M. Holmquist et al. / Journal of the European Ceramic Society 20(2000)599-606 the wall temperature of a CMC liner may be kept in the associated with creep and sintering due to the high range of 1200-1400C while ensuring a suitable uniform fusivities of oxides compared to SiC. Single crystal oxi- combustion des, particularly complex oxides(e.g. YAG), are known The envisaged application of CMC in combustor to have significantly better creep behaviour than poly liners requires that the component resist thermal loads. crystals, although fabricating fibres remains a challenge. These applications are designed to have minimal However, a single crystal alumina fibre(SaphikonM)is requirements for components to withstand pressure or commercially available having a higher temperature other mechanical loads. In these cases, the failure strain capability and better creep resistance than polycrystal of the composite is the most important measure of its line alumina fibres. This fibre, which is produced by the damage tolerance. The causes of thermally induced edge film fed growth method from molten alumina, has strains include thermal mismatch with surrounding the disadvantage of being monofilament(diameter 125 components, temperature gradients within the compo- um)which prevents weaving and forming into complex nent and transient strains during temperature cycling.2 shapes, and a very high cost(due to the manufacturing he expected properties needed are temperature stabi- method). Several concepts for fibre coatings that would lity to 1400oC, oxidation resistance, mechanical stabi- improve the damage tolerance of oxide CMCs have been demonstrated in model composites. 1oe have only lity, chemical stability, damage tolerance and thermal been proposed but the performance of these have only shock resistance. These properties must be maintained for long times(>10, 000 h)and under cyclic conditions Two classes of CMCs are considered for this applica tion: oxide and non-oxide materials. Most development 2. Objective work has been done on non-oxide materials and com- mercial variants are usually based on silicon carbide The objective of the"Novel Oxide Ceramic Compo- (SiC)fibres (i.e. Nicalon"M or Tyranno TM)with a Sic sites "programme was to develop an all-oxide composite or an oxide (i.e. Al2O3)matrix. They have a fibre/matri for long life-time applications(>10,000 h)at tempera- interphase of carbon(C)or boron nitride(Bn) that tures above 1400 C in oxidising environments. Design contain weakly bonded planes of atoms, providing a and development of an oxide interphase has been weak debond layer which imparts a non-brittle fracture reported previously. 14-16 paper reports on the behaviour. Non-oxide CMCs have attractive high tem- development and scale-up of a composite fabrication perature properties, such as creep resistance and micro- process, results from mechanical testing as well as fab- structural stability. They also show high thermal rication and combustor rig tests of a model component conductivity and low thermal expansion, which reduces ermally induced strains. However, the oxidation sen- sitivity of the interphase will cause embrittlement of the 3. Experimental composite after service at high temperatures for long times. Embrittlement is most severe with cyclic mechan-.. Materials ical and thermal loading beyond the proportional limit, because oxygen that penetrates via the matrix crack Single crystal continuous a-Al2O3 sapphire fibres with created will react locally with the fibres and fibre coat- a nominal diameter of 125 um( Saphikon, USA)were ings to form oxide products. These reaction products chosen for this project since they have a thermodyn will create strong bonds between the fibre and matrix, stability compatible with the temperature goal and which prevent crack deflection and suppress internal forming limitations were not an issue for flat combustor frictional mechanisms that otherwise give toughness tiles. However, this single crystal fibre may not be the A way to avoid the oxidation problem is to use all-oxide ultimate candidate for this application due to its limited composites(i.e. the composites consist of oxide fibres, high-temperature temperature properties and sensitivity to slow oxide interfacial coatings and oxide matrices ). The com- crack growth, 3 but rather provide a model material on ponents are fully oxidised and further damage by oxida- to which to base an oxide/oxide CMC. Alumina was tion can be avoided, even at high temperatures and after chosen as a matrix to minimise thermally induced stres matrix cracking. Oxide composites also have the attrac- ses between the fibre and the matrix and limit undesired tive feature of potentially low cost. The primary difficul- chemical reactions. A small amount of zirconia was ties with this approach are the lack of suitable oxide fibre added to control matrix grain growth at elevated tem- reinforcement and the lack of oxide based "debond peratures. Zirconia has been identified as a suitable layer analogous to carbon or boron nitride. Most oxides interphase material in alumina based composites since also have low thermal conductivity and high thermal the system is thermochemically stable. 