Y Liu et al. Materials Science and Engineering A 498(2008)430-436 500 Pa, yielding a thickness of 200 nm. Thirdly, two layers of (a) SiC matrices were infiltrated among monofilaments. Subsequently the two layers of Sic and two layers of BCx were infiltrated by alternate ICVI in different reactors. Each layer of Sic matrix was chieved at 1000C for 80 h at reduced pressure of 2 kPa by using methyltrichlorosilane(MTS, CH3 SiCl3) with a H2: MTS molar ratio of 10. This was achieved by bubbling hydrogen gas through the MTS. The argon diluent was used to slow down the chemical reaction rate during deposition. BCx matrix deposition condi- tions were as follows: temperature 950C, pressure 1 kPa, time Oh, boron trichloride(BCl3 299.99 vol% and iron< 10 ppm)flow 20ml min-, methane(CH4 >99.95 vol %)flow 10 ml min-I, hydro- gen(H2 >99.999 vol %)flow 60 ml min-, Argon(Ar>99.99 vol%) ow 60 ml min-I. Then, the as-received composite was machined nd polished, and 3 mm x 4 mm x 30 mmsubstrates were obtained Finally, the specimens were coated with three layers CVD SiC coat ing. The conditions for CVD Sic coating were the same as the sic sa17cmm×06 matrix except for the deposition time, which was 30 ha time. On the other hand, in order to compare the mechanical properties, the C/ Sic (b)F. composites were prepared under the same deposition conditions, which had six layers Sic matrix and three layers Sic coatings 2. 2. Characterization and measurements of the composites The morphologies of the BCx layer and the modified com posites were observed with scanning electron microscope(SEM, JSM6700F). The flexural strength of the composite specimens before and after oxidation was measured by a three-point bend- and the loading rate was O5 nure. The span dimension was 20 mm ing method at room tempel Fracture toughness was measured by the single-edge notch beam method with samples of 3 mm x 5 mm x 40 mm. Five samples vere tested. The notch was produced by electro-discharge machin- ing with a depth and a width of 2.5 and 0.2 mm, respectively. the span dimension was 30 mm and the loading rate was 0.5 mm min- m x2.00k The value of fracture toughness(Klc) was calculated by using of the American ASTME 399-74 expression (1) 后)=2(0)2-4829)+2189() 382(1)+23829( where Y is the geometrical factor for an edge crack in a three-point bend beam, calculated by Eq (2): Pc is the fracture load; C is the notch depth; S is the span dimension, H and B are the thickness and broadness of the sample, respectively. Fracture work was calculated by the following formula [19 here ac is characteristic area of fracture curve which refers to the area under load-displacement curve above 90% stress: H and B are Fig. 1. Morphologies of the multilayer matrices of the as-fabricated modified the thickness and broadness of the sample, respectively composites:(a) multilayer matrices among fiber bundles; (b)Sic matrix among mono-filament; (c)EDAX graph of multilayer matrices. The monotonic tensile tests were performed at room temper ature to determine the tensile properties of the material, namel the ultimate tensile stress on a servo-hydraulic machine(Model INSTRON 1196 from INSTRON Ltd, England). Five samples were Tensile strength and tensile modulus was calculated by the fol- tested. The load increased at a constant rate of 0.2 mm min-1 lowing formulas: to fracture of the specimens. The displacement, load were moni- red Strain to failure was measured through strain gage(INSTRON CATALOGUE 2620-601) BH (4)Y. Liu et al. / Materials Science and Engineering A 498 (2008) 430–436 431 of 500 Pa, yielding a thickness of 200 nm. Thirdly, two layers of SiC matrices were infiltrated among monofilaments. Subsequently the two layers of SiC and two layers of BCx were infiltrated by alternate ICVI in different reactors. Each layer of SiC matrix was achieved at 1000 ◦C for 80 h at reduced pressure of 2 kPa by using methyltrichlorosilane (MTS, CH3SiCl3) with a H2:MTS molar ratio of 10. This was achieved by bubbling hydrogen gas through the MTS. The argon diluent was used to slow down the chemical reaction rate during deposition. BCx matrix deposition condi￾tions were as follows: temperature 950 ◦C, pressure 1 kPa, time 20 h, boron trichloride (BCl3 ≥ 99.99 vol.% and iron ≤ 10 ppm) flow 20 ml min−1, methane (CH4 ≥ 99.95 vol.%) flow 10 ml min−1, hydro￾gen (H2 ≥ 99.999 vol.%) flow 60 ml min−1, Argon (Ar ≥ 99.99 vol.%) flow 60 ml min−1. Then, the as-received composite was machined and polished, and 3 mm × 4 mm × 30 mm substrates were obtained. Finally, the specimens were coated with three layers CVD SiC coat￾ing. The conditions for CVD SiC coating were the same as the SiC matrix except for the deposition time, which was 30 h a time. On the other hand, in order to compare the mechanical properties, the C/SiC composites were prepared under the same deposition conditions, which had six layers SiC matrix and three layers SiC coatings. 2.2. Characterization and measurements of the composites The morphologies of the BCx layer and the modified com￾posites were observed with scanning electron microscope (SEM, JSM6700F). The flexural strength of the composite specimens before and after oxidation was measured by a three-point bend￾ing method at room temperature. The span dimension was 20 mm and the loading rate was 0.5 mm min−1. Five samples were tested. Fracture toughness was measured by the single-edge notch beammethod with samples of 3 mm × 5 mm × 40 mm. Five samples were tested. The notch was produced by electro-discharge machin￾ing with a depth and a width of 2.5 and 0.2 mm, respectively. The span dimension was 30 mm and the loading rate was 0.5 mm min−1. The value of fracture toughness (K1c) was calculated by using of the American ASTME 399-74 expression: K1c = PC B S H3/2 f C H (1) f C H = 2.9 C H 1/2 − 4.62.9 C H 3/2 + 21.82.9 C H 5/2 −37.62.9 C H 7/2 + 38.72.9 C H 9/2 (2) where Y is the geometrical factor for an edge crack in a three-point bend beam, calculated by Eq. (2); PC is the fracture load; C is the notch depth; S is the span dimension, H and B are the thickness and broadness of the sample, respectively. Fracture work was calculated by the following formula [19]: W = Ac BH (3) where Ac is characteristic area of fracture curve, which refers to the area under load–displacement curve above 90% stress; H and B are the thickness and broadness of the sample, respectively. The monotonic tensile tests were performed at room temper￾ature to determine the tensile properties of the material, namely, the ultimate tensile stress on a servo-hydraulic machine (Model INSTRON 1196 from INSTRON Ltd., England). Five samples were tested. The load increased at a constant rate of 0.2 mm min−1 up to fracture of the specimens. The displacement, load were moni￾tored. Strain to failure was measured through strain gage (INSTRON CATALOGUE 2620-601). Fig. 1. Morphologies of the multilayer matrices of the as-fabricated modified composites: (a) multilayer matrices among fiber bundles; (b) SiC matrix among mono-filament; (c) EDAX graph of multilayer matrices. Tensile strength and tensile modulus was calculated by the fol￾lowing formulas: T = P BH (4)