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 chemical 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 theoretical 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 stabilized 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 composition. 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 functionally 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 materials it is important that all the simultaneously deposited components be of the same charge polarity and the same electrophoretic mobility because in that case the deposit composition 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 particles 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 content of defects. The aim of the experiments was to describe the application of this suspension in the preparation of layered, 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 electrophoretic 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 established as optimal for the deposition of one-component Al2O3 and ZrO2 systems [11]. Two-component suspensions 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 depositions were conducted under constant current (5 mA) conditions. 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 deposition 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 deposition 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 composites were denoted PC 75/25, PC 50/50 and PC 25/75) were prepared by interrupted deposition, i.e. several consecutive 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 interrupted electrophoretic deposition, i.e. a sequence of depositions from suspensions of slightly changed composition. Electrophoretic deposition started with pure Al2O3 suspension (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 suspension (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 temperature 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 dilatometer (L75/50, Linseis, Germany). Coefficient of thermal expansion (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 relative 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 materials 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 50m, 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 individual phases. However, these inhomogeneities were responsible 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 composite 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 abbreviations 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 considerations in the introduction to this chapter, namely that the appearance of cracks in ZrO2 layers more than 50m 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 layers 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 values, 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 dangerous. 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 individual pure components it could be seen that there were at least two sources of the stress introduced into the composite during 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