B Wilshire, M.R. Bache /Journal of the European Ceramic Sociery 27(2007)4603-4611 4605 130 MPa 120 MPa 1051△scA203 0.015 100 MPa 110 MPa ▲ HNSICf-A203 0.010 0.005 aooEEcs 0.000 Time(ks) Stress(MPa) are Em measurements reported for Nicalon TM NLM202 fibres HNSiC-Sicll and HNSiCr-SiBC samples tested in air at C sips for Fig. 1. The variations of the creep strain with time for HNSiCr-SiBC samples sted at various stresses in air at 1300oC iCe-Al, O3 an to the stress axes occupy approximately one fifth of the testpiece The presence of 25 vol %o Sic whiskers significantly cross-sectional areas. Thus, the load-bearing capabilities of the increases the creep resistance of alumina. The porous Al2O3 alumina-matrix composites are governed by the longitudinal matrices formed by in situ oxidation of liquid aluminium must therefore have creep strengths much lower than that of the fully s are ss lev- or While the creep resistance of the fibres clearly exceeds that dense SiCw-Al2O3 samples. Yet, at comparable stress lev- of the matrices with the SiCr-Al2O3 and HNSiCr-Al2O3 sam- els, the Em values for the SiCw-Al2O3 specimens are several ples, it has been claimed that the matrices are more creep orders of magnitude faster than those for the HNSiCf-Al2O3 resistant than the fibres with the SiC-SiC, SiCf-SiBCO and and SiCr-Al2O3 products Consequently, the alumina matrices HNSiCr-SiC materials However, as evident from Fig 3, the make little contribution to the overall creep resistance of the Em values recorded for the SiCr-A12O3 testpieces are equal to fibre-reinforced composites. those for the SiCf-SiBC and lower than those for the SiCf-Sic <It is also apparent from Fig. 2 that stresses about five times specimens, with all three materials being reinforced with com- gher must be applied to the Nicalon NLM202 and Hi- able volume fractions of 0/90 NicalonMNLM202 fibre NicalonM fibres to obtain creep rates comparable with those for Similarly, with equivalent Hi-Nicalon reinforcement, the Em val the SiCf-Al2O3 and HNSiCr-Al2O3 specimens, respectively. ues for HNSiCf-A12O3 are equal to those for HNSiCf-SiBC and This result would be expected because these composites contain lower than those for the HNSiCr-SiC sample(Fig 3). He O vol. of interwoven 0/900 fibre bundles, so the fibres parallel porous SiC and SiBC matrices must be characterized by a creep resistance at least as poor as that for the weak A12O3 matrices Irrespective of whether the present series of CFCMCs were produced with A12O3, SiC or SiBC matrices, it is therefore clear oSiCw-Al203 that the creep strengths of the matrices are markedly inferior to those of the fibres. On this basis, from the results presented Cao- Mao in Figs. 3 and 4, the improvement in creep and creep rupture resistance achieved by replacing Nicalon NLM202 with Hi- Nicalon fibres can be quantified easily 3. 2. Creep data comparisons CFCMCs obviously display a stochastic strength response because of their essentially brittle character, coupled with the Stress(MPa) near-random nature of the size and distribution of the macro- scopic and microscopic flaws which are present. Even so, Fig. 2. The stress dependences of the minimum creep rates recorded for the recognizing that the em and tf measurements in Figs. 3 and 4 HNSiCr-Al2O3 composite in air at 1300 C compared with data available for SiCr-Al20,7 and a SiC whisker-reinforced composite, SiCw-Al2O3.15 Also were obtained for a range of composites tested in two diffe included are results reported at 1300C for Nicalon TM NLM202 6 and Hi. ent laboratories 6-11 the recorded data sets reveal remarkably NicalonTM fibres 17 In addition, stress-creep rate values at 1200.C are shown consistent patterns of property variation as the fibre-matrix com- for synthetic Cao-50% Mgo samples(Table 2).B. Wilshire, M.R. Bache / Journal of the European Ceramic Society 27 (2007) 4603–4611 4605 Fig. 1. The variations of the creep strain with time for HNSiCf–SiBC samples tested at various stresses in air at 1300 ◦C. are ε˙m measurements reported for NicalonTM NLM202 fibres16 and one ε˙m value found for Hi-NicalonTM fibres.17 The presence of 25 vol.