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Mechanical properties of PZT ceramics with tailored microstructure
Materials Chemistry and Physics 61 (1999) 24±30
Mechanical properties of PZT ceramics with tailored microstructure
$
R.A. Pferner, G. Thurn*, F. Aldinger
Max-Planck-Institut fuÈr Metallforschung and UniversitaÈt Stuttgart, Institut fuÈr Nichtmetallische Anorganische Materialien, Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, 70569 Stuttgart, Germany
Abstract PZT ceramics with tailored microstructure were fabricated using 3 and 5 valence ions as dopants. It was shown that the type of additive does not in¯uence the microstructural development. Vacancies in the Pb-lattice are being formed for all dopants to assure charge equilibrium. Thus, Pb-vacancies are identi®ed to dominate the microstructural development of PZT ceramics. Samples with 2 mol% additive which were sintered to near theoretical density exhibit a homogeneous microstructure with an average grain size of 2 mm. The morphotropic phase boundary (MPB) of doped PZT is shifted towards higher PbZrO3 content as compared to undoped PZT. This effect is explained in terms of crystallographic considerations. The investigations on the mechanical properties are focused on the area of the MPB that is relevant for industrial applications due to the extraordinary high electrical properties. For the ®rst time an entire evaluation of the mechanical properties have been conducted considering the fracture toughness of poled and unpoled samples, R-curve behavior, temperature dependent bending strength and ferroelastic behavior. From these measurements the in¯uence of domain switching processes on the mechanical properties are deduced and a correlation between mechanical, ferroelastic and ferroelectric behavior and the microstructure is supposed. # 1999 Elsevier Science S.A. All rights reserved. Keywords: PZT ceramics; Microstructure; Fracture toughness; Bending strength; Modulus of elasticity; Domain switching processes
1. Introduction PZT ceramics have been used as acoustic transducers, pressure sensors and large displacement actuators. High relