Study On Chromium Diboride Matrix Composite Coatings

Chromium boride(CrB₂)has wide application prospects in refractories,hard tool materials and wear-resistant parts due to its excellent wear resistance,corrosion resistance,high hardness and high temperature oxidation resistance.However,there are few studies on chromium boride coating prepared by plasma spraying at home and abroad.Hence,microstructure and properties of CrB₂-based composite coatings prepared by Direct Plasma Spraying CrB₂-Cr₃C₂and CrB₂-Cr₃C₂-Al₂O₃and Reactive Plasma Spraying Cr-B₄C and Cr₂O₃-B₄C-Al were studied comparatively based on the principle that the reactions between Cr and B₄C,Al,Cr₂O₃and B₄C can respectively take place in this paper.And reaction mechanism of Cr-B₄C and Cr₂O₃-B₄C-Al systems in plasma spraying process and formation mechanism of composite coatings were revealed.In addition,the effects of raw material powder ratio on the structure and properties of composite coatings in reactive plasma spraying Cr₂O₃-B₄C-Al system were studied.The results show that phase of the coating obtained by direct plasma spraying of CrB₂-Cr₃C₂and CrB₂-Cr₃C₂-Al₂O₃composite powders is basically the same as that of the raw material powders,and only a small amount of CrB₂and Cr₃C₂have been decomposed and oxidized.While main phases of plasma sprayed composite powders of Cr-B₄C system are CrB₂,CrB,Cr₃C₂and Cr₇C₃,with a certain amount of residual raw materials Cr and B₄C.And main phases of plasma sprayed composite powders of Cr₂O₃-B₄C-Al system are CrB₂,Al₂O₃,Cr₃C₂and CrB.It shows that the reactions between raw materials are quite sufficient.Comparing coatings structure of four systems,it is found that the coatings obtained by plasma spraying Cr₂O₃-B₄C-Al system composite powders have uniform and compact structure,obvious layered structure,and the quality of the coatings is obviously better than that of the other three systems.The reaction mechanism of Cr-B₄C system in plasma flame can be described by mechanism that"solid state reaction-melting-diffusion-reaction".Solid-state reaction of Cr and B₄C composite powders first occurs before melting by using the heat of plasma flame flow.After the melting of Cr powder,CrB₂and Cr₃C₂are formed by diffusion reaction of B₄C particles coated with Cr.Under the action of high temperature flame flow,most of the CrB₂and Cr₃C₂particles melt to form eutectic structure,while some of the unmelted particles disperse in the gap of coating structure.The reaction mechanism of Cr₂O₃-B₄C-Al is"melting-diffusion-reaction",that is,Al melts firstly,then Cr₂O₃and B₄C particles were wrapped by melted Al in plasma flame,and liquid Al reacts with Cr₂O₃particles to form Cr and Al₂O₃.Ultimately,CrB₂and Cr₃C₂are formed by reaction of Cr and B₄C.Eutectic structure was formed by melting CrB₂,Cr₃C₂and Al₂O₃particles under the action of exothermic reaction and dual heat of plasma flame flow.By comparing the structure and properties of the coatings obtained from different raw materials,it is found that when Cr₂O₃:B₄C:Al=62:16:22,the reaction of the three raw materials is complete,and the coatings obtained have better quality,the lowest porosity and the highest hardness.Comparing with CrB₂-Cr₃C₂,Cr-B₄C and CrB₂-Cr₃C₂-Al₂O₃coatings,it is found that the coatings prepared by reactive plasma Cr₂O₃-B₄C-Al system have higher hardness(1230.15HV_0.1)and toughness,scratch resistance.Because of the relatively high content of Al₂O₃phase produced by the reaction,its friction and wear properties are not as good as the Cr-B₄C system.The thermal shock life of Cr₂O₃-B₄C-Al coatings reached 304 times under water quenching at 850oC and the structure of the coatings expanded less.The failure of the coatings was caused by the oxidation cracking of the bottom layer,which had obvious better thermal shock resistance.At 850oC and 1000oC,the oxidation weight gain of Cr₂O₃-B₄C-Al coatings is relatively large and the structure after oxidation is more compact.So,the protection of the internal structure of the coatings is improved,and the oxidation depth is not obvious.Therefore,the high temperature oxidation resistance of Cr₂O₃-B₄C-Al coating is better.