H. Hadraba et al /Ceramics International 30(2004)853-863 change in the composition of the material is a change in the sitions were conducted under constant current (5 mA)con- physical or chemical properties of the material in a certain ditions direction The layered alumina/zirconia composite material (LC In electrophoretic deposition of FGM composite materi- HP/3Y in the following) consisted of 30 layers of Al2O als it is important that all the simultaneously deposited com- and 29 layers of ZrO2, which alternated regularly in the ponents be of the same charge polarity and the same elec- composite and were prepared by interrupted electrophoretic trophoretic mobility because in that case the deposit com- deposition, i.e. several consecutive depositions, alternatel osition corresponds to the suspension composition [2] with the AlzO3 and the ZrOz suspension. The total deposi- In the works of Cihlar et al. [10] and Maca et al. tion time of layered composite materials was 130 min. Since [11, the electrokinetic behavior of Al2O3 and ZrO2 par- the particle concentration in the suspension decreased with ticles in isopropanol suspensions and the deposition of time, it was necessary to increase continually the deposi single-component deposits were studied. This experience tion time of individual layers so that they were of constant was applied in the present work to the preparation of thickness. To do this, the relations derived in a previous two-component composite ceramic materials with a low con- work [11] were employed tent of defects. The aim of the experiments was to describe Particle composite materials with a constant ratio of type he application of this suspension in the preparation of lay- HP AlO3 and type 3Y Zro( depending on the level of ered, particle and functionally gradient composite material volume concentrations of individual components these com- posites were denoted PC 75/25, PC 50/50 and PC 25/75) were prepared by interrupted deposition, i.e. several consec 2. Experimental utive depositions with one suspension, between which the spension was stirred. Each partial deposition consumed 2.. Materials 5 min and the total deposition time was 140 min. The two types of FGM composite were prepared by inter- The same ceramic powder materials were used as in the rupted electrophoretic deposition, i.e. a sequence of depo- preceding work [11]: Al2O3 of the RC HP DB HP sitions from suspensions of slightly changed composition in the following, manufacturer Malakoff Ind, USA) and Electrophoretic deposition started with pure Al2O3 suspen- ZrO2 of theTz-3Y type(3Y in the following, manufacturer sion(of type HP). Every 5 min the deposition was interrupted Tosoh, Japan). A more detailed description of these powders and 2 ml(FGM HP/3Y-2 deposit) or 5ml(FGM HP/3Y-5 is given in previous work [11] deposit) suspension in the cell were replaced with ZrO2 sus- Isopropanol(p a, Onex, Czech Republic) was used as pension(of type 3Y). With every interruption, the electrodes the dispersion medium for the preparation of suspensions were taken out and the suspension was stirred manually, both of Al2O3 and Zro2 powders while monochloroacetic acid before and after changing the suspension composition. The (MCAA)(p a, Lachema, Czech Republic) was used as the total deposition time was again 140 min stabilizing and dispersing agent. The content of water in the suspensions was reduced to a minimum(0.01%)using the 2.4. Evaluation of deposit properties same procedure as in previous work [11 When the deposition was finished, all the deposits with 2. 2. Suspension composition the electrode were first dried for 24 h at room tempera ture and for I h at a temperature of 70C and then taker One-component suspensions (employed for elec- down from the electrode, annealed (800C/1 h)and sintered trophoretic deposition of layered and functionally gradient (1500C/2 h)in air atmosphere. The course of sintering materials)were prepared by mixing 15 wt. of powder process was monitored using a high-temperature dilatome (either Al2O3 or ZrO2) and 12.75 wt. of MCAA in ter(L75/50, Linseis, Germany ) Coefficient of thermal ex 72.25 wt of isopropanol. This composition was estab- pansion(CTE) was calculated from the cooling curves of lished as optimal for the deposition of one-component sintered materials. The relative density of annealed deposit Al2O and ZrOz systems [11]. Two-component suspen-(Prel-goo)was established from soaking capacity and the rel- sions employed in electrophoretic deposition of particle ative density of sintered deposit(Prel-1500)was found by the composite materials contained 15 wt. of powder mixture Archimedes method (EN 623-2) (made up of Al2O3 and ZrO2 in volume ratios of 75: 25, Polished specimens of sintered deposits were thermally 50:50 and 25: 75), 12.75 wt. of MCAA and 72.25 wt of etched. The size of sintered ceramic grains was established by computer image analysis(Atlas Software, Tescan, Czech Republic) from microphotographs prepared by scannin 2.3. Electrophoretic deposition electron microscopy(Philips XL30, The Netherlands) The chemical composition of functionally gradient ma- a detailed description of the horizontal electrophoretic terials was determined by electron microprobe analysis ell is given in previous work [11]. All electrophoretic depo(Philips, The Netherlands)854 H. Hadraba et al. / Ceramics