Study On ZrB₂-ZrC-Al₂O₃ Composite Coatings

As an ultra-high temperature ceramic material,zirconium boride(ZrB₂)has excellent properties such as high melting point,high hardness,low density,good chemical stability and high thermal conductivity,but its high temperature oxidation resistance and ablation resistance are poor.At present,the preparation of ZrB₂ coating and its composite coating by plasma spraying has broad application prospects,but this method has many problems such as many coating defects,high porosity and low density.In this paper,ZrB₂-ZrC-Al₂O₃ composite coating was prepared by plasma spraying method,and SiC was added as a reinforcing phase in the sprayed composite powder.In this paper,four kinds of raw materials such as plasma sprayed ZrB₂-ZrC-Al₂O₃ and ZrB₂-ZrC-Al₂O₃-SiC and reaction system ZrO₂-B₄C-Al and ZrO₂-B₄C-Al-SiCwere studied.The microstructure and properties(hardness,toughness,friction and wear properties,thermal shock resistance,high temperature oxidation resistance and ablation resistance)of ZrB₂-ZrC-Al₂O₃ composite coatings prepared by the raw material system were studied.The thermodynamic reaction conditions of ZrO₂-B₄C-Al-SiCsystem were calculated.The reaction mechanism of ZrO₂-B₄C-Al-SiC system in plasma spraying process and the formation mechanism of coating are revealed.The coatings obtained by plasma spraying ZrB₂-ZrC-Al₂O₃ and ZrB₂-ZrC-Al₂O₃-SiC did not undergo obvious phase change,only partial oxidation occurred,the coating had no obvious layered structure,and the coating morphology showed a coated and coated morphology.There are many holes and the density is low.The two reaction systemsof ZrO₂-B₄C-Al and ZrO₂-B₄C-Al-SiC coatings reacted during the spraying process,and ZrB₂ and ZrC were formed.The coating structure was dense and the coating showed obvious river pattern,and the coating quality was significantly better than the two direct spraying system coatings;The addition of phase SiC can inhibit grain growth and make the coating grains finer.The reaction mechanism of the ZrO₂-B₄C-Al-SiC system is: the first melted Al in the composite powder wraps ZrO₂,B₄ C and SiC particles,and the molten Al reacts with ZrO₂ particles to form Al₂O₃ and active Zr atoms,then the active Zr atoms react with B₄ C to form ZrB₂ and ZrC,while SiC is present in these new phases as a second phase.The reaction product is impinged on the surface of the substrate at a high speed under the carrying of a high-speed,high-heat plasma flame stream,and is deposited on the surface of the substrate to form a coating.The results show that the microhardness of ZrB₂-ZrC-Al₂O₃-SiC composite coating is the highest(1280.85 HV0.1),and the dispersion of ZrO₂-B₄C-Al-SiC coating is the best(9.54).The composite coating of ZrO₂-B₄C-Al-SiC system has the best toughness,and the addition of SiC plays a positive role in improving the toughness of the coating.Moreover,the friction and wear properties and thermal shock resistance of the ZrO₂-B₄C-Al-SiC composite coating are superior to those of the other three composite coatings.The oxidation resistance of ZrO₂-B₄C-Al-SiC and ZrO₂-B₄C-Al systems is better than that of ZrB₂-ZrC-Al₂O₃ and ZrB₂-ZrC-Al₂O₃-SiC systems.The four system composite coatings exhibit different ablation mechanisms under plasma ablation.The ablation resistance of the ZrB₂-ZrC-Al₂O₃-SiC system coating is better than that of the other three systems.The addition of SiC improves the ablation resistance of the ZrB₂-ZrC-Al₂O₃ coating,but does not improve the ablation resistance of the coating of the reaction system.