《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_fabrication of Al2O3-zRo2

CERAMICS INTERNATIONAL SEVIER Ceramics International 30(2004)853-863 www.elsevier.com/locate/ceramint Electrophoretic deposition of alumina and zirconia II. Two-component systems Hynek Hadraba*, Karel Maca, Jaroslav Cihlar Received 30 May 2003: received in revised form 17 August 2003; accepted 25 September 2003 Abstract The similar electrophoretic mobility of Al2O and ZrOz in the isopropanol suspensions containing monochloroacetic acid enabled a controlled preparation of layered and particle composites Al2O3/ZrO2 as well as functionally gradient materials with gradual composition transition from Al2O3 to ZrO2. In view of the negative charge of Al2 O3 and Zro particles in the isopropanol suspensions used, all the prepared types of composite were deposited on the anode and thus they were not affected by possible solvent electrolysis, which contributed to their defect-free and low-porosity structure. Phenomena related to the deposition kinetics of these composites as well as some properties of as-sintered composites are described in the paper. o2003 Elsevier Ltd and Techna Group s r l. All rights reserved eywords: B Composites, D. Al2O3; D ZrO2; Electrophoretic deposition 1. Introduction could not find in the available literature any experimental work on electrophoretic deposition of Al2O3/ZrO2 particle Depending on the geometry of the reinforcement phas composites. Only Wang et al. [3 described the preparation composite materials can be subdivided into particle, fiber of a particle composite based on Al2O3/Zro2, which was and layered composites[1]. The technique of electrophoretic formed when a deposit of Al/ZrO2 particles was sintered in deposition appears to be of much promise in the preparation oxygen atmosphere of particle and layered composites Layered composite materials are produced by alternating In electrophoretic deposition of particle composites it electrophoretic depositions of suspensions of different com- is necessary that all the simultaneously deposited phases position. The preparation of Al2O3/ZrO2 layered composite should have identical charge polarity and electrophoretic via alternating electrophoretic deposition from aqueous [4, 51 mobility in order to obtain a homogeneous deposit [2]. To but more often from ethanol [6-8] suspension of Al2O3 and obtain maximum density and homogeneity of the chem- Z O2 particles was reported in the literature. The preparation ical composition of deposit it is also important that the of ceramic layered composites made up of thin layers poses deposited phases be thoroughly dispersed in the suspension problems from the viewpoint of the appearance of stresses retical possibility of electrophoretic deposition of particle on the interface of two different materials. These stresses and not coagulated. Deliso et al. [2] predicted the theo- appear in the process of deposit drying and sintering due te composite based on Al2O3 and ZrO2 as early as 1988 different deposit green densities, and also due to different B hen they found that Al2O and ZrOz particles in aqueous thermal expansion of materials while the sintered composite lized by ammonium polyacrylate. Although in the literature of the dng sit These stresses may lead to the defor mation One of the ways of avoiding the problems related to the Al2O3/ZrO composites and also on particle composites harp transition between two layers is to introduce weak of other than Al203/ZrO2 composition, the present authors interfaces between them, another way is to prepare function ally gradient responding author. Fax: +420-541 143202 in this paper understood as heterogeneous multi-component ail address: hadraba a umi. fme. vutbr cz(H. Hadraba) materials with composition gradient. The consequence of a 0272-8842/$30.00 0 2003 Elsevier Ltd and Techna Group S r l. All rights reserved doi:10.1016/ ceramist2003.09.020

Ceramics International 30 (2004) 853–863 Electrophoretic deposition of alumina and zirconia II. Two-component systems Hynek Hadraba∗, Karel Maca, Jaroslav Cihlar Department of Ceramics, Brno University of Technology, Brno 616 69, Czech Republic Received 30 May 2003; received in revised form 17 August 2003; accepted 25 September 2003 Available online 20 March 2004 Abstract The similar electrophoretic mobility of Al2O3 and ZrO2 in the isopropanol suspensions containing monochloroacetic acid enabled a controlled preparation of layered and particle composites Al2O3/ZrO2 as well as functionally gradient materials with gradual composition transition from Al2O3 to ZrO2. In view of the negative charge of Al2O3 and ZrO2 particles in the isopropanol suspensions used, all the prepared types of composite were deposited on the anode and thus they were not affected by possible solvent electrolysis, which contributed to their defect-free and low-porosity structure. Phenomena related to the deposition kinetics of these composites as well as some properties of as-sintered composites are described in the paper. © 2003 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Composites; D. Al2O3; D. ZrO2; Electrophoretic deposition 1. Introduction Depending on the geometry of the reinforcement phase, composite materials can be subdivided into particle, fiber and layered composites [1]. The technique of electrophoretic deposition appears to be of much promise in the preparation of particle and layered composites. In electrophoretic deposition of particle composites it is necessary that all the simultaneously deposited phases should have identical charge polarity and electrophoretic mobility in order to obtain a homogeneous deposit [2]. To obtain maximum density and homogeneity of the chem￾ical composition of deposit it is also important that the deposited phases be thoroughly dispersed in the suspension and not coagulated. Deliso et al. [2] predicted the theo￾retical possibility of electrophoretic deposition of particle composite based on Al2O3 and ZrO2 as early as 1988, when they found that Al2O3 and ZrO2 particles in aqueous medium had the same electrophoretic mobility if stabi￾lized by ammonium polyacrylate. Although in the literature much data is given on electrophoretic preparation of layered Al2O3/ZrO2 composites and also on particle composites of other than Al2O3/ZrO2 composition, the present authors ∗ Corresponding author. Fax: +420-541143202. E-mail address: hadraba@umi.fme.vutbr.cz (H. Hadraba). could not find in the available literature any experimental work on electrophoretic deposition of Al2O3/ZrO2 particle composites. Only Wang et al. [3] described the preparation of a particle composite based on Al2O3/ZrO2, which was formed when a deposit of Al/ZrO2 particles was sintered in oxygen atmosphere. Layered composite materials are produced by alternating electrophoretic depositions of suspensions of different com￾position. The preparation of Al2O3/ZrO2 layered composite via alternating electrophoretic deposition from aqueous [4,5] but more often from ethanol [6–8] suspension of Al2O3 and ZrO2 particles was reported in the literature. The preparation of ceramic layered composites made up of thin layers poses problems from the viewpoint of the appearance of stresses on the interface of two different materials. These stresses appear in the process of deposit drying and sintering due to different deposit green densities, and also due to different thermal expansion of materials while the sintered composite is cooling [9]. These stresses may lead to the deformation of the deposit [5] or even to the appearance of cracks [9]. One of the ways of avoiding the problems related to the sharp transition between two layers is to introduce weak interfaces between them, another way is to prepare function￾ally gradient materials (FGM in the following), which are in this paper understood as heterogeneous multi-component materials with composition gradient. The consequence of a 0272-8842/$30.00 © 2003 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2003.09.020

