B. wilshire, M.R. Bache /Journal of the European Ceramic Society 27(2007)4603-4611 compatibility of Cao-MgO ceramics with carbon in aggres- sive oxidizing environments at 1600c and above is evident from the dominant selection of carbon-bearing doloma lin- ings for the original Thomas or basic Bessemer process and 0.08 graphite-bearing magnesia linings for modern basic oxygen steel-making vessels The potential success of Cr-doloma composites would epend critically on fabrication methods being devised to obtain low-porosity matrices, free from the inter-connected po which would provide channels for oxygen ingress. If low porosity creep damage-resistant Ca0-45 wt% MgO matrices can be developed, the impressive creep properties of the Cr-Sic material in argon(Fig. 10)may be attainable with Cr-doloma composites in oxidizing atmospheres. Fig. 12. The effect of variations in MgO content on the creep strain/time curves 4. Conclusions recorded for synthetic CaO-MgO samples tested under compressive stresses of 62 MPa at1327°C3 The improvements in creep and creep rupture strength achieved through different fibre-matrix combinations are decaying creep curves were always recorded, i.e. after loading, assessed for composites produced with Sic or SiBC matrices the creep rate decreases gradually and creep cracking was not (with carbon interfaces )or Al2O3 matrices(with double BN/Sic discernible29-3 after true creep strains up to 0. 25(when the tests interfaces ) reinforced with either Nicalon TM NLM202 or Hi- were discontinued ). On this basis, doloma appears to represent Nicalon"M fibres. This comparison of tensile data sets recorded a creep damage-resistant option as a matrix for fibre-reinforced in air at 1300 C demonstrates the substantial performance gains derived when hi-Nicalon fibres are used with either sibc e Al2O3 matrices. 3.7. Potential doloma-matrir composites For the product range considered, the fibres aligned parallel to the tensile stress axes govern the rates of creep strain accumula Although creep damage resistance is a critical requirement, tion and crack growth. However, in non-protective atmospheres, several additional property features are relevant to consideration the principal creep life-limiting phenomenon is premature fibre of doloma as a candidate matrix material for high-performance failure associated with oxygen penetration as cracks develop in CFCMCS the brittle matrices. Yet, while crack formation occurs during creep of the Sic, SiBC and Al2O3 matrix materials, as well (a) With synthetic CaO-MgO ceramics, the creep resistance with most monolithic ceramics, such damage is not discernible increases with increasing MgO content,3I with the natural with two-phase Cao 45 wt% MgO(doloma). Provided that pro- product having a lower cree th than the heti cess routes can be devised to obtain fine-grain lot Cao-50% MgO material due to the marginally lower Mgo doloma matrices, free from interconnected pores allowing oxy- level, higher impurity levels and finer average crystal size gen ingress, Cr-doloma composites could represent a relatively Table 2). At temperatures around 1300C, the synthetic inexpensive creep damage-resistant option for high temperature Cao-50% MgO samples exhibit creep strengths better than applications involving long-term service under load in oxidizing those recorded for the SiCw-Al2O3 ceramic, as evident environmen from Fig. 1. However, the present analysis confirms that the weak SiC, SiBC and al2 O3 matrices make little contri- References bution to the overall creep resistance of the present set of 1. Ruffles. P. Aerospace Structural Materials: Present and Future. The Insti- SiC fibre-reinforced composites, 8 so the creep strength of tute of Materials. London. 1995 the Cao-45 wt %o MgO samples is non-critical 2. Miller, S, Advanced materials means advanced engines. Interdiscipl. Sci. (b) Calcined natural doloma products can deteriorate by ' per- Re,1996,21,2 ishing'during storage, which occurs as a result of moisture 3. Parlier, M and Ritt, M H. State of the art and perspectives for oxide-oxide attacking the lime phase. Yet, with fully sintered samples 4.Heredia, E E McNulty, JC, Zok, E.Wand Evans,A.G,Oxidation of both natural and synthetic doloma(Table 2), no evidence of surface attack was apparent even after several years of 1995,78,2097 atmospheric exposure at room temperature. 5. Jones, R. H, Henager Jr, C. H. and Windish Jr, C. F, High temperature (e) Unfortunately, doloma is likely to react with both silicon Mater Sci eng. 4. 1995.A198103 carbide and oxide fibres at high temperatures, indicating 6. wilshire. B. Carreno. E and Percival, M G. L, Tensile creep and creep that CaO-MgO matrices would be better employed with acture of a fibre-reinforced SiC-SiC composite. Sc Mater, 1998, 39, less-expensive carbon fibre reinforcement. Certainly, the4610 B. Wilshire, M.R. Bache / Journal of the European Ceramic Society 27 (2007) 4603–4611 Fig. 12. The effect of variations in MgO content on the creep strain/time curves recorded for synthetic CaO–MgO samples tested under compressive stresses of 62 MPa at 1327 ◦C.31 decaying creep curves were always recorded, i.e. after loading, the creep rate decreases gradually and creep cracking was not discernible29–31 after true creep strains up to 0.25 (when the tests were discontinued). On this basis, doloma appears to represent a creep damage-resistant option as a matrix for fibre-reinforced composites. 