Preparation And Properties Investigation Of TaC Coated Graphite By Disproportionation Reaction In Molten Salts

Graphite,as crucible and other supporting materials commonly used in the preparation of the third generation semiconductor,has an important effect on the performance of the prepared semiconductor.Graphite is currently the most widely used material for semiconductor preparation,but it still has shortcomings such as releasing impurities and reacting with the reactant gas source during use,which affects the performance and benefits of the prepared semiconductor.As an ultra-high temperature ceramic material,TaC has good chemical stability in high temperature and strong corrosive environment,and exhibits excellent protective performance in the preparation of third-generation semiconductors.Combining the advantages of the molten salt method in the preparation of complexshaped components,this topic obtained TaC pre-coating through disproportionation reaction in the molten salt,and obtained TaC coating material after further vacuum high temperature heat treatment.The effects of molten salt process parameters and the characteristics of the graphite substrate on TaC pre-coating were studied,and the reaction mechanism of forming the TaC pre-coating by disproportionation reaction in molten salt was discussed.The changes of precoating before and after vacuum heat treatment were analyzed,and the intrinsic properties of TaC coating were characterized.At the same time,the corrosion resistance of TaC coating in SiC corrosion environment and SiC crystal growth environment was tested,providing reference for the preparation of TaC coating on graphite surface.(1)The reaction system of TaC pre-coating prepared on the graphite surface was designed,with K2TaF7 and Ta as reactants in NaCl and KCl molten salt.The effects of reaction temperature,reaction time,ratio of reactants K2TaF7 and Ta,and molten salt content on the preparation of TaC pre-coating were studied.It was found that the thickness of TaC pre-coating increased with the increase of reaction temperature,reaction time and Ta content in reactants,and the change of molten salt content had no obvious effect on TaC pre-coating.When the reaction temperature was 1200℃,holding for 3 h,the ratio of reactants was K1Ta3,the mass ratio of mixed salt and Ta source(K2TaF7 and Ta)was 5:1,a continuous,uniform TaC pre-coating bonded tightly with the graphite substrate can be obtained.The influence of different pretreatment methods on the graphite surface on the TaC pre-coating was explored,and it was found that the thickness of the TaC pre-coating could be effectively increased after the surface pretreatment.(2)The phase structure of TaC pre-coating was studied by XRD,and the reaction mechanism of the TaC pre-coating was analyzed.TaC pre-coating was composed of TaC phase and Ta2C phase.In the molten salt,The TaF72-ions from the decomposed K2TaF7 reacted with Ta in molten salt to form TaF52-ions,which diffused to the surface of graphite sbustrate.The disproportionation reaction of TaF52-occurred on the surface of the graphite substrate.Part of the reaction product formed TaF72-,and part of the reaction product formed TaC with the graphite sbustrate.Ta in molten salt reacted with the generated TaC to form Ta2C.Therefore,there are both TaC phase and Ta2C phase in the TaC pre-coating.As the reaction proceeded,the TaC pre-coating gradually thickened and the content of C in the surface layer of the TaC pre-coating was less.At this time,Ta was more likely to react with TaC to form Ta2C.(3)After the TaC pre-coating prepared by disproportionation reaction in the molten salt was heat-treated at 2300℃ in a vacuum environment for 3 h,the final TaC coating can be obtained.The coating had no other phase but TaC and exhibited the growth characteristic of free orientation.The surface quality of the coating was good,without penetrating cracks and perforations.The heat treatment process also helped to remove impurities such as residual salts in the graphite substrate.(4)The mechanical properties of the TaC coating were characterized.The bonding strength between the TaC coating and the graphite substrate was measured by the pull-off method.The bonding strength was about 9.49 MPa.The hardness and elastic modulus of the TaC coating characterized by nanoindentation were 14.42 GPa and 123.32 GPa,respectively,and the Weibull distribution analysis of the hardness and elastic modulus of the TaC coating was carried out.(5)The corrosion resistance of TaC coating in SiC corrosion environment was tested in the simulated environment in the laboratory and SiC crystal growth environment.The coating showed good service performance in a SiC crystal growth environment(above 2000℃for 150 h,of which 130 h was above 2300℃).The overall structure of the surface was complete,and only the edges and corners were peeled off.The coating mass loss rate was about 0.41 g/m2·h.