F. Qian-Gang et al /Ceramics International 36(2010)1463-1466 b:阝-sic Pressing head Pack powders C/C composites Fig. 1. Sketch of preparing SiC coating for C/C composites by HPRs slight argon flow. During this process, Si will melt and react with C/C composites to form SiC coating. The sketch of Fig. 2. XRD patterns of the surface of the C/C samples with coatings prepared preparing SiC coating for C/C composites by HPRS is shown in by PRS (a) and HPRS (b) Fig. 1. For comparison, another kind of Sic coating was prepared by pressure-less reactive sintering(PRS) with the same starting powders and heat-treatment temperature. During the preparation of coating, melted Si will react with C/C To investigate the thermal stress resistance of the coatings, to form SiC coating. Under pressure, some of liquid Si in the the thermal cycling test between 1773 K and room temperature pack powders was congregated to the surface of the samples was performed. The samples were weighed at room tempera- and left in the SiC coating, which is advantageous to relax the ture by electronic balance with a sensitivity of +0. I mg during stress at the end of the cracks and heal up the cracks in the thermal cycling tests. To evaluate the mechanical properties of coating the samples, three-point bending tests were carried out in a Fig. 3(a) displays SEM image of the coating prepared by servohydraulic machine of 8871 (INSTRON CO, Ltd, USA). PRS. This coating has a porous structure with some The span was 40 mm and the crosshead speed was 0.5 mm/min. microcracks, resulted from bigger coefficient of thermal Five samples for each kind of sample were tested and the final expansion of Sic coating than that of C/C composites Due flexural properties were obtained by computing the average to its loose structure, this coating might provide a poor values of five samples. oxidation protective ability for C/C composites. From Fig. 3(b) The morphologies and crystalline structures of the coatings the as-received coating prepared by HPRS possesses a dense were analyzed by JSM-6460 scanning electron microscopy and crack-free structure With pressure, the coatings will have (SEM) and Rigaku D/max-3C X-ray diffraction (XRD) compressive stress, which can effectively make up for the by the shrinkage of SiC coating during 3. Results and discussion cooling process from high temperature to room temperature The thermal stress in SiC coatings can be relaxed by the Fig. 2 shows XRD patterns of the surface of the coated introduction of pressure samples. From Fig. 2(a), the diffraction peaks of graphite and SEM images of the cross-section of the SiC coated samples cubic B-Sic are detected from the surface of the coating are shown in Fig 4 From Fig 4(a), there are large numbers of obtained by PRS. Graphite is corresponding to the C/c holes in the SiC coating prepared by PRS, owing to the difficult ubstrate, and B-SiC comes from the coating. Fig. 2(b)displays sintering of SiC ceramic without pressure From Fig 4(b), Sic that a new phase of Si was generated in the coating by HPRS. coating prepared by HPRs is denser than that without pressure, Fig. 3. SEM images of the surface of the coatings prepared by PRS (a) and HPRS(b).slight argon flow. During this process, Si will melt and react with C/C composites to form SiC coating. The sketch of preparing SiC coating for C/C composites by HPRS is shown in Fig. 1. For comparison, another kind of SiC coating was prepared by pressure-less reactive sintering (PRS) with the same starting powders and heat-treatment temperature. To investigate the thermal stress resistance of the coatings, the thermal cycling test between 1773 K and room temperature was performed. The samples were weighed at room tempera￾ture by electronic balance with a sensitivity of 0.1 mg during thermal cycling tests. To evaluate the mechanical properties of the samples, three-point bending tests were carried out in a servohydraulic machine of 8871 (INSTRON CO., Ltd., USA). The span was 40 mm and the crosshead speed was 0.5 mm/min. Five samples for each kind of sample were tested and the final flexural properties were obtained by computing the average values of five samples. The morphologies and crystalline structures of the coatings were analyzed by JSM-6460 scanning electron microscopy (SEM) and Rigaku D/max-3C X-ray diffraction (XRD). 3. Results and discussion Fig. 2 shows XRD patterns of the surface of the coated samples. From Fig. 2(a), the diffraction peaks of graphite and cubic b-SiC are detected from the surface of the coating obtained by PRS. Graphite is corresponding to the C/C substrate, and b-SiC comes from the coating. Fig. 2(b) displays that a new phase of Si was generated in the coating by HPRS. During the preparation of coating, melted Si will react with C/C to form SiC coating. Under pressure, some of liquid Si in the pack powders was congregated to the surface of the samples and left in the SiC coating, which is advantageous to relax the stress at the end of the cracks and heal up the cracks in the coating. Fig. 3(a) displays SEM image of the coating prepared by PRS. This coating has a porous structure with some microcracks, resulted from bigger coefficient of thermal expansion of SiC coating than that of C/C composites. Due to its loose structure, this coating might provide a poor oxidation protective ability for C/C composites. From Fig. 3(b), the as-received coating prepared by HPRS possesses a dense and crack-free structure. With pressure, the coatings will have compressive stress, which can effectively make up for the tensile stress induced by the shrinkage of SiC coating during the cooling process from high temperature to room temperature. The thermal stress in SiC coatings can be relaxed by the introduction of pressure. SEM images of the cross-section of the SiC coated samples are shown in Fig. 4. From Fig. 4(a), there are large numbers of holes in the SiC coating prepared by PRS, owing to the difficult sintering of SiC ceramic without pressure. From Fig. 4(b), SiC coating prepared by HPRS is denser than that without pressure, Fig. 1. Sketch of preparing SiC coating for C/C composites by HPRS. Fig. 3. SEM images of the surface of the coatings prepared by PRS (a) and HPRS (b). Fig. 2. XRD patterns of the surface of the C/C samples with coatings prepared by PRS (a) and HPRS (b). 1464 F. Qian-Gang et al. / Ceramics International 36 (2010) 1463–1466