Preparation And Performance Study Of Graphite Based On Surface Modified SiC Coatings

With the booming development of third generation semiconductors,gallium nitride（GaN）has an important role to play in the field of LED lighting.Currently GaN-based LED epitaxial growth mainly uses Metal Organic Chemical Vapor Deposition（MOCVD）technology.As an important consumable for MOCVD equipment,the graphite base is susceptible to powder failure due to corrosion by the reactive atmosphere NH₃ during using,so a coating needs to be prepared on the graphite base surface.Silicon carbide（SiC）coating is the material of choice for graphite base surface coating due to its good oxidation resistance,thermal shock resistance,thermal conductivity,chemical stability and matching the thermal expansion coefficient of graphite base.In this paper,SiC coatings were prepared on the surface of graphite substrates using trichloromethylsilane（CH₃SiCl₃,MTS）as the raw material and the Chemical Vapour Deposition（CVD）technique with the MTS-H₂ reaction system.In the first part of the experiments,the SiC coatings were prepared under different parameters to investigate the influence of process parameters such as deposition position,deposition temperature,H₂ to MTS flow ratio and deposition pressure on the coatings.In the second part of the experiment,the graphite substrate was first modified with oxygen plasma and then the SiC coating was prepared.The effect of the graphite surface oxygen plasma modification on the properties of the CVD SiC coating was investigated by means of XRD,SEM,Roman,nanoindentation,oxidation analysis and thermal shock analysis.In this thesis,it is concluded that the deposition rate of CVD SiC coating decreases as the graphite substrate is further away from the gas inlet,the<111>optimal orientation of SiC grains is weakened,and the denseness of the coating decreases.As the deposition temperature rises,the deposition rate of the CVD SiC coating gradually increases and the<111>selective orientation of the SiC grains increases,but when the deposition temperature reaches 1300°C,the SiC is broken and the denseness of the CVD SiC coating decreases;as the deposition pressure increases,the deposition rate of the CVD SiC coating first increases and then decreases,and the shape of the SiC coating changes from cusp-like to gravel-like.The denseness of the CVD SiC coating decreases continuously;when the H₂/MTS flow rate is low,more free carbon appears in the CVD SiC coating due to the lower H₂ concentration and more precursors of carbon decomposed by the MTS.As the H₂/MTS flow rate ratio increases,the free carbon gradually decreases and the denseness of the coating increases.However,when the H₂/MTS flux ratio reaches4.5,the denseness of the coating decreases.In addition,it is concluded that after the oxygen plasma treatment of the graphite substrate,the surface activity of the graphite substrate increases,which facilitates the deposition reaction and improves the denseness of the CVD SiC coating.In addition,after oxygen plasma treatment,the number of hydrophilic oxygen-containing functional groups on the graphite substrate surface increases,which effectively improves the hydrophilicity and adsorption of the graphite substrate surface and increases the bonding strength between the CVD SiC coating and the graphite substrate.The microhardness,Young’s modulus,oxidation resistance and thermal shock resistance of the CVD SiC coated specimens were significantly improved after the oxygen plasma treatment on the graphite surface.