SiC whisker-toughened MoSi_2-SiC-Si coating to protect carbon/carbon composites against oxidation

摘要

The ultrahigh strength of carbon/carbon (C/C) composites at temperatures above 2273 K make them promising candidate materials for high temperature structural applications [1,2]. However, they are oxidized when used in an oxidizing environment at high temperature, leading to a decrease in mechanical properties [3]. A multi-layer protective coating, using SiC as an internal buffer layer, has been proposed [4-6] and MoSi_2, which has a high melting point and excellent oxidation resistance, could be a useful outer-layer material. However, the mismatch of thermal expansion coefficient between the MoSi_2 outer-layer and the SiC bonding layer usually leads to cracking of the MoSi_2 coating, which produces channels for oxygen attack. Therefore, pure MoSi_2 cannot be used as the outer-layer, and a multi-composition coating might be better. The toughness of MoSi_2 can be improved by adding SiC and Si [7], and the toughness of the coating can be increased by adding SiC whiskers (SiCw) [8]. To date, there are no reports of using MoSi_2-SiC-Si toughened with SiCw as the coating material and such is examined here. To obtain a dense MoSi_2-SiC-Si coating with a uniform distribution of SiCw, a two-step technique was used. An inner-layer of porous SiC with distributed SiCw and a dense outer-layer of MoSi_2-SiC-Si were prepared by slurry and pack cementation, respectively. The microstructures and oxidation protection ability of the coatings were investigated.