REBILLAT et al: SiC/SIC COMPOSITES 4611 Table 1. Interfacial shear stress (MPa) measured using various methods on 2D-SiC/SiC composites with Py C based fiber coatings and reinforced with either as-received or treated fibers SiC/C/SiC )n/SiC loops)[24] domain Untreated fibers 21-115 4080 12-10 Treated fibers PyC(o 1) 165-273 PyC/SiC)4 2). The selected processing conditions have been shown to cause minimum damage to the fibers and to improve adhesion of the BN coating onto the fib- ers, and the microstructure [13]. In the bi-layered coating(referred to as BN4, Table 2 and Fig. 3), the first sublayer on the fiber is BN2 type(poorly crystallized), whereas the second one is BNI type (highly crystallized). Processing of BN2 involved the less aggressive gaseous phase, which led to a better contact between the fiber and the coating. The pro- essing conditions of BNI were found to be aggress- ive against the fibers [13]- (a) 3.2. Push-out tests Fiber bonding and frictional sliding in the 2D SiC/BN/SiC composites were investigated by means of single fiber push-out tests [14-17].500 um thick wedges were prepared using standard metallographic ONERA, France)was used. The load was applied to the top of the fiber using a flat bottom diamond cone (at a constant displacement rate of 0.1 um/s). The nterface characteristics were extracted from the experimental stress-fiber end displacement curves by fitting the push-out model of Hsueh [ 15], as discussed n a previous paper [16] Only a few push-out experiments could be carried out on the microcomposites owing to the difficulties involved in microcomposite handling, preparation and testing. Parallel-faced strips were cut out of the Fig 3. Images of BN coatings:(a)TEM-image and DEAS. microcomposites which had been previously embed- picture of the BNI coating showing the three-dimensional ded in glass [18](microcomposites 2)or in a ceramic xagonal structure; (b) SEM-image of bi-layered BN ement [19](microcomposites 2 and 4). A first series BN4)in a SiC/BN/SiC microcomposite showing dered BN2 layer and the BN1 layer with a three dimensional ordered hexagonal structure Table 2. Main characteristics ber coatings [13] Batch BN coating Number Coating of push out tests on microcomposites 2 used a Vickers zation adhesion diamond probe [18).A correction for indentor dis- placement was done. Most of the tests were push in rather than push-out tests, Push-out could not be BN1+BN2 Strong achieved on the specimens with a thickness exceeding 200 um. A number of push-in curvesREBILLAT et al.: SiC/SiC COMPOSITES 4611 Table 1. Interfacial shear stress (MPa) measured using various methods on 2D-SiC/SiC composites with PyC based fiber coatings and reinforced with either as-received or treated fibers SiC/C/SiC Interphase Crack spacing Crack spacing Tensile tests Tensile tests Push-out tests Push-out tests SiC/(C/SiC)n/SiC [30] [31] (hysteresis (curved (curved (plateau) loops) [24] domain) domain) [8, 29] [7, 24] [8, 29] Untreated fibers 2D woven PyC (0.1) 12 8 0.7 Microcomposites PyC(0.1) 3 4–20 Minicomposites PyC(0.1) 21–115 40–80 2D woven PyC(0.5) 4 14–16 12–10 (PyC/SiC)2 2 31 19.3 (PyC/SiC)4 9 28 12.5 Treated fibers 2D woven PyC(0.1) 203 140 190 165–273 PyC(0.5) 370 100–105 (PyC/SiC)2 150 133 (PyC/SiC)4 90 90 2). The selected processing conditions have been shown to cause minimum damage to the fibers and to improve adhesion of the BN coating onto the fib￾ers, and the microstructure [13]. In the bi-layered coating (referred to as BN4, Table 2 and Fig. 3), the first sublayer on the fiber is BN2 type (poorly crystallized), whereas the second one is BN1 type (highly crystallized). Processing of BN2 involved the less aggressive gaseous phase, which led to a better contact between the fiber and the coating. The pro￾cessing conditions of BN1 were found to be aggress￾ive against the fibers [13]. 3.2. Push-out tests Fiber bonding and frictional sliding in the 2D￾SiC/BN/SiC composites were investigated by means of single fiber push-out tests [14–17]. 500 µm thick wedges were prepared using standard metallographic techniques. An interfacial test system (designed by ONERA, France) was used. The load was applied to the top of the fiber using a flat bottom diamond cone (at a constant displacement rate of 0.1 µm/s). The interface characteristics were extracted from the experimental stress–fiber end displacement curves by fitting the push-out model of Hsueh [15], as discussed in a previous paper [16]. Only a few push-out experiments could be carried out on the microcomposites owing to the difficulties involved in microcomposite handling, preparation and testing. Parallel-faced strips were cut out of the microcomposites which had been previously embed￾ded in glass [18] (microcomposites 2) or in a ceramic cement [19] (microcomposites 2 and 4). A first series Table 2. Main characteristics of the BN fiber coatings [13] Batch BN coating Number of Degree of Coating BN layers crystallization adhesion 1 BN1 1 High Strong 2 BN2 1 Low Weak 4 BN4 2 BN11BN2 Strong Fig. 3. Images of BN coatings: (a) TEM-image and DEAS￾picture of the BN1 coating showing the three-dimensional ordered hexagonal structure; (b) SEM-image of bi-layered BN interphase (BN4) in a SiC/BN/SiC microcomposite showing poorly ordered BN2 layer and the BN1 layer with a three￾dimensional ordered hexagonal structure. of push out tests on microcomposites 2 used a Vickers diamond probe [18]. A correction for indentor dis￾placement was done. Most of the tests were push￾in rather than push-out tests. Push-out could not be achieved on the specimens with a thickness exceeding 200 µm. A number of push-in curves was unusable