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)

H. Hadraba et al /Ceramics International 30(2004)853-863 500m Fig. 1. Microphotographs of alumina/zirconia layered composite LC HP/3Y. The fracture toughness of sintered deposits was measured First, a layered deposit with varied ZrO2 layer thickness by the indentation method(according to Japanese Industrial and constant Al2O3 layer thickness(70 um) was prepared Standard JIs R 1607) on a Vickers indentor at a loading ZrO2 layers of more than 60 um in thickness exhibited force of 98N cracks. These cracks were of large opening displacement Similar cracks were reported by Hillman et al. [9], who showed that such cracks had developed already in the 3. Results and discussion stage of drying and sintering due to the different green densities of individual layers. The following deposition 3.1. Electrophoretic deposition of layered composite were therefore performed such that the thickness of Al2O3 and ZrO2 layers in the produced layered composites were All depositions were conducted at a constant current of less than 50 um, which was the thickness of the layers in which the cracks did not appear. Layered composite mA, with the particles depositing on the anode as in the receding works [10, 11] prepared in this way was really free from these defects (Fg.1) ZrO, ALO bbzro AlyO 500m 3 um Fig. 2. Microstructure of alumina/zirconia interface in layered composite LC HP/3Y at(a)low and(b) high magnification

H. Hadraba et al. / Ceramics International 30 (2004) 853–863 855 Fig. 1. Microphotographs of alumina/zirconia layered composite LC HP/3Y. The fracture toughness of sintered deposits was measured by the indentation method (according to Japanese Industrial Standard JIS R 1607) on a Vickers indentor at a loading force of 98 N. 3. Results and discussion 3.1. Electrophoretic deposition of layered composite materials All depositions were conducted at a constant current of 5 mA, with the particles depositing on the anode as in the preceding works [10,11]. Fig. 2. Microstructure of alumina/zirconia interface in layered composite LC HP/3Y at (a) low and (b) high magnification. First, a layered deposit with varied ZrO2 layer thickness and constant Al2O3 layer thickness (70
m) was prepared. ZrO2 layers of more than 60
m in thickness exhibited cracks. These cracks were of large opening displacement. Similar cracks were reported by Hillman et al. [9], who showed that such cracks had developed already in the stage of drying and sintering due to the different green densities of individual layers. The following depositions were therefore performed such that the thickness of Al2O3 and ZrO2 layers in the produced layered composites were less than 50
m, which was the thickness of the layers in which the cracks did not appear. Layered composite prepared in this way was really free from these defects (Fig. 1)

