Study On Preparation Technology And Friction Properties Of SiC Coatings By CVD Method

Graphite is widely used in the graphite components of high temperature gas-cooled reactors because of its high melting point,excellent nuclear properties,mechanical and chemical stability.During the operation of the reactor,frequent friction between graphite components may lead to the generation of a large amount of debris,which will block pipes,affect equipment maintenance,and even lead to the failure of graphite components,which will affect the normal operation of the whole reactor.So it is of great significance to study its friction and wear.At the same time,because of its light weight and high strength,C/C composite materials are widely used in friction fields such as braking,but its direct application is limited by its low friction coefficient and no wear-resisting.As a kind of excellent wear-resisting material,SiC coating has good nuclear properties.And SiC coating has good compatibility with graphite and C/C composites.Therefore,SiC coating is an ideal candidate material to effectively solve the above problems.SiC coatings were prepared on graphite surface by chemical vapor deposition（CVD）using SiCl₄-CH₄-H₂-N₂ system.Firstly,the influences of deposition temperature,deposition pressure and C/Si molar ratio on deposition rate,phase composition and microstructure of CVD-SiC coating were systematically studied.Based on this,the influences of deposition temperature,deposition pressure and C/Si molar ratio on the tribological properties of SiC coating were investigated.The main conclusions are as follows:（1）The deposition rate and crystallinity of SiC coatings increase first and then decrease with the increase of deposition temperature.The silicon phase in the coating gradually disappears with increasing temperature.With the increase of deposition temperature,the coating changes from fibrous to cellularity,and the SiC grains grow to<111>preferred orientation when the temperature reaches a certain level.If the temperature continues to rise,the substrate surface will appear the phenomenon of grain accumulation.（2）With the increase of deposition pressure,the deposition rate and crystallinity of SiC coatings increase first and then decrease.Under the influence of the concentration of reactive gas and the game between"loss effect",the SiC coating particles turn into fine leaf-like,four-ridged,hexagon-like spheroid and sheet structures with the increase of deposition pressure.（3）The deposition rate of SiC coating increases with the increase of the C/Si molar ratio.When the C/Si molar ratio is 1.4:1,the preferred orientation of SiC coating<111>is very obvious,and the crystallinity reaches 89%.（4）The tribological properties of SiC coatings are greatly affected by the preparation process and testing conditions.The high crystallinity of SiC coating usually has high friction coefficient and low wear rate,and low crystallinity or impurity phase may reduce the friction coefficient to a certain extent.The tribological and adhesive wear mechanism of SiC coating will be aggravated under the influence of load and ambient temperature.