% SiC whiskers significantly increases the creep resistance of alumina.15 The porous A12O3 matrices formed by in situ oxidation of liquid aluminium must therefore have creep strengths much lower than that of the fully dense SiCw–A12O3 samples. Yet, at comparable stress lev￾els, the ε˙m values for the SiCw–A12O3 specimens are several orders of magnitude faster than those for the HNSiCf–A12O3 and SiCf–A12O3 products. Consequently, the alumina matrices make little contribution to the overall creep resistance of the fibre-reinforced composites.8 It is also apparent from Fig. 2 that stresses about five times higher must be applied to the NicalonTM NLM202 and Hi￾NicalonTM fibres to obtain creep rates comparable with those for the SiCf–A12O3 and HNSiCf–A12O3 specimens, respectively. This result would be expected because these composites contain 40 vol.% of interwoven 0/90◦ fibre bundles, so the fibres parallel Fig. 2. The stress dependences of the minimum creep rates recorded for the HNSiCf–Al2O3 composite in air at 1300 ◦C compared with data available for SiCf–Al2O3 7 and a SiC whisker-reinforced composite, SiCw–Al2O3. 15 Also included are results reported at 1300 ◦C for NicalonTM NLM20216 and Hi￾NicalonTM fibres.17 In addition, stress–creep rate values at 1200 ◦C are shown for synthetic CaO–50% MgO samples (Table 2). Fig. 3. Comparisons of the stress/minimum creep rate relationships for SiCf–Al2O3 7 and HNSiCf–Al2O3, as well as for SiCf–SiC,6,9 SiCf–SiBC,10 HNSiCf–SiC11 and HNSiCf–SiBC samples tested in air at 1300 ◦C. to the stress axes occupy approximately one fifth of the testpiece cross-sectional areas. Thus, the load-bearing capabilities of the alumina–matrix composites are governed by the longitudinal (0◦) fibres.8 While the creep resistance of the fibres clearly exceeds that of the matrices with the SiCf–A12O3 and HNSiCf–A12O3 sam￾ples, it has been claimed that the matrices are more creep resistant than the fibres with the SiCf–SiC,9 SiCf–SiBC10 and HNSiCf–SiC11 materials. However, as evident from Fig. 3, the ε˙m values recorded for the SiCf–A12O3 testpieces are equal to those for the SiCf–SiBC and lower than those for the SiCf–SiC specimens, with all three materials being reinforced with com￾parable volume fractions of 0/90◦ NicalonTM NLM202 fibres. Similarly, with equivalent Hi-Nicalon reinforcement, the ε˙m val￾ues for HNSiCf–A12O3 are equal to those for HNSiCf–SiBC and lower than those for the HNSiCf–SiC sample (Fig. 3). Hence, the porous SiC and SiBC matrices must be characterized by a creep resistance at least as poor as that for the weak A12O3 matrices. Irrespective of whether the present series of CFCMCs were produced with A12O3, SiC or SiBC matrices, it is therefore clear that the creep strengths of the matrices are markedly inferior to those of the fibres.8 On this basis, from the results presented in Figs. 3 and 4, the improvement in creep and creep rupture resistance achieved by replacing NicalonTM NLM202 with Hi￾NicalonTM fibres can be quantified easily. 3.2. Creep data comparisons CFCMCs obviously display a stochastic strength response because of their essentially brittle character, coupled with the near-random nature of the size and distribution of the macro￾scopic and microscopic flaws which are present. Even so, recognizing that the ε˙m and tf measurements in Figs. 3 and 4 were obtained for a range of composites tested in two differ￾ent laboratories,6–11 the recorded data sets reveal remarkably consistent patterns of property variation as the fibre–matrix com￾binations are changed