H. Hadraba et al /Ceramics International 30(2004)853-863 3Y ZrO2-T --- HP AL.O-T LC HP/Y-T OTHOzu-> 王-12+4P4Q,T=10% TE(HP ALO3T)=9010°K 16 C HP/3Y-T)=-1478% TE(LC HP/3Y-T)=9.710*KT Yzo2T)=2559% 02004006008001000120014001600 TEMPERATURE [C] ce of relative length change of alumina/zirconia layered composite LC HP/3Y and single-component alumina and zirconia deposits on sintering temperature. The relative density of layered composite LC HP/3Y was cause of residual thermal stresses(or) in the layers. The 7%TD Single-component deposits had a relative density magnitude of such stress in ZrO2 can be calculated from the of 99.2%(type HP AlO3) or 99.9%TD(type 3Y ZrO2) relation [91 11. The higher porosity of the laminated composite was probably due to the inhomogeneities at the interface of in- orZo,&(CTEzrO2-CTEAlO3)ATEZrO2 dividual phases. However, these inhomogeneities were re- 1一vrO2 sponsible for about 2% of the porosity, the bond of layers in tzrO EzrO,/(I-vZ the composite was thus of good quality, as shown in Fig. 2 Fig. 3 gives for the layered alumina/zirconia compos- ite the dependence of shrinkage in parallel direction to the where tZrO2 and tAl203 are the average layer thickness val- layers(transversal direction-T)on the temperature in the ues, VZrO, and VAl2O, are the Poisson ratios and Ezron and course of sintering and cooling. In this work the abbrevia- EAl, are the elasticity moduli of ZrO and Al,O3 of the tions t(transversal direction) and L (longitudinal direction) composite(the stress in the Al2O3 phase, orAb, is obtained will have the same meaning as in the previous paper [11]. by interchanging the subscripts of the quantities) The shrinkage of the alumina/zirconia layered composite in Employing the values given in Table 1, we obtain for the this direction was given by the shrinkage of Al2O3, which Al2 O3 layer ssion stress orAl203 =-362 MPa and shrinks less than Zro(for comparison, the sintering curves for ZrO tensile stress orZrO2 for one-component deposits in transversal direction are also are parallel to the interface of Al2 O3 and ZrOz layers,the given in Fig. 3). This result lends support to the consider- tensile stresses(which originated in ZrO2) are more dan ations in the introduction to this chapter, namely that the gerous. By analysing Eq (I)it can be shown that in zro2 appearance of cracks in ZrO2 layers more than 50 Hm thick residual stress increases with decreasing thickness of the was due to different green densities of individual layers. A ZrO2 layer. By contrast, in the case of stresses appearing an be seen from Fig 3, in the course of sintering the ZrO2 in the course of sintering it was shown that with increasing layer should shrink more than the surrounding Al2O3 lay thickness of the ZrO2 layer the danger of cracks appearing ers in the composite permitted, which led to tensile stress in this layer increased. From the viewpoint of defect-free in the layer In the case of thicker layers this tension led in layer a layered composite must thus have a certain optimum turn to the appearance of cracks. Whether in thinner layers his stress remained or relaxed at the sintering temperature Properties of the ceramic materials being deposited (for example, via diffusion processes on grain boundaries) remains unanswered in this paper Al O3 e The slope or the cooling curve was used to calculate Elasticity module, E(GPa)[8] 380.0 210.0 CTE. As is obvious from Fig. 3, the CTE of the LC Poisson ratio,v[8 0.31 HP/3Y composite was roughly an average of CTEAL,O, and CTE(x10-K)[11] CTEzrO, which differed by about 13%. This fact was the Average layer thickness, I(um)

856 H. Hadraba et al. / Ceramics International 30 (2004) 853–863 Fig. 3. Dependence of relative length change of alumina/zirconia layered composite LC HP/3Y and single-component alumina and zirconia deposits on sintering temperature. The relative density of layered composite LC HP/3Y was 97%TD. Single-component deposits had a relative density of 99.2% (type HP Al2O3) or 99.9%TD (type 3Y ZrO2) [11]. The higher porosity of the laminated composite was probably due to the inhomogeneities at the interface of in￾dividual phases. However, these inhomogeneities were re￾sponsible for about 2% of the porosity, the bond of layers in the composite was thus of good quality, as shown in Fig. 2. Fig. 3 gives for the layered alumina/zirconia compos￾ite the dependence of shrinkage in parallel direction to the layers (transversal direction—T) on the temperature in the course of sintering and cooling. In this work the abbrevia￾tions T (transversal direction) and L (longitudinal direction) will have the same meaning as in the previous paper [11]. The shrinkage of the alumina/zirconia layered composite in this direction was given by the shrinkage of Al2O3, which shrinks less than ZrO2 (for comparison, the sintering curves for one-component deposits in transversal direction are also given in Fig. 3). This result lends support to the consider￾ations in the introduction to this chapter, namely that the appearance of cracks in ZrO2 layers more than 50
m thick was due to different green densities of individual layers. As can be seen from Fig. 3, in the course of sintering the ZrO2 layer should shrink more than the surrounding Al2O3 lay￾ers in the composite permitted, which led to tensile stress in the layer. In the case of thicker layers this tension led in turn to the appearance of cracks. Whether in thinner layers this stress remained or relaxed at the sintering temperature (for example, via diffusion processes on grain boundaries) remains unanswered in this paper. The slope of the cooling curve was used to calculate the CTE. As is obvious from Fig. 3, the CTE of the LC HP/3Y composite was roughly an average of CTEAl2O3 and CTEZrO2 , which differed by about 13%. This fact was the cause of residual thermal stresses (σr) in the layers. The magnitude of such stress in ZrO2 can be calculated from the relation [9]: σrZrO2 = (CTEZrO2 − CTEAl2O3 ) TEZrO2 1 − νZrO2 ×
1 + tZrO2 tAl2O3 EZrO2 /(1 − νZrO2 ) EAl2O3 /(1 − νAl2O3 ) −1 (1) where tZrO2 and tAl2O3 are the average layer thickness val￾ues, νZrO2 and νAl2O3 are the Poisson ratios and EZrO2 and EAl2O3 are the elasticity moduli of ZrO2 and Al2O3 of the composite (the stress in the Al2O3 phase, σrAl2O3 is obtained by interchanging the subscripts of the quantities). Employing the values given in Table 1, we obtain for the Al2O3 layer compression stress σrAl2O3 = −362 MPa and for ZrO2 tensile stress σrZrO2 = +373 MPa. These stresses are parallel to the interface of Al2O3 and ZrO2 layers; the tensile stresses (which originated in ZrO2) are more dan￾gerous. By analysing Eq. (1) it can be shown that in ZrO2 residual stress increases with decreasing thickness of the ZrO2 layer. By contrast, in the case of stresses appearing in the course of sintering it was shown that with increasing thickness of the ZrO2 layer the danger of cracks appearing in this layer increased. From the viewpoint of defect-free layer a layered composite must thus have a certain optimum Table 1 Properties of the ceramic materials being deposited Al2O3 ZrO2 Elasticity module, E (GPa) [8] 380.0 210.0 Poisson ratio, ν [8] 0.26 0.31 CTE (×10−6 K−1) [11] 9.0 10.3 Average layer thickness, t (
m) 41.5 42.8

