M. Holmquist et al. / Journal of the European Ceramic Society 20(2000)599-606 optimised zirconia coating based on a powder slurry 3.3. Evaluation technique was developed using a proprietary pro- cess.5,6 The zirconia slurry contai Tensile testing was done using 200 mm long test bars, carbon black powder(ZrO2/C volume ratio was 1: 1). gauge length 40 mm, at room temperature and elevated The matrix slurry was prepared by dispersing alumina temperatures(800, 1200 and 1400 C). An induction powder(SM8, Baikowski, France)and 5 vol% unstabi- furnace with a susceptor was used lised zirconia powder (Degussa, Germany) in water ture testing. The test temperature was controlled by a using classical dispersion techniques thermocouple on the specimen in its centre. Tensile testing was also carried out on 100 mm long samples 3.2. Composite processing with 20 mm gauge length at room temperature in the as- rece ed condition, after static thermal ageing for 100 a process based on prepreg technique was developed 1000 h at 1400 C in air and after cyclic thermal Fig. 1. Single crystal alumina fibres were first passed ageing tests. The cyclic thermal ageing was carried out through the zirconia slurry. After drying, the coated using a 17 min cycle: 20%C-20oC. Forced fibre was wound around an alumina /zirconia powder air-cooling was used and the sample experienced tape placed on top of a large diameter spool. An alu severe cooling regime because it was only cooled from mina/zirconia layer was tape-cast directly onto the one side. A servo-hydraulic MTs testing system with spool and allowed to dry. Prepregs were then cut and hydraulic collet grips and a cross-head displacement stacked to form cross-ply green composite preforms. rate of 0.5 mm/min was used. Strain was monitored by Composites were hot pressed in a graphite die under uniaxial or biaxial extensometers. Microstructural fea- nitrogen atmosphere according to the temperature tures of composite cross-sections and fracture surfaces schedule established previously(1400C, 10 MPa, 70 were characterised using optical and scanning electron min)and finally heat-treated at 1250 C to remove the microscopy carbon in order to form the porous zirconia interphase. Composite plates ranging in size from 50x50 mm- to 3. 4. Component testing 180x 200 mm were made using this manufacturing method. Results from two full size plates(plates 4 and The applicability of the composite as a material for 5, 180x200 mm2)are presented and discussed in this uncooled combustor walls was assessed by evaluation in report. Interphase coating process conditions were a combustor test rig operating at conditions realistic of slightly changed from plate 4 to plate 5. The purpose a gas turbine combustor. The rig consists of a square was to make a thinner coating(but keeping the same frame with effusion cooled nickel alloy walls which are porosity level) in order to increase the fibre/matrix load attached with bolts. One side of the combustor has cut transfer. This was achieved by decreasing the slurry outs where flat tiles can be mounted. Ceramic tiles viscosity. A [0/90].s stacking sequence was used to (approximately 90x 50x3 mm)were fabricated using obtain a symmetrical and balanced composite plate. the process described above and tested in these cut-outs, Hot isostatic pressing was evaluated as an alternative to the front(higher temperature)tile having no holes and pressing process. Specialised tooling was he rear (lower temperature) having two air dilution designed to keep the plates fat during processing and holes(Fig 9). In order to predict the temperature dis- suitable encapsulation technique developed. Results ribution in the combustor walls, a computational fluid from HIPing are reported separately. dynamics (CFD)calculation was carried out using FLUent code. The thermal stresses in the tiles were then calculated by finite element(FE)methods(ANSYS code)using the temperature predictions AlO tape precasting Furnace 4. Results and discussion 4.1. Microstructure of composites wheel Fibre volume fractions around 30% were achieved and the plates were approximately 2.80 mm thick with densities around 87% of theoretical. Initially there were some delamination problems. A solution was found by Hot-pressing increasing the pressure during final sintering to 15 MPa Fig 1. Schematic illustration of fabrication process of ceramic matrix and performing a gentle heat-treatment to 1250"C after the plates had been cut to test bars. A cross-section isoptimised zirconia coating based on a powder slurry technique was developed using a proprietary pro￾cess.15,16 The zirconia slurry contained a binder and carbon black powder (ZrO2/C volume ratio was 1:1). The matrix slurry was prepared by dispersing alumina powder (SM8, Baikowski, France) and 5 vol% unstabi￾lised zirconia powder (Degussa, Germany) in water using classical dispersion techniques. 