Study On Preparation And Properties Of Anti-oxidation Coatings For Silicon Carbide Electric Heating Element

Lower oxide diffusion coefficient and thermal expansion coefficient of silicon dioxide make Silicon Carbide Electric heating Element have excellent oxidation resistance in high temperature. But the volume change in different crystallographic form of silicon dioxide during heating and cooling condition 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. Using slime process condition, many composite coatings which were mainly composed by mullite were prepared on the surface of Silicon Carbide Electric heating Element. The amortize structure was controlled by adjusting to the raw material of coating proportion in slime.In this paper, mullite/SiC/Al2O3/Si composite coating was selected mainly by analyzing the thermal expansion coefficient, volume effect of phase change, oxide diffusion coefficient and stability between coating and based materials in high temperature and contrast experiments. The performance of coating was evaluated by the measures of thermal shock recycle and calculating the increasing mass from oxidation. The influence for the different process conditions on the properties of coating was studied. The form mechanism and oxidation resistant performance of coating were studied by observing the microstructure and the change of based materials weight after thermal shock recycle. The different was researched between the based materials with no coatings and composite coatings.According to thermodynamics analyses, the oxidation of the coating can be in (1) Si ( s)+ O2 (g)=SiO2(s),(2) SiC ( s)+ 2O2 (g)=SiO2(s)+CO2(g), and oxidation outcome is SiO2 in a reasonable temperature range. The oxidation weight increasing rate for the Electric heating Element which were coated in oxidation condition around 1200～1400℃is less than those without coating.To sum up , the optimizing infiltration compositions and process are: 50%mullite+20%Si+20%SiC+10%Al2O3,holding 4 hours at 1400℃,and then the coating is sealed by Si(OC2H5)4.The best anti-oxidation effect for the sealed coating is 0.036% increasing weight after oxidation for 60 hours in 1400℃air. The surface for the coating one appear slight crack after thermal shock recycle for 40 times and the oxidation weight increasing for the coating is only (1.492%)1/18 compared with Element without coating.