L. Cheng et al./ Materials Science and Engineering 4300(2001)219-225 Table 3 Arrhenius relations of oxidation in different temperature ranges for C/C and C/SiC compoistes C/C or C/SiC composites with a coating Arrhenius relations of weight change with temperature Diffusion through cracks diffusion through oxide ln△W= +20.776 AP-CVI C/SiC +15.984ln△W=128l5/T-9.5335In△W= +13.818 LFCⅵIC/SiC ln△W=-1 +17.667ln△W=15786/T-13.416lnAW= +1389 the PyC interlayer was oxidized out than after Consid- 5. Conclusions ering that the oxidation led to a porosity change and rated the reaction of carbon with with oxygen, the Oxidation behavior of the carbon fiber renfo C/Sic composite with a thicker PyC interlayer lost carbon or ceramic matrix composites with a coi weight more rapidly than that with a thinner one with could be discribed by a continuous function over the increasing the oxidation time. Thus, it could be easily full temperature range, no matter how the composites understood that the maximum weight loss of the LP- were prepared CVI C/SiC composite was much higher than that of the The Sic matrix made the coating cracking tempera- AP-CVI C/SiC composite. The LP-CVI C/SiC com- ture of the AP-CVI C/SiC being 100 C lower than that posite with a thicker PyC interlayer had a better oxida of the C/C, and the PyC interlayer with a different tion resistance than the C/c composite because th thickness made the coating cracking temperature and maximum weight loss of the latter would be much matrix cracking temperature of the LP-CVI C/SiC be- larger than that of the former if the oxidation time was ing 100oC lower than those of the AP-CVI C/SIC. increased furthermore There was an optimum thickness of the Pyc inter- tyer for improving the oxidation resistance of C/Sic composites. On one hand, the thicker the PyC inter- cracking temperature, the activation energy of diffusion layer, the lower the transition temperature, but the the relation of weight change with temperature for the larger the maximum weight loss. On the other hand, the thicker the PyC interlayer, the higher the mechanical C/C composite because it was nonlinear. The activation properties energy for diffusion through the matrix cracks in LP- Below the transition temperatures, the activation en- VI C/SiC composite was calculated to be higher than ergies of oxidation through reaction for the C/C and that in the AP-CVI C/SiC composite. This indicated the C/Sic composites determined by the Arrhenius that the matrix cracks in the former was smaller in relations of weight change with temperature varied width than those in the latter at the same temperature. little Above the coating cracking temperature, the C/C Above the coating temperatures, the Si-W layer and AP-CVI C/SiC composite should exhibited the gher activation energy for diffusion than the si same oxidation behavior because their coating had the layer because the silica film formed on the former had same outer layer and the oxidation took place only on a higher resistance to oxygen diffusion than the oxide their coating surfaces. However, the activation energy film formed on the latter for diffusion through the oxide film on the former coating surface was a little higher than that on the latter coating surface. Because the AP-CVI C/SiC sub- References strates were not machined, their surface was very rough and then the prepared Si-w layer was non-uniform [1] F. Lamouroux, X. Bourrat, R. Naslain, Carbon 31(1993)1273- When the si-w layer was oxidized for 5 h, the CVD 21 amouroux. G. Camus. J. Amer. Ceram. Soc. 77(1994) Sic layer was oxidized at the area where the Si-W 2057 layer was too thin. The oxidation behavior of the B] F. Lamouroux, G. Camus, J. Amer. Ceram Soc. 77(1994) 2058-2068. AP-CVI C/SiC composite above its coating cracking (4)RJ Kerans, Control of fiber-matrix interface properties in ce- temperature was considered to be changed by oxidation amic composites, in: R. Naslain(Ed h Temperatur of the CVD SiC layer. The Si-W layer had higher ramic Matrix Composites. Woodhead Publications, Bordeaux, activation energy for diffusion than the Si-Zr layer 1993 301-312. This indicated that the silica film formed on the former 5R.A. Lowden, K L More, O.J. Schwarz, N L. Vaughn, Improved fiber-matrix interlayers for Nicalon/ Sic composites, in: R had a higher resistance to oxygen diffusion than the Naslain(Ed. ) High Temperature Ceramic Matrix Compos oxide film formed he latter Woodhead Publications, Bordeaux, 1993, pp. 345-352.224 L. Cheng et al. / Materials Science and Engineering A300 (2001) 219–225 Table 3 Arrhenius relations of oxidation in different