W. Yang et al/Ceramics International 31(2005)525-531 but multiple PyC sub-layers, has been developed [5,10]. volume ratio 1: 10 and total hydrogen flow rate 1000 sccm tion resistance of several Hi-Nicalon fiber The preforms were kept at 1273 K during the CVI process reinforced SiC/SiC composites with such multilayers have The matrix densification process continued for 17 h under been reported [10,11]. reduced pressure(total reaction pressure)of 14.7 kPa. Prior Although SiC/SiC composites with(PyC-SiCn multi- to the matrix densification, fiber/matrix interfacial coatings layers showed promise of improving the oxidation with designed structures of 100 nm PyC and 50 nm PyC+ sistance,it remains unclear whether such stiff SiC sub- 150 nm SiC 50 nm Pyc were deposited on the fiber layer in the multilayers will cause the degradation of the surfaces in the two performs, respectively, using the same mechanical performances of the composites, especially for CVI system. CH4 was used as source gas for PyC layer with those reinforced with Tyranno-SA fiber, which is a newly processing conditions as: temperature, 1223 K; total developed advanced SiC fiber, with near stoichiometric C-Si pressure, 14.7 kPa; CH4 flow rate, 200 sccm. The deposition chemistry and highly crystalline structure [12]. This fiber conditions for the Sic sub-layer in the tri-interlayer exhibits excellent mechanical properties, much improved composite were the same as the matrix densification. After thermal conductivity and thermal stability, and relatively the deposition of each sub-layer, the CVI system low fabrication cost compared with the old-generation Sic- maintained at the deposition temperatures for about 10 min based fibers such as Nicalon-CG and Hi-Nicalom [8], to make sure a full reaction of the residual source gas in the although the Tyranno-SA fibers display lower failure strain furnace, then, moved to next sub-layer deposition (related to its high modulus), which limits the non-linear Upon completing the fabrication process, the micro- stress-strain domain of the SiC/SiC composites. The Sic/ structure of the composites, the thickness and space site reinforced with Tyranno-SA fibers also uniformities of the deposited interlayers were inspected showed much higher statistic reliability of flexural strength using scanning electron microscopy (SEM, JEOL JSM- [13]. Its Weibull modulus of strength upon three-point 6700F). The interlayer thickness was measured on high ending was 10.2 versus those of the Nicalon/SiC (2. 1)and magnification SEM images from six areas over the cross- Hi-Nicalon/SiC (7. 4)composites. Therefore, it is important section of each composite with an estimated resolution of issue to make an understanding of the effects of the carbon 10 nm and/or(PyC-SiC)n interlayers on the interfacial propertie and mechanical properties of the advanced Tyranno-SA/SiC 2. 2. Mechanical tests Previous researches indicated that flexural strengths of The mechanical properties and fracture behaviors were investigated bending (w thickness of carbon interlayer up to 100 nm, beyond which span of 18 mm). Three tests were conducted for each no obvious change of the strength was observed [14]. In this composite. Specimens were cut parallel to one of the fiber study, two new Tyranno-SA/SiC composites with 100 nm bundle directions of the fabric cloth. Both the tensile and PyC single-interlayer(TSA-SL) and 50 nm PyC 150 nm compression surfaces of each specimen were carefully SiC +50 nm PyC multilayers(TSA-ML)were designed and ground using diamond slurry to eliminate the effects of fabricated to investigate the effects of the SiC sub-layer on surface CVD-SiC layers, which were formed at the end of the interfacial shear strength(ISs)and mechanical proper- the CVi process. The final dimension of the specimen w ties of the composites. Because of the limited number of L30 mm x w4.0 mm x T1.5 mm. The crosshead speed was specimens for each composite, only comparative three-point 0.0083 mm/s. The load/displacement data were recorded bending tests were performed for comparison with previous Proportional limit stress(PLS)and ultimate flexural strength results (UFS) were derived from the load/displacement curves according to ASTM C 1341-97[15]. The fracture surfaces were observed with interfacial debonding and fiber pullouts 2. Experimental using the SEM. Specimens that did not completely separate during the bending tests were carefully broken apart by 2. 1. Interlayer deposition and composite fabrication hands so that the fracture surfaces could be examined As mentioned before, interfacial bonding strength is The composites were fabricated using a chemical vapor critical on determining the mechanical properties of SiC/SiC infiltration(CVI) system. Detailed process information can composites. Interfacial shear strength is associated with the be found elsewhere [14]. In brief, two composite preforms fiber bond strength and represents the stress required were prepared with 2D plain-woven Tyranno-sA overcome the chemical bonding and static coefficient of (as-received) in 0/90. No surface pre-treatment was friction between the fiber and the fiber coating [16]. The rformed to the fibers. The volume loads of the fibers ISSs of both composites were investigated by single fiber for both preforms were 43%. The preforms were densified pushout technique, which is a widely used technique for with SiC matrix through thermal decomposition deriving Iss in SiC/Sic composites because of simplicit CH3SiCI3 (MTS). Mrs was carried by hydrogen with a easy in sample preparation, and relatively easy in obtaininbut multiple PyC sub-layers, has been developed [5,10]. Improved oxidation resistance of