temperature, input C/Hf ratio and concentration of H2on the deposition rate, phase structure, micro structure and mechanical properties were investigated. The influencing mechanisms were also analyzed.At1400～1600℃, the deposition rates increases with the deposition temperature, and the preferred orientation changes from (220) to (200). With the increased temperature, the diffusion ability of deposition atoms enhances and the size of columnar crystals increases. With the increased input C/Hf ratio, the deposition rate increases, and the preferred orientation changes from (220) to random orientation. The cross-section of HfC coating changes from fine-columnar to particle stacked. The deposition rate increases with the increased concentration of H2, the preferred orientation of HfC crystal changes from random to (200). The microstructure changes from particle stacked to columnar crystal.When the deposition process is controlled by surface reaction mechanism, the coating growth is dominated by the nucleation of HfC. While the deposition process is controlled by mass transport mechanism, the coating growth is dominated by crystal growth of HfC. The deposition process consists of the C-deposition and Hf-deposition, and the C-deposition is the dominated process.The mechanical properties of HfC coatings are mainly determined by the microstructure and density. The HfC coating with particle-stacked structure has the lowest mechanical properties, with hardness of4.03～14.64GPa and Young’s modulus of62.61～199.10GPa, respectively. The fine-columnar coating has the best mechanical properties, with the hardness of16.62～20.29GPa and Young’s modulus of247.35～282.34GPa, respectively. Meanwhile, the HfC coating with coarse columnar structure has the moderated mechanical properties.