Study On Zirconium Diboride Matrix Composite Coatings

Zirconium diboride（ZrB₂）has high melting point,strength and hardness,as well as a low coefficient of thermal expansion.In addition,ZrB₂ has excellent chemical stability and heat-resistant ablation and other special properties.ZrB₂ coating prepared by plasma spraying has broad application prospects.However,the ZrB₂ composite coating prepared by direct plasma spraying has the problems of high porosity and low density,and the quality and performance needs to be further improved.In order to solve the above problems,ZrB₂ matrix composite coatings were prepared by reactive plasma spraying in this study.In this paper,ZrB₂ matrix composite coatings were successfully prepared by direct plasma spraying ZrB₂-ZrC system and ZrB₂-ZrC-Al₂O₃ system,as well as reactive plasma spraying Zr-B₄C system and ZrO₂-B₄C-Al system.The microstructure and propoties（hardness,toughness,bonding strength,wear resistance,thermal shock resistance and ablation resistance）of four system composite coatings（ZrB₂-ZrC,Zr-B₄C,ZrB₂-ZrC-Al₂O₃ and ZrO₂-B₄C-Al）were investigated comparatively.The reaction mechanism of Zr-B₄C and ZrO₂-B₄C-Al systems in the process of plasma spraying and the formation mechanism of composite coatings were revealed.The influences of spraying power,spraying distance,particle size of composite podwer,solid content of slurry,raw material rate on the ZrO₂-B₄C-Al composite coating were studied.The phases of ZrB₂-ZrC and ZrB₂-ZrC-Al₂O₃ composite coating prepared by plasma spraying were consistent with the raw materials,with only a few oxidized phases appearing.Zr-B₄C and ZrO₂-B₄C-Al system had sufficient reaction during plasma spraying,the main phases of Zr-B₄C system were ZrB₂ and ZrC,and the main phases of ZrO₂-B₄C-Al system were ZrB₂,ZrC and Zr_0.9Al_0.1O_1.95.By comparing the ZrB₂-ZrC system,Zr-B₄C system,ZrB₂-ZrC-Al₂O₃ system and ZrO₂-B₄C-Al system,it was found that the ZrO₂-B₄C-Al coating had the highest density and the most obvious lamellar structure with fine grained ZrB₂ and ZrC distributed uniformly.The quality of ZrO₂-B₄C-Al system composite coating was obviously better than that of the other three composite coatings.The reactive mechanisms of Zr-B₄C and ZrO₂-B₄C-Al system during plasma spraying are melting-diffusion-reaction.When Zr-B₄C system come into the plasma flame Zr dissolved firstly and wrapped the B₄C particles,diffusion occurred between Zr and B₄C,and reacted ZrB₂ and ZrC.About ZrO₂-B₄C-Al system,Al dissolved firstly and wrapped the ZrO₂ and B₄C particles,the diffusion reaction occurred between Al and ZrO₂to generate Zr and Al₂O₃.The intermediate product Zr continued to react with B₄C to form ZrB₂ and ZrC,and the dense lamellar coating structure was formed during solidification.According to the orthogonal experiment and the influences of ZrO₂-B₄C-Al composite coating,the optimum parameters of the ZrO₂-B₄C-Al system coating were the spraying power 35kW,the spraying distance10cm,the solid content of the slurry 30%,the particle size of the spraying feedstocks-200 mesh³00 mesh,and the rate of ZrO₂:B₄C:Al is 65:12:23.By comparing the ZrB₂-ZrC system,Zr-B₄C system,ZrB₂-ZrC-Al₂O₃ system and ZrO₂-B₄C-Al system,it was found that the ZrO₂-B₄C-Al system composite coating had better hardness(1278 HV_0.1)and uniformity,toughness,bonding strength（26.7 MPa）and wear resistance（wear rate:0.822）.The composite coating prepared by reactive plasma spraying ZrO₂-B₄C-Al system had better thermal shock resistance and ablation resistance than the composite coating prepared by direct plasma spraying ZrB₂-ZrC-Al₂O₃ system.