J Mater Sci(2007)42:5046-5056 5049 large grains incorporating the small one by grain size, which was expected to have a high diffusivity at boundary diffusion. Especially, for grains with small grain boundaries and result in a large grain size as size, the grain boundary diffusion operates much more annealing at high temperatures. However, an unex readily. As observed in two Nippon Carbon fibers, the pected phenomenon was observed between two Nip grain coarsening is more obvious in annealed state than pon Carbon fibers. This can be attributed to the excess those of as-received state (tEM observations have carbon in HNL fiber. TEM observation revealed that revealed that grain size is about 5 nm for HNL fiber heat treatment of the hNL fiber results in a gradual [17], 20 nm for HNLS fiber [18], 200 nm for TySA fiber organization of the free carbon phase in terms of the [11). On the other hand, the residual trace oxygen may size of the carbon layer and the number of stacked lay a role in the Si and c grain boundary transport by layers as increasing temperatures [17. Takeda et al. accelerating diffusion [17], because the oxygen is not [20] have investigated the properties of polycarbosilen necessarily eliminated from the fiber as reported in the derived silicon carbide fibers with various C/Si com literature [8, even for the HNLS fiber which was positions, and revealed that microstructure and fabricated at very high temperature mechanical properties are quite dependent on the C/si onsidering the starting temperature for grain composition. Grain growth is suppressed with increase coarsening in Fig. 2, the grain size might be related in excess carbon. In other studies [21, 22), the carbon primarily to the maximum temperature at which the suppressing growth and coalescence of the Sic micro- fibers were fabricated. The fabrication temperatures crystals was also observed. Sasaki [22] found that car- have been presented for two Nippon Carbon fibers bon disappeared above 1,500C heat treatment in Sic (HNL: 1,350C, HNLS: 1, 600C) and for TySA fiber fiber using Raman study And then an abrupt increase (about 1, 800C)in the literature[19]. From the Fig. 2, of crystal size at 1, 500C was observed it can be seen that the crystallite size increased when For the TySA fiber, this fiber originally has a annealing temperature is above the fabrication tem- large crystallite size. In previous studies[8-10,a perature as expected. For hNL fiber, the grain coars- bon-rich core was revealed in TySA fiber, which results ening occurred at relatively low temperature is due to from the production process. Colomban et al. esti the decomposition of amorphous phase at about mated carbon grain size and Sic grain size in TySA 1,300C. On the other hand, the thermally activated fiber from Raman spectroscopy 9, 10]. The Carbon diffusion plays an important role on the grain coars- grains appear approximately 2-3 times smaller on the ening of Sic materials at high temperatures. fiber's core(0.9-1.7 nm)than on periphery (1 If we make a further comparison in crystallite size 2.6 nm). The grain size of Sic in fiber core is much between two Nippon Carbon fibers again, we can see smaller than edge region. Likely, this is due to that that a large difference in crystallite size was observed carbon suppressed the grain growth of B-SiC.Fur for two fibers annealed at same temperature. As above thermore, this fiber contains the small amount of alu mentioned, the HNL fiber has an small starting grain mina(less than 1 wt %)as sintering additive, which will also inhibit the grain growth of Sic. As a result, the TySA fiber showed an excellent thermal stability in Ar HNL atmosphere ATySA Tensile strength Tensile strengths were obtained by a single filament tensile test technique at room temperature. If plotting the tensile data into the Fig. 2. it is obvious that the strengths of annealed fiber were related to B-SiC crystallite size(Dsic) as shown in Fig 3. The Fig 3 confirmed the fact that generally materials with large grain size have low strengths. The growth of Sic crystals reduces the bonding forces at the grain Initial12001400160018002000 boundaries. Since the manufactures are always seeking Annealing temperature, T/c the optimal fabrication temperature at which the uperior thermal stability and excellent mechanical Fig 2 Apparent crystallite size of B-SiC for Sic fibers annealed strength can be obtained simultaneously, thus, the at elevated temperatures in Ar for 1 h upper fabrication temperatures are typically fixed bylarge grains incorporating the small