H. Hadraba et al /Ceramics International 30(2004)853-863 AL2O, Zro Zro2 alo 20 Am 20 uLm Fig. 4. Microphotographs of indentation cracks in alumina/zirconia layered composite LC HP/Y propagated perpendicular to the interface of alumina and zirconia(crack initiated in(a)alumina and in(b) zirconia) thickness range. The composites prepared as part of the densities, and their different thermal expansion. Both these present work satisfied this requirement phenomena introduce tensile stresses into Zro2 layers and From a comparison of dilatometric curves of the Al203 compression stresses into Al2O3 layers. What effect these and Zro layered composite with the curves of the individ- stresses have on the mechanical properties of the composite ual pure components it could be seen that there were at least is shown in the following paragraph two sources of the stress introduced into the composite dur- Figs. 4-6 are microphotographs of indentation cracks in ing the sintering process: the different sintering kineti yered composite materials. The effect of layer interface on individual components resulting from their different gi indentation crack propagation was studied. The indentation Ao」 ZrO2 0 20 um f indentation cracks in alumina/zirconia layered composite LC HP/Y pro d askew to terface of alumina an (a) alumina and in(b)zirconia)

H. Hadraba et al. / Ceramics International 30 (2004) 853–863 857 Fig. 4. Microphotographs of indentation cracks in alumina/zirconia layered composite LC HP/3Y propagated perpendicular to the interface of alumina and zirconia (crack initiated in (a) alumina and in (b) zirconia). thickness range. The composites prepared as part of the present work satisfied this requirement. From a comparison of dilatometric curves of the Al2O3 and ZrO2 layered composite with the curves of the individ￾ual pure components it could be seen that there were at least two sources of the stress introduced into the composite dur￾ing the sintering process: the different sintering kinetics of individual components resulting from their different green Fig. 5. Microphotographs of indentation cracks in alumina/zirconia layered composite LC HP/3Y propagated askew to the interface of alumina and zirconia (crack initiated in (a) alumina and in (b) zirconia). densities, and their different thermal expansion. Both these phenomena introduce tensile stresses into ZrO2 layers and compression stresses into Al2O3 layers. What effect these stresses have on the mechanical properties of the composite is shown in the following paragraph. Figs. 4–6 are microphotographs of indentation cracks in layered composite materials. The effect of layer interface on indentation crack propagation was studied. The indentation

