Study On The Microstructure And Properties Of Cvd-SiC Coating On The Surface Of Graphite Base

As a key component of the metal organic chemical vapor deposition equipment,the graphite base will fail due to the corrosion off powder during the service process,and the fallen powder will also contaminate the chips.Surface coating technology is an effective means to solve this problem.Silicon carbide（SiC）has a series of excellent properties such as good thermodynamic stability,excellent thermal conductivity,oxidation resistance and corrosion resistance,combined with the good coverage and controllability of chemical vapor deposition（CVD）technology,CVD-SiC coating becomes an ideal choice for protective coating on the surface of graphite base.In this paper,low-pressure chemical vapor deposition method was used to prepare SiC coating on the surface of high-purity graphite substrate,combined with analysis and testing methods such as thermodynamic analysis,scanning electron microscopy,energy spectrum analysis,XRD analysis,oxidation,vacuum thermal cycling,surface roughness,and the changes of the main gas phase components during the SiC deposition process were analyzed,the effects of deposition process parameters on structure and performance of the coatings were systematically studied,the formation mechanism of the preferred orientation of SiC coating was discussed,and the relationship between the preferred orientation of SiC coating and the surface morphology of the coating,and the relationship between the performance and microstructure of the coating were analyzed.The main conclusions are as follows:（1）The main gas phase components in the deposition process of SiC were CH₄,Si HCl₃ and SiCl₄ at low temperature,and SiCl₂ and C₂H₂ at high temperature.（2）The deposition rates of coatings increased rapidly with the increase of temperature,and was controlled by the surface chemical reaction.At this time,β-SiC was easy to grow along the（111）crystal plane,thus forming the<111>preferred orientation.With the increase of the deposition temperature,the preference for the<111>growth direction of the grains gradually increased,and its sandy growth characteristics also increased.As the deposition temperature increased,the average grain sizes of the coatings increased,and the variability of the grain sizes increased,resulting in an increase in the surface roughness of the coatings.When the deposition temperature was low,the coating was rich in carbon,and its oxidation resistance was poor.As the temperature increased,the coating had no obvious defects,and its oxidation resistance increased.As the temperature further increased,the coating appeared to be demarcated,its anti-oxidation performance decreased.（3）With the increase of the distance between the substrate and the air distribution plate,the preference for the<111>growth direction of the grains first strengthened and then weakened,and the gravel-like growth characteristics also showed a changing law that first strengthened and then weakened.When the substrate position was in the range of 50～650 mm,as the distances between the substrates and the air distribution plate increased,the average grain sizes and the difference in the grain sizes of the coatings surface increased,resulting in an increase in the surface roughness of the coatings;When the distance increased to 850 mm,the gravel-like growth characteristics of the grains were weaker,and the surface roughness was smaller.As the distances between the substrates and the air distribution plate increased,the concentration of nucleating substances decreased,the etching effect of HCl and the depletion effect of reactants increased,resulting in a decrease in the deposition rate of the coating.When the substrate was closer to the air distribution plate,the structure of coating became loose.As the distances increased,the loose structure disappeared,and the oxidation resistance and thermal fatigue resistance of the coatings increased.With the further increase of the distances,the demarcation and carbon richness began to appear,the oxidation resistance and thermal fatigue resistance of the coatings greatly reduced.（4）As the deposition pressure increased,the preference for the<111>growth direction of the grains first strengthened and then weakened,and theirs gravel-like growth characteristics also changed accordingly.When the deposition pressure was in the range of 0.5～5 k Pa,as the deposition pressures increased,the average grain sizes and the difference in the grain sizes of the coatings increased,resulting in an increase in the surface roughness of the coatings.When the pressure increased to 20 k Pa,the gravel-like growth characteristics of the grains were weaker,and the surface roughness was smaller.With the increase of deposition pressure,on the one hand,the increase of reactant concentration would increase the deposition rate;on the other hand,the increase of reactant depletion effect and the decrease in the diffusion rate of the gas-phase reactants to the surface of the substrate would decrease the deposition rate,the opposite effects of these two aspects made the deposition rates present a non-monotonic change that first rose,then fell and then rose.When the deposition pressure was small,the coating had a boundary phenomenon.As the pressure increased,the boundary phenomenon disappeared,and its anti-oxidation and thermal fatigue resistance properties increased.When the deposition pressure increased to 5 k Pa,the coating became thinner and the boundary phenomenon appeared again,and its performance became worse.As the deposition pressure further increased to 20 k Pa,the boundary was replaced by layers,and its properties of anti-oxidation and thermal fatigue resistance rebounded slightly.（5）With the increase of the molar ratio of H₂/MTS,the preference for the<111>growth direction of grains first strengthened and then weakened,causing the gravel-like growth characteristics of grains to increased first and then weakened.As the molar ratio of H₂/MTS increased,the average grain sizes of the coatings decreased,and the variability of the grain sizes decreased,resulting in a decrease in the surface roughness of the coatings.As the molar ratio of H₂/MTS increased,the MTS concentration per unit volume decreased,which in turn caused the deposition rates of coatings to decrease.As the molar ratio of H₂/MTS increased,the oxidation resistance and thermal fatigue resistance of the coatings both increased first and then decreased,when the molar ratio of H₂/MTS was 15,the performance of the coating was better.