1632 C. Kaya et al. Joumal of the European Ceramic Sociery 29(2009)1631-1639 pressing of the coated and impregnated fibre mats for consoli ation of the composites in multilayer form. This process leads to the reduction of the overall porosity and hence to improved mechanical properties. Damage mechanisms of the optimised CMCs during tensile strength tests were analysed using in situ acoustic emission technique 2. Experimental work 2. Matrix material 000 High purity a-alumina powder (Tai-Micron, Japan) with an - average particle size of 160 nm and surface area of 14.3 m/g was used as the matrix material. Alumina powder was first dis- persed in distilled water with the additions of dispersant, liquid binder, which is a water based acrylic polymer(Duramax B1014, Chesham Chemicals Ltd, UK), and 0.5 wt. colloidal Y203 Fig. 1. The EPD coating unit connected to the power supply with an average particle size of 10 nm(Nyacol Corp, USA)as sintering aid. The suspension was magnetically stirred for 2h fibre surface. The EPD coating cell shown in Fig. I was used followed by mechanical ball-milling with alumina balls for 8h to coat the fibre mats using an electrode separation of 20 mm and finally ultrasonication for 0.5h. The solid-loading of the and a deposition voltage of 10V d.c. for 3 min. The coating suspension was 85 wt. and the pH was adjusted to be around chamber made of Perspex contains two stainless steel electrodes (30 cm x 20cm) which are connected to a power supply. The coating suspensions(NdPO4 and ZrO2) were used separately. 2. 2. Interface materials Coated fibre mats were then heat treated at 600oC for 0.5h to increase the adhesion between the fibre and the coating layer NdPO4 and Zro were used as interface materials to coat the woven fibre mats using electrophoretic deposition. The tech- 2. 4. Composite processing niques for the synthesis of these powders were reported in detail elsewhere., 8 Briefly, NdPO4 was prepared by the neu- It is very important to obtain ceramic composites with tral reaction of neodymium nitrate with ammonium di-hydrogen the lowest possible porosity to achieve superior mechanical phosphate(ADPH) at room temperature. Equimolar amounts of properties. The approach followed here to improve composite Nd(NO3 )3 and ADPH were dissolved in water to make 0. 25 M densification was to impregnate the electrophoretically coated solutions. The two solutions were mixed by vigorous stirring fibres with nano-sized AlO3 particles in suspension. A device and heating, followed by filtration. The resultant gel filtrate was was developed in which suspension flow onto the fibre mat was en dried and calcined at 1000C for 3 h to yield stoichiometric automatically controlled, as shown in Fig. 2a. Each coated layer NdPO4 monazite powder with an average particle size of 60 nm. of fibres is taken in turn and placed on a clean sheet of glass. A A 15 wt %o aqueous suspension was prepared by ball milling for controlled amount of suspension is then poured onto the fibre 4 h with the pH value adjusted to be 3. Hydrothermal process- mat, where upon a roller is used to ensure an even and thor- ing was used to synthesise Zroz powders. Zirconium acetate ough impregnation. The fibre mat is then carefully turned over precursor was processed in an autoclave at 220C under auto- and the process repeated. Once one layer has been so treated geneous pressure for 2 h. The resultant sol was dried at 110C it was carefully laid on a sheet of filter paper. The next layer for I h and calcined at 600C for 4 h to remove excess acetates of fibres was subjected to the same treatment, and then neatly and obtain zirconia particles(with an average particle size of stacked on top of the first layer in parallel condition. In this way 20 nm). Zirconia particles were re-dispersed in distilled water the desired number of layers(12 layers in the present work to with solid-loading of 15 wt %o and a pH value of 2. obtain 40 vol. fibre loading in the composite)was built up. The green bodies"containing 12 impregnated fibre mats were then 2.3. Fibre coating by EPD stacked in a warm pressing device shown in Fig 2b to compress and consolidate the multilayer structure at 180C for 2 h. To Eight-harness satin woven mullite fibre mats with 1200 sin- ensure that the layers of fibres would not adhere to the supports, gle filaments in a bundle(Nextel 720, 3M, USA) were used aluminium foil was introduced below the layer of filter paper as reinforcement. The fibre mats were pre-treated by desiz- already supporting the fibres. Once the sample was fully con- ing at 500C for 1 h to remove the organic protection layer structed, spacers were placed at either side in order to obtain from the fibre surface. The fibre mats were then immersed in a composite thickness of 3 mm. The warm pressed specimens an ammonia based solution, consisting of an ammonium salt were finally pressureless sintered at 1200C for 2 h in air. The of polymethacrylic acid ( Versical KA21, pH: 9, Allied colloids, sintered composite plates were then cut to fabricate specimens UK)in order to create a strong negative surface charge on the for mechanical testing and microstructural observations. Fig 31632 C. Kaya et al. / Journal of the European Ceramic Society 29 (2009) 1631–1639 pressing of the coated and impregnated fibre mats for consoli￾dation of the composites in multilayer form. This process leads to the reduction of the overall porosity and hence to improved mechanical properties. Damage mechanisms of the optimised CMCs during tensile strength tests were analysed using in situ acoustic emission technique. 