3.7. Potential doloma–matrix composites Although creep damage resistance is a critical requirement, several additional property features are relevant to consideration of doloma as a candidate matrix material for high-performance CFCMCs. (a) With synthetic CaO–MgO ceramics, the creep resistance increases with increasing MgO content,31 with the natural product having a lower creep strength than the synthetic CaO–50% MgO material due to the marginally lower MgO level, higher impurity levels and finer average crystal size (Table 2). At temperatures around 1300 ◦C, the synthetic CaO–50% MgO samples exhibit creep strengths better than those recorded for the SiCw–Al2O3 ceramic, as evident from Fig. 1. However, the present analysis confirms that the weak SiC, SiBC and Al2O3 matrices make little contri￾bution to the overall creep resistance of the present set of SiC fibre-reinforced composites,8 so the creep strength of the CaO–45 wt% MgO samples is non-critical. (b) Calcined natural doloma products can deteriorate by ‘per￾ishing’ during storage, which occurs as a result of moisture attacking the lime phase. Yet, with fully sintered samples of both natural and synthetic doloma (Table 2), no evidence of surface attack was apparent even after several years of atmospheric exposure at room temperature. (c) Unfortunately, doloma is likely to react with both silicon carbide and oxide fibres at high temperatures, indicating that CaO–MgO matrices would be better employed with less-expensive carbon fibre reinforcement. Certainly, the compatibility of CaO–MgO ceramics with carbon in aggres￾sive oxidizing environments at 1600 ◦C and above is evident from the dominant selection of carbon-bearing doloma lin￾ings for the original Thomas or basic Bessemer process and graphite-bearing magnesia linings for modern basic oxygen steel-making vessels. The potential success of Cf–doloma composites would depend critically on fabrication methods being devised to obtain low-porosity matrices, free from the inter-connected pores which would provide channels for oxygen ingress. If low￾porosity creep damage-resistant CaO–45 wt% MgO matrices can be developed, the impressive creep properties of the Cf–SiC material in argon (Fig. 10) may be attainable with Cf–doloma composites in oxidizing atmospheres. 4. Conclusions The improvements in creep and creep rupture strength achieved through different fibre–matrix combinations are assessed for composites produced with SiC or SiBC matrices (with carbon interfaces) or Al2O3 matrices (with double BN/SiC interfaces), reinforced with either NicalonTM NLM202 or Hi￾NicalonTM fibres. This comparison of tensile data sets recorded in air at 1300 ◦C demonstrates the substantial performance gains derived when Hi-NicalonTM fibres are used with either SiBC or Al2O3 matrices. For the product range considered, the fibres aligned parallel to the tensile stress axes govern the rates of creep strain accumula￾tion and crack growth. However, in non-protective atmospheres, the principal creep life-limiting phenomenon is premature fibre failure associated with oxygen penetration as cracks develop in the brittle matrices. Yet, while crack formation occurs during creep of the SiC, SiBC and Al2O3 matrix materials, as well as with most monolithic ceramics, such damage is not discernible with two-phase CaO–45 wt% MgO (doloma). Provided that pro￾cess routes can be devised to obtain fine-grain low-porosity doloma matrices, free from interconnected pores allowing oxy￾gen ingress, Cf–doloma composites could represent a relatively inexpensive creep damage-resistant option for high temperature applications involving long-term service under load in oxidizing environments. References 1. Ruffles, P., Aerospace Structural Materials: Present and Future. The Insti￾tute of Materials, London, 1995. 2. Miller, S., Advanced materials means advanced engines. Interdiscipl. Sci. Rev., 1996, 21, 2. 3. Parlier, M. and Ritti, M. H., State of the art and perspectives for oxide–oxide composites. Aerospace Sci. Tech., 2003, 14, 1. 4. Heredia, F. E., McNulty, J. C., Zok, F. W. and Evans, A. G., Oxidation embrittlement probe for ceramic–matrix composites. J. Am. Ceram. Soc., 1995, 78, 2097. 5. Jones, R. H., Henager Jr., C. H. and Windish Jr., C. F., High temperature corrosion and crack growth of SiC–SiC at variable oxygen partial pressures. Mater. Sci. Eng. A, 1995, A198, 103. 6. Wilshire, B., Carreno, F. and Percival, M. G. L., Tensile creep and creep ˜ fracture of a fibre-reinforced SiC–SiC composite. Scr. Mater., 1998, 39, 729