S Jacques et al. Journal of the European Ceramic Society 20(2000)1929-1938 (1+vEmEr +(1-2v)Eol 2.43. B microstructure and texture Erl(+DEr+(I-v)Eol Thin longitudinal sections of minicomposites after tensile test were studied by transmission electron microscopy(TEM: Topcon 002B, Japan) using bright- where o is the maximum stress of the cycle, Eo is the field, high resolution(HR) and selected area diffraction initial Youngs modulus of the minicomposite, Er(280 (SAD) techniques. The samples were embedded in a GPa) and Em (400 GPa)are the Youngs moduli of ceramic cement and mechanically thinned. The thin he fibre and the matrix, respectively, v is the SiC Pois- sheets were then ion-milled(600 Duo Mill from Gatan sons ratio (vavevm0. 2), and R is the fibre radius USA)until the electron transparency (R7 um).84 was measured on the last cycle befor the failure. N is the number of matrix cracks. It has be measured by optical microscopy on polished long itudinal sections of the failed minicomposites after che- mical etching(Murakami reactant) in order to reveal 3. 1. Mechanical behaviour the matrix microcracks closed during unloading The second method considers the matrix crack spacing 3. .. Tensile tests distance Is(=N/L,. t is given by Eq( 2). 9. 0 Average tensile mechanical characteristics at ambient temperature for the four batches of minicomposites are os Re Err (2) Presented in Table 1. Fig 3 displays a typical force-strain Table verage tensile mechanical characteristics of each batch of mini- where os is the applied stress at matrix cracking satura composites. ion and Vm the matrix volume fraction(Vml-Ve) In Batch r (m/h) EPL (%) FPL(N EF(% FF(N) he present case, because of a low fibre volume fraction (Ve s0. 2), the minicomposites failed before reaching the 0(PyC) 6 56 cracking saturation. Therefore, os was similar to the 2 After ultimate failure, the morphology of the fracture EPL and FPL are the proportional limit and force, EF and FF are surfaces was observed with a scanning electron microscope the strain and force at failure (SEM)(HITACHI S800) 2. 2. Oxidation test 180 Interphase 0 In order to study the oxidation resistance of the inter- phases, the minicomposites were submitted to thermal ageing in air under a static loading following a procedure 120 similar to that described by Lebrun et al. Each speci- F(N men, with a 50 mm gauge length, was vertically main- tained in the hot area of a furnace between two alumina tubes with an alumina-based adhesive(Armco ref. 603 SA). At each alumina rod ends, hooks allowed device self-alignment. Once the temperature reached the set point of 700oC, a 9.5 kg load(corresponding to a force E(%) E(%) 180 of 93 N)was very carefully hooked to the bottom end of Interphase 2 the alumina tube in the cold area. The specimen lifetime (i.e. the time before failure) was automatically measured 120 by means of a switch connected with a timer which F(N FON detects the load fall. The test originality lied in the pos sibility to expose the minicomposite either to a dry air stream(from the top to the bottom obtained from liquid air evaporation or to a moist air obtained by sparging in 40 C liquid water. In the case of moist air, the air inlet pipe was maintained at 50 C between the sparger and the furnace; this device ensured constant Fig 3. Typical tensile force-strain curves with unloading-reloading moisture content for each experiment cycles for minicomposites with different kinds of interphaseb2 ˆ …1 ‡ †Em‰ Š Ef ‡ …1 ÿ 2†E0 Ef‰ Š …1 ‡ †Ef ‡ …1 ÿ †E0 where p is the maximum stress of the cycle, E0 is the initial Young's modulus of the minicomposite, Ef (280 GPa) and Em (400 GPa) are the Young's moduli of the ®bre and the matrix, respectively,  is the SiC Pois￾son's ratio (nnfnm0.2), and Rf is the ®bre radius (Rf7 mm). 
was measured on the last cycle before the failure. N is the number of matrix cracks. It has been measured by optical microscopy on polished long￾itudinal sections of the failed minicomposites after che￾mical etching (Murakami reactant) in order to reveal the matrix microcracks closed during unloading. The second method considers the matrix crack spacing distance ls (=N/Lg).  is given by Eq. (2).9,10  ˆ sRf 2Vfls 1 ‡ EfVf EmVm   …2† where s is the applied stress at matrix cracking satura￾tion and Vm the matrix volume fraction (Vm1ÿVf). In the present case, because of a low ®bre volume fraction (Vf 0.2), the minicomposites failed before reaching the cracking saturation. Therefore, s was similar to the failure stress. After ultimate failure, the morphology of the fracture surfaces was observed with a scanning electron microscope (SEM) (HITACHI S800). 2.4.2. Oxidation tests In order to study the oxidation resistance of the inter￾phases, the minicomposites were submitted to thermal ageing in air under a static loading following a procedure similar to that described by Lebrun et al.11 Each speci￾men, with a 50 mm gauge length, was vertically main￾tained in the hot area of a furnace between two alumina tubes with an alumina-based adhesive (Aremco ref. 603, USA). At each alumina rod ends, hooks allowed device self-alignment. Once the temperature reached the set￾point of 700C, a 9.5 kg load (corresponding to a force of 93 N) was very carefully hooked to the bottom end of the alumina tube in the cold area. The specimen lifetime (i.e. the time before failure) was automatically measured by means of a switch connected with a timer which detects the load fall. The test originality lied in the pos￾sibility to expose the minicomposite either to a dry air stream (from the top to the bottom) obtained from liquid air evaporation or to a moist air obtained by sparging in 40C liquid water. In the case of moist air, the air inlet pipe was maintained at 50C between the sparger and the furnace; this device ensured constant moisture content for each experiment. 2.4.3. BN microstructure and texture Thin longitudinal sections of minicomposites after tensile test were studied by transmission electron microscopy (TEM: Topcon 002B, Japan) using bright- ®eld, high resolution (HR) and selected area di€raction (SAD) techniques. The samples were embedded in a ceramic cement and mechanically thinned. The thin sheets were then ion-milled (600 Duo Mill from Gatan, USA) until the electron transparency. 3. Results 3.1. Mechanical behaviour 3.1.1. Tensile tests Average tensile mechanical characteristics at ambient temperature for the four batches of minicomposites are presented in Table 1. Fig. 3 displays a typical force±strain Fig. 3. Typical tensile force±strain curves with unloading-reloading cycles for minicomposites with di€erent kinds of interphases. Table 1 Average tensile mechanical characteristics of each batch of mini￾composites.a Batch r (m/h) "PL (%) FPL (N) "F (%) FF (N) 0 (PyC) 6 0.09 111 0.56 169 1 2 0.04 59 0.35 128 2 2.5 0.10 119 0.53 167 3 3 0.06 83 0.55 137 a "PL and FPL are the proportional limit and force, "F and FF are the strain and force at failure S. Jacques et al. / Journal of the European Ceramic Society 20 (2000) 1929±1938 1931