International 30 (2004) 853–863 change in the composition of the material is a change in the physical or chemical properties of the material in a certain direction. In electrophoretic deposition of FGM composite materi￾als it is important that all the simultaneously deposited com￾ponents be of the same charge polarity and the same elec￾trophoretic mobility because in that case the deposit com￾position corresponds to the suspension composition [2]. In the works of Cihlar et al. [10] and Maca et al. [11], the electrokinetic behavior of Al2O3 and ZrO2 par￾ticles in isopropanol suspensions and the deposition of single-component deposits were studied. This experience was applied in the present work to the preparation of two-component composite ceramic materials with a low con￾tent of defects. The aim of the experiments was to describe the application of this suspension in the preparation of lay￾ered, particle and functionally gradient composite materials. 2. Experimental 2.1. Materials The same ceramic powder materials were used as in the preceding work [11]: Al2O3 of the RC HP DBM type (HP in the following, manufacturer Malakoff Ind., USA) and ZrO2 of theTZ-3Y type (3Y in the following, manufacturer Tosoh, Japan). A more detailed description of these powders is given in previous work [11]. Isopropanol (p.a., Onex, Czech Republic) was used as the dispersion medium for the preparation of suspensions of Al2O3 and ZrO2 powders while monochloroacetic acid (MCAA) (p.a., Lachema, Czech Republic) was used as the stabilizing and dispersing agent. The content of water in the suspensions was reduced to a minimum (0.01%) using the same procedure as in previous work [11]. 2.2. Suspension composition One-component suspensions (employed for elec￾trophoretic deposition of layered and functionally gradient materials) were prepared by mixing 15 wt.% of powder (either Al2O3 or ZrO2) and 12.75 wt.% of MCAA in 72.25 wt.% of isopropanol. This composition was estab￾lished as optimal for the deposition of one-component Al2O3 and ZrO2 systems [11]. Two-component suspen￾sions employed in electrophoretic deposition of particle composite materials contained 15 wt.% of powder mixture (made up of Al2O3 and ZrO2 in volume ratios of 75:25, 50:50 and 25:75), 12.75 wt.% of MCAA and 72.25 wt.% of isopropanol. 2.3. Electrophoretic deposition A detailed description of the horizontal electrophoretic cell is given in previous work [11]. All electrophoretic depo￾sitions were conducted under constant current (5 mA) con￾ditions. The layered alumina/zirconia composite material (LC HP/3Y in the following) consisted of 30 layers of Al2O3 and 29 layers of ZrO2, which alternated regularly in the composite and were prepared by interrupted electrophoretic deposition, i.e. several consecutive depositions, alternately with the Al2O3 and the ZrO2 suspension. The total deposi￾tion time of layered composite materials was 130 min. Since the particle concentration in the suspension decreased with time, it was necessary to increase continually the deposi￾tion time of individual layers so that they were of constant thickness. To do this, the relations derived in a previous work [11] were employed. Particle composite materials with a constant ratio of type HP Al2O3 and type 3Y ZrO2 (depending on the level of volume concentrations of individual components these com￾posites were denoted PC 75/25, PC 50/50 and PC 25/75) were prepared by interrupted deposition, i.e. several consec￾utive depositions with one suspension, between which the suspension was stirred. Each partial deposition consumed 5 min and the total deposition time was 140 min. The two types of FGM composite were prepared by inter￾rupted electrophoretic deposition, i.e. a sequence of depo￾sitions from suspensions of slightly changed composition. Electrophoretic deposition started with pure Al2O3 suspen￾sion (of type HP). Every 5 min the deposition was interrupted and 2 ml (FGM HP/3Y-2 deposit) or 5 ml (FGM HP/3Y-5 deposit) suspension in the cell were replaced with ZrO2 sus￾pension (of type 3Y). With every interruption, the electrodes were taken out and the suspension was stirred manually, both before and after changing the suspension composition. The total deposition time was again 140 min. 2.4. Evaluation of deposit properties When the deposition was finished, all the deposits with the electrode were first dried for 24 h at room tempera￾ture and for 1 h at a temperature of 70 ◦C and then taken down from the electrode, annealed (800 ◦C/1 h) and sintered (1500 ◦C/2 h) in air atmosphere. The course of sintering process was monitored using a high-temperature dilatome￾ter (L75/50, Linseis, Germany). Coefficient of thermal ex￾pansion (CTE) was calculated from the cooling curves of sintered materials. The relative density of annealed deposit (ρrel-800) was established from soaking capacity and the rel￾ative density of sintered deposit (ρrel-1500) was found by the Archimedes method (EN 623-2). Polished specimens of sintered deposits were thermally etched. The size of sintered ceramic grains was established by computer image analysis (Atlas Software, Tescan, Czech Republic) from microphotographs prepared by scanning electron microscopy (Philips XL30, The Netherlands). The chemical composition of functionally gradient ma￾terials was determined by electron microprobe analysis (Philips, The Netherlands)