| It is important to improve the oxidation resistance of graphite materials at high temperature in industrial production.In this paper,in order to improve the oxidation resistance of graphite materials at high temperature,SiC was used as the modified material.SiC matrix modified graphite was prepared by adding SiC or Si into graphite raw material,and the oxidation resistance of graphite was improved preliminarily.On this basis,the SiC coating was prepared on the surface of SiC matrix modified graphite by embedding silicon infiltration method.The SiC matrix-coating modified graphite material was prepared,further improving the oxidation resistance of graphite material.In order to solve the problems of hard machining caused by more residual Si on the surface of SiC coating prepared by the embedded silicon infiltration method,the BN powder was introduced into the silicon source used in the embedding method to solve the problem of excessive adhesion of Si on the surface of SiC coating.The effects of raw material ratio,molding pressure,isostatic pressing pressure and Si doping on microstructure and properties of SiC modified graphite materials were investigated.At the same time,by optimizing the design of silicon source,the influence of sintering temperature,sintering vacuum degrees and matrix porosity on the microstructure and properties of SiC coating were studied by optimizing the design of BN powder doped in the silicon source used in the embedding method.The main conclusions were as follows:The effect of raw material ratio on the composition and performance of SiC modified graphite material was studied by orthogonal test.The results showed that the graphite dosage was the main factor influencing SiC modified graphite material porosity and bending strength of the SiC dosage was the main factor influencing SiC resistivity of graphite materials and antioxidant strength.Excessive amount of phenolic resin would increase the porosity of the material,reduce the oxidation resistance and the flexural strength of the composites.When flake graphite: SiC: phenolic resin =12:8:1,SiC matrix modified graphite had the best oxidation resistance at 1300 degrees,and the weight loss rate of 2h in static air was 18.78%.The effects of molding and isostatic pressing pressure on the composition,mechanism and properties of SiC modified graphite were studied.The results showed that with the increase of molding pressure,the porosity of SiC modified graphite prepared by molding and isostatic pressing process decreased,the bending strength increases,the resistivity decreased,and the oxidation resistance increased.In the same molding pressure,molding material had better performance than the static pressure molding process.When the isostatic pressing molding pressure was 200 MPa,the SiC matrix modified graphite had the smallest pore opening rate and the best oxidation resistance at 1300 degrees.At 1300 degrees,the weight loss rate of 3h in static air was 24.47%.However,when the isostatic pressing pressure was 200 MPa,there was a large elastic aftereffect phenomenon,which showed that the bending strength of the material decreases.The effects of the Si content in the raw materials of Composition,structure and performance of SiC modified graphite were studied The results showed that the binding force of flake graphite and SiC formed by the reaction between Si and C was weak.So,with the increase of Si content in raw material,the opening rate of SiC matrix modification of the graphite material increased,bending strength decreased,the resistivity increased.The oxidation resistance of SiC matrix modified graphite material increased with the increase of Si content because the strengthening effect of SiC on oxidation resistance was greater than that of pore and defectIn order to improve the preparation technique of SiC coated graphite materials and reduce the content of residual Si on the surface of the SiC coating,the effect of BN addition on the composition and properties of SiC coating prepared by silicon infiltration process was investigated.The results showed that with the liquid Si permeating into the matrix,a certain thickness of BN layer would be formed on the surface of the substrate.The BN layer had a strong hindering effect on the Si premeating into the matrix,which could effectively reduce the residual Si content on the surface of the SiC coating.When the content of BN was too much,the thickness of SiC coating prepared was thin and the density was low.When the content of BN is too little,the residual Si content of SiC coating was much more,and the machinability and corrosion resistance of the coating were low.The optimum content of BN is 12%.Under this condition,the residual Si content of SiC coating was less,and the coating was thicker.The effects of sintering temperature and vacuum degrees on the structure and properties of the prepared silicified graphite were investigated in the process of silicon infiltration by embedding method.Since the permeability of gaseous Si in the matrix was better than that of liquid Si,so increasing the sintering temperature and vacuum,the liquid Si evaporation into gaseous Si,could effectively improve the bonding strength of the density of SiC coating and substrate.The optimum mass fraction of BN was 12%.Under this condition,the residual Si content on the SiC coating was less,and the coating was thick.The effect of matrix porosity on the composition and properties of the prepared silicified graphite in the process of silicon infiltration by embedding method was studied.When the silicon source contains BN,the amount of Si entering into the matrix was controlled by BN content.When the amount of Si in the matrix was less,the porosity of the SiC coating was worse because of the larger porosity of the matrix.At 1600 degreess and 19~26Pa vacuum degrees,the thickness of SiC coating was the largest,the thickness of which was 80.24 microns.The modification of SiC coating had a good effect on improving the high temperature oxidation resistance of graphite materials.The weight loss rate of isostatic pressing graphite was 89.96% oxidation without coating treatment at 1300 degrees for 6 hours.After coating treatment,the weight loss of the same oxidation condition was only 1.39%.SiC matrix and coating modified to further improve the oxidation resistance of graphite materials.SiC matrix modified graphite was prepared by mass fraction of flake graphite: SiC: phenolic resin =12:8:1 and isostatic pressing pressure of 200 Mpa,and the SiC coating was modified by silicon infiltration at 1600 degreess and 19~26Pa vacuum degrees,which showed good antioxidant properties.It had a good oxidation resistance in the static air of 1300 degreess Celsius,and the weight loss rate is only 0.1% after 6 hours of oxidation. |
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