ABSTRACT
Due to excellent high-temperature performance, carbon materials were widely used in many industries. However, the deadly problem for the application of these materials lay in their quick oxidation at high temperature in the presence of oxygen, which seriously affected the performance and service life of carbon materials. Thus, preparing anti-oxidation coating on the surface of the matrix was the ideal method to improve oxidation resistance and mechanical property of carbon materials without bad influence on the matrix's own performance. The article centered on preparation of oxidation resistance coating, and characterized the organization structure, composition,thermal property,and chemical structure by field emission scanning electron microscopy (FESEM), X-Ray Diffraction (XRD), transmission electron microscope (TEM), thermo-analytical instrument (TG/DSC), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman).
Based on the above, the failure mechanism of pure SiC coating,effect of the reinforcement on the performance of the coating, preparation technology of the coating, and the introduction methods of the reinforcement were investigated.
We prepared pure SiC coating on carbon materials by embedding method,and contrasted the morphology and structure changes before and after oxidation test and thermal shock test, then analyzed the protection and failure mechanism of the coating to the matrix. The results showed that the pure SiC coating had a certain antioxidant effect on graphite matrix,but its effect was not significant. The oxidation weightless could not satisfy the practical application, with oxidation for 2h at 1400oC after three cycles. Pure SiC coating had a certain degree of oxidation resistance for carbon materials and showed some self-healing ability, but self-consumption of the coating was bigger in the self-healing process, which caused a degree of negative influence to its protection. In thermal shock test, the failure was mainly due to its own low density, which induced the coating not able to play well protection at low temperature, and the cracking of the coating caused by thermal mismatch was more serious. Pure SiC coating in the oxidation test was mainly controlled by two stages: When the temperature was low,the oxidation process was controlled by both oxidation and diffusion reaction, and homogeneous oxidation reaction occurred on the surface of the matrix; at higher temperature,the oxidation process was controlled by the diffusion rate,and uneven oxidation reaction occurred on the surface of the matrix. The crack mechanisms of the pure SiC coatings were different in high temperature oxidation test and thermal shock test. The micro crack formation was on outside the surface of coating in high temperature oxidation test, and with the oxidation test repeated, the crack could be self-healing. However, in thermal shock test, the cracks were through the coating to the matrix surface, and as the experiment progressed,the crack didn't heal, eventually led to fall off.
Taking the research on failure mechanism of the pure SiC coating as the foundation, through the introduction of SiC whisker layer, we improved the performance of SiC coating prepared by the embedding method. Comparing the morphology and structural changes before and after the oxidation tests, we analyzed the protection mechanism of the coating to the matrix. Introduction of SiC whisker with H-PSO as a raw material before preparation of the final coating, which made the coating showed good oxidation resistance at high temperature. In the process of oxidation, micro pores of the coating surface could heal. The oxidation weight loss was only 0.71% after oxidation at 1400°C for 4h. The presence of SiC whisker had a certain inhibition for excessive penetration of silicon, but because there was still a micro porous coating, the oxidation resistance of coating still needed to be improved.
We prepared the composite coating on surface of graphite matrix. The coating was reinforced by MoSi2 through physical doping. Comparing the morphology and structure changes before and after the oxidation and thermal shock tests, the protection mechanism of the coating to the matrix was analyzed. The best preparation process of the composite coating was studied by uniform design experiment. The results showed that the most important parameter influence on composite coating prepared by embedding method was the holding time. According to the experimental results and conditions permissible range, we chose the test parameters were: heat temperature was 1600°C, holding time was 2h, and gas flow rate was 150ml/min. After cyclic static air oxidation test, the coating showed better antioxidant property, and its weightlessness was lower than that of pure SiC coating, after oxidation at 1400°C for 8h,and the oxidation weight loss was less than 4%. The introduction of MoSi2 was beneficial to thermal stress adjustment between coating and matrix, and reduced the coating cracks due to thermal stress generated in the process of preparation. Because of high temperature plasticity and volume expansion, MoSi2 made the density of the coating increased. However, as strengthen body, the introduction of MoSi2 was not obvious for thermal shock effect at low temperature.
With H-PSO as a pioneer body, we introduced SiC whisker layer on the graphite or C/C matrix surface by dipping sintering process. The preparation of SiC whisker was simple and low cost. A SiC/Si/MoSi2 coating for carbon materials was prepared by subsequent embedding technique. The thickness of the coating was about 300|im. In addition, the coating combined with matrix well, and the surface was continuous and dense. Through oxidation test at 1000℃or 1400℃and thermal shock test at 800℃,the performance of the coating was investigated. For graphite matrix coated by the SiC/Si/MoSi2 coating, the oxidation pretreatment experiment was carried out in the static air at 1400°C for 4h before oxidation test and the sample had 0.045% weight gain.
Subsequent oxidation test showed that, the SiC/Si/MoSi2 multi-coating had excellent anti-oxidation property, which could protect graphite materials from oxidation at 1000°C in air for 12h and the corresponding weight loss was below 1%. Based on the surface morphology changes, oxidation pretreatment experiment and oxidation test enhanced densification of multi-coating and the coating had a certain self-healing ability. SiC whisker layer on the matrix surface could effectively regulated stress and reduced the generation of cracks.
In addition, the existence of the whisker layer could effectively inhibit excess infiltration of molten Si and control the thickness of the coating. The introduction of MoSi2 could effectively improve oxidation resistance of the coating. The formation of SiO2 during oxidation process of MoSi2 could fill the pores of the coating itself, showing the self-healing property. MoSi2 had better effect compared with Mo,which could protect graphite materials from oxidation at 1400oC in air for l0h and the corresponding weight loss was about 1%. For C/C composite,introduction of SiC whisker layer could inhibit excess infiltration of molten Si at a certain degree,and adjust thermal expansion differences between coating and matrix, but anti-oxidation property at 1400°C needed to be improved. In addition,preparation of SiC/Si/MoSi2 composite coating on graphite matrix could effectively improve the thermal shock resistance of the coating.
Same with H-PSO as the pioneer body,we introduced of SiC whisker layer on the graphite matrix surface by powder method, and used orthogonal test method to research the influence of preparation technology on growth of SiC whisker, with the crystalline fraction as target. The growth mechanism of SiC whisker was studied through comparing the changes of structure and morphology of the products at different temperatures. In addition, introducing SiC whisker layer by the new method, the final composite coating was formed on graphite matrix after subsequent embedding technique. By the corresponding oxidation test and thermal shock test, with the help of the test method to observe the changes of the coating morphology and phase,we got the following conclusion: The heat treatment temperature (T) was the most important factor that influenced the growth of SiC whisker, far exceeding the other factors. High temperature, higher gas flow, lower porosity and longer holding time were favorable to whisker generation. The SiC whisker was of core-shell structure with SiC phase as core and silicon oxide as the shell,growing along the direction of (111). The morphology of the whisker was uniform, distributing on the matrix surface with long straight state and larger length-diameter ratio. The existence of carbon matrix was beneficial to the growth of SiC whisker, and the growth of SiC whisker had followed defect-vapor-vapor and vapor-liquid-defect-solid mechanism. Introduction of SiC whisker layer by the new method, and then the composite coating prepared by embedding method enhanced the anti-oxidation property of the materials obviously. The coating could protect graphite materials from oxidation at 1400°C in air for lOh and the corresponding weight loss was below 1%.
Keywords: carbon materials; coating; SiC whisker; oxidation resistance;thermal shock resistance
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