Preparation Of Cubic Silicon Carbide Coatings By Chloride-based Chemical Vapor Deposition

Cubic silicon carbide??-SiC?ceramic coatings possess excellent radiation resistance when facing high temperature plasma,high flux neutron and severe physical and chemical damage,it will also play an important role in improving the safety and reliability as the first-wall materials for magnetic nuclear fusion power systems.Herein,a chloride-based chemical vapor deposition?CVD?technique is utilized for?-SiC coatings fabrication.The aim of this study is to fast fabricate stoichiometric,millimeter-sized thickness?-SiC coatings with a controllable preferred orientation and microstructure,and to provide the theoretical basis for its application in first-wall materials.In present study,the effects of deposition parameters,including deposition temperature(T_dep),total pressure(P_tot),molar ratio of C and Si precursor(R_C/Si)and carbon sources,on the preferred orientation,microstructure,crystallinity and deposition rate(R_dep)were systematically investigated.The key factors that affecting the above results,the formation mechanism of fiber oriented SiC were also analyzed,and the relationship between the preferred orientation and its microstructure was established.Results indicated that the preferred orientation of SiC is mainly affected by T_depep and P_tot,it's easy to obtain<110>-oriented SiC under high T_depep and low P_tot,while<111>-oriented SiC is easily obtained under opposite condition.In contrast,it showed no obvious influence when changing R_C/Si/Si and carbon sources.With increasing T_dep,the microstructure of SiC changed from the compact dense thin films to the cauliflower-like equiaxed structure,and then to the roof-like columnar structure.The reduction of P_totot can lead to the formation of loose SiC whisker.The high R_C/Si/Si is in favorable for the development of thin columnar to thick columnar crystal with a roof or ridge-like surface.The crystallinity of SiC is improved with increasing T_dep,and become the finest when P_tot=4 kPa.The most appropriate R_C/Si/Si for the growth of stoichiometric SiC with high crystallinity is supposed to be in the range of 0.86 to1.00,a high or low R_C/Si/Si will increase the defect concentration in terms of massive twins.C₂H₂ is suitable for the growth of good crystallinity<110>-oriented SiC at high T_dep,while CH₄ and C₃H₈ are for<111>-oriented SiC growth under low T_dep.The R_depep of SiC coatings with the T_depep can be expressed in terms of Arrhenius equation,when T_dep?1723 K,the growth of SiC is controlled by gas phase chemical reaction regime?CRR?,R_depep increases exponentially with T_dep.However,when T_depep rises to above 1723¹773 K,surface mass transport regime?MTR?is in charge,R_depep increase slowly or even drop down when R_C/Si?0.86 due to the higher etching rates.Particularly,for P_tot=4 kPa,0.86?R_C/Si?1.00,the R_depep of SiC coatings will reach200¹227?m h^-1(1673?T_dep?1773 K).Finally,through analysis the fiber orientation of<111>-and<110>-oriented SiC based on twin planes reentrant corner effect?TRPE?,it was found that<110>-oriented SiC showed a high formation rates and a more coarse surface microstructure than that of<111>-oriented SiC.The main reason may include that,according to Langmuir diffusion model,the?110?plane can provide greater atomic adhesion probability,and more twin plane reentrant corner growth sites with a lower surface formation energy.In general,the hexagonal pyramid structure formed by the reverse domain is a typical feature of the<111>-oriented SiC grains,while the<110>-oriented SiC mainly exhibits a four-fold pyramid or a ridge-like structure.