Study On Preparation And Properties Of Anti-oxidation Grade Coatings For Silicon Carbide Based Composite Materials

Lower oxide diffusion coefficient and thermal expansion coefficient of silicon dioxide make silicon carbide have excellent oxidation resistance in high temperature. But the volume change in different crystallographic form of silicon dioxide makes the thin film craze, and accelerate the oxidation rate of silicon carbide. Oxidation and corrosion resistant coating on the surface of silicon carbide based composite materials is one of the best measures to improve the performance of materials in high temperature. Mullite/mullite whisker grade composite coating with amortize structure was prepared on the surface of re-crystallized silicon carbide, using slime and sol-gel combining slime. The amortize structure was controlled by adjusting to the raw material of coating proportion in slime or sol-gel combining slime.In this paper, mullite coating with grade thermal expansion coefficient was designed by analyzing mainly the thermal expansion coefficient, volume effect of phase change, oxide diffusion coefficient and stability between coating and based materials in high temperature. Mullite whiskers were in-situ synthesized in oxidation resistant coating. The performance of coating was evaluated by the measures of thermal recycle shock and calculating the increasing mass from oxidation. Comparing to the mullite coating of traditional process, the form mechanism and oxidation resistant performance of grade coating were studied by observing the microstructure and the change of based materials weight after thermal recycle shock. The difference was researched among the based materials with no coatings, traditional coatings, and mullite/mullite whisker grade composite coating. The thermal shock was carried under the temperature that in this rage alpha quartz can be changed alpha cristobalite reciprocally with much volume change. It is discussed the mechanism of mullite whisker in coating growing, strengthening and toughening. It also analyzed the failure of coating and discussed the mechanism of improving silicon carbide performance by whisker grade composite coating.It is showed that there should be less silicon dioxide in mullite coating, because the volume change of the silicon dioxide during the process of phase change is the main factor for the failure of coating. The amortize structure and mullite whisker canmake the effect on coating of thermal expansion of different raw materials lessen and prolong silicon carbide life. It is also gained that there is different mechanism of growth between whisker in space and whisker in coating. The former is gas - liquid -solid and the later is gas - solid. It is noticed that mullite whisker in coating can change to columnar crystal or be molten if the applying temperature is higher than 1500°C for a long time.The mullite/mullite whisker grade composite coating is dense, homogeneous and no convex and there are no pores in coating and the interface between coating and based materials. The length diameter ratio is from 1:10 to 1:15 and the thickness of coating is from 1 to 3um. None crack was found in the composite coating and adding weight is only 0.2609 milligram per square centimeter after thermal shocking 40 times. With the times of thermal shocking more, the superiority of composite coating is more.It is proved that mullite/mullite whisker grade composite coating can improved the thermal shock resistant performance of based materials much more, basing on the designing of oxide diffusion coefficient and thermal expansion coefficient of raw materials. The oxidation resistant ability is more excellent than the traditional mullite coating, especially thermal recycle shock under 1400°C.