A.R. Boccaccini et al./Composites: Part A 32(2001)997-1006 diphasic (or mixed) sols(e.g. silica-alumina dual sols of mullite composition), the process becomes more compl cated than with the single species (e.g. single silica or alumina sols). This is because it is necessary to control the mobility and zeta potential of both species in order for both of them to migrate to the same electrode and co-deposit without segregation under EPD conditions. In particular, for the fabrication of mullite matrices, it is necessary to main tain the initial silica-alumina proportion in the deposit material to warrant stoichiometric mullite composition Several authors have shown ways to engineer the surface sPTo84、eeU charge of the particles by the addition of surfactants [44]or to control short range particle-particle interactions and M micrograph of EPD-infiltrated Nicalon'fibre mat using a rheological characteristics of the colloidal suspensions by f mullite Details of the EPD technique used are careful variation of the particle size, solids-loading and pH iginal work [38). A high level of matrix infiltration is seen. It [48, 49]. In the case of diphasic(mixed sols of mullite nown that the deposited material fairly kept the original mulli composition, the pH is chosen so t sitely charged, i.e. the alumina and silica particles are posi tively and negatively charged, respectively. Thus, placed between the deposition electrode and the non heterocoagulated particle clusters are formed, which move onducting fibre preform [35, 41, 42]. On applying a rela- as single, composite particles [38, 43]. An example of a ively high voltage (e.g. 60V for alumina sol [35]), the Nicalon fibre mat infiltrated by a mixed sol of mullite alumina particles migrate and deposit on to the membrane composition using EPD is shown in Fig. 2 [38]. A high from one direction only, through the fibre preform, until a level of matrix infiltration is seen. It was shown in this sufficient matrix thickness which envelopes the preform is work that the deposited material fairly kept the original achieved. The high voltage causes hydrogen evolution at the mullite stoichiometric composition anode but the gas is prevented from becoming part of the One of the most critical processing steps that compact by the presence of the filter membrane. In this way optimised, as emphasised by most authors, is the drying of alumina-alumina [35] and mullite-mullite [45] woven fibre the infiltrated fibre preforms. This is because extensive reinforced ceramic matrix composites could be fabricated. microcracking of the gelled ceramic matrix can occur on An analysis of the published work shows that the quality drying, as usually occurs in sol-gel processing. Cracking of the infiltration also depended strongly on the architecture frequently develops due to the differential shrinkage of the of the fibre preform employed but, in general, EPD was able gel network generating tensile stresses at the surface, which to infiltrate even the very tight woven fibre mats used. The may lead to the catastrophic growth of microscopic flaw parameters of the EPD infiltration []. However, thin films(<1-2 um)can be dried without applied and deposition time, were optimised in the different cracking because the tensile stresses developed when they studies to obtain a high solids-loading in the intra-tow shrink are insufficient to cause the growth of cracks [50] regions and firm, adherent, ceramic deposits. When using Thus, a careful control of the thickness of the matrix mate rials deposited by EPD is required, as highlighted in the literature [4, 35]. Fig. 3 shows the different cracking devel- opment upon drying in two Nicalon fibre mats which have been EPD-infiltrated with mullite composition sol [34]. Th sample with thicker deposit exhibits extensive microcrack- ing, while the optimisation of deposit thickness leads to a minimisation of microcracking The analysis of the literature reveals that when the ePd and drying conditions were optimised, the infiltrated fibre fabrics were of sufficient quality (high infiltration, no macroporosity and minimal microcrack development)to be used as preforms for the fabrication of ceramic or glass 25 mm matrix composites The EPD process has also been successfully applied to infiltrate metallic fibre fabrics. Boehmite. silica and titania Fig. 3. Macrograph showing the development of microcracking in SiC Nicalon fibre mats EPD- infiltrated with mullite composition mixed sol nanoparticles have been used as precursors for the ceramic pon drying. Extensive microcracking is developed in thick deposits(left) matrices [22,,, 52]. 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