ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS 4 1-porous mould(EPD)or porous green body(EPl) 2-cathode(platinum hite 68 3-anode(platinum or graphite) 4-DC power supply 5-suspension 6-bidistillied water(+ electrolyte 7-pH meter with PC interface 8- interface for data acquisition and conditioning 9-measuring gages 5 measurement 10-PC for process control and data analyzing 25 mm 10 mm 10 m Figure I Experimental set-up for the electrophoretic deposition/impregnation process from aqueous suspensions the electrodes) were separated. Thus no gas bubbles on basis of SEM images by means of image analysis were found within the deposit. One chamber was filled software(Image C). For this purpose, SEM pictures of with the suspension, the other with bidistilled water not impregnated silica green bodies were taken as ref- containing different amounts of TMAH. The width of erence first. The densification of the impregnated green the different chambers was optimized after measur- bodies was determined on the basis of SEM pictures ing the effective electric field strength within the elec- with the same magnification, that were taken equidis trophoresis cell [34]. This set-up was used to determine tantly(300 um) from the surface in contact with the deposition rate, green density and pore size. For more suspension during EPI towards the bulk. A densifica complex shaped components the set-up had to be re- tion of 0%o means that the same porosity is visible as adjusted concerning the shape of the porous mould and for the not impregnated references. a densification of the electrodes. The applied voltage was varied between 100%o means, that at the given magnification no porosity I and 15 V/cm. Deposition time was 3 min in case of is visible. These values were correlated finally to abse silica and zirconia and 30 s in case of the SiC coatings. lute values of the density and pore size, achieved from In case of electrophoretic impregnation, the same ex- Archimedes method and mercury porosimetry. Fur- perimental set-up was used, but instead of the porous thermore, Raman microscopy was used for a position polymer mould a cast silica green body with open sensitive characterization of graded composites pores(26% porosity, mean pore size 1. 8 um)was used. The electric field strength was varied between 1.5 and 6 V/cm and deposition time was 30 and 60 s, 3. Results and discussion respectively The deposition rate for the electrophoretic deposition After shaping, the green bodies and coated substrates of the different powders used was determined first, be were dried in air under ambient humidity. No cracking cause only an exact control of this parameter can guar occurred for the green bodies and no cracking was ob- antee a reproducible manufacturing of components with served for the Sic coatings for a thickness of up to given specifications concerning wall thickness and tol- 200 um Sintering of the compacts was carried out ei- trances. The deposition rate for an aqueous suspension ther in vacuum(SiO2 powder mixtures, 1500-1600 C), containing 30 wt% of nanosized fumed silica(OX50) argon atmosphere(SiC, 1700-2100 C), air(zirconia, is shown in Fig. 2(dark line, circles) as function of the 1300-1700C)or in a zone-sintering furnace(OX50, applied electric field strength for a deposition time of 1320°C 3 min A linear dependency was observed. The green density of the corresponding silica green bodies was not influenced by the electric field strength and was 2. 3. Characterization determined by Archimedes method to be 39.4%TD. Density and size distribution of green and sin- This means that e.g., a green body with a thickness of tered samples were measured by Archimedes method 12 mm was deposited within 3 min for a field strength and mercury porosimetry(Porotec Pascal P140, P440), of 10 V/cm respectively. Microstructural homogeneity was investi- For nanosized zirconia a similar dependency of the gated on basis of SEM and high resolution SEM deposition rate from the electric field strength was ob SEM)images. Densification due to EPi was determined served but the absolute values are lower than for silica 805ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS Figure 1 Experimental set-up for the electrophoretic deposition/impregnation process from aqueous suspensions. the electrodes) were separated. Thus no gas bubbles were found within the deposit. One chamber was filled with the suspension, the other with bidistilled water containing different amounts of TMAH. The width of the different chambers was optimized after measur￾ing the effective electric field strength within the elec￾trophoresis cell [34]. This set-up was used to determine deposition rate, green density and pore size. For more complex shaped components the set-up had to be re￾adjusted concerning the shape of the porous mould and the electrodes. The applied voltage was varied between 1 and 15 V/cm. Deposition time was 3 min in case of silica and zirconia and 30 s in case of the SiC coatings. In case of electrophoretic impregnation, the same ex￾perimental set-up was used, but instead of the porous polymer mould a cast silica green body with open pores (26% porosity, mean pore size 1.8 µm) was used. The electric field strength was varied between 1.5 and 6 V/cm and deposition time was 30 and 60 s, respectively. After shaping, the green bodies and coated substrates were dried in air under ambient humidity. No cracking occurred for the green bodies and no cracking was ob￾served for the SiC coatings for a thickness of up to 200 µm. Sintering of the compacts was carried out ei￾ther in vacuum (SiO2 powder mixtures, 1500–1600◦C), argon atmosphere (SiC, 1700–2100◦C), air (zirconia, 1300–1700◦C) or in a zone-sintering furnace (OX50, 1320◦C). 2.3. Characterization Density and pore size distribution of green and sin￾tered samples were measured by Archimedes method and mercury porosimetry (Porotec Pascal P140, P440), respectively. Microstructural homogeneity was investi￾gated on basis of SEM and high resolution SEM (HR￾SEM) images. Densification due to EPI was determined on basis of SEM images by means of image analysis software (ImageC). For this purpose, SEM pictures of not impregnated silica green bodies were taken as ref￾erence first. The densification of the impregnated green bodies was determined on the basis of SEM pictures with the same magnification, that were taken equidis￾tantly (300 µm) from the surface in contact with the suspension during EPI towards the bulk. A densifica￾tion of 0% means that the same porosity is visible as for the not impregnated references. A densification of 100% means, that at the given magnification no porosity is visible. These values were correlated finally to abso￾lute values of the density and pore size, achieved from Archimedes method and mercury porosimetry. Fur￾thermore, Raman microscopy was used for a position￾sensitive characterization of graded composites. 3. Results and discussion The deposition rate for the electrophoretic deposition of the different powders used was determined first, be￾cause only an exact control of this parameter can guar￾antee a reproducible manufacturing of components with given specifications concerning wall thickness and tol￾erances. The deposition rate for an aqueous suspension containing 30 wt% of nanosized fumed silica (OX50) is shown in Fig. 2 (dark line, circles) as function of the applied electric field strength for a deposition time of 3 min. A linear dependency was observed. The green density of the corresponding silica green bodies was not influenced by the electric field strength and was determined by Archimedes method to be 39.4%TD. This means that e.g., a green body with a thickness of 12 mm was deposited within 3 min for a field strength of 10 V/cm. For nanosized zirconia a similar dependency of the deposition rate from the electric field strength was ob￾served, but the absolute values are lower than for silica. 805