B. Wilshire, M.R. Bache/ Journal of the European Ceramic Sociery 27(2007)4603-4611 Research emphasis has also been directed to oxide fibre reinforced composites, with the oxide fibres seemingly less 5um liable to oxidation-assisted failure Yet, as expected for the 2.5D Cr-Sic product, creep data sets reported for an Al2O3-fibre reinforced SiC-matrix composite have emphasized the marked reduction in creep life and ductility caused by testing in air rather than vacuum.Moreover, in seeking to identify suitable oxide-oxide composites, the creep strengths of currently avail able oxide fibres are inferior to the values for the established sic fibres Clearly, irrespective of the fibre type chosen, the princi- pal creep life-limiting phenomenon encountered with woven CFCMCs operating under load in non-protective atmospheres at high temperatures is premature fibre failure associated with oxygen ingress as cracks develop in the brittle matrices. For this reason,to protect the vulnerable fibres and fibre-matrix inter- faces, it is now proposed that major benefits could be realized by considering composites fabricated with creep damage-resistant matrices, i.e. high-melting point ceramic matrices not prone to creep crack formation d 3.6. Creep of Cao-MgO ceramics Just as cracks develop in the brittle SiC, SiBC and Al2O matrices of the SiC fibre-reinforced composites, during tensile creep, intergranular cracks form extensively on the transverse grain boundaries of most polycrystalline ceramics produced i monolithic form as found for both sintered silicon carbide26 and alumina. 2 Even during creep in compression, intergran- ular damage accumulates on boundaries experiencing tensile Fig. Il. (a) Intergranular cracking of porous magnesia after a strain of 0.05 at hoop and radial stresses, in line with the stress distributions pre- 75 MPa and 1327 C and (b)the microstructure of natural doloma after a strain dicted using finite element methods. Yet, while creep cracks of 0.08 at 62MPa and 1 127C.The compression axis is vertical are evident after compressive creep strains of only a few per- cent with polycrystalline MgO(Fig. 1 la), as well as with tions, with testpieces containing 0, 25, 75 and 100% MgO,the Cao, crack formation was not- during compressive decaying primary stages give way to accelerating tertiary defor- creep of two-phase CaO-Mgo ( doloma)specimens containing mation as the crack incidence increases with increasing creep M42-50 wt% MgO (Fig. llb) strain. However, in the absence of crack formation or any other The Cao-Mgo system is a simple eutectic, with an eutectic damage process which can cause a tertiary acceleration with the omposition of Cao-32 wt%MgO and an eutectic temperature microstructurally stable Ca0-50% MgO samples, continuously of -2300 C With increasing MgO content, the microstructure changes from alime crystal matrix enclosing equiaxed magnesia Table 2 grains to a periclase grain network surrounding equiaxed lime Fabrication procedures, analyses(wt%)and microstructures of natural and syn- crystals. For a series of CaO-MgO samples varying in composi- thetic doloma tion from0 to 100 wt% MgO, 3 cracks were not discernible in specimens containing around 50 wt% MgO, whereas inter- Synthetic doloma granular damage was readily apparent with samples produced Starting material Whitwell dolomite with 0, 25, 75 and 100 wt% MgO. Thus, cracks evolve prefer- CaCO3·MgCO2) Mg(oh) CaCO3 entially on Cao-Cao and Mgo-Mgo boundaries rather than Sintering temperature(C) 1600 on Cao-MgO interfaces. In this context, it should be noted that porosity of sintered bars(%64-6 ~4-6 doloma testpieces produced with around 40-50 wt%o MgO were Average crystal size(um) 3-5 2-16 not prone to creep crack formation, irrespective of whether the Composition samples were fabricated using synthetic CaO and MgO powders Cao or natural dolomite( CacO3 MgCO3), as listed in Table 2 The composition dependence of creep crack development the synthetic Cao-Mgo ceramics was confirmed31 by differ- A2 03 ences in the shapes of the E/t trajectories observed at 1327C Other oxides as illustrated in Fig. 12. Even under compressive creep condiB. Wilshire, M.R. Bache / Journal of the European Ceramic Society 27 (2007) 4603–4611 4609 Research