Improved oxidation resistance of chemical vapor reaction SiC coating modified with silica for carbon/carbon composites

Abstract

To protect carbon/carbon (C/C) composites from oxidation, a SiC coating modified with SiO 2 was prepared by a complex technology. The inner SiC coating with thickness varying from 150 to 300 µm was initially coated by chemical vapor reaction (CVR): a simple and cheap technique to prepare the SiC coating via siliconizing the substrate that was exposed to the mixed vapor (Si and SiO 2 ) at high temperatures (1 923−2 273 K). Then the as-prepared coating was processed by a dipping and drying procedure with tetraethoxysilane as source materials to form SiO 2 to fill the cracks and holes. Oxidation tests show that, after oxidation in air at 1 623 K for 10 h and thermal cycling between 1 623 K and room temperature 5 times, the mass loss of the CVR coated sample is up to 18.21%, while the sample coated with modified coating is only 5.96%, exhibiting an obvious improvement of oxidation and thermal shock resistance of the coating. The mass loss of the modified sample is mainly contributed to the reaction of C/C substrate with oxygen diffusing through the penetrating cracks formed in thermal shock tests.

A study of surface-coated SiC whiskers on carbon fiber substrates and their properties for diesel particulate filter applications

“…Further, filters that had been coated for more than 20 min showed rapid oxidation around 800 8C, indicating that the additional surface coating, deposited for more than 20 min, improved the oxidation resistance temperature by up to 200 8C, as compared to the bare substrate filter. However, the oxidation behavior of our surface-coated filter, when compared with that of other studies of SiC coating [14], was lower than expected. This could be attributable to a mismatch between the SiC coating and the carbon fiber substrate.…”

In-situ synthesis of SiC nanofibers for improving the oxidation resistance of graphite

“…On the other hand, the SiO 2 phase was obtained from tetraethyl orthosilicate according to Eqs. (10) and (11) [16].…”

“…Because the SiO 2 glassy phase has a melting point in the range of 1373-1923 K [28] and the vaporizing rate of this phase is low [16], the liquid SiO 2 phase could fill the coating pores and since the coefficient of oxygen diffusion to the SiO 2 glassy phase at 1773 K is very low (5.3 Â 10 À 13 cm 2 /s) [29], the SiO 2 liquid phase could prevent the diffusion of oxygen to the coating. Therefore, the oxidation process was controlled by the diffusion of oxygen through the coating pores and the coating oxidation was prevented [16]. Also the ZrO 2 phase could react with the SiO 2 produced in the secondary coating, thus forming a refractory Zircon phase (ZrSiO 4 ) [22].…”

“…These samples were labeled as ISP. Similar to the initial coating, the heat treatment of the secondary coating was performed at 1873 K for 2 h. In order to fill the pores in the coating obtained by the slurry painting method, the surfaces were covered with tetraethyl orthosilicate for up to 4 times and then dried [16].…”

Preparation of a nanostructured SiC-ZrO2 coating to improve the oxidation resistance of graphite

“…Because the SiO 2 glass phase has a melting point in the range of 1373-1923 K and a low vaporizing rate [28], the liquid SiO 2 phase could fill the coating pores and since the coefficient of oxygen diffusion to the SiO 2 glass phase at 1773 K is very low [29], the SiO 2 liquid phase could prevent the diffusion of oxygen to the coating. Therefore, the oxidation process was controlled by the diffusion of oxygen through the coating pores and the coating oxidation was prevented [30]. Also, the oxidation of ZrB 2 by reaction (6) can produce molten-B 2 O 3 and ZrO 2 .…”