H. Hadraba et al /Ceramics International 30 (2004)853-863 AL, O3 20 um AO」 20 um Fig. 6. Microphotographs of indentation cracks in alumina zirconia layered composite LC HP/3Y propagated parallel with the interface of alumina and zirconia(crack initiated in(a)alumina and(b) zirconia). was conducted such that the appearing crack propagated terial, again parallel with the interface(even a crack initiated from the Al]O3 or ZrO2 material in three different directions: in the ZrOz layer got deflected into the Al2O3 layer).As erpendicular, askew and parallel to the interface mentioned above, the direction of compression (in Al2O3) If the crack propagated perpendicular to the interface and and tensile (in ZrO2) residual thermal stresses in the lay was initiated in Al2O3 or ZrO2(see Fig 4), it propagated ers is parallel with the layer interface. In the tensile stress through the Al2o3/ZrO2 interface without changing the di- field, a crack initiated in ZrOz parallel with the layer in- rection of propagation terface got in the Zroz layer deflected perpendicular to the When the indentation crack propagated askew to the layer interface, where its propagation was made easier due Al2O3/ZrO2 interface (see Fig. 5) the direction of crack to the crack opening out in the tensile field. After crossing propagation got deflected. When passing through the inter- the interface it propagated in the Al2O3 layer parallel with face, a crack that was initiated in Al2O3 was deflected in the tensile stresses in this layer. Another possible explana the Zro layer from the layer interface( towards the normal tion for the crack deflection from ZrO2 into Al2O3 is the to the interface area) and thus its path in the Zroz layer was deformation of the stress field close to the Al2O3/ZrO2 in- shortened. A crack that was initiated in ZrO2 and was propa- terface. As can be seen from Figs. 5 and 6, near the interface gating askew to the Zro2/Al2O3 interface deflected towards the crack is not initiated from the indentation corner, it is he layer interface when passing through the interface shifted towards the Al2O3 layer (in the case of symmetri The authors of recent papers [6, 7] also described a simi- cal indentations with respect to interface position(Fig. 4)it lar indentation crack propagation askew to the interface of is initiated from the indentation corner). As Youngs mod- Al2O3 and Zro2 layers. Hatton and Nicholson [8] explaine ulus of AlO3 is almost twice that of ZrO(see Table 1) is behavior of Al2O3/ZrO2-based layered composites by the stress induced by the indentation tool was in Al2O3 al- the presence of residual thermal stresses in composite lay- most twice that in Zro2. It is thus possible that maximum ers. In the compression field, the crack propagated parallel stress(and, consequently, the point of crack initiation)was to the direction of compression while in the tensile field not in the indentation corner. In the case of indentation it propagated perpendicular to the direction of tension [8]. in ZrO2, the crack could have initiated even in the al2o This also shows in the behavior of indentation cracks pass- layer at a point close to the indentation corner. The crack ing through the AlO3/ZrO2 interface described above: in then propagated on the one hand towards the indentation(in the Zro2 layer the crack was deflected from the layer inter- ZrO2)and, on the other hand, parallel with the interface(in face and propagated perpendicular to the tensile tension in Al2O3)in order not to be closed by the acting compressive the layer while in the AlzO3 layer it was deflected towards stress. It is not the aim of the present work to give a de- the layer interface, i.e. parallel with the compression tension tailed description of mechanical properties of layered com- in the layer posites but important knowledge has been obtained through A crack initiated parallel with the Al2O3/ZrO2 interface the changes in the trajectory of a crack propagating at the see Fig. 6)always propagated preferably in the Al2O3 ma- la ayer interface. It is a proof of the strength of the bond of

858 H. Hadraba et al. / Ceramics International 30 (2004) 853–863 Fig. 6. Microphotographs of indentation cracks in alumina/zirconia layered composite LC HP/3Y propagated parallel with the interface of alumina and zirconia (crack initiated in (a) alumina and (b) zirconia). was conducted such that the appearing crack propagated from the Al2O3 or ZrO2 material in three different directions: perpendicular, askew and parallel to the interface. If the crack propagated perpendicular to the interface and was initiated in Al2O3 or ZrO2 (see Fig. 4), it propagated through the Al2O3/ZrO2 interface without changing the di￾rection of propagation. When the indentation crack propagated askew to the Al2O3/ZrO2 interface (see Fig. 5) the direction of crack propagation got deflected. When passing through the inter￾face, a crack that was initiated in Al2O3 was deflected in the ZrO2 layer from the layer interface (towards the normal to the interface area) and thus its path in the ZrO2 layer was shortened. A crack that was initiated in ZrO2 and was propa￾gating askew to the ZrO2/Al2O3 interface deflected towards the layer interface when passing through the interface. The authors of recent papers [6,7] also described a simi￾lar indentation crack propagation askew to the interface of Al2O3 and ZrO2 layers. Hatton and Nicholson [8] explained this behavior of Al2O3/ZrO2-based layered composites by the presence of residual thermal stresses in composite lay￾ers. In the compression field, the crack propagated parallel to the direction of compression while in the tensile field it propagated perpendicular to the direction of tension [8]. This also shows in the behavior of indentation cracks pass￾ing through the Al2O3/ZrO2 interface described above: in the ZrO2 layer the crack was deflected from the layer inter￾face and propagated perpendicular to the tensile tension in the layer while in the Al2O3 layer it was deflected towards the layer interface, i.e. parallel with the compression tension in the layer. A crack initiated parallel with the Al2O3/ZrO2 interface (see Fig. 6) always propagated preferably in the Al2O3 ma￾terial, again parallel with the interface (even a crack initiated in the ZrO2 layer got deflected into the Al2O3 layer). As mentioned above, the direction of compression (in Al2O3) and tensile (in ZrO2) residual thermal stresses in the lay￾ers is parallel with the layer interface. In the tensile stress field, a crack initiated in ZrO2 parallel with the layer in￾terface got in the ZrO2 layer deflected perpendicular to the layer interface, where its propagation was made easier due to the crack opening out in the tensile field. After crossing the interface it propagated in the Al2O3 layer parallel with the tensile stresses in this layer. Another possible explana￾tion for the crack deflection from ZrO2 into Al2O3 is the deformation of the stress field close to the Al2O3/ZrO2 in￾terface. As can be seen from Figs. 5 and 6, near the interface the crack is not initiated from the indentation corner, it is shifted towards the Al2O3 layer (in the case of symmetri￾cal indentations with respect to interface position (Fig. 4) it is initiated from the indentation corner). As Young’s mod￾ulus of Al2O3 is almost twice that of ZrO2 (see Table 1), the stress induced by the indentation tool was in Al2O3 al￾most twice that in ZrO2. It is thus possible that maximum stress (and, consequently, the point of crack initiation) was not in the indentation corner. In the case of indentation in ZrO2, the crack could have initiated even in the Al2O3 layer at a point close to the indentation corner. The crack then propagated on the one hand towards the indentation (in ZrO2) and, on the other hand, parallel with the interface (in Al2O3) in order not to be closed by the acting compressive stress. It is not the aim of the present work to give a de￾tailed description of mechanical properties of layered com￾posites but important knowledge has been obtained through the changes in the trajectory of a crack propagating at the layer interface. It is a proof of the strength of the bond of

