Preparation Of Graphite Ceramic Coatings

Graphite and its composite materials which exhibited excellent physical and chemical properties in the metallurgical,mechanical,chemical,aerospace and other fields are widely used.With the continuous development of science and technology,we put forward higher requirements on the performance of graphite materials.The graphite materials are easily oxidized in the high temperature oxygen atmosphere so that affected the using of graphite materials at high temperatures seriously.Therefore,the preparation of anti-oxidation coating which prevented the diffusion of oxygen into the interior of the graphite substrate at high temperatures and extended the time of graphite materials at the high temperature of oxygen atmosphere has become the main research direction.In this paper,the research status and problems of the preparation of anti-oxidation coatings on the surface of graphite substrates by gel gel method,chemical vapor deposition method and slurry method were reviewed and the main research directions were put forward.The SiC/Si₃N₄ composite coating was prepared on the surface of graphite substrate by means of chemical vapor deposition and slurry method.First,a layer of SiC coating was prepared on the surface of graphite by chemical vapor deposition.The effects of deposition temperature,deposition time and the flow rate of mixed gas on the densification and oxidation resistance of SiC coating were discussed.Then,a single SiC coating was prepared on the surface of graphite by slurry method.The effect of sintering temperature on the densification and oxidation resistance of SiC coating was discussed.On the basis of this,the effect of sintering temperature on microstructure and compactness of Si₃N₄ coating prepared on the surface of SiC coating by slurry method and the effect on the densification and antioxidant properties of graphite / SiC / Si₃N₄ composites were studied.The results show that the temperature of deposition affected the rate of deposition and the size of SiC grains directly.In addition the temperature of deposition has great influence on the morphology and strength of the coating.In the deposition temperature range of 900 ? 1200 ?,the rate of deposition increases with the deposition temperature increasing,and the thickness of SiC coating increases from 11.72?m?900 ??to 14.70?m?1200 ??),the bonding strength of SiC coating and graphite become more closely.The mass loss ratio of the samples decreased from 21.21% to 13.87% after oxidation in air at 1200 ? for 20 h.The flow of trichloromethylsilane?MTS?+ H₂ mixed gas has a great influence on the morphology and size of SiC particles.When the mixed gas flow increases from 80 L / h to 140 L / h,the particle size of the SiC coating increases,and the surface of the coating is dense and smooth.If the mixed gas flow is too large,the roughness of Surface increases.The hardness of the sample decreases with the increase of the mixed gas flow from 1.18 GPa to 1.31 GPa and then decreases to 1.27 GPa.When the mixed gas flow rate is 120 L / h,the porosity reached a minimum value of 25.54%.When the mixed gas flow rate is 120 L / h,the mass loss ratio of the sample reaches a minimum value of 13.87%.The time of deposition mainly affected the thickness of the coating and the size of the SiC particles.Under the deposition temperature of 1200 ?,the flow of Ar is 120 L / h and the flow of mixed gas is 120 L / h,the thickness of SiC coating increases from 13.64?m to 80.70?m with the time of deposition from 2 h to 6 h.The porosity of the sample decreased from 18.54% to 9.32%,the microhardness increased from 1.93 GPa by 0.83 GPa,and the oxidation weight loss rate decreased from 13.87% to 6.23% after 20 h in the oxidation atmosphere at 1200 ?.The porosity of the samples prepared by the slurry method increased with the increasing of the sintering temperature,and the porosity of the samples decreased first and then decreased from 1550 ? to 43.4%.The porosity was 29.82%,and then increased to 1750 ? porosity of 32.56%.The mass loss vatio of the sample decreases with the increasing of the sintering temperature.When the sintering temperature reaches 1650 ?,the mass loss vatio of the sample reached the lowest value of 39.25% after oxidation in air.The Si₃N₄ coating prepared by slurry method can improve the oxidation resistance of the sample effectively.The mass loss vatio of the sample which have a single SiC coating prepared by the slurry method was reduced from 39.25% to 27.31% after oxidation in air.The mass loss vatio of the sample which have a single SiC coating prepared by chemical vapor deposition was reduced from 13.87% to 5.85% after oxidation in air.