4608 B. wilshire, M.R. Bache /Journal of the European Ceramic Society 27(2007)4603-4611 w HNSiCf-SiBC 口-SC1400C b E10 200pm 125150175200225 Fig. 10. The stress dep the minimum creep rates recorded for the HNSiCr-AlzO3 and HNSiCp-SiBC composites in air at 1300C, compared with results reported for 2. 5D Cr-Si les" tested under low pressure argon at 1200-1400°C. ognized as showing creep brittle behaviour, i.e. ErlEp= l where Ep is the primary creep strain and Ef is the total creep strain to failure 20 (3)Although large cracks can develop before creep failure finally occurs under low applied stresses, so that the present CFCMCs can be regarded as 'crack tolerant, these creep brittle materials have low creep damage tolerance values 500pm a), where A can be defined2las Fig 9. Scanning electron micrographs showing: (a)nucleation of cracks normal to the tensile axis in the SiBC surface layer and (b)regions of fibre pull-out on the fracture surface of the HNSiCr-SiBC composite tested in air at 1300C so A=l because Et =0(Figs. 1 and 7). The creep dam- age tolerance value is important in practical situations when well as by the HNSiCf-Al2O3 and HNSiCr-SiBC composites materials must withstand local strain concentrations, say, in regions where a change in component cross-section leads to stress concentrations. Values of A in the range 5-10 are 3.5. Creep in oxidizing environments then thought to ensure that the strain concentrations encoun- tered during service will not lead to premature failure. 22 As demonstrated in Figs. 3 and 4, product development ini Hence, with the SiCr-reinforced composites now consid- tiatives such as the introduction of Hi-NicalonTM fibres and ered, a values near unity may represent a severe design partially self-sealing SiBC matrices have resulted in substan constraint. For this reason. alternative fibre-matrix combi tial improvements in the creep and creep fracture strength of nations should be considered for safety-critical aeroengine SiC fibre-reinforced composites. Even so, for aeroengine and and related applications related applications, the resulting component life enhancement be as impressive as the increased product costs might Interestingly, even when compared with the results now imply for several reasons. documented for the high-performance HNSiCr-Al2O3 and HNSiCr-SiBC products, the impressive creep resistance of a (1)With the problem of oxidation-assisted fibre failure caused carbon fibre-reinforced Sic-matrix composite-44 is illustrated predominantly by cracking of the weak porous matrices, a in Fig. 10. The carbon fibres were again introduced as 2D low design stress limit must be imposed for components woven bundles, but with the carbon fabric layers interlinked erving in non-protective atmospheres to avoid delamination, giving 2. 5D Cr-SiC testpieces. However, (2)Because NicalonM NLM202, Hi-Nicalon"M and other the data sets for the 2. D Cr-Sic samples were determined under types of SiC fibres display continuously decaying creep low-pressure argon. Hence, a major reduction in creep life and curves, 6, I7 similar curve shapes are exhibited by SiC fibre- ductility will occur when crack development in the brittle Sic reinforced materials(Figs. I and7). In the absence of clearly matrix allows oxygen penetration during creep in non-protective defined tertiary stages, these woven composites must be rec- atmospheres.4608 B. Wilshire, M.R. Bache / Journal of the European Ceramic Society 27 (2007) 4603–4611 Fig. 9. Scanning electron micrographs showing: (a) nucleation of cracks normal to the tensile axis in the SiBC surface layer and (b) regions of fibre pull-out on the fracture surface of the HNSiCf–SiBC composite tested in air at 1300 ◦C. well as by the HNSiCf–Al2O3 and HNSiCf–SiBC composites (Figs. 3 and 4). 3.5. Creep in oxidizing environments As demonstrated in Figs. 3 and 4, product development ini￾tiatives such as the introduction of Hi-NicalonTM fibres and partially self-sealing SiBC matrices have resulted in substan￾tial improvements in the creep and creep fracture strength of SiC fibre-reinforced composites. Even so, for aeroengine and related applications, the resulting component life enhancement may not be as impressive as the increased product costs might imply for several reasons. (1) With the problem of oxidation-assisted fibre failure caused predominantly by cracking of the weak porous matrices, a low design stress limit must be imposed for components serving in non-protective atmospheres. (2) Because NicalonTM NLM202, Hi-NicalonTM and other types of SiC fibres display continuously decaying creep curves,16,17 similar curve shapes are exhibited by SiC fibre￾reinforced materials (Figs. 1 and 7). In the absence of clearly defined tertiary stages, these woven composites must be rec￾Fig. 10. The stress dependences of the minimum creep rates recorded for the HNSiCf–Al2O3 and HNSiCf–SiBC composites in air at 1300 ◦C, compared with results reported for 2.5D Cf–SiC samples24 tested under low pressure argon at 1200–1400 ◦C. ognized as showing creep brittle behaviour, i.e. εf/εp ∼= 1, where εp is the primary creep strain and εf is the total creep strain to failure.20 (3) Although large cracks can develop before creep failure finally occurs under low applied stresses, so that the present CFCMCs can be regarded as ‘crack tolerant’, these creep brittle materials have low creep damage tolerance values (λ), where λ can be defined21 as λ = 1 + εt ε˙mtf  (3) so λ = 1 because εt ∼= 0 (Figs. 1 and 7). The creep dam￾age tolerance value is important in practical situations when materials must withstand local strain concentrations, say, in regions where a change in component cross-section leads to stress concentrations.22 Values of λ in the range 5–10 are then thought to ensure that the strain concentrations encoun￾tered during service will not lead to premature failure.22 Hence, with the SiCf-reinforced composites now consid￾ered, λ values near unity may represent a severe design constraint. For this reason, alternative fibre–matrix combi￾nations should be considered for safety-critical aeroengine and related applications. Interestingly, even when compared with the results now documented for the high-performance HNSiCf–Al2O3 and HNSiCf–SiBC products, the impressive creep resistance of a carbon fibre-reinforced SiC–matrix composite23,24 is illustrated in Fig. 10. The carbon fibres were again introduced as 2D woven bundles, but with the carbon fabric layers interlinked to avoid delamination, giving 2.5D Cf–SiC testpieces. However, the data sets for the 2.5D Cf–SiC samples were determined under low-pressure argon. Hence, a major reduction in creep life and ductility will occur when crack development in the brittle SiC matrix allows oxygen penetration during creep in non-protective atmospheres.