Y Liu et al. /Materials Science and Engineering A 475(2008)217-22 Agglomerati Particle eration pores Intra-particle pores 10 and preform prepared by silicon nitride particles(a) particle agglomerations; (b) particle preform; (c)the glomeration pores; and (d)the intra-agglomeration pores (1)[30 ent that a large amount of Sic is infiltrated in the inter- and intra-agglomerations pores. The reinforcing SiC particles P and agglomerations are enveloped by CVI SiC. The resid (1) ual pore inter- and intra-agglomeration is unavoidable,but the amount of pore decreases obviously. The residual pore where k is Boltzmann's constant. k=1. 10-23 J/K: Tis the size inter-agglomeration is about 200-400 um, which is rather smaller than that before infiltration. The residual pore size intra- temperature, K; oi or oj is the effective diameter of molecule i agglomeration is about 2-4um, which is also smaller than or j, m: P is partial pressure of gas, Pa; m; or m; is the molecule that before infiltration. The size changes of inter-and intra- nass of i or j specimen gas. As an example, the a values of all gas agglomeration pores before and after infiltration shows that the in SiCl4-NH3-H2-Ar system are listed in Table 1. Therefore, amount of deposit in inter-agglomeration pore is more than that e can conclude that the gas diffusion mechanism is transition of intra-agglomeration pore diffusion in the inter-agglomeration pores due to the d=5-89A Interface microstructure observed by tEM shows a thin SiC The diffusion mechanism belongs to Knudsen diffusion in the layer can be deposited on the near surface of SiC reinforcing intra- agglomeration pores due to d≤0.1λ particle during CVI(see Fig. 2(c). The incomplete burnout of residual carbon detaches from SiC reinforcing particle and exists 3.2. Microstructure and properties of SiCpysic composites between the CVI column SiC and the common SiC matrix, which will exist as a defect layer, resulting in weak interfacial bonding Fig. 2 shows typical microstructures of SiC(Py/SiC compos- strength for SiC(P)/SiC composites CvI SiC whiskers are found ites fabricated by CVI. Fig. 2(a)and(b)show the cross-section in some intra-agglomeration pores as shown in Fig. 2(d). The morphologies of the composites infiltrated for 500 h. It is appar- diameters of the whiskers are almost the same, about several Table Mean free paths of reactant gases and other coefficients in the SiCL-NH3-H2-Ar system Gas species Flux(ml min-) a(nm) M(gmol-) Partial pressure(Pa) 0.2915 0.5484 99 0.2900 17Y. Liu et al. / Materials Science and Engineering A 475 (2008) 217–223 219 Fig. 1. Typical morphologies of particle agglomerations and preform prepared by silicon nitride particles (a) particle agglomerations; (b) particle preform; (c) the inter-agglomeration pores; and (d) the intra-agglomeration pores. (1) [30]. λi = ⎡ ⎣ n j=1  1 + mi mj π σi + σj 2 2 P kT ⎤ ⎦ (1) where k is Boltzmann’s constant, k = 1.38 × 10−23 J/K; T is the temperature, K; σi or σj is the effective diameter of molecule i or j, m; P is partial pressure of gas, Pa; mi or mj is the molecule mass ofi orjspecimen gas. As an example, the λ values of all gas in SiCl4–NH3–H2–Ar system are listed in Table 1. Therefore, we can conclude that the gas diffusion mechanism is transition diffusion in the inter-agglomeration pores due to the d = 5–89λ. The diffusion mechanism belongs to Knudsen diffusion in the intra-agglomeration pores due to d ≤ 0.1λ. 3.2. Microstructure and properties of SiC(p)/SiC composites Fig. 2 shows typical microstructures of SiC(P)/SiC compos￾ites fabricated by CVI. Fig. 2(a) and (b) show the cross-section morphologies of the composites infiltrated for 500 h. It is appar￾ent that a large amount of SiC is infiltrated in the inter￾and intra-agglomerations pores. The reinforcing SiC particles and agglomerations are enveloped by CVI SiC. The resid￾ual pore inter- and intra-agglomeration is unavoidable, but the amount of pore decreases obviously. The residual pore size inter-agglomeration is about 200–400m, which is rather smaller than that before infiltration. The residual pore size intra￾agglomeration is about 2–4 m, which is also smaller than that before infiltration. The size changes of inter- and intra￾agglomeration pores before and after infiltration shows that the amount of deposit in inter-agglomeration pore is more than that of intra-agglomeration pores. Interface microstructure observed by TEM shows a thin SiC layer can be deposited on the near surface of SiC reinforcing particle during CVI (see Fig. 2(c)). The incomplete burnout of residual carbon detaches from SiC reinforcing particle and exists between the CVI column SiC and the common SiC matrix, which will exist as a defect layer, resulting in weak interfacial bonding strength for SiC(P)/SiC composites. CVI SiC whiskers are found in some intra-agglomeration pores as shown in Fig. 2(d). The diameters of the whiskers are almost the same, about several Table 1 Mean free paths of reactant gases and other coefficients in the SiCl4–NH3–H2–Ar system Gas species Flux (ml min−1) σ (nm) M (g mol−1) Partial pressure (Pa) λ (m) H2 100 0.2915 2 270 37.7 SiCl4 30 0.5484 169.9 81 50.6 NH3 40 0.2900 17 108 90.1 Ar 200 0.3432 40 540 90.1