K. Shimoda et al./ Composites Science and Technology 68(2008)98-105 Table l al Sic/Sic composites for structural materials and corresponding issues Category Advanced fiber CvI Advanced fiber MI Crystallized PIP NITE Fiber Tyranno-SA/Hi-Nicalon Type-S Interphase PyC/SiC Matrix 3C-SIC 3C-SiC 3C-SiC Excess Si l06~ None Excess C Occationally Other phase(s) None Oxides. <I0% Moderate Low Moderate-high Thermal conductivity High Low-moderate Moderate-high Good interfaces arres st,deflect and branch the propagating crack that have initiated at outer or pore surfaces of the matrix The propagating of cracks deflected along the interface enables energy dissipation through friction, and conse- quently allows a pseudo-ductile macroscopic fracture behavior in ceramic composites [17-19]. pyrolytic carbon (PyC) has conventionally been employed as interfaces in SiC/SiC composites. Therefore, mechanical properties and fracture behavior might be highly improved by the processing control of interface. This paper focused on the influences of forming process and thickness of Pyc inter face on the density, process induced damage, mechanical 100nm properties and fracture behavior of unidirectional SiC/ Sic composites by NItE process ig. I. TEM micrograph of as-received SiC nano-powd 2. Experimental procedure TyrannoTM-SA grade-3 polycrystalline SiC fiber tows Y203=9 wt%(Al2O3: Y203=60: 40) and SiO2=3 wt%) / be Industries Ltd, Ube, Japan)were used as reinforce- Those prepared green sheets were unidirectionally stacked ent. General characteristics of the fiber are listed in th the fiber volume fraction of about 42-55% in a Table 2. The thickness of Py C layer deposited by chemical graphite fixture, and then hot-pressed at 1800C for 2 h vapor deposition(CVD) was highly-accurately controlled in Ar atmosphere under a pressure of 20 MPa after at about 0.25, 0.50 and 1.00 um. Ultra-fine B-SiC nano- drying powder (Sumitomo Osaka Cement Co. Ltd, Japan, T-1 Density and porosity of hot-pressed composites were grade)with a mean particle diameter of 30 nm was used determined by the Archimedes principle, using distilled or matrix formation, and Al2O3(Sumitomo Chemical water as the immersion medium. Theoretical density was Industries Ltd, Japan, 99.99% pure) with a mean particle calculated by following the ratio of a mixture of Sic diameter of 0.3 um, Y,O3(Kojundo Chemical Laboratory nano-powder, sintering additives and fiber volume fraction Co Ltd, Japan, 99.99% pure)with a mean particle diam- Three-point bending test(test bars 4.0 Wx25x20mm' eter of 0.4 um and SiO,(Kojundo Chemical Laboratory was carried out at room-temperature, with crosshead speed Co.Ltd, Japan,99.9%pure)with a mean particle diame- of 0.5 mm/min and outer support span of 18 mm in an ter of I um were used as sintering additives. Fig. I shows INSTRON 5581 test machine, using the number of 3-5 TEM image of SiC nano-powder as-received. Unidirec- specimens. Bending bars were cut parallel to the fiber axis tional uncoated and PyC coated fiber tows were Flexural stress(o), flexural strain(e) and modulus of elas impregnated in SiC "nano-slurry, which is mixture of ticity in bending(E)were calculated by the following Eqs Sic nano-powder and sintering additives (Al,O3+ (1H3), respectively Table 2 Properties of Tyranno M-SA fiber(grade-3 SiC fiber Atomic ratio(C/Si) Diameter(um) Density(mg/m,) Filaments/yarn Tensile strength(GPa) Elastic modulus(GPa) TyrannoM-SA 1.08 3.10interfaces arrest, deflect and branch the propagating cracks that have initiated at outer or pore surfaces of the matrix. The propagating