Research On The Influence Of Pre-carbon Layer On The Structure And Properties Of SiC Coating On Carbon Material Surfac

As a large agricultural country,with the progress of science and technology,the demand for modern agricultural facilities is increasing.High temperature thermal equipment plays an important role in the preparation of agricultural equipment.C/C composites and graphite materials in carbon materials are widely used in high temperature thermal equipment because of their high temperature stability,low thermal expansion coefficient and good corrosion resistance.However,the service life of carbon materials decreases significantly in aerobic environment.In order to solve this problem,SiC coatings are usually prepared on the surface of carbon materials to improve the oxidation resistance.However,due to the difference in thermal expansion coefficient between SiC coatings and carbon matrix materials,the prepared SiC coatings crack or even fall off.In this paper,the slurry brushing method and physical activation method are used to introduce a pre-carbon layer on the surface of the carbon material,and then the SiC coating is prepared by the embedding method.A gradient coating is formed on the surface of the substrate carbon material to alleviate the thermal stress caused by the difference in the thermal expansion coefficient.Cracking or even shedding of the coating,to achieve long-term application of carbon materials.When the pre-carbon layer is prepared by slurry coating method,the type and thickness of the pre-carbon layer are controlled by the change of slurry ratio and coating times,so as to produce the pre-carbon layer with different composition and thickness.On the basis of this pre-carbon layer,SiC coating is prepared by embedding method,and each layer is characterized and analyzed.The effect of the pre-carbon layer prepared by slurry coating on the structure and properties of the subsequent SiC coating was investigated.The results show that the optimum process parameters for preparing pre-carbon layer by slurry coating method are as follows:the mass ratio of carbon powder to phenolic resin is 1:2,and the coating times is 3;at this time,the introduction of the pre-carbon layer increased the bonding strength between the SiC coating and the matrix graphite and C/C composites by nearly 34%and 45%,respectively,and the oxidation weight loss rate of the SiC coating sample prepared after the introduction of the pre-carbon layer was significantly lower than that of the sample without the pre-carbon layer.It is found that the pre-carbon layer of activated carbon material is more suitable for graphite material matrix in a variety of pre-carbon layer ratio,and the oxidation weight loss rate of the sample is about 40%lower than that of the sample without pre-carbon layer.When the pre-carbon layer is used for the C/C composite matrix,the oxidation weight loss rate of the sample is only reduced by about 25%.The main reason is that the SiC coating prepared on the surface of graphite material with activated carbon pre-carbon layer is oxidized to form a denser Si O₂ film,which makes it have better oxidation resistance.When the pre-carbon layer was introduced by physical activation method,the effects of activation temperature,activation gas flow rate and activation time on the surface morphology and functional groups of the pre-carbon layer were studied.On this basis,the subsequent preparation of SiC coating was characterized and analyzed,and the influence of the preparation process of physical activation method on the structure and properties of SiC coating was investigated.The results show that the pre-carbon layer prepared by physical activation method increases the bonding strength of SiC coating on the surface of graphite and C/C composites by about 70%and 83%respectively under the optimum process conditions.The surface morphology of the pre-carbon layer and the content of each functional group are affected by the activation temperature,activation time and activation gas flow rate.The increase of activation temperature will make the surface morphology rough and increase the percentage of oxygen-containing functional groups.The extension of activation time and the increase of activation gas flow rate will further increase the roughness of the pre-carbon layer,but will reduce the proportion of oxygen-containing functional groups on the surface.The percentage content of oxygen-containing functional groups on the surface of pre-carbon layer has an important influence on the oxidation resistance of SiC coating.The higher the percentage of oxygen-containing functional groups on the surface of the pre-carbon layer,the higher the activity of the pre-carbon layer and the stronger the bonding ability with SiC coating.The introduction of pre-carbon layer by physical activation method can effectively improve the oxidation resistance of SiC coating on the surface of carbon materials.After preparing SiC coating on the surface of C/C composites and graphite materials,the oxidation weight loss rate is reduced by about 60%and 37%respectively compared with the sample without pre-carbon layer.For the pre-carbon layer obtained by different preparation methods,the effect of pre-carbon layer on the subsequent preparation of SiC coating was systematically studied.The results show that compared with the slurry brushing method,the bonding strength of the subsequent SiC coating and the substrate was increased by about 27%and 26%respectively after the pre-carbon layer was introduced on the surface of graphite and C/C composites by physical activation method.The oxidation resistance at 1200℃was increased by about 16%and10%respectively.The introduction of pre-carbon layer by physical activation method is more conducive to the formation of strong bonding and dense and complete surface morphology between SiC coating and substrate on the surface of carbon materials,which can more effectively realize the long-term application of carbon materials.