emphasis has also been directed to oxide fibre￾reinforced composites, with the oxide fibres seemingly less liable to oxidation-assisted failure.3 Yet, as expected for the 2.5D Cf–SiC product, creep data sets reported for an Al2O3-fibre￾reinforced SiC–matrix composite have emphasized the marked reduction in creep life and ductility caused by testing in air rather than vacuum.25 Moreover, in seeking to identify suitable oxide–oxide composites, the creep strengths of currently avail￾able oxide fibres are inferior to the values for the established SiC fibres.3 Clearly, irrespective of the fibre type chosen, the princi￾pal creep life-limiting phenomenon encountered with woven CFCMCs operating under load in non-protective atmospheres at high temperatures is premature fibre failure associated with oxygen ingress as cracks develop in the brittle matrices. For this reason, to protect the vulnerable fibres and fibre–matrix inter￾faces, it is now proposed that major benefits could be realized by considering composites fabricated with creep damage-resistant matrices, i.e. high-melting point ceramic matrices not prone to creep crack formation. 3.6. Creep of CaO·MgO ceramics Just as cracks develop in the brittle SiC, SiBC and Al2O3 matrices of the SiC fibre-reinforced composites, during tensile creep, intergranular cracks form extensively on the transverse grain boundaries of most polycrystalline ceramics produced in monolithic form, as found for both sintered silicon carbide26 and alumina.27 Even during creep in compression, intergran￾ular damage accumulates on boundaries experiencing tensile hoop and radial stresses, in line with the stress distributions pre￾dicted using finite element methods.28 Yet, while creep cracks are evident after compressive creep strains of only a few per￾cent with polycrystalline MgO (Fig. 11a), as well as with CaO, crack formation was not observed29–31 during compressive creep of two-phase CaO–MgO (doloma) specimens containing ∼42–50 wt% MgO (Fig. 11b). The CaO–MgO system is a simple eutectic, with an eutectic composition of CaO–32 wt% MgO and an eutectic temperature of ∼2300 ◦C. With increasing MgO content, the microstructure changes from a lime crystal matrix enclosing equiaxed magnesia grains to a periclase grain network surrounding equiaxed lime crystals. For a series of CaO–MgO samples varying in composi￾tion from 0 to 100 wt% MgO,31 creep cracks were not discernible in specimens containing around 50 wt% MgO, whereas inter￾granular damage was readily apparent with samples produced with 0, 25, 75 and 100 wt% MgO. Thus, cracks evolve prefer￾entially on CaO–CaO and MgO–MgO boundaries rather than on CaO–MgO interfaces. In this context, it should be noted that doloma testpieces produced with around 40–50 wt% MgO were not prone to creep crack formation, irrespective of whether the samples were fabricated using synthetic CaO and MgO powders or natural dolomite (CaCO3·MgCO3), as listed in Table 2. The composition dependence of creep crack development in the synthetic CaO–MgO ceramics was confirmed31 by differ￾ences in the shapes of the ε/t trajectories observed at 1327 ◦C, as illustrated in Fig. 12. Even under compressive creep condi￾Fig. 11. (a) Intergranular cracking of porous magnesia after a strain of 0.05 at 75 MPa and 1327 ◦C and (b) the microstructure of natural doloma after a strain of 0.08 at 62 MPa and 1127 ◦C.29 The compression axis is vertical. tions, with testpieces containing 0, 25, 75 and 100% MgO, the decaying primary stages give way to accelerating tertiary defor￾mation as the crack incidence increases with increasing creep strain. However, in the absence of crack formation or any other damage process which can cause a tertiary acceleration with the microstructurally stable CaO–50% MgO samples, continuously Table 2 Fabrication procedures, analyses (wt%) and microstructures of natural and syn￾thetic doloma30 Natural doloma Synthetic doloma Starting material Whitwell dolomite (CaCO3·MgCO2) Analar Mg(OH)2CaCO3 Calcination temperature (◦C) 1300 1300 Sintering temperature (◦C) 1600 1800 Porosity of sintered bars (%) ∼4–6 ∼4–6 Average crystal size (m) 3–5 12–16 Compositions CaO 58.02 56.53 MgO 40.53 42.32 SiO2 0.49 0.76 Al2O3 0.03 0.10 Fe2O3 0.77 0.13 Other oxides 0.15 0.16