of cracks deflected along the interface enables energy dissipation through friction, and conse￾quently allows a pseudo-ductile macroscopic fracture behavior in ceramic composites [17–19]. pyrolytic carbon (PyC) has conventionally been employed as interfaces in SiC/SiC composites. Therefore, mechanical properties and fracture behavior might be highly improved by the processing control of interface. This paper focused on the influences of forming process and thickness of PyC inter￾face on the density, process induced damage, mechanical properties and fracture behavior of unidirectional SiC/ SiC composites by NITE process. 2. Experimental procedure TyrannoTM-SA grade-3 polycrystalline SiC fiber tows (Ube Industries Ltd., Ube, Japan) were used as reinforce￾ment. General characteristics of the fiber are listed in Table 2. The thickness of PyC layer deposited by chemical vapor deposition (CVD) was highly-accurately controlled at about 0.25, 0.50 and 1.00 lm. Ultra-fine b-SiC nano￾powder (Sumitomo Osaka Cement Co. Ltd., Japan, T-1 grade) with a mean particle diameter of 30 nm was used for matrix formation, and Al2O3 (Sumitomo Chemical Industries Ltd., Japan, 99.99% pure) with a mean particle diameter of 0.3 lm, Y2O3 (Kojundo Chemical Laboratory Co. Ltd., Japan, 99.99% pure) with a mean particle diam￾eter of 0.4 lm and SiO2 (Kojundo Chemical Laboratory Co. Ltd., Japan, 99.9% pure) with a mean particle diame￾ter of 1 lm were used as sintering additives. Fig. 1 shows TEM image of SiC nano-powder as-received. Unidirec￾tional uncoated and PyC coated fiber tows were impregnated in SiC ‘‘nano’’-slurry, which is mixture of SiC nano-powder and sintering additives (Al2O3 + Y2O3 = 9 wt% (Al2O3:Y2O3 = 60:40) and SiO2 = 3 wt%). Those prepared green sheets were unidirectionally stacked with the fiber volume fraction of about 42–55% in a graphite fixture, and then hot-pressed at 1800 C for 2 h in Ar atmosphere under a pressure of 20 MPa after drying. Density and porosity of hot-pressed composites were determined by the Archimedes principle, using distilled water as the immersion medium. Theoretical density was calculated by following the ratio of a mixture of SiC nano-powder, sintering additives and fiber volume fraction. Three-point bending test (test bars 4.0w · 25L · 2.0T mm3 ) was carried out at room-temperature, with crosshead speed of 0.5 mm/min and outer support span of 18 mm in an INSTRON 5581 test machine, using the number of 3–5 specimens. Bending bars were cut parallel to the fiber axis. Flexural stress (r), flexural strain (e) and modulus of elas￾ticity in bending (E) were calculated by the following Eqs. (1)–(3), respectively: Table 1 List of developmental SiC/SiC composites for structural materials and corresponding issues Category Advanced fiber CVI Advanced fiber MI Crystallized PIP NITE Fiber Tyranno-SA/Hi-Nicalon Type-S Interphase PyC PyC/SiC Varied PyC Matrix Base phase 3C–SiC 3C–SiC 3C–SiC 3C–SiC Porosity 10% 0% 20% 0% Excess Si Occationally 10% None None Excess C None Occationally 10% None Other phase (s) None None None Oxides, <10% General issues Strength Moderate Moderate Low Moderate-high Thermal conductivity Moderate High Low-moderate Moderate-high Hermeticity Poor Poor Poor Good Table 2 Properties of TyrannoTM-SA fiber (grade-3) SiC fiber Atomic ratio (C/Si) Diameter (lm) Density (mg/m3 ) Filaments/yarn Tensile strength (GPa) Elastic modulus (GPa) TyrannoTM-SA 1.08 7.5 3.10 1600 2.51 409 Fig. 1. TEM micrograph of as-received SiC nano-powder. K. Shimoda et al. / Composites Science and Technology 68 (2008) 98–105 99