temperature ranges for C/C and C/SiC compoistes C/C or C/SiC composites with a coating Arrhenius relations of weight change with temperature Reaction Diffusion through cracks Diffusion through oxide C/C ln DW=−13790/T+16.086 ln DW=−38147/T+20.776 AP-CVI C/SiC ln DW=−13141/T+15.984 ln DW=12815/T−9.5335 ln DW=−23102/T+13.818 LP-CVI C/SiC ln DW=−13906/T+17.667 ln DW=15786/T−13.416ln DW=−23040/T+13.898 the PyC interlayer was oxidized out than after. Consid￾ering that the oxidation led to a porosity change and accelerated the reaction of carbon with oxygen, the C/SiC composite with a thicker PyC interlayer lost weight more rapidly than that with a thinner one with increasing the oxidation time. Thus, it could be easily understood that the maximum weight loss of the LP￾CVI C/SiC composite was much higher than that of the AP-CVI C/SiC composite. The LP-CVI C/SiC com￾posite with a thicker PyC interlayer had a better oxida￾tion resistance than the C/C composite because the maximum weight loss of the latter would be much larger than that of the former if the oxidation time was increased furthermore. From the transition temperature to the coating cracking temperature, the activation energy of diffusion through the coating cracks could not be calculated by the relation of weight change with temperature for the C/C composite because it was nonlinear. The activation energy for diffusion through the matrix cracks in LP￾CVI C/SiC composite was calculated to be higher than that in the AP-CVI C/SiC composite. This indicated that the matrix cracks in the former was smaller in width than those in the latter at the same temperature. Above the coating cracking temperature, the C/C and AP-CVI C/SiC composite should exhibited the same oxidation behavior because their coating had the same outer layer and the oxidation took place only on their coating surfaces. However, the activation energy for diffusion through the oxide film on the former coating surface was a little higher than that on the latter coating surface. Because the AP-CVI C/SiC sub￾strates were not machined, their surface was very rough and then the prepared Si–W layer was non-uniform. When the Si–W layer was oxidized for 5 h, the CVD SiC layer was oxidized at the area where the Si–W layer was too thin. The oxidation behavior of the AP-CVI C/SiC composite above its coating cracking temperature was considered to be changed by oxidation of the CVD SiC layer. The Si–W layer had higher activation energy for diffusion than the Si–Zr layer. This indicated that the silica film formed on the former had a higher resistance to oxygen diffusion than the oxide film formed on the latter. 5. Conclusions Oxidation behavior of the carbon fiber renforced carbon or ceramic matrix composites with a coating could be discribed by a continuous function over the full temperature range, no matter how the composites were prepared. The SiC matrix made the coating cracking tempera￾ture of the AP-CVI C/SiC being 100°C lower than that of the C/C, and the PyC interlayer with a different thickness made the coating cracking temperature and matrix cracking temperature of the LP-CVI C/SiC be￾ing 100°C lower than those of the AP-CVI C/SiC. There was an optimum thickness of the PyC inter￾layer for improving the oxidation resistance of C/SiC composites. On one hand, the thicker the PyC inter￾layer, the lower the transition temperature, but the larger the maximum weight loss. On the other hand, the thicker the PyC interlayer, the higher the mechanical properties. Below the transition temperatures, the activation en￾ergies of oxidation through reaction for the C/C and the C/SiC composites determined by the Arrhenius relations of weight change with temperature varied little. Above the coating temperatures, the Si–W layer had higher activation energy for diffusion than the Si–Zr layer because the silica film formed on the former had a higher resistance to oxygen diffusion than the oxide film formed on the latter. References [1] F. Lamouroux, X. Bourrat, R. Naslain, Carbon 31 (1993) 1273– 1288. [2] F. Lamouroux, G. Camus, J. Amer. Ceram. Soc. 77 (1994) 2049–2057. [3] F. Lamouroux, G. Camus, J. Amer. Ceram. Soc. 77 (1994) 2058–2068. [4] R.J. Kerans, Control of fiber-matrix interface properties in ce￾ramic composites, in: R. Naslain (Ed.), High Temperature Ce￾ramic Matrix Composites, Woodhead Publications, Bordeaux, 1993, pp. 301–312. [5] R.A. Lowden, K.L. More, O.J. Schwarz, N.L. Vaughn, Improved fiber-matrix interlayers for Nicalon/SiC composites, in: R. Naslain (Ed.), High Temperature Ceramic Matrix Composites, Woodhead Publications, Bordeaux, 1993, pp. 345–352