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 was

H. 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

H. Hadraba et al /Ceramics International 30 (2004)853-863 3 um Fig. 7. Microstructure of alumina/zirconia particle composite of type PC 50/50 I FGM HP/3Y -2 500pm FGM HP/3Y-5 Fig 8 Microphotographs of alumina/zirconia functionally gradient materials of type FGM HP/3Y-2 and FGM HP/3Y- 2=5bozo 8○us9> 1.0 2.5 DISTANCE [mm] Fig 9. Dependence of alumina volume concentration in alumina/zirconia fuctionally gradient material FGM HP/5Y-5 on distance from anode

860 H. Hadraba et al. / Ceramics International 30 (2004) 853–863 Fig. 7. Microstructure of alumina/zirconia particle composite of type PC 50/50. Fig. 8. Microphotographs of alumina/zirconia functionally gradient materials of type FGM HP/3Y-2 and FGM HP/3Y-5. Fig. 9. Dependence of alumina volume concentration in alumina/zirconia fuctionally gradient material FGM HP/3Y-5 on distance from anode

H. Hadraba et al /Ceramics International 30(2004)853-863 。 25bz=0z9z6u25°> 100 120140 TIME [min ig. 10. Dependence of alumina volume concentration in alumina/zirconia suspension in alumina/zirconia functinally gradient material FGM HP/SY-5 on deposition time was calculated using the relation ticles on the electrode. To express the change in suspen- (h/2.78)-1 sion composition caused by particle deposition the relations 0.0338 (2) and values were used that had been found for the deposi- tion of one-component deposits [11]. The good agreement which is inverse to the relation for one-component deposit between the theoretical suspension composition and the de- Al203 [11]. Applying this relation also in the case of the posit composition established(see Fig. 10) implies the va- functionally gradient composite Al2O3/ZrO2 is justified lidity of all the assumptions made: electrophoretic mobility view of the fact that identical electrophoretic mobility of of both components in the suspension was identical [10], the Al2O3 and ZrO2 particles in isopropanol suspensions stabi- relations derived in the preceding work [11] for deposition lized with MCAA was established [10] and the validity of kinetics are valid and they also hold for the two-component this equality has also been confirmed in the case of a mix system ture of Al2O3 and ZrO2 particles( Section 3.2) As reported in the preceding work [ll the length of The theoretical time dependence of the suspension com- indentation cracks propagating from the corners of inden- position was calculated on the basis of an iterative pro- tation by Vickers indentor to the material surface provides cedure, when for every interruption of deposition the new for conclusions as to the size of fracture toughness of the volume concentrations Al2O3(C1) and ZrO2(ch+)) in the material. Using this method, Nicholson[17] demonstrated solution were calculated from the initial volume concentra- that the fracture toughness of functionally gradient material tions Al2O3(cA)and ZrO2(c)according to the relations is directly proportional to its composition. Fig. 11 gives CVI-ciVo 0.0338△r the fracture toughness of functionally gradient material (3) FGM HP/Y-5 as a function of its composition. As can be seen from Fig. 11, the fracture toughness in transversal c2,=W0-2=2v(1-e-03842)+c4 direction is proportional to the composition of functionally (4) gradient composite and it is higher than the fracture tough ness of pure phases. The fracture toughness in longitudina ere Vo is the total suspension volume, Vi is the volume of direction deflects from the linear dependence. As stated noved and added ZrOz suspension when the deposition is above, due to the different green densities throughout the interrupted (5 ml)and At is the time between individual de- deposit, the deposit was deformed during drying and sin- position interruptions (5 min). The initial Al2 O3 suspensio tering. This deformation was caused by internal stresses concentration was =3.61 voL %, 6=0 and the con- acting in parallel to the electrode, i.e. in the transversal centration of added ZrO2 suspension was cs= 2.40 vol %. direction. The part of deposit containing predominantly Relations(3)and(4)express both the change in suspen- ZrO2 was exposed to tensile stresses in transversal direc sion composition, which is given by the removal and ad- tion. These stresses opened up the crack in longitudinal dition of a small amount of suspension, and the change in direction and the fracture toughness established was in this this composition, which is given by the deposition of par- direction lower than in transversal direction. Conversely, in

