Reserch On Performance Of TiB₂-Based High Temperature Ceramics Coating By Tig Cladding

In this experiment, in-situ synthesized TiB₂-based ceramics coatings on graphite matrix were made by means of gas tungsten arc cladding, with Ti, B₄C and SiC powders as the raw materials. According to the analysis, we designed Ti-B₄C and Ti-B₄C-SiC two series researches including ten schemes, therefore the best cladding parameters of every constituent were determined by extensive experiments. The microstructure and phase composition were researched by use of SEM and XRD. The interfacial bonding strength, ablation resistance and oxidation resistance of coating were also examined.The results showed that continuous and uniform protective coatings had been deposited by a novel tungsten inert gas cladding technique on graphitic carbon substrates. The shapes of TiB₂ depended on the concentration of titanium and boron atoms during solidification process. So different proportions of the titanium and boron ingredients has been a influence on the growth morphology such as leaflike, needle and irregular strip. Meanwhile hypodispersion of grains in the coatings would materially improve strengthening. With SiC added, the ablation resistance and oxidation resistance of coating were improved significantly.Microscopy cross-section morphology showed that an excellent bond was formed at the coating/substrate interface. The bond zone between the substrate and the coatings was wholly metallurgical bonding, and part of coating materials had already penetrated into substrate. The interfacial bonding strength between the coating and the C/C composite material was measured, with the value up to 8.8MPa.The oxidation behavior was studied at the high temperature of 1200℃for 11 hours. The best ratio of the specimens of Ti-B₄C in oxidation resistance was n(Ti):n(B₄C)=1:1, and internal coating had been completely oxidized and weight loss was 34.4mg/cm2. With SiC added, there was a kind of glass-ceramic covered on the surface of coating during the oxidation, which could slow penetration of oxygen atoms. The coating with Ti:B₄C:SiC mole ratios of 1:5:3 performed a good oxidation resistance. The oxidized depth and the lost weight of the oxidation were only 10μm and 2.8mg/cm2;Loose and porous structure of graphite lead to poor ablation resistance, whose ablative rate was 29.33mg/s. Compared to graphite, the coating with Ti:B₄C:SiC mole ratios of 1:5:4 performed a good ablation resistance, whose ablative rate was only 1.18mg/s, increasing 25 times. The specimens of Ti-B₄C series performed a poor ablation resistance. With SiC added, the ablation resistance of coating improved significantly, because the SiO₂ and TiO₂ glass-ceramic appeared during the ablation. It could attach to the surface with free-flowing, which would fill and heal the holes effectively, and performed an excellent ablation resistance.