4604 B. wilshire, M.R. Bache /Journal of the European Ceramic Society 27(2007)4603-4611 eliminate the amorphous silicon oxy-carbide phase (SiC:O,) operations, all matrices had porosities of 15% or more Specifi which reduces the creep strength of NicalonM NLM20 ly, small pores were present in the matrix regions within fibres 12, 3 fibre bundles, with large pores(termed macro-pores) between To consider the effects of changing the matrix type, one of the plies and at yarn intersections within the plies the present Hi- NicalonM-reinforced composites had an alu Having selected composites with similar'macro-structures mina matrix, giving a material referred to as HNSiCr-Al2O3. comparable test procedures were used to determine the creep and The other composite was prepared with an 'enhanced'Sic creep fracture properties. Thus, for the present HNSiCr-A1203 matrix, containing boron-based additives which form a sealant and HNSiCr-SiBC materials, tensile creep tests were carried out glass to limit oxygen penetration into the material during creep in air at 1300 C using a servo-hydraulic machine in load control exposure.+ This product is designated as HNSiCr-SiBC mode. The load train included hydraulic wedge grips, an align- For tensile creep tests carried out in air at 1300C, ment fixture, a twin-zone split furnace and a high-temperature the behaviour patterns displayed by the HNSiCr-Al2O3 and extensometer. The tests were undertaken using flat specimens HNSiCr-SiBC samples are analyzed in relation to data obtained of 2 mm thickness and mm width, with 40 mm gauge lengths under the same conditions for two groups of CFCMCs machined such that the tensile stress axes were parallel (0o) to one of the 0/90 fibre directions. These procedures are the (a)The consequences of changing only the fibre type Same as those adopted to test the SiCr-SiC, SiCr-SiBCIO and arent by the creep properties of HNSiCr-Sic products. The present experimental methods HNSiCe-AlO with those of an alumina-matrix com- also gave results indistinguishable from those reported when posite reinforced with NicalonTM NLM202 fibres. called constant-load creep machines were used with the SiCe-sice SiCf-Al2O3. With both materials, the fibres were given and SiCt-A12O3 composites a thin boron nitride coating before 5 um thick SiC coat ings were deposited by chemical vapour infiltration(CVI), 3. Results and discussion resulting in double BN/SiC interfaces. The A12O3 matrices were then formed by in situ directional oxidation of liq Over the stress ranges investigated at 1300 C for the uid aluminium, as previously described for the SicCr-Al2O3 HNSiCr-Al3O3 and HNSiCr-SiBC samples, the general man- ner in which the creep strain(E)increases with time (t)is (b) The HNSiCr-SiBC samples relate to three different com- similar to that reported previously for all other CFCMCs now posites with SiC fibres reinforcing SiC matrices. All of these considered. 6- Thus, as illustrated by the e/t trajectories for the products were fabricated with carbon interfaces(0.5 um HNSiCf-SiBC material in Fig. 1, following the initial loading thick), before CVI processing to introduce the polycrys- strain, the creep rate decays continuously, reaching a minimum alline SiC matrices. However, the fibre-matrix combination rate(Em) just prior to failure. Little or no period of accelerat- differed. with ing tertiary creep is then apparent before fracture occurs after a Nicalon NLM202 fibres reinforcing,a standard Sic time(tr)when the total creep strain reaches the limiting creep matrix(called Sicr-Sic), ductility(Et). Hence, to compare the creep and creep fracture Nicalon M NLM202 fibres reinforcing an'enhanced properties of CFCMCs produced with NicalonM NLM202 or SiC matrix(called SiCr-SiBC)and Hi-NicalonTM fibres reinforcing Al2 O3, SiC or SiBC matrices Hi-Nicalon"M fibres reinforcing"a standard SiC matrix the values of Em, tr and ef were determined from each creep (called HNSiCr-SiC). curve The principal distinguishing features of these CFCMCs are 3. 1. Relative strengths of fibres and matrices summarized in Table 1. Each composite contained approx imately 40 vol %o fibres with average diameters of 15 um, Fig. 2 shows the variations of the mi incorporated as bundles of about 500 fibres woven to obtain rate with stress at 1300 C for SiCr-Al2O3'and the 2D layers of fabric. The woven layers or plies were then aligned present HNSiCr-AlO3 material, together with results for and stacked to produce preforms having balanced 0/90 archi- an alumina-matrix composite reinforced with 25 vol. Sic tectures. Moreover, after the fibre coating and densification whiskers,now termed SiCw-Al2O3. Also included in Fig. 2 Distinguishing features of fibre-reinforced composites Material designation Fibre type Matrix material Interface type Reference Al O3 Al2O3 BN/SIC Al2O3 BN/SiC Hi-Nicalon Enhanced SiC Carbon Carbon SiCe-SIBC Nicalon NLM 20 Enhanced SiC Carbon HNSiCe-SiC Hi-NicalonTM Carbon4604 B. Wilshire, M.R. Bache / Journal of the European Ceramic Society 27 (2007) 4603–4611 eliminate the amorphous