several Hi-Nicalon fiber reinforced SiC/SiC composites with such multilayers have been reported [10,11]. Although SiC/SiC composites with (PyC-SiC)n multi￾layers showed promise of improving the oxidation resistance, it remains unclear whether such stiff SiC sub￾layer in the multilayers will cause the degradation of the mechanical performances of the composites, especially for those reinforced with Tyranno-SA fiber, which is a newly developed advanced SiC fiber, with near stoichiometric C-Si chemistry and highly crystalline structure [12]. This fiber exhibits excellent mechanical properties, much improved thermal conductivity and thermal stability, and relatively low fabrication cost compared with the old-generation SiC￾based fibers such as Nicalon-CG and Hi-NicalomTM [8], although the Tyranno-SA fibers display lower failure strain (related to its high modulus), which limits the non-linear stress–strain domain of the SiC/SiC composites. The SiC/ SiC composite reinforced with Tyranno-SA fibers also showed much higher statistic reliability of flexural strength [13]. Its Weibull modulus of strength upon three-point bending was 10.2 versus those of the Nicalon/SiC (2.1) and Hi-Nicalon/SiC (7.4) composites. Therefore, it is important issue to make an understanding of the effects of the carbon and/or (PyC-SiC)n interlayers on the interfacial properties and mechanical properties of the advanced Tyranno-SA/SiC composites. Previous researches indicated that flexural strengths of Tyranno-SA/SiC composites were very sensitive to the thickness of carbon interlayer up to
100 nm, beyond which no obvious change of the strength was observed [14]. In this study, two new Tyranno-SA/SiC composites with 100 nm PyC single-interlayer (TSA-SL) and 50 nm PyC + 150 nm SiC + 50 nm PyC multilayers (TSA-ML) were designed and fabricated to investigate the effects of the SiC sub-layer on the interfacial shear strength (ISS) and mechanical proper￾ties of the composites. Because of the limited number of specimens for each composite, only comparative three-point bending tests were performed for comparison with previous results. 2. Experimental 2.1. Interlayer deposition and composite fabrication The composites were fabricated using a chemical vapor infiltration (CVI) system. Detailed process information can be found elsewhere [14]. In brief, two composite preforms were prepared with 2D plain-woven Tyranno-SA fiber cloths (as-received) in 0/908. No surface pre-treatment was performed to the fibers. The volume loads of the fibers for both preforms were 43%. The preforms were densified with SiC matrix through thermal decomposition of CH3SiCl3 (MTS). MTS was carried by hydrogen with a volume ratio 1:10 and total hydrogen flow rate 1000 sccm. The preforms were kept at 1273 K during the CVI process. The matrix densification process continued for 17 h under reduced pressure (total reaction pressure) of 14.7 kPa. Prior to the matrix densification, fiber/matrix interfacial coatings with designed structures of 100 nm PyC and 50 nm PyC + 150 nm SiC + 50 nm PyC were deposited on the fiber surfaces in the two performs, respectively, using the same CVI system. CH4 was used as source gas for PyC layer with processing conditions as: temperature, 1223 K; total pressure, 14.7 kPa; CH4 flow rate, 200 sccm. The deposition conditions for the SiC sub-layer in the tri-interlayer composite were the same as the matrix densification. After the deposition of each sub-layer, the CVI system was maintained at the deposition temperatures for about 10 min to make sure a full reaction of the residual source gas in the furnace, then, moved to next sub-layer deposition. Upon completing the fabrication process, the micro￾structure of the composites, the thickness and space uniformities of the deposited interlayers were inspected using scanning electron microscopy (SEM, JEOL JSM- 6700F). The interlayer thickness was measured on high magnification SEM images from six areas over the cross￾section of each composite with an estimated resolution of
10 nm. 2.2. Mechanical tests The mechanical properties and fracture behaviors were investigated by three-point bending tests (with a support span of 18 mm). Three tests were conducted for each composite. Specimens were cut parallel to one of the fiber bundle directions of the fabric cloth. Both the tensile and compression surfaces of each specimen were carefully ground using diamond slurry to eliminate the effects of surface CVD-SiC layers, which were formed at the end of the CVI process. The final dimension of the specimen was L30 mm W4.0 mm T1.5 mm. The crosshead speed was 0.0083 mm/s. The load/displacement data were recorded. Proportional limit stress (PLS) and ultimate flexural strength (UFS) were derived from the load/displacement curves according to ASTM C 1341-97 [15]. The fracture surfaces were observed with interfacial debonding and fiber pullouts using the SEM. Specimens that did not completely separate during the bending tests were carefully broken apart by hands so that the fracture surfaces could be examined. As mentioned before, interfacial bonding strength is critical on determining the mechanical properties of SiC/SiC composites. Interfacial shear strength is associated with the fiber bond strength and represents the stress required to overcome the chemical bonding and static coefficient of friction between the fiber and the fiber coating [16]. The ISSs of both composites were investigated by single fiber pushout technique, which is a widely used technique for deriving ISS in SiC/SiC composites because of simplicity, easy in sample preparation, and relatively easy in obtaining 526 W. Yang et al. / Ceramics International 31 (2005) 525–531