YAN Z hi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009)61-64 A Si-Mo layer was prepared by painting the mixture spectroscopy (EDS) slurry of Si and Mo powders and silica sol on the surface of substrate then drying and sintering at 1 500 C for 10 3 Results and discussion min. The above process was repeated for 3 times to form three-layer Si-Mo coating. The coating was about 1 3.1 Oxidation of c/c-sic substrate thick and the details were reported in Ref[8] morphologies of C/C-SiC composites before and For SiC/Si-Mo multilayer coating, a SiC layer was after oxidation are shown in Fig. 1. Pores in the origin first deposited on the substrate for 6 h at 1 100 C in C/C porous preforms are well infiltrated by the reaction- MrS(methyltrichlorosilane )-H2-Ar system. Then, a formed SiC and residual Si. C/C-SiC composites have Si-Mo layer was made by slurry painting. Thes se high density(Fig. 1(a)) and the open porosity is less than processes were repeated until the coating structure, form 3%(2], which indicates that MSI is a rapid process to inside to outside, was SiC-Si-Mo-SiC--Si-Mo-SiC. fabricate dense C/C-SiC composites, and to retain high This coating was about 100 um thick too, and the detail residual Si content(about 14.2%( mass fraction)[2D) information was reported in Ref.191 The C/C-Sic substrate has a network structure and the fiber bundles are distributed in the continuous 2.2 Oxidation tests network framework consisting of SiC and Si(Fig. 1(b)) The substrate and two coated samples atl500℃ corundum tube furnace to Therefore, when being oxidized at or below 1 400 C C/C-SiC composites al ways exhibit good shape retention investigate the isothermal thermal cycling behavior. The though carbon in the substrate is utterly burnt out. It is samples were put inside or taken out of the furnace found that the substrate does not distort after 10 h directly to air within several seconds. Cumulative mass oxidation at 1"C. While after oxidation at 1 500 C changes of the samples were measured at room for I h, it is badly distorted and many lumps appear on temperature by an electronic balance with a sensitivity of ±0.1mg. The mass change(△m) of the samples is calculated by the following equation the substrate becomes very loose and many microcracks -mo)/mo×100% (1) emerge By EDS analysis, these lumps contain mainly (1) and O, suggesting that they should be the molten Sio where mo and mn are the mass of the samples before and glass agglomeration after oxidation, respectively Above the Si melting point(1 410 C), residual Si in sapa t he morphologies and crystalline structures of the the substrate melts evaporates into Si(g) and diffuses ples were analyzed by scanning electron microscopy toward the surface of sample. Si(g) is rapidly oxidized (SEM, Jeol-6300LV)equipped with energy dispersive into SiOz and deposits on the surface. It is concluded that d Fig 1 Morphologies of C/C-SiC composites:(a),(b)Before oxidation; (c),(d)Oxidation at 1 500 C for I h62 YAN Zhi-qiao, et al/Trans. Nonferrous Met. Soc. China 19(2009) 61í64 A Si-Mo layer was prepared by painting the mixture slurry of Si and Mo powders and silica sol on the surface of substrate then drying and sintering at 1 500 ć for 10 min. The above process was repeated for 3 times to form three-layer Si-Mo coating. The coating was about 100 μm thick and the details were reported in Ref.[8]. For SiC/Si-Mo multilayer coating, a SiC layer was first deposited on the substrate for 6 h at 1 100 ć in MTS (methyltrichlorosilane)-H2-Ar system. Then, a Si-Mo layer was made by slurry painting. These processes were repeated until the coating structure, form inside to outside, was SiCėSi-MoėSiCėSi-MoėSiC. This coating was about 100 μm thick too, and the detail information was reported in Ref.[9]. 2.2 Oxidation tests The substrate and two coated samples were heated at 1 500 ć in air in a corundum tube furnace to investigate the isothermal thermal cycling behavior. The samples were put inside or taken out of the furnace directly to air within several seconds. Cumulative mass changes of the samples were measured at room temperature by an electronic balance with a sensitivity of ±0.1 mg. The mass change (ǻm) of the samples is calculated by the following equation: ǻm=(m1ím0)/m0h100% (1) where m0 and m1 are the mass of the samples before and after oxidation, respectively. The morphologies and crystalline structures of the samples were analyzed by scanning electron microscopy (SEM, Jeolí6300LV) equipped with energy dispersive spectroscopy(EDS). 3 Results and discussion 3.1 Oxidation of C/C-SiC substrate Morphologies of C/C-SiC composites before and after oxidation are shown in Fig.1. Pores in the original C/C porous preforms are well infiltrated by the reaction￾formed SiC and residual Si. C/C-SiC composites have high density (Fig.1(a)) and the open porosity is less than 3%[2], which indicates that MSI is a rapid process to fabricate dense C/C-SiC composites, and to retain high residual Si content (about 14.2% (mass fraction) [2]). The C/C-SiC substrate has a network structure and the fiber bundles are distributed in the continuous network framework consisting of SiC and Si (Fig.1(b)). Therefore, when being oxidized at or below 1 400 ć, C/C-SiC composites always exhibit good shape retention though carbon in the substrate is utterly burnt out. It is found that the substrate does not distort after 10 h oxidation at 1 400 ć. While after oxidation at 1 500 ć for 1 h, it is badly distorted and many lumps appear on the surface (Fig.1(c)). SEM image (Fig.1(d)) shows that the substrate becomes very loose and many microcracks emerge. By EDS analysis, these lumps contain mainly Si and O, suggesting that they should be the molten SiO2 glass agglomeration. Above the Si melting point (1 410 ć), residual Si in the substrate melts evaporates into Si(g) and diffuses toward the surface of sample. Si(g) is rapidly oxidized into SiO2 and deposits on the surface. It is concluded that Fig.1 Morphologies of C/C-SiC composites: (a), (b) Before oxidation; (c), (d) Oxidation at 1 500 ćfor 1 h