et al./Ce International 31(2005)47-52 L。PLs△UFs 嗑 叫a 30 Present results 0100200300400500600 Displacement/ Fig. 6. Graphite interlayer thickness dependence of the PLS and UFS of Fig. 5. Load-displacement curves. fibers failed after the specimens achieved their load maxi- tow to act like ropes thus providing sufficient reinforcement mums, beyond which the specimens showed certain levels to carry the load at the onset of matrix load [15]. Further of down-hill side load till large displacement. The fracture studies on the composites revealed that the thickness of this surfaces showed interfacial debonding and sound fiber pull- layer is also important in determining the performance of outs fracture behaviors, as typically shown in the inserted the materials 16,7]. A recent efforts on a systematic study SEM image in Fig. 5 on the effects of graphite interlayer thickness on the flex The proportional limit stress(PLS)and ultimate flex- ural properties of 2D CVI-Hi-Nicalon/SiC composites has ral strength(UFS)of the composite were derived from the been reported [7]. Those composites were fabricated using load-displacement curves and are listed in Table 1. The UFS MTS. The graphite interlayers of different thickness in the was determined according to simple beam theory as composites were deposited using an isothermal C VI coating 3 PL process prior to the fabrication of the composites Including UFS= 2 WT2 (2) the strengths of present composite in their results to relate the PlS and ufS to the graphite layer thickness resulted in here P is the flexural load. L is the load-supporting span Fig. 6, which indicates a just-fit PLS but a slightly lower UFS (18 mm). W and T are the width and thickness of the spec- of the present composite according to the strength-graphite imen, respectively. The PLS is the stress corresponding to thickness trends. However, considering the relatively large a 0.01% offset strain [14]. The average PLS and UFS are error bars, such a difference in the UFS is not signifi 450±65 MPa and567±75MPa, respectively. cant. This indicates that SiC/SiC composites from ETS would not suffer from significant change/loss in strength 3.3. Effect of the graphite interlayer but with the advantage of spontaneous graphite interlayer formation Tough ceramic matrix composites require a compl enforcement/matrix interlayer. Initially, there was little concern with regard to the thickness of the graphite layer in 3.4. Interfacial shear strength and its effects on PLS a SiC/SiC composite. It was assumed that the fibers were The composite exhibited incatastrophic failure behavior long and the entanglements in the bundles would allow the (Fig. 5)with interfacial debonding and sound fiber pullouts at the fracture surface indicating a reasonable interfacial bonding strength. To confirm this, the Iss was investigated 崇≡ ructures, and mechanical properties of the composites using single fiber pushout tests, and the result is given in Flexural strength ISS (MPa) Table 1. The average Iss is 86+19 MPa, which is far smaller than that of a Hi-Nicalon/SiC composite(505+91 MPa) Specimen PLS(MPa) UFS (MPa) from MTs, in which no interlayer was deposited [8].The high ISs of latter composite caused a brittle failure of the No.2433 material with rather low and same value of pls and uFs No. 3 91+18 MPa. While, the ductile fracture behavior of present Average450±65567±7586±19 ETS-composite with markedly improved strengths, 450+ (graphite) 65 MPa and 567+ 75 MPa for PLS and UFS, respectively50 W. Yang et al. / Ceramics International 31 (2005) 47–52 Fig. 5. Load–displacement curves. sliding, and failures of the fibers. A significant fraction of fibers failed after the specimens achieved their load maxi￾mums, beyond which the specimens showed certain levels of down-hill side load till large displacement. The fracture surfaces showed interfacial debonding and sound fiber pull￾outs fracture behaviors, as typically shown in the inserted SEM image in Fig. 5. The proportional limit stress (PLS) and ultimate flexu￾ral strength (UFS) of the composite were derived from the load–displacement curves and are listed in Table 1. The UFS was determined according to simple beam theory as: UFS = 3 2 PL WT2 (2) where P is the flexural load. L is the load-supporting span (18 mm). W and T are the width and thickness of the spec￾imen, respectively. The PLS is the stress corresponding to a 0.01% offset strain [14]. The average PLS and UFS are 450 ± 65 MPa and 567 ± 75 MPa, respectively. 3.3. Effect of the graphite interlayer Tough ceramic matrix composites require a compliant reinforcement/matrix interlayer. Initially, there was little concern with regard to the thickness of the graphite layer in a SiC/SiC composite. It was assumed that the fibers were long and the entanglements in the bundles would allow the Table 1 Density, interfacial structures, and mechanical properties of the composites Density (mg/m3) Interlayer (nm) Flexural strength ISS (MPa) Specimen PLS (MPa) UFS (MPa) No. 1 521 637 No. 2 433 577 No. 3 395 487 2.42 180 (graphite) Average 450 ± 65 567 ± 75 86 ± 19 Fig. 6. Graphite interlayer thickness dependence of the PLS and UFS of various Hi-Nicalon/SiC composites. tow to act like ropes thus providing sufficient reinforcement to carry the load at the onset of matrix load [15]. Further studies on the composites revealed that the thickness of this layer is also important in determining the performance of the materials [6,7]. A recent efforts on a systematic study on the effects of graphite interlayer thickness on the flex￾ural properties of 2D CVI-Hi-Nicalon/SiC composites has been reported [7]. Those composites were fabricated using MTS. The graphite interlayers of different thickness in the composites were deposited using an isothermal CVI coating process prior to the fabrication of the composites. Including the strengths of present composite in their results to relate the PLS and UFS to the graphite layer thickness resulted in Fig. 6, which indicates a just-fit PLS but a slightly lower UFS of the present composite according to the strength-graphite thickness trends. However, considering the relatively large error bars, such a difference in the UFS is not signifi- cant. This indicates that SiC/SiC composites from ETS would not suffer from significant change/loss in strength but with the advantage of spontaneous graphite interlayer formation. 3.4. Interfacial shear strength and its effects on PLS The composite exhibited incatastrophic failure behavior (Fig. 5) with interfacial debonding and sound fiber pullouts at the fracture surface, indicating a reasonable interfacial bonding strength. To confirm this, the ISS was investigated using single fiber pushout tests, and the result is given in Table 1. The average ISS is 86±19 MPa, which is far smaller than that of a Hi-Nicalon/SiC composite (505 ± 91 MPa) from MTS, in which no interlayer was deposited [8]. The high ISS of latter composite caused a brittle failure of the material with rather low and same value of PLS and UFS, 91±18 MPa. While, the ductile fracture behavior of present ETS-composite with markedly improved strengths, 450 ± 65 MPa and 567 ± 75 MPa for PLS and UFS, respectively