| Carbon/carbon (C/C) composites exhibits many excellent properties. Such as low density, low thermal expansion coefficient, high thermal conductivity, large strength-to-weight ratio, high modulus. What's more, the higher the temperature is, the better its strength is. Thus the C/C composites have been widely used in aircraft and aerospace, nuclear industry even in the biomedical. However, the oxidation of these composites at temperatures above 643 K great limits use in inert atmosphere(when the weight loss reach 2%-5%, the corresponding mechanical properties would decrease 40%-50%). To solve this problem, the anti-oxidation coating is considered a good choice.The mullite coating and mullite whiskers-toughened mullite coating were prepared on the sample of C/C composite material with SiC coating by pulsed arc discharge (PADD). The influence of deposition voltage, pulse frequency, particle size and the content of the mullite whisker on the micro structure and oxidation resistance of the as-prepared coatings were investigated. The phase composition, microstructure of the surface and cross-section of the as-prepared coatings were characterized by XRD, SEM and EDS. The results are as follows:The mullite coating was prepared by PADD. The optimal technological parameters are:pulse voltage of 400 V, pulse frequency of 2000 Hz, iodine content of 3.0 g/L, deposition time of 20 min, hydrothermal temperature of 393 K, suspension powder of mullite keep 20 g/L, pulse duty ratio of 50%. The mullite/SiC multilayer coatings can effectively protect C/C composites from oxidation in air at 1773 K for 156 h with a weight loss rate of 4.50×10-5 g·cm-2·h-1. The mullite whiskers-toughened mullite coating can effectively protect C/C composites from oxidation in air at 1773 K for 254 h with a weight loss of only 5.90×10-3 g-cm-2 and a corresponding weight loss rate of 2.32×10-5 g·cm-2·h-1. The failure of the double-layer coatings is attributed to the generation of oxidation holes and microcracks, which may be due to the volatilization of the borosilicate glass layer and the escaping of CO and CO2.The mullite coating was prepared on the sample of C/C composite material with SiC coating by hot dipping. The mullite whiskers-toughened mullite coating was prepared by molten-salt method and in-situ method. The slurry intensity and the heat treatment temperature on the micro structure and oxidation resistance of the as-prepared coatings were studied. It was found that the best process conditions for preparing the coatings were the slurry intensity keep 0.2 g/ml, the heat treatment temperature was 473 K, the volume ratio of SiO2 sol and the distilled water reached 1:3. The mullite/SiC multilayer coatings can effectively protect C/C composites from oxidation in air at 1773 K for 168 h with a weight loss rate of 3.90×10-5 g·cm-2·h-1.The introduction of whiskers improved the oxidation resistance of SiC-mullite coating, whose mass loss decreased from 4.4% to 1.7% after isothermal oxidation at 1773 K for 254 h. Meanwhile, the thermal shock resistance of coating was improved significantly by introducing the whiskers, whose mass loss decreased from 4% to 0.97% after 50 thermal cycles between 1773 K and room temperature. The whiskers could effectively alleviate the thermal stress generated from thermal shock and increase the toughness of coating, resulting in the decrease of crack sizes. |
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