338 C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 curve red fibre surfaces was increased resulting in enhanced coating only on the fibre surface 18.32 Under hese conditions, NdPO4 particles were deposited onto he fibres by improved electrostatic attraction leading to the formation of the coating layer whilst the fibre mat was immersed in the coating suspension, as shown in ig. 3c. The effectiveness of fibre surface treatment is thus demonstrated in Fig. 3b and c, indicating the homogeneous and full coverage of the fibre surface and woven fibre mat layer by the monazite particles due to the strong mutual electrostatic attraction The results presented in Figs. 2 and 3 provide addi- tional information about the relationship between the nature of monazite interface and the coating quality. In a previous work, it has been shown that the needle-lik morphology and coarse particle size(400 nm)of NdPO4 nhibit the infiltration process and result in poor coating of mullite fibres. It appears from the present results that the very fine(30 nm) and more equiaxed morphol- ogy of coating particles enhances the homogeneous coating, proving that dip-coating n appropriate method for coating woven fibre fabric layers rather than individual fibres. Fibre surface pre-treatment with a wetting agent prior to dip-coating is also shown to be a critical step in order to obtain a high-quality coating 3.3. Electrophoretic deposition and pressure filtration The SEM micrographs in Fig. 4 show the micro structures of the EPD-infiltrated and pressure filtrated mullite fibre-reinforced mullite composites with a NdPO4 interphase. NdPO4-coated mullite woven fibre mats were nearly fully infiltrated by the mullite powders during EPD and pressure filtration as the green micro- structure in Fig. 4a demonstrates. As can be seen the intra-tow regions of the woven fibre mat are well infil- trated. The sintered microstructure is shown in Fig. 4b indicating the absence of any drying or sintering cracks within the composite after sintering at 1200oC for 3 h, resulting in a relatively high density of 86% of theore- tical density(TD) Matrix phase composition was found to be stoichiometric 3: 2 mullite plus clinic form) using XRD, as shown in Fig 5a. No resi- dual cristobalite was found under the detection limit of XRD. The sintered microstructure of mullite plus 5 wt% zirconia matrix is shown in Fig. 5b. The image was taken from a sample fractured at room temperature indicating the presence of homogeneously distributed Fig 3 SEM micrographs of mullite fibres after dip-coating in NdPO4 very fine(50-350 nm in diameter)spherical pores within suspension, showing:(a)poor coating quality if the fibre surface is not the mullite grains and also on the grain boundaries pre-treated with a high pH sol prior to coating, (b) homogeneous This microstructural feature is due to the hydrothermal Oa coating around the mul urface obtained by dip coat- processing of the mullite powders and it is considered to hen the fibre surface is pre e fibre mat with very fine d and(c)successfully coated be a convenient feature to obtain both good thermal particles. Note that coated shown here have been fired at 600 oC and the average coating shock resistance and high temperature damage-tolerant behacurved fibre surfaces was increased resulting in enhanced coating only on the fibre surface.18,32 Under these conditions, NdPO4 particles were deposited onto the fibres by improved electrostatic attraction leading to the formation of the coating layer whilst the fibre mat was immersed in the coating suspension, as shown in Fig.3c.The effectiveness of fibre surface treatment is thus demonstrated in Fig.3b and c, indicating the homogeneous and full coverage of the fibre surface and woven fibre mat layer by the monazite particles due to the strong mutual electrostatic attraction. The results presented in Figs.2 and 3 provide addi￾tional information about the relationship between the nature of monazite interface and the coating quality.In a previous work, it has been shown that the needle-like morphology and coarse particle size (400 nm) of NdPO4 inhibit the infiltration process and result in poor coating of mullite fibres.14 It appears from the present results that the very fine (30 nm) and more equiaxed morphol￾ogy of coating particles enhances the homogeneous coating, proving that dip-coating is an appropriate method for coating woven fibre fabric layers rather than individual fibres.Fibre surface pre-treatment with a wetting agent prior to dip-coating is also shown to be a critical step in order to obtain a high-quality coating layer. 3.3. Electrophoretic deposition and pressure filtration The SEM micrographs in Fig.4 show the micro￾structures of the EPD-infiltrated and pressure filtrated mullite fibre-reinforced mullite composites with a NdPO4 interphase.NdPO4-coated mullite woven fibre mats were nearly fully infiltrated by the mullite powders during EPD and pressure filtration as the green micro￾structure in Fig.4a demonstrates.As can be seen the intra-tow regions of the woven fibre mat are well infil￾trated.The sintered microstructure is shown in Fig.4b indicating the absence of any drying or sintering cracks within the composite after sintering at 1200
C for 3 h, resulting in a relatively high density of 86% of theore￾tical density (TD).Matrix phase composition was found to be stoichiometric 3:2 mullite plus zirconia (mono￾clinic form) using XRD, as shown in Fig.5a.No resi￾dual cristobalite was found under the detection limit of XRD.The sintered microstructure of mullite plus 5 wt.% zirconia matrix is shown in Fig. 5b. The image was taken from a sample fractured at room temperature indicating the presence of homogeneously distributed very fine (50–350 nm in diameter) spherical pores within the mullite grains and also on the grain boundaries. This microstructural feature is due to the hydrothermal processing of the mullite powders and it is considered to be a convenient feature to obtain both good thermal shock resistance and high temperature damage-tolerant behaviour. Fig.3. SEM micrographs of mullite fibres after dip-coating in NdPO4 suspension, showing: (a) poor coating quality if the fibre surface is not pre-treated with a high pH sol prior to coating, (b) homogeneous NdPO4 coating around the mullite fibre surface obtained by dip coat￾ing when the fibre surface is pre-treated and (c) successfully coated mullite fibre mat with very fine NdPO4 particles.Note that coated fibres shown here have been fired at 600
C and the average coating thickness is about 2 mm. 2338 C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342