H. Hadraba et al /Ceramics International 30(2004)853-863 Table 2 Densities, hardness, fracture toug nd grain sizes of particle composite materials prepared by electrophoretic deposition Composite designation Relative density Hardness, HV(GPa) Fracture toughness, Mean grain size(um) Pe-800(%) (%) KiC(MPam /) dg-Al2O3 HP AlO3 [11 62.2 PC HP50/3Y-50 3.8 97 353 16.1 0.42 154 l08 0.61 PC HP25/3Y-75 99 44 8.1 0.77 3Y ZrO2 [11] 12.9 9.6 0.5 electrophoretically deposited layers. The possibility of mak- is new is the possibility of preparing the Al2O3/ZrO2 parti- ing use of this behavior to improve the mechanical proper- cle composite of controlled composition by the method of ties of ceramic materials is quite promising [9, 12]. A posi- electrophoretic deposition. A necessary condition of con- tive result of the present work can be seen in the defectless trolling the deposition process is the same electrophoretic structure of composites with low porosity(compared, for ex- mobility of the two components in the suspension. Identical mple, with some deposits prepared from aqueous [4, 5] electrophoretic mobility of Al2O3 and ZrO2 in isopropanol ethanol [6,7] suspensions). This is probably due to the fact suspensions stabilized by monochloroacetic acid was estab- that in the isopropanol suspensions used both Al2O3 and lished in work of Cihlar et al. [10] and the homogeneous dis- Zro2 particles were charged negatively and thus deposited tribution of individual components over the whole deposit on the anode, which eliminated some negative phenomena cross-section has confirmed it(see Fig. 7) (e.g. electrolysis of the solvent) during deposition on the cathode 3. 2. Electrophoretic deposition of particle composite materials Also electrophoretic deposition of functionally gradient materials was conducted at a constant current of 5 mA and Particle composites were prepared from suspensions con- the deposit was formed on the anode taining 15 wt. of a mixture of alumina and zirconia. The A microphotograph of sections through deposits is shown composites were prepared with 75 vol. Al2O3(PC 75/25), Fig. 8. Although the structure of the specimens prepared 50 vol %Al2O3(PC 50/50)and 25 vol %Al2O(PC 25/75). was layered, it featured the smooth concentration transition Electrophoretic deposition was performed at a constant elec- from Al2O3 to ZrO2, as can be seen from Fig 9. The reason tric current of 5 mA, with the suspension regularly stirred in probably lay in the thorough mixing of suspension, where 5 min intervals to prevent the suspension from sedimenting the electrodes were removed during mixing [11 It is obvious from Fig. 8 that the deposit FGM HP/3Y-2 While the relative density of as-annealed deposit de- developed crack. The crack ran parallel with the electrode creased almost linearly with increasing ZrOz content in the plane. Deposit cracking was probably due to the differ deposit, the final density had a minimum for composites ential shrinkage of individual parts of the composite. At with 75 and 50 vol % concentrations of Al2O3, whose den- the beginning of deposition pure Al2O3 was deposited(see sities were only 97.3 and 97.5%TD, respectively (Table 2). Fig 9), whose green density exceeded 60%TD [11] while The hardness values established for particle composites at the opposite end of deposit the majority phase was ZrO2 are given in Table 2. With increasing ZrOz content in the whose green density was less than 50%TD [11]. Thus in omposite the deposit hardness decreased. Table 2 also gives the course of sintering, individual parts of the deposit at- the fracture toughness values of particle composite materials. tained differential shrinkage, which generally leads to di It can be seen from the table that even 25 vol. ZrO2(PC mensional deformations [15]. In the present case they re- 75/25) had a positive effect on the fracture toughness of sulted in cracked deposit. To prevent cracking, it would thus AlO3 be necessary to find such combinations of powder materi- It can be seen from the mean sizes of Al2O3 and ZrO2 als and deposition conditions that would lead to green bod grains in the composites(Table 2)that the particles of mi- ies of comparable green density but also of similar CtE nority component in the particle composite practically did [16] not increase in size in the course of sintering. The growth of Fig. 10 gives a comparison of the composition of deposi the majority component grain was, of course, also hindered with the composition of suspension. The composition of by the surrounding minority phase the composite FGM HP/3Y-5 was measured over the whole Limiting the growth kinetics of matrix grains(be it Al2O3 cross-section by linear(point-by-point)X-ray microanalysis or ZrO2)in Al2O3/ZrO2 particle composite is no new knowl- in equidistant steps of 0. 1 mm. The time t(min) in which edge, it was described as early as the 1980s [13, 14]. what the layer in the distance h(mm) from the electrode wasH. Hadraba et al. / Ceramics International 30 (2004) 853–863 859 Table 2 Densities, hardness, fracture toughness and grain sizes of particle composite materials prepared by electrophoretic deposition Composite designation Relative density Hardness, HV (GPa) Fracture toughness, KIC (MPa m1/2) Mean grain size (
