| Carbon materials have the advantages of high temperature resistance,chemical corrosion resistance,high specific strength and low coefficient of thermal expansion（CTE）.It has been widely used in petroleum,chemical industry,metallurgy and other fields,such as graphite rotor for liquid aluminum refining.The working temperature of aluminum melt is about 750℃,but graphite will oxidize above 400℃,and the oxidation reaction rate will accelerate with the rise of temperature.The phosphate coating system can reduce the oxidation active sites of graphite and provide good oxidation resistance.In this paper,the aluminum phosphate coating was designed and prepared by solution impregnation technology,and modified by different ions(Y3+,Zr4+and Ti4+).The oxidation resistance and thermal shock cycle resistance of different phosphate coating systems in static air environment of 500-1000℃were studied,and their oxidation protection mechanism and failure mechanism were revealed.The main research contents and conclusions are as follows:（1）The phase transformation and micro morphology evolution of Al（PO3）3 coating during heat treatment and oxidation were studied,and the oxidation resistance of Al（PO3）3 coating under static air environment of500-1000℃was investigated.At low temperature,the improvement effect of phosphate coating samples is not obvious,but with the rise of temperature,the protective ability of the coating becomes more prominent.After oxidation at 800℃for 5 h,the weight loss of graphite and sample A0 were 5.212×10-3 g·cm-2 and 1.781×10-3 g·cm-2,respectively.When the temperature was increased to 1000℃,the weight loss of the coating sample was only 9.152×10-3 g·cm-2,lower than 40.430×10-3 g·cm-2 of the graphite matrix.The coating samples were subjected to thermal shock test for 16 times at 800℃-room temperature and 1000℃-room temperature respectively,and the weight loss range was 0.800×10-3-3.480×10-3 g·cm-2 and 7.513×10-3-14.913×10-3 g·cm-2.With the rise of temperature,its weight loss increases and impact resistance decreases.（2）The phase transformation and micro morphology evolution of Y3+modified aluminum phosphate coating during heat treatment and oxidation were analyzed.The modified coating samples had better oxidation resistance and thermal shock resistance in the air environment below800℃.After constant temperature oxidation at 800-1000℃for 5 h,the weight loss of sample AY5 was 0.565×10-3 g·cm-2,5.070×10-3 g·cm-2 and7.321×10-3 g·cm-2,respectively.It had undergone 16 times thermal shock experiments at 800℃-room temperature and 1000℃-room temperature,respectively,with a weight loss range of 0.523×10-3-2.287×10-3 g·cm-2and 8.267×10-3-12.161×10-3 g·cm-2.The wettability of Y3+modified aluminum phosphate impregnation solution is better,but the CTE of the coating increases.With the rise of temperature,its weight loss increases and its protection capability reduces,so its anti-oxidation/thermal shock resistance below 800℃is better.（3）The phase transformation and microstructure changes of Zr4+modified aluminum phosphate coatings during heat treatment and oxidation were investigated,and the modified coatings had excellent oxidation/thermal shock resistance in air environment above 800℃.After constant temperature oxidation at 800-1000℃for 5 h,the weight loss of sample AZ10 was 0.932×10-3 g·cm-2,4.301×10-3 g·cm-2 and 5.940×10-3g·cm-2,respectively.16 thermal shock tests were carried out at 800℃-room temperature and 1000℃-room temperature,respectively.The weight loss range of the samples AZ10 was 0.891×10-3-3.613×10-3 g·cm-2 and 6.643×10-3-9.923×10-3 g·cm-2.The wettability of Zr4+modified aluminum phosphate impregnation solution becomes worse and the coating thickness adds.Meanwhile,because of the unique crystal structure of ZrP2O7,the CTE of the modified coating is reduced,which is more compatible with the graphite matrix and can withstand the thermal shock of higher temperature.（4）The phase and microstructure evolution of Ti4+modified aluminum phosphate coating during heat treatment and oxidation were discussed.TiP2O7 and ZrP2O7 belong to the AB2O7 group compounds,so some of their physical and chemical properties are similar;However,due to the small difference in crystal structure,the CTE of Ti4+modified phosphate coating increases,and the protection ability of sample AT10coating to the matrix is lower than that of sample AZ10. |
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