Study On Niobium Diboride Composite Coatings

NbB₂has excellent properties such as high melting point,high strength,high electrical conductivity and high chemical stability,which give NbB₂broad application prospects.However,the current preparation of NbB₂coating has the disadvantages of low deposition efficiency,difficulty in controlling the thickness of the coating,and uneven coating structure.In this paper,NbB₂composite coatings were prepared by plasma spraying NbB₂-NbC and NbB₂-NbC-Al₂O₃composite powders and plasma spraying Nb-B₄C and Nb₂O₅-B₄C-Al composite powders.Comparative study on the microstructure and properties（hardness,toughness,scratch resistance,friction and wear properties,high temperature corrosion resistance,thermal shock and ablation resistance）of composite coatings prepared from four system materials.Reveals the formation mechanism of NbB₂composite coatings.And studied the effect of raw material composition on the microstructure and properties of Nb₂O₅-B₄C-Al composite coating.The phase of the coating prepared by plasma spraying NbB₂-NbC and NbB₂-NbC-Al₂O₃composite powders did not change significantly.However,a significant reaction occurred during the plasma spraying of Nb-B₄C and Nb₂O₅-B₄C-Al composite powders.The main phases of Nb-B₄C system were NbB₂and NbC,and the main phases of Nb₂O₅-B₄C-Al coating were NbB₂,NbC andγ-Al₂O₃.The coating of the plasma sprayed Nb₂O₅-B₄C-Al composite powders is uniform and dense,and the fine-grained NbB₂and NbC are dispersed,and the coating quality is superior to the other three system coatings.When the mass ratio of Nb₂O₅:B₄C:Al is 64:13:23,the obtained coating has the most NbB₂content,and the coating has lower porosity,higher hardness,better toughness and friction and wear properties.Only simple melt-deposition and no phase change occurred during the coating process of plasma sprayed NbB₂-NbC and NbB₂-NbC-Al₂O₃composite powders.The reaction of the plasma sprayed Nb-B₄C composite powders during the coating process is obvious,and the formation mechanism is mainly solid phase diffusion-reaction-melting-deposition:Nb and B₄C undergo solid phase diffusion reaction to form NbB₂and NbC,NbB₂and NbC are melted by reaction exothermic and high-speed gas flow and deposited on the surface of the substrate to form a coating.The formation mechanism in the coating process of plasma sprayed Nb₂O₅-B₄C-Al composite powders is melt-diffusion-reaction-deposition:heat-melted Al encapsulates Nb₂O₅and B₄C particles,and forms Al₂O₃and active Nb atoms by solid-liquid diffusion reaction.And the active Nb atom reacts with B₄C to form NbB₂and NbC,the melt impinges on the surface of the substrate under the action of a high velocity gas stream and rapidly cools to solidify to form a coating.Compared with the NbB₂composite coatings obtained by plasma spraying NbB₂-NbC,Nb-B₄C,NbB₂-NbC-Al₂O₃composite powders,the NbB₂composite coating obtained by plasma spraying Nb₂O₅-B₄C-Al composite powders has the highest hardness(1231.4 HV_0.1)and better hardness uniformity,toughness and scratch resistance.The coating obtained by plasma spraying Nb₂O₅-B₄C-Al composite powders has the lowest and stable friction coefficient and the minimum wear rate,showing the best friction and wear performance.Comparing the high temperature performance of the composite coatings of the four systems,it is found that the coatings obtained by plasma spraying Nb₂O₅-B₄C-Al composite powders have better high temperature corrosion resistance,thermal shock resistance and ablation resistance than those of the other three system composite coatings.