H. Hadraba et al. / Ceramics International 30 (2004) 853–863 861 Fig. 10. Dependence of alumina volume concentration in alumina/zirconia suspension in alumina/zirconia functinally gradient material FGM HP/3Y-5 on deposition time. deposited was calculated using the relation: t = ln (h/2.78) − 1 0.0338 (2) which is inverse to the relation for one-component deposit Al2O3 [11]. Applying this relation also in the case of the functionally gradient composite Al2O3/ZrO2 is justified in view of the fact that identical electrophoretic mobility of Al2O3 and ZrO2 particles in isopropanol suspensions stabi￾lized with MCAA was established [10] and the validity of this equality has also been confirmed in the case of a mix￾ture of Al2O3 and ZrO2 particles (Section 3.2). The theoretical time dependence of the suspension com￾position was calculated on the basis of an iterative pro￾cedure, when for every interruption of deposition the new volume concentrations Al2O3(cA i+1) and ZrO2(cZ i+1) in the solution were calculated from the initial volume concentra￾tions Al2O3(cA i ) and ZrO2(cZ i ) according to the relations: cA i+1 = cA i V0 − cA i V1 − cA i V0(1 − e−0.0338 t) V0 (3) cZ i+1 = cZ i V0 − cZ i V1 − cZ i V0(1 − e−0.0338 t) + cZ s V1 V0 (4) where V0 is the total suspension volume, V1 is the volume of removed and added ZrO2 suspension when the deposition is interrupted (5 ml) and t is the time between individual de￾position interruptions (5 min). The initial Al2O3 suspension concentration was cA 0 = 3.61 vol.%, cZ 0 = 0 and the con￾centration of added ZrO2 suspension was cZ s = 2.40 vol.%. Relations (3) and (4) express both the change in suspen￾sion composition, which is given by the removal and ad￾dition of a small amount of suspension, and the change in this composition, which is given by the deposition of par￾ticles on the electrode. To express the change in suspen￾sion composition caused by particle deposition the relations and values were used that had been found for the deposi￾tion of one-component deposits [11]. The good agreement between the theoretical suspension composition and the de￾posit composition established (see Fig. 10) implies the va￾lidity of all the assumptions made: electrophoretic mobility of both components in the suspension was identical [10], the relations derived in the preceding work [11] for deposition kinetics are valid and they also hold for the two-component system. As reported in the preceding work [11] the length of indentation cracks propagating from the corners of inden￾tation by Vickers indentor to the material surface provides for conclusions as to the size of fracture toughness of the material. Using this method, Nicholson [17] demonstrated that the fracture toughness of functionally gradient material is directly proportional to its composition. Fig. 11 gives the fracture toughness of functionally gradient material FGM HP/3Y-5 as a function of its composition. As can be seen from Fig. 11, the fracture toughness in transversal direction is proportional to the composition of functionally gradient composite and it is higher than the fracture tough￾ness of pure phases. The fracture toughness in longitudinal direction deflects from the linear dependence. As stated above, due to the different green densities throughout the deposit, the deposit was deformed during drying and sin￾tering. This deformation was caused by internal stresses acting in parallel to the electrode, i.e. in the transversal direction. The part of deposit containing predominantly ZrO2 was exposed to tensile stresses in transversal direc￾tion. These stresses opened up the crack in longitudinal direction and the fracture toughness established was in this direction lower than in transversal direction. Conversely, in

H. Hadraba et al /Ceramics International 30 (2004)853-863 le-component deposits [11] e-FGM-transversal direction E00u29°-u VOLUME CONCENTRATION OF ZIRCONIA [ % Fig. 11. Dependence of fracture toughness of functionally gradient material FGM HP/Y-5 on zirconia volume concentration in depo the layers with predominant Al2O3 of higher green density Acknowledgements here appeared compression stresses that were closing the crack and this was reflected in the higher value of fracture This work was supported by the Czech Ministry of Edu toughne cation by the grant no. VZ CEZ: J22/98 4. Conclusion References Layered composite materials were prepared by alter- [M.P. Harmer, H.M. Chan, G.A. Miller, Unique opportunities for mi- lating deposition from isopropanol suspensions of Al2O3 structural engineering with duplex and laminar ceramic compos- and ZrO2 and subsequent sintering. The particles at the s,J.Am. Ceram.Soce.75(1992)1715-1728 layer interface were properly sintered and the layer inter- [2JE.M. Deliso, J. Kowalski, J.W.R. Cannon, Application of electroki- face did not contain an increased amount of pores. Thanks netic properties to the fabrication of an alumina-zirconia composite, Adv Ceram. Mater. 3(1988)407-410 to this strong bond between layers and thanks to residual [3]Z Wang, P. Xiao, J. Shemilt, Fabrication of composite coatings using stresses introduced into the material in the course of dry a combination of electrochemical methods and reaction bonding ing, sintering and cooling from the sintering temperature process, J. Eur. Ceram Soc. 20(2000)1469-1473 here occurred changes in the propagation of cracks in the []T. Uchikoshi, K. Ozawa, B D. Hatton, Y. Sakka, Dense, bubble-free composite, which makes the material interesting from the amic deposits from aqueous suspensions by electrophoretic depo- sition, J. Mater. Res. 16(2001)321-324. viewpoint of possible increase in fracture toughness and []B. Ferrari, A.J. Sanchez-Herencia, R. Moreno, Aqueous elec- strength trophoretic deposition of Al] O3/ZrO2 layered ceramics, Mater. Lett. Using electrophoretic deposition of a mixture of Al2O3 35(1998)370-374 and ZrO2 from isopropanol suspensions stabilized with [6]P Sarkar, X. Haung, P.S. Nicholson, Structural ceramic microlami- monochloroacetic acid it was possible to prepare particle ates by electrophoretic deposition, J. Am. Ceram. Soc. 75(1999) composite materials with a constant controlled Al2O3/Zro2 [7O. Prakash, P. Sarkar, P.S. Nicholson, Crack deflection in ce ratio. Thanks to the similar electrophoretic mobility of amic/ceramic laminates with strong interfaces, J. Am. Ceram. Soc. Al]O3 and Zro2 particles in the suspension the composite 78(1995)1125-1127 composition was constant throughout the whole deposit [8]B. Hatton, P.S. Nicholson, Design and fracture of layered volume Am. Ceram.Soc.84(2001)571-576 Functionally gradient materials with a smooth concentra [9]C. Hillman, Z Suo, F.E. Lange, Cracking of laminate tion transition from Al2O3 to ZrO2 were prepared by elec axial tensile stresses, J. Am. Ceram. Soc. 79(1996) 2133 trophoretic deposition of suspensions whose composition ihlarova, H. Hadraba, Influence of was changed in the course of deposition. The deposit com- electrophoretic behavior of alcoholic suspensions of alumina and position copied the suspension composition and the deposi zirconia, submitted for publicatio [11]K Ma Hadraba, J. Cihlar, Electrophoretic deposition of alumina tion kinetics corresponded to the theoretical models derived d zirconia-l. Sing ponent systems, Ceram. Int. 30(2004) for one-component deposits