3.2. Composite processing A process based on prepreg technique was developed, Fig. 1. Single crystal alumina ®bres were ®rst passed through the zirconia slurry. After drying, the coated ®bre was wound around an alumina/zirconia powder tape placed on top of a large diameter spool. An alu￾mina/zirconia layer was tape-cast directly onto the spool and allowed to dry. Prepregs were then cut and stacked to form cross-ply green composite preforms. Composites were hot pressed in a graphite die under nitrogen atmosphere according to the temperature schedule established previously (1400C, 10 MPa, 70 min15) and ®nally heat-treated at 1250C to remove the carbon in order to form the porous zirconia interphase. Composite plates ranging in size from 5050 mm2 to 180200 mm2 were made using this manufacturing method. Results from two full size plates (plates 4 and 5, 180200 mm2 ) are presented and discussed in this report. Interphase coating process conditions were slightly changed from plate 4 to plate 5. The purpose was to make a thinner coating (but keeping the same porosity level) in order to increase the ®bre/matrix load transfer. This was achieved by decreasing the slurry viscosity. A [0/90]8,s stacking sequence was used to obtain a symmetrical and balanced composite plate. Hot isostatic pressing was evaluated as an alternative to the hot pressing process. Specialised tooling was designed to keep the plates ¯at during processing and suitable encapsulation technique developed. Results from HIPing are reported separately.17 3.3. Evaluation Tensile testing was done using 200 mm long test bars, gauge length 40 mm, at room temperature and elevated temperatures (800, 1200 and 1400C). An induction furnace with a susceptor was used in the high tempera￾ture testing. The test temperature was controlled by a thermocouple on the specimen in its centre. Tensile testing was also carried out on 100 mm long samples with 20 mm gauge length at room temperature in the as￾received condition, after static thermal ageing for 100 and 1000 h at 1400C in air and after cyclic thermal ageing tests. The cyclic thermal ageing was carried out using a 17 min cycle; 20C!1200C!20C. Forced air-cooling was used and the sample experienced a severe cooling regime because it was only cooled from one side. A servo-hydraulic MTS testing system with hydraulic collet grips and a cross-head displacement rate of 0.5 mm/min was used. Strain was monitored by uniaxial or biaxial extensometers. Microstructural fea￾tures of composite cross-sections and fracture surfaces were characterised using optical and scanning electron microscopy. 3.4. Component testing The applicability of the composite as a material for uncooled combustor walls was assessed by evaluation in a combustor test rig operating at conditions realistic of a gas turbine combustor.1,19 The rig consists of a square frame with e€usion cooled nickel alloy walls which are attached with bolts. One side of the combustor has cut￾outs where ¯at tiles can be mounted. Ceramic tiles (approximately 90503 mm3 ) were fabricated using the process described above and tested in these cut-outs, the front (higher temperature) tile having no holes and the rear (lower temperature) having two air dilution holes (Fig. 9). In order to predict the temperature dis￾tribution in the combustor walls, a computational ¯uid dynamics (CFD) calculation was carried out using FLUENT code. The thermal stresses in the tiles were then calculated by ®nite element (FE) methods (ANSYS code) using the temperature predictions. 4. Results and discussion 4.1. Microstructure of composites Fibre volume fractions around 30% were achieved and the plates were approximately 2.80 mm thick with densities around 87% of theoretical. Initially there were some delamination problems. A solution was found by increasing the pressure during ®nal sintering to 15 MPa and performing a gentle heat-treatment to 1250C after the plates had been cut to test bars. A cross-section is Fig. 1. Schematic illustration of fabrication process of ceramic matrix composites. M. Holmquist et al. / Journal of the European Ceramic Society 20 (2000) 599±606 601