YAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009)61-64 C/C-SiC composites with residual Si cannot be directly FANG[II] reported a Si-Mo slurry coating on C/C used at1500℃ composites. The coating consisted of Si and MoSi2. It worked well at 1 370 C. but soon became invalid at 3.2 Oxidation of C/C-SiC substrate with three-layer 1 450 C In a conclusion, coatings with residual Si can Si-Mo coating not be used at 1 500 C directly SiC, Si, MoSi2 and Sio2 are found in the three-layer Si-Mo coating. After 140 h oxidation at 1 400 C, the 3.3 Oxidation of C/C-SiC substrate with SiC/Si-Mo mass loss of the coated sample is 1. 34%, showing that multilayer coating the coating can provide longtime oxidation protection for Based on the above analysis it can be inferred that C/C-SiC composites at that temperature[ 8]. While after the evaporation of residual Si and higher chemical I h oxidation at 1 500 C, the mass loss reaches 0. 76% activity of Si-Mo layer lead to the failure of the C/C-SiC (Fig. 2(a)) and many bubbles appear on the surface. The substrate with and without three-layer Si-Mo coating surface SEM image, shown in Fig. 2(b), reveals that Our purpose is thus to develop the coating system with continuous SiO2 glass has been formed accompanied dense barriers. The designed SiC/Si-Mo multilayer with bubbles. The phase formation Mot2Si-MoSi2 is coating, from inside to outside, was Sic-Si-Mo-SiC accompanied by a volume reduction of about 27%[10].-Si-Mo-SiC Even after 150 h oxidation at 1 400 C, This reduction produces certain pores and interface in the the mass loss of the coated sample was 0.25%[9].When Si-Mo layer and leads to higher chemical activity. At being oxidized at 1 500 C, the coated sample kept m I 500 C, SiC, Si and MoSi are rapidly oxidized into gaining during 105 h, which suggests that the coating can SiO2 glass and CO. Simultaneously, residual Si in the protect the C/C-SiC substrate from oxidation at that coating melts and evaporates. Gases of Si(g) and Co temperature. Surface SEM image(Fig 3(b) shows a gather in the coating and bubbles are formed in the Sio2 glassy SiO2 layer covering the surface well except the glass when the pressure exceeds the critical value. Then existence of several microcracks. During the oxidation he coating is out of service. Therefore, three-layer test, the sample was taken out of the furnace directly into Si-Mo coating cannot provide oxidation protection for air within several seconds for weighing. The cooling C-SiC composites at 1 500C rate from 1 500C to room temperature was very quick. 0.10 0.20.40.60.81.0 80100120 Oxidation time/h ep 500m Fig 2 Oxidation of C/C-SiC composites with three-layer Si-Mo Fig 3 Oxidation of C/c-Sic composites with SiC/Si-Mo coating at 1 500 C:(a) Isothermal oxidation curve;(b) sEm multilayer coating at 1 500 C: (a) Isothermal oxidation curve image after I h oxidation (b) SEM image after 105 h oxidationYAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009) 61í64 63 C/C-SiC composites with residual Si cannot be directly used at 1 500 ć. 3.2 Oxidation of C/C-SiC substrate with three-layer Si-Mo coating SiC, Si, MoSi2 and SiO2 are found in the three-layer Si-Mo coating. After 140 h oxidation at 1 400 ć, the mass loss of the coated sample is 1.34%, showing that the coating can provide longtime oxidation protection for C/C-SiC composites at that temperature[8]. While after 1 h oxidation at 1 500 ć, the mass loss reaches 0.76% (Fig.2(a)) and many bubbles appear on the surface. The surface SEM image, shown in Fig.2(b), reveals that continuous SiO2 glass has been formed accompanied with bubbles. The phase formation Mo+2SiėMoSi2 is accompanied by a volume reduction of about 27%[10]. This reduction produces certain pores and interface in the Si-Mo layer and leads to higher chemical activity. At 1 500 ć, SiC, Si and MoSi2 are rapidly oxidized into SiO2 glass and CO. Simultaneously, residual Si in the coating melts and evaporates. Gases of Si(g) and CO gather in the coating and bubbles are formed in the SiO2 glass when the pressure exceeds the critical value. Then the coating is out of service. Therefore, three-layer Si-Mo coating cannot provide oxidation protection for C/C-SiC composites at 1 500 ć. Fig.2 Oxidation of C/C-SiC composites with three-layer Si-Mo coating at 1 500 ć: (a) Isothermal oxidation curve; (b) SEM image after 1 h oxidation FANG[11] reported a Si-Mo slurry coating on C/C composites. The coating consisted of Si and MoSi2. It worked well at 1 370 ć, but soon became invalid at 1 450 ć. In a conclusion, coatings with residual Si can not be used at 1 500 ć directly. 3.3 Oxidation of C/C-SiC substrate with SiC/Si-Mo multilayer coating Based on the above analysis, it can be inferred that the evaporation of residual Si and higher chemical activity of Si-Mo layer lead to the failure of the C/C-SiC substrate with and without three-layer Si-Mo coating. Our purpose is thus to develop the coating system with dense barriers. The designed SiC/Si-Mo multilayer coating, from inside to outside, was SiCėSi-MoėSiC ėSi-MoėSiC. Even after 150 h oxidation at 1 400 ć, the mass loss of the coated sample was 0.25%[9]. When being oxidized at 1 500 ć, the coated sample kept mass gaining during 105 h, which suggests that the coating can protect the C/C-SiC substrate from oxidation at that temperature. Surface SEM image (Fig.3(b)) shows a glassy SiO2 layer covering the surface well except the existence of several microcracks. During the oxidation test, the sample was taken out of the furnace directly into air within several seconds for weighing. The cooling rate from 1 500 ć to room temperature was very quick. Fig.3 Oxidation of C/C-SiC composites with SiC/Si-Mo multilayer coating at 1 500 ć: (a) Isothermal oxidation curve; (b) SEM image after 105 h oxidation