3 However, in expansion, leading to high thermally induced strains order for the interphase to behave as a debond layer, Polycrystalline oxide based fibres(e.g. Nextel 610 or is necessary to reduce the strength by introducing por- Nextel 720)have temperature limitations <1100 C osity (a porosity level of 30% has been suggested").Anthe wall temperature of a CMC liner may be kept in the range of 1200±1400C while ensuring a suitable uniform combustion temperature. The envisaged application of CMC in combustor liners requires that the component resist thermal loads. These applications are designed to have minimal requirements for components to withstand pressure or other mechanical loads. In these cases, the failure strain of the composite is the most important measure of its damage tolerance. The causes of thermally induced strains include thermal mismatch with surrounding components, temperature gradients within the compo￾nent and transient strains during temperature cycling.2 The expected properties needed are temperature stabi￾lity to 1400C, oxidation resistance, mechanical stabi￾lity, chemical stability, damage tolerance and thermal shock resistance. These properties must be maintained for long times (>10,000 h) and under cyclic conditions. Two classes of CMCs are considered for this applica￾tion: oxide and non-oxide materials. Most development work has been done on non-oxide materials and com￾mercial variants are usually based on silicon carbide (SiC) ®bres (i.e. NicalonTM or TyrannoTM) with a SiC or an oxide (i.e. Al2O3) matrix. They have a ®bre/matrix interphase of carbon (C) or boron nitride (BN) that contain weakly bonded planes of atoms, providing a weak debond layer which imparts a non-brittle fracture behaviour. Non-oxide CMCs have attractive high tem￾perature properties, such as creep resistance and micro￾structural stability. They also show high thermal conductivity and low thermal expansion, which reduces thermally induced strains. However, the oxidation sen￾sitivity of the interphase will cause embrittlement of the composite after service at high temperatures for long times. Embrittlement is most severe with cyclic mechan￾ical and thermal loading beyond the proportional limit, because oxygen that penetrates via the matrix cracks created will react locally with the ®bres and ®bre coat￾ings to form oxide products. These reaction products will create strong bonds between the ®bre and matrix, which prevent crack de¯ection and suppress internal frictional mechanisms that otherwise give toughness. A way to avoid the oxidation problem is to use all-oxide composites (i.e. the composites consist of oxide ®bres, oxide interfacial coatings and oxide matrices). The com￾ponents are fully oxidised and further damage by oxida￾tion can be avoided, even at high temperatures and after matrix cracking. Oxide composites also have the attrac￾tive feature of potentially low cost. The primary dicul￾ties with this approach are the lack of suitable oxide ®bre reinforcement and the lack of oxide based ``debond'' layer analogous to carbon or boron nitride. Most oxides also have low thermal conductivity and high thermal expansion, leading to high thermally induced strains. Polycrystalline oxide based ®bres (e.g. NextelTM 610 or Nextel 720) have temperature limitations <1100C associated with creep and sintering due to the high dif￾fusivities of oxides compared to SiC. Single crystal oxi￾des, particularly complex oxides (e.g. YAG), are known to have signi®cantly better creep behaviour than poly￾crystals, although fabricating ®bres remains a challenge. However, a single crystal alumina ®bre (SaphikonTM) is commercially available having a higher temperature capability and better creep resistance than polycrystal￾line alumina ®bres. This ®bre, which is produced by the edge ®lm fed growth method from molten alumina, has the disadvantage of being mono®lament (diameter 125 mm) which prevents weaving and forming into complex shapes, and a very high cost (due to the manufacturing method). Several concepts for ®bre coatings that would improve the damage tolerance of oxide CMCs have been proposed but the performance of these have only been demonstrated in model composites.3±10 2. Objective The objective of the ``Novel Oxide Ceramic Compo￾sites'' programme was to develop an all-oxide composite for long life-time applications (>10,000 h) at tempera￾tures above 1400C in oxidising environments.11 Design and development of an oxide interphase has been reported previously.14±16 This paper reports on the development and scale-up of a composite fabrication process, results from mechanical testing as well as fab￾rication and combustor rig tests of a model component. 3. Experimental 3.1. Materials Single crystal continuous a-Al2O3 sapphire ®bres with a nominal diameter of 125 mm (Saphikon, USA) were chosen for this project since they have a thermodynamic stability compatible with the temperature goal and forming limitations were not an issue for ¯at combustor tiles. However, this single crystal ®bre may not be the ultimate candidate for this application due to its limited high-temperature properties12 and sensitivity to slow crack growth,13 but rather provide a model material on to which to base an oxide/oxide CMC. Alumina was chosen as a matrix to minimise thermally induced stres￾ses between the ®bre and the matrix and limit undesired chemical reactions. A small amount of zirconia was added to control matrix grain growth at elevated tem￾peratures. Zirconia has been identi®ed as a suitable interphase material in alumina based composites since the system is thermochemically stable.3 However, in order for the interphase to behave as a debond layer, it is necessary to reduce the strength by introducing por￾osity (a porosity level of 30% has been suggested4 ). An 600 M. Holmquist et al. / Journal of the European Ceramic Society 20 (2000) 599±606