Study On Reaction Plasma Synthesis Mechanism And Spraying Technology Of ZrB₂-SiC-ZrC Coating

ZrC is widely used in important fields such as aerospace due to its excellent performance.ZrB₂-SiC-ZrC composite coating has excellent oxidation resistance and ablation resistance.In order to solve the rapid oxidation behavior of ZrC at high temperature,raw powders such as Zr H₂,Si and B₄C were used in this paper.Reactive plasma spraying（RPS）technology was employed to prepare ZrB₂-SiC-ZrC composite coating on ZrC substrate.The effect of different processing parameters on the phase compositions and the microstructures of the coating was studied.At the same time,the reaction mechanism of RPS ZrB₂-SiC-ZrC coating was analyzed by combining thermal analysis technology with the quenching test which the RPS products at different flight distances was obtained.In this paper,the original powder was mixed by a ball milling process,and then mixed powders was sprayed to obtain a spherical powder with a particle size of 50μm to 100μm.DSC analysis results showed that the system was endothermic from 326°C to 738°C,and Zr H₂ was decomposed state.Zirconium oxide（Zr₃O）exists at 700°C to 1100°C due to the presence of oxygen-containing substances in the system.With the increasing of the temperature,C atoms were freed from B₄C,and thus Zr₃O was reduced by C to form ZrC.At the same time,a small amount of Si melted and reacted with Zr to form Zr Si.When the temperature continued to increase,B from B₄C entered into Zr-Si-C system to form Zr-Si-C-B system,and small-sized ZrB₂ gradually aggregated and grew.The phases and microstructures of the particles at different quenching distances were analyzed.The results show that the four XRD curves appeared similar shapes,and they were composed of Zr O₂ and ZrB₂.However,the main phase was Zr O₂,which was caused by the oxidation of the powders after they entered the water.The melting degree of the particles was different during the plasma spraying process due to the size difference of the original particles,and thus the wall thicknesses of the agglomerated particles was also different.The phase compositions of the coatings showed no change with different spray current.The phase types did not change when the spraying distance increased,but the content of the phase was various.The surface,cross and fracture sections of the coating were observed.It was found that the spraying powder melted more fully,and the coating surface was smoother when the spray current was increased.The disc-like splats were increased,and the coating was denser.The microcracks and pores were reduced,and the porosity of the coating decreases.When the spraying distance increased,the spraying powder melted more fully,and the disc-like splats on the coating surface increased.However,the impact energy and impact speed of the spraying powder would be reduced when they reached the substrate.The coating was loose,and the number of holes increased,resulting in that the porosity increased and the densification density was low.Therefore,it was concluded that the optimal spraying process parameters are:spray current was 600A and spray distance was 80mm,according to the parameters range in this paper.