862 H. Hadraba et al. / Ceramics International 30 (2004) 853–863 Fig. 11. Dependence of fracture toughness of functionally gradient material FGM HP/3Y-5 on zirconia volume concentration in deposit. the layers with predominant Al2O3 of higher green density there appeared compression stresses that were closing the crack and this was reflected in the higher value of fracture toughness. 4. Conclusion Layered composite materials were prepared by alter￾nating deposition from isopropanol suspensions of Al2O3 and ZrO2 and subsequent sintering. The particles at the layer interface were properly sintered and the layer inter￾face did not contain an increased amount of pores. Thanks to this strong bond between layers and thanks to residual stresses introduced into the material in the course of dry￾ing, sintering and cooling from the sintering temperature, there occurred changes in the propagation of cracks in the composite, which makes the material interesting from the viewpoint of possible increase in fracture toughness and strength. Using electrophoretic deposition of a mixture of Al2O3 and ZrO2 from isopropanol suspensions stabilized with monochloroacetic acid it was possible to prepare particle composite materials with a constant controlled Al2O3/ZrO2 ratio. Thanks to the similar electrophoretic mobility of Al2O3 and ZrO2 particles in the suspension the composite composition was constant throughout the whole deposit volume. Functionally gradient materials with a smooth concentra￾tion transition from Al2O3 to ZrO2 were prepared by elec￾trophoretic deposition of suspensions whose composition was changed in the course of deposition. The deposit com￾position copied the suspension composition and the deposi￾tion kinetics corresponded to the theoretical models derived for one-component deposits. Acknowledgements This work was supported by the Czech Ministry of Edu￾cation by the grant no. VZ CEZ: J22/98. References [1] M.P. Harmer, H.M. Chan, G.A. Miller, Unique opportunities for mi￾crostructural engineering with duplex and laminar ceramic compos￾ites, J. Am. Ceram. Soc. 75 (1992) 1715–1728. [2] E.M. Deliso, J. Kowalski, J.W.R. Cannon, Application of electroki￾netic properties to the fabrication of an alumina–zirconia composite, Adv. Ceram. Mater. 3 (1988) 407–410. [3] Z. Wang, P. Xiao, J. Shemilt, Fabrication of composite coatings using a combination of electrochemical methods and reaction bonding process, J. Eur. Ceram. Soc. 20 (2000) 1469–1473. [4] T. Uchikoshi, K. Ozawa, B.D. Hatton, Y. Sakka, Dense, bubble-free ceramic deposits from aqueous suspensions by electrophoretic depo￾sition, J. Mater. Res. 16 (2001) 321–324. [5] B. Ferrari, A.J. Sánchez-Herencia, R. Moreno, Aqueous elec￾trophoretic deposition of Al2O3/ZrO2 layered ceramics, Mater. Lett. 35 (1998) 370–374. [6] P. Sarkar, X. Haung, P.S. Nicholson, Structural ceramic microlami￾nates by electrophoretic deposition, J. Am. Ceram. Soc. 75 (1999) 2907–2909. [7] O. Prakash, P. Sarkar, P.S. Nicholson, Crack deflection in ce￾ramic/ceramic laminates with strong interfaces, J. Am. Ceram. Soc. 78 (1995) 1125–1127. [8] B. Hatton, P.S. Nicholson, Design and fracture of layered Al2O3/TZ3Y composites produced by electrophoretic deposition, J. Am. Ceram. Soc. 84 (2001) 571–576. [9] C. Hillman, Z. Suo, F.F. Lange, Cracking of laminates subjected to biaxial tensile stresses, J. Am. Ceram. Soc. 79 (1996) 2127–2133. [10] J. Cihlar, Z. Cihlarova, H. Hadraba, Influence of weak acids on electrophoretic behavior of alcoholic suspensions of alumina and zirconia, submitted for publication. [11] K. Maca, H. Hadraba, J. Cihlar, Electrophoretic deposition of alumina and zirconia—I. Single-component systems, Ceram. Int. 30 (2004) 843–851