ZrB₂-ZrC-Matrix Composite Coating

The ZrB₂-ZrC-based composite coating,because of the excellent mechanical property,good chemical stability and high-temperature stability,can be applied to the the important part of which working temperature is very high,such as the nose cone of the intercontinental missile warhead,the solid rocket nozzle,the nose cone of the spacecraft and the leading edge of the wing.However,with the increasing speed of the aircraft,the operating temperature of these parts is also higher and higher,even more than 2000°C.Therefore,there are more higher requirements on the high temperature resistance of the ZrB₂-ZrC based composite coating.At present,plasma sprayed ZrB₂-ZrC based composite coatings have many problems such as high porosity,low density,poor oxidation resistance and corrosion resistance at high temperature.The quality and properties of the coatings need to be improved.In order to solve these problems,in this paper,four reaction systems including Zr-B₄C,Zr-B₄C-SiC,Zr-B₄C-Al and Zr-B₄C-Al-SiC were designed to prepare the ZrB₂-ZrC based composite coating by reactive plasma spraying（RPS）.Comparative study on micro-structure and Properties of ZrB₂-ZrC coating prepared by Direct Plasma spraying were conducted to help analyse the effects of Al and SiC additives on micro-structure and properties（hardness,toughness,high temperature oxidation resistance,ablation resistance and thermal shock resistance）of ZrB₂-ZrC based composite coatings.The formation mechanism of the six kinds of the ZrB₂-ZrC based composite coatings were revealed.The ZrB₂-ZrC based composite coating prepared by the spraying system has more defects,whose structure is loose and density is low.The reactive spraying systems go through three reaction stages:solid-solid reaction,liquid-solid reaction and liquid-liquid reaction to form the ZrB₂-ZrC based composite coatings,in which the ZrB₂and ZrC are uniformly distributed,and the structure is dense.The quality of the coating prepared by RPS is obviously better than that prepared by direct plasma spraying.Fine grain structure appeared in the ZrB₂-ZrC composite coatings prepared by RPS with Zr-B₄C-Al powder and Zr-B₄C-Al-SiC powder,the quality of which was obviously superior to that of other coatings.The additive Al,SiC can refine the grain and improve the quality of coating obviously.The hardness and toughness of the coating prepared by RPS are higher than that prepared by direct spraying system.The ZrB₂-ZrC based composite coatings prepared by RPS using Zr-B₄C-Al and Zr-B₄C-Al-SiC systems show the best hardness and toughness.Al additives can improve the hardness and toughness of ZrB₂-ZrC based composite coatings significantly.Among the six kinds of coatings studied in this paper,the ZrB₂-ZrC based composite coatings prepared with Zr-B₄C-Al-SiC system have the best resistance to oxidation and ablation at high temperature.SiC additive can obviously improve the high temperature performance of the coating.The coating of Zr-B₄C-Al system has good thermal shock resistance as well as the coating of Zr-B₄C-Al-SiC system.Al,SiC can significantly improve the thermal shock resistance of ZrB₂-ZrC based composite coatings.