C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 2337 deposition behaviour during EPD will be enhanced i,g nd electrophoretic deposition as the particle mobility an weak interfaces), leading to the desired fibre debond- -30 ing, crack deflection and fibre pull-out mechanisms The electrophoretic mobility data in terms of particle Fig 3a shows a SEM microstructure of a fibre, which net surface charge and mean particle size for the mullite as been dip-coated wi ted with NdPO4 without fibre pre- powder and the NdPOa aqueous sol suspensions at a treatment, indicating the poor quality of the coating as constant pH value of 3 are shown in Table 1. From a result of the absence lectrostatic attraction these data, it is evident that both particles are positively- between the fibre and coating particles. However, when charged at a pH value of 3, therefore if any particles are the fibre surface is conditioned with a high pH solution required to be attracted to the target material (for prior to coating in order to create a strong negative example to the electrodes during EPD or to the fibre surface charge, successful results of mullite fibres coated surface during dip coating), the surface charge of such with NdPO4 particles are achievable, as shown in Fig. 3b substrates has to be controlled to be opposite of the after thermal treatment at 600oC for 0.5 h. The image deposited material. The net surface charge around a indicates that the coating is homogeneous over the fibre particle is a very critical parameter in EPD and dip- surface and its thickness is approximately 2.5 um. One coating processes in order to obtain maximum deposi- of the significant disadvantages associated with dip tion and coating efficiency using the"mutual electro- coating is the excess deposition of the coating particles static attraction concept in the inter-fibre regions, generally resulting from the presence of large flocculated chains within the coating 3. 2. Characterisation of the interphase material (NdPO4) sol. As shown in Fig 3c, NdPO4 particles did not fill the regions between individual fibres within the woven mat The key issues in the dip coating process are the structure and during dip coating under the application xtent of wetting of the fibres by the sol and the electro- of vacuum, the capillary pressure difference across the static attraction between the fibre surface and coating particles. 16, 18.31 However, the characteristics of the coating particles dispersed in a suspension in terms of heir shape, size and surface charge have to be taken into consideration as they affect the coating behaviour significantly. The TEM micrograph shown in Fig. 2a indicates that the synthesised NdPO particles are of near spherical shape and the particle size is in the range of 10-70 nm, with a mean particle size of 30 nm(as determined by particle size analysis). In order to deter- mine the microstructure of the interphase after sintering, a sample prepared from the coating suspension contain- ing NdPO4 powders was prepared using simple die press- ing and then sintered at 1200 C for 3 h( the same sintering temperature used for the fin al composite). The SEM micrograph of the sintered microstructure obtained is shown in Fig. 2b. It is seen that the NdPO4 body has a plate-like grain microstructure with the presence of no intra- or inter-granular porosity indicating the dense nature of the material. This microstructure provides when used as an interphase in a composite, a weak bonding with both matrix and fibre (i.e. the required Table Surface charge properties and mean particle size of the hydrothermally nthesised mullite plus 5 wt %o zirconia and NdPOa coating powders dispersed in water at a pH value of 3 Electropho Mean particle mobility (10-mVs-) Mullite+5 wt%0 +1.97 Fig. 2.(a)TEM micrograph of synthesised NdPO4 ZrO2, pH: 3 pherical shape and (b) sEM micre NdPO4, PH: 3 +2.7 30 nm sintered microstructure of NdPO4 material after sintering at for 3 helectrophoretic deposition as the particle mobility and deposition behaviour during EPD will be enhanced.7,29,30 The electrophoretic mobility data in terms of particle net surface charge and mean particle size for the mullite powder and the NdPO4 aqueous sol suspensions at a constant pH value of 3 are shown in Table 1.From these data, it is evident that both particles are positively￾charged at a pH value of 3, therefore if any particles are required to be attracted to the target material (for example to the electrodes during EPD or to the fibre surface during dip coating), the surface charge of such substrates has to be controlled to be opposite of the deposited material.The net surface charge around a particle is a very critical parameter in EPD and dip￾coating processes in order to obtain maximum deposi￾tion and coating efficiency using the ‘‘mutual electro￾static attraction’’ concept. 3.2. Characterisation of the interphase material (NdPO4) The key issues in the dip coating process are the extent of wetting of the fibres by the sol and the electro￾static attraction between the fibre surface and coating particles.16,18,31 However, the characteristics of the coating particles dispersed in a suspension in terms of their shape, size and surface charge have to be taken into consideration as they affect the coating behaviour significantly.The TEM micrograph shown in Fig.2a indicates that the synthesised NdPO4 particles are of near spherical shape and the particle size is in the range of 10–70 nm, with a mean particle size of 30 nm (as determined by particle size analysis).In order to deter￾mine the microstructure of the interphase after sintering, a sample prepared from the coating suspension contain￾ing NdPO4 powders was prepared using simple die press￾ing and then sintered at 1200
C for 3 h (the same sintering temperature used for the final composite).The SEM micrograph of the sintered microstructure obtained is shown in Fig.2b.It is seen that the NdPO4 body has a plate-like grain microstructure with the presence of no intra- or inter-granular porosity indicating the dense nature of the material.This microstructure provides, when used as an interphase in a composite, a weak bonding with both matrix and fibre (i.e. the required weak interfaces),1 leading to the desired fibre debond￾ing, crack deflection and fibre pull-out mechanisms. Fig.3a shows a SEM microstructure of a fibre, which has been dip-coated with NdPO4 without fibre pre￾treatment, indicating the poor quality of the coating as a result of the absence of electrostatic attraction between the fibre and coating particles.However, when the fibre surface is conditioned with a high pH solution prior to coating in order to create a strong negative surface charge, successful results of mullite fibres coated with NdPO4 particles are achievable, as shown in Fig.3b after thermal treatment at 600
C for 0.5 h. The image indicates that the coating is homogeneous over the fibre surface and its thickness is approximately 2.5 mm.One of the significant disadvantages associated with dip coating is the excess deposition of the coating particles in the inter-fibre regions, generally resulting from the presence of large flocculated chains within the coating sol.As shown in Fig.3c, NdPO4 particles did not fill the regions between individual fibres within the woven mat structure and during dip coating under the application of vacuum, the capillary pressure difference across the Table 1 Surface charge properties and mean particle size of the hydrothermally synthesised mullite plus 5 wt.% zirconia and NdPO4 coating powders dispersed in water at a pH value of 3 Electrophoretic mobility (108 m2 V.s1 ) Mean particle size Mullite+5 wt.% ZrO2, pH: 3 +1.97 2.65 mm NdPO4, pH: 3 +2.78 30 nm Fig.2. (a) TEM micrograph of synthesised NdPO4 particles indicating their near spherical shape and (b) SEM micrograph showing the dense sintered microstructure of NdPO4 material after sintering at 1200
C for 3 h. C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342 2337