2. Experimental work 2.1. Matrix material High purity -alumina powder (Tai-Micron, Japan) with an average particle size of 160 nm and surface area of 14.3 m2/g was used as the matrix material. Alumina powder was first dis￾persed in distilled water with the additions of dispersant, liquid binder, which is a water based acrylic polymer (Duramax B1014, Chesham Chemicals Ltd., UK), and 0.5 wt.% colloidal Y2O3 with an average particle size of 10 nm (Nyacol Corp., USA) as sintering aid. The suspension was magnetically stirred for 2 h followed by mechanical ball-milling with alumina balls for 8 h and finally ultrasonication for 0.5 h. The solid-loading of the suspension was 85 wt.% and the pH was adjusted to be around 4. 2.2. Interface materials NdPO4 and ZrO2 were used as interface materials to coat the woven fibre mats using electrophoretic deposition. The tech￾niques for the synthesis of these powders were reported in detail elsewhere.2,8 Briefly, NdPO4 was prepared by the neu￾tral reaction of neodymium nitrate with ammonium di-hydrogen phosphate (ADPH) at room temperature. Equimolar amounts of Nd(NO3)3 and ADPH were dissolved in water to make 0.25 M solutions. The two solutions were mixed by vigorous stirring and heating, followed by filtration. The resultant gel filtrate was then dried and calcined at 1000 ◦C for 3 h to yield stoichiometric NdPO4 monazite powder with an average particle size of 60 nm. A 15 wt.% aqueous suspension was prepared by ball milling for 4 h with the pH value adjusted to be 3. Hydrothermal process￾ing was used to synthesise ZrO2 powders. Zirconium acetate precursor was processed in an autoclave at 220 ◦C under auto￾geneous pressure for 2 h. The resultant sol was dried at 110 ◦C for 1 h and calcined at 600 ◦C for 4 h to remove excess acetates and obtain zirconia particles (with an average particle size of 20 nm). Zirconia particles were re-dispersed in distilled water with solid-loading of 15 wt.% and a pH value of 2. 2.3. Fibre coating by EPD Eight-harness satin woven mullite fibre mats with 1200 sin￾gle filaments in a bundle (NextelTM 720, 3M, USA) were used as reinforcement. The fibre mats were pre-treated by desiz￾ing at 500 ◦C for 1 h to remove the organic protection layer from the fibre surface. The fibre mats were then immersed in an ammonia based solution, consisting of an ammonium salt of polymethacrylic acid (Versical KA21, pH:9, Allied colloids, UK) in order to create a strong negative surface charge on the Fig. 1. The EPD coating unit connected to the power supply. fibre surface. The EPD coating cell shown in Fig. 1 was used to coat the fibre mats using an electrode separation of 20 mm and a deposition voltage of 10 V d.c. for 3 min. The coating chamber made of Perspex contains two stainless steel electrodes (30 cm × 20 cm) which are connected to a power supply. The coating suspensions (NdPO4 and ZrO2) were used separately. Coated fibre mats were then heat treated at 600 ◦C for 0.5 h to increase the adhesion between the fibre and the coating layer. 2.4. Composite processing It is very important to obtain ceramic composites with the lowest possible porosity to achieve superior mechanical properties. The approach followed here to improve composite densification was to impregnate the electrophoretically coated fibres with nano-sized Al2O3 particles in suspension. A device was developed in which suspension flow onto the fibre mat was automatically controlled, as shown in Fig. 2a. Each coated layer of fibres is taken in turn and placed on a clean sheet of glass. A controlled amount of suspension is then poured onto the fibre mat, where upon a roller is used to ensure an even and thor￾ough impregnation. The fibre mat is then carefully turned over and the process repeated. Once one layer has been so treated, it was carefully laid on a sheet of filter paper. The next layer of fibres was subjected to the same treatment, and then neatly stacked on top of the first layer in parallel condition. In this way the desired number of layers (12 layers in the present work to obtain 40 vol.% fibre loading in the composite) was built up. The “green bodies” containing 12 impregnated fibre mats were then stacked in a warm pressing device shown in Fig. 2b to compress and consolidate the multilayer structure at 180 ◦C for 2 h. To ensure that the layers of fibres would not adhere to the supports, aluminium foil was introduced below the layer of filter paper already supporting the fibres. Once the sample was fully con￾structed, spacers were placed at either side in order to obtain a composite thickness of 3 mm. The warm pressed specimens were finally pressureless sintered at 1200 ◦C for 2 h in air. The sintered composite plates were then cut to fabricate specimens for mechanical testing and microstructural observations. Fig. 3