m) ρrel-800 (%) ρrel-1500 (%) dg-Al2O3 dg-ZrO2 HP Al2O3 [11] 62.2 99.2 17.8 5.2 1.93 – PC HP75/3Y-25 57.2 97.3 16.1 8.0 1.14 0.42 PC HP50/3Y-50 53.8 97.5 15.4 9.8 1.08 0.61 PC HP25/3Y-75 46.8 99.3 14.4 8.1 0.87 0.77 3Y ZrO2 [11] 41.6 99.9 12.9 9.6 – 0.55 electrophoretically deposited layers. The possibility of mak￾ing use of this behavior to improve the mechanical proper￾ties of ceramic materials is quite promising [9,12]. A posi￾tive result of the present work can be seen in the defectless structure of composites with low porosity (compared, for ex￾ample, with some deposits prepared from aqueous [4,5] or ethanol [6,7] suspensions). This is probably due to the fact that in the isopropanol suspensions used both Al2O3 and ZrO2 particles were charged negatively and thus deposited on the anode, which eliminated some negative phenomena (e.g. electrolysis of the solvent) during deposition on the cathode. 3.2. Electrophoretic deposition of particle composite materials Particle composites were prepared from suspensions con￾taining 15 wt.% of a mixture of alumina and zirconia. The composites were prepared with 75 vol.% Al2O3 (PC 75/25), 50 vol.% Al2O3 (PC 50/50) and 25 vol.% Al2O3 (PC 25/75). Electrophoretic deposition was performed at a constant elec￾tric current of 5 mA, with the suspension regularly stirred in 5 min intervals to prevent the suspension from sedimenting [11]. While the relative density of as-annealed deposit de￾creased almost linearly with increasing ZrO2 content in the deposit, the final density had a minimum for composites with 75 and 50 vol.% concentrations of Al2O3, whose den￾sities were only 97.3 and 97.5%TD, respectively (Table 2). The hardness values established for particle composites are given in Table 2. With increasing ZrO2 content in the composite the deposit hardness decreased. Table 2 also gives the fracture toughness values of particle composite materials. It can be seen from the table that even 25 vol.% ZrO2 (PC 75/25) had a positive effect on the fracture toughness of Al2O3. It can be seen from the mean sizes of Al2O3 and ZrO2 grains in the composites (Table 2) that the particles of mi￾nority component in the particle composite practically did not increase in size in the course of sintering. The growth of the majority component grain was, of course, also hindered by the surrounding minority phase. Limiting the growth kinetics of matrix grains (be it Al2O3 or ZrO2) in Al2O3/ZrO2 particle composite is no new knowl￾edge, it was described as early as the 1980s [13,14]. What is new is the possibility of preparing the Al2O3/ZrO2 parti￾cle composite of controlled composition by the method of electrophoretic deposition. A necessary condition of con￾trolling the deposition process is the same electrophoretic mobility of the two components in the suspension. Identical electrophoretic mobility of Al2O3 and ZrO2 in isopropanol suspensions stabilized by monochloroacetic acid was estab￾lished in work of Cihlar et al. [10] and the homogeneous dis￾tribution of individual components over the whole deposit cross-section has confirmed it (see Fig. 7). 3.3. Electrophoretic deposition of functionally gradient materials Also electrophoretic deposition of functionally gradient materials was conducted at a constant current of 5 mA and the deposit was formed on the anode. A microphotograph of sections through deposits is shown in Fig. 8. Although the structure of the specimens prepared was layered, it featured the smooth concentration transition from Al2O3 to ZrO2, as can be seen from Fig. 9. The reason probably lay in the thorough mixing of suspension, where the electrodes were removed during mixing. It is obvious from Fig. 8 that the deposit FGM HP/3Y-2 developed crack. The crack ran parallel with the electrode plane. Deposit cracking was probably due to the differ￾ential shrinkage of individual parts of the composite. At the beginning of deposition pure Al2O3 was deposited (see Fig. 9), whose green density exceeded 60%TD [11] while at the opposite end of deposit the majority phase was ZrO2, whose green density was less than 50%TD [11]. Thus in the course of sintering, individual parts of the deposit at￾tained differential shrinkage, which generally leads to di￾mensional deformations [15]. In the present case they re￾sulted in cracked deposit. To prevent cracking, it would thus be necessary to find such combinations of powder materi￾als and deposition conditions that would lead to green bod￾ies of comparable green density but also of similar CTE [16]. Fig. 10 gives a comparison of the composition of deposit with the composition of suspension. The composition of the composite FGM HP/3Y-5 was measured over the whole cross-section by linear (point-by-point) X-ray microanalysis in equidistant steps of 0.1 mm. The time t (min) in which the layer in the distance h (mm) from the electrode was