silicon oxy-carbide phase (SiCxOy) which reduces the creep strength of NicalonTM NLM202 fibres.12,13 To consider the effects of changing the matrix type, one of the present Hi-NicalonTM-reinforced composites had an alu￾mina matrix, giving a material referred to as HNSiCf–A12O3. The other composite was prepared with an ‘enhanced’ SiC matrix, containing boron-based additives which form a sealant glass to limit oxygen penetration into the material during creep exposure.14 This product is designated as HNSiCf–SiBC. For tensile creep tests carried out in air at 1300 ◦C, the behaviour patterns displayed by the HNSiCf–A12O3 and HNSiCf–SiBC samples are analyzed in relation to data obtained under the same conditions for two groups of CFCMCs. (a) The consequences of changing only the fibre type becomes apparent by comparing the creep properties of HNSiCf–A12O3 with those of an alumina–matrix com￾posite reinforced with NicalonTM NLM202 fibres, called SiCf–A12O3. 7 With both materials, the fibres were given a thin boron nitride coating before ∼5m thick SiC coat￾ings were deposited by chemical vapour infiltration (CVI), resulting in double BN/SiC interfaces. The A12O3 matrices were then formed by in situ directional oxidation of liq￾uid aluminium, as previously described for the SiCf–Al2O3 specimens.7 (b) The HNSiCf–SiBC samples relate to three different com￾posites with SiC fibres reinforcing SiC matrices. All of these products were fabricated with carbon interfaces (∼0.5m thick), before CVI processing to introduce the polycrys￾talline SiC matrices. However, the fibre–matrix combination differed, with • NicalonTM NLM202 fibres reinforcing6,9 a standard SiC matrix (called SiCf–SiC), • NicalonTM NLM202 fibres reinforcing10 an ‘enhanced’ SiC matrix (called SiCf–SiBC) and • Hi-NicalonTM fibres reinforcing11 a standard SiC matrix (called HNSiCf–SiC). The principal distinguishing features of these CFCMCs are summarized in Table 1. Each composite contained approx￾imately 40 vol.% fibres with average diameters of ∼15m, incorporated as bundles of about 500 fibres woven to obtain 2D layers of fabric. The woven layers or plies were then aligned and stacked to produce preforms having balanced 0/90◦ archi￾tectures. Moreover, after the fibre coating and densification operations, all matrices had porosities of 15% or more. Specifi- cally, small pores were present in the matrix regions within the fibre bundles, with large pores (termed macro-pores) between the plies and at yarn intersections within the plies.7 Having selected composites with similar ‘macro-structures’, comparable test procedures were used to determine the creep and creep fracture properties. Thus, for the present HNSiCf–A12O3 and HNSiCf–SiBC materials, tensile creep tests were carried out in air at 1300 ◦C using a servo-hydraulic machine in load control mode. The load train included hydraulic wedge grips, an align￾ment fixture, a twin-zone split furnace and a high-temperature extensometer. The tests were undertaken using flat specimens of 2 mm thickness and 8 mm width, with 40 mm gauge lengths machined such that the tensile stress axes were parallel (0◦) to one of the 0/90◦ fibre directions. These procedures are the same as those adopted to test the SiCf–SiC,9 SiCf–SiBC10 and HNSiCf–SiC products.11 The present experimental methods also gave results indistinguishable from those reported when constant-load creep machines6 were used with the SiCf–SiC6 and SiCf–A12O3 composites.7 3. Results and discussion Over the stress ranges investigated at 1300 ◦C for the HNSiCf–A13O3 and HNSiCf–SiBC samples, the general man￾ner in which the creep strain (ε) increases with time (t) is similar to that reported previously for all other CFCMCs now considered.6–11 Thus, as illustrated by the ε/t trajectories for the HNSiCf–SiBC material in Fig. 1, following the initial loading strain, the creep rate decays continuously, reaching a minimum rate (ε˙m) just prior to failure. Little or no period of accelerat￾ing tertiary creep is then apparent before fracture occurs after a time (tf) when the total creep strain reaches the limiting creep ductility (εf). Hence, to compare the creep and creep fracture properties of CFCMCs produced with NicalonTM NLM202 or Hi-NicalonTM fibres reinforcing A12O3, SiC or SiBC matrices, the values of ε˙m, tf and εf were determined from each creep curve. 3.1. Relative strengths of fibres and matrices Fig. 2 shows the variations of the minimum creep rate with stress at 1300 ◦C for SiCf–A12O3 7 and the present HNSiCf–Al2O3 material, together with results for an alumina–matrix composite reinforced with 25 vol.% SiC whiskers,15 now termed SiCw–A12O3. Also included in Fig. 2 Table 1 Distinguishing features of fibre-reinforced composites Material designation Fibre type Matrix material Interface type Reference HNSiCf–Al2O3 Hi-NicalonTM Al2O3 BN/SiC SiCf–Al2O3 NicalonTM NLM 202 Al2O3 BN/SiC 7 HNSiCf–SiBC Hi-NicalonTM Enhanced SiC Carbon SiCf–SiC NicalonTM NLM 202 SiC Carbon 6,9 SiCf–SIBC NicalonTM NLM 202 Enhanced SiC Carbon 10 HNSiCf–SiC Hi-NicalonTM SiC Carbon 11