one by grain boundary diffusion. Especially, for grains with small size, the grain boundary diffusion operates much more readily. As observed in two Nippon Carbon fibers, the grain coarsening is more obvious in annealed state than those of as-received state (TEM observations have revealed that grain size is about 5 nm for HNL fiber [17], 20 nm for HNLS fiber [18], 200 nm for TySA fiber [11]). On the other hand, the residual trace oxygen may play a role in the Si and C grain boundary transport by accelerating diffusion [17], because the oxygen is not necessarily eliminated from the fiber as reported in the literature [8], even for the HNLS fiber which was fabricated at very high temperature. Considering the starting temperature for grain coarsening in Fig. 2, the grain size might be related primarily to the maximum temperature at which the fibers were fabricated. The fabrication temperatures have been presented for two Nippon Carbon fibers (HNL: 1,350 C, HNLS: 1,600 C) and for TySA fiber (about 1,800 C) in the literature [19]. From the Fig. 2, it can be seen that the crystallite size increased when annealing temperature is above the fabrication tem￾perature as expected. For HNL fiber, the grain coars￾ening occurred at relatively low temperature is due to the decomposition of amorphous phase at about 1,300 C. On the other hand, the thermally activated diffusion plays an important role on the grain coars￾ening of SiC materials at high temperatures. If we make a further comparison in crystallite size between two Nippon Carbon fibers again, we can see that a large difference in crystallite size was observed for two fibers annealed at same temperature. As above mentioned, the HNL fiber has an small starting grain size, which was expected to have a high diffusivity at grain boundaries and result in a large grain size as annealing at high temperatures. However, an unex￾pected phenomenon was observed between two Nip￾pon Carbon fibers. This can be attributed to the excess carbon in HNL fiber. TEM observation revealed that heat treatment of the HNL fiber results in a gradual organization of the free carbon phase in terms of the size of the carbon layer and the number of stacked layers as increasing temperatures [17]. Takeda et al. [20] have investigated the properties of polycarbosilen￾derived silicon carbide fibers with various C/Si com￾positions, and revealed that microstructure and mechanical properties are quite dependent on the C/Si composition. Grain growth is suppressed with increase in excess carbon. In other studies [21, 22], the carbon suppressing growth and coalescence of the SiC micro￾crystals was also observed. Sasaki [22] found that car￾bon disappeared above 1,500 C heat treatment in SiC fiber using Raman study. And then an abrupt increase of crystal size at 1,500 C was observed. For the TySA fiber, this fiber originally has a very large crystallite size. In previous studies [8–10], a car￾bon-rich core was revealed in TySA fiber, which results from the production process. Colomban et al. esti￾mated carbon grain size and SiC grain size in TySA fiber from Raman spectroscopy [9, 10]. The Carbon grains appear approximately 2–3 times smaller on the fiber’s core (0.9–1.7 nm) than on its periphery (1.7– 2.6 nm). The grain size of SiC in fiber core is much smaller than edge region. Likely, this is due to that carbon suppressed the grain growth of b-SiC. Fur￾thermore, this fiber contains the small amount of alu￾mina (less than 1 wt%) as sintering additive, which will also inhibit the grain growth of SiC. As a result, the TySA fiber showed an excellent thermal stability in Ar atmosphere. Tensile strength Tensile strengths were obtained by a single filament tensile test technique at room temperature. If plotting the tensile data into the Fig. 2, it is obvious that the strengths of annealed fiber were related to b-SiC crystallite size (DSiC) as shown in Fig. 3. The Fig. 3 confirmed the fact that generally materials with large grain size have low strengths. The growth of SiC crystals reduces the bonding forces at the grain boundaries. Since the manufactures are always seeking the optimal fabrication temperature at which the superior thermal stability and excellent mechanical strength can be obtained simultaneously, thus, the upper fabrication temperatures are typically fixed by 0 5 10 15 20 25 30 35 1200 1400 1600 1800 2000 HNL HNLS TySA Initial Annealing temperature, T/C Fig. 2 Apparent crystallite size of b-SiC for SiC fibers annealed at elevated temperatures in Ar for 1 h J Mater Sci (2007) 42:5046–5056 5049 123