LA.H. Al-Dawery, E.G. Butler/Composites: Part A 32(2001)1007-1012 size of 30 nm. Aqueous sols containing this powder were Nextel 720 Fibre Mat prepared. Sols with different solid loading were prepared and investigated. These include sols with 10, 15 and 20 wt% ZrO2 powder As an alternative, zirconia sol was also prepared in-house using hydrothermal synthesis. The process was carried out FibreSurface Conditioning in the medium size autoclave. 750 ml of zirconium acetate solution was placed in the autoclave, where a temperature of 220C was reached. After 2 h of reaction, taking place at this temperature, a white slurry(zirconium oxide and acetic Room Temperature Drying acid)was produced. The following step was to dry the slurry In and residual zirconium acetate was then obtained Calcina- on o tion was carried out at 600C for 4 h. The resultant zirco- and Matrix Mat nium powder was then ball milled in water. Initial coating experiments were carried out using nethod previously developed at the IRC/Birmingham University for coating Almaxwoven fabric [6]. The Cold/arm Pressure Infiltration process begins with conditioning of fibres surfaces by immersing the fibre mat in an ammonia based surfactant with a concentration of 1. 5% and a ph of 11.5. The fibre mat was left in surfactant over night and then it was dried in Room Temperature Drying air. To ensure removal of entrapped air, treated fibre mats were degassed under vacuum. They were then immersed in the interphase sol. This was followed by degassing in an ultrasonic bath. Afterwards, octanol was added on the top Pressureless Sintering surface of the sol. Fibre mats were then pulled out of the sol/ octanol. The octanol removes excess sol from the fibre tow to prevent bridging between fibres. Finally, interphase Re-Infiltration with Sol coated fibre mats were left to dry at room temperature in air, before being fired at 700C for 1 h At a later stage, an interphase coating process has been conducted using a simple dip coating technique, without Fibre-Reinforced ceramic pre-conditioning the fibre mats. After dipping in the sol the fibre mat is left under vacuum for 20 min to ensure the Fig. 1. Flow chart for processing oxide/oxide fibre-reinforced ceramic removal of the entrapped air within the fibre mat and hence matrix composite. increasing the chance of the sol material to fully surround the bulk of the fibres The coated fibre mat is then subjected 2.E to a room temperature drying over night. 2. Fibre conditioning 2.3. Vacuum-assisted impregnation of fibre tows with Experimental procedures applied for processing CMCs ceramic sols are illustrated in Fig. 1. Organic sizing on the Nextel720 Electrophoretic deposition(EPD) process was introduced fibre mats was removed by heating, in static air atmosphere, with the aim of ensuring the existence of sufficient matrix to a temperature of 600C for 2 h. During this process, the like material filling the voids within fibre tows [7]. At a later colour of the fibres changes from light green to dark and stage, comprehensive investigation on the whole process finally to a light cream-like colour. The above condition has shown that the process was ineffective when processi were found to be beneficial to allow even and sufficient large-sized samples and that densification should take place de-sizing over relatively large areas of fibre mats. by hot-pressing [8]. Thus, an alternative process has been developed here. Simple vacuum impregnation of fibre mats 2. 2. Processing and application of zirconia interphases with sols containing mullite composition and other suitable materials was found to be the best alternative approach to The first work on interphase coating begun by using a achieve full fibre-tow infiltration. The sols used here contai commercially available type VP zirconia powder(Degussa nano-metre sized particles. The dipping process was Ltd). This is a hydrophilic powder with an average particle assisted by considerably high vacuum levels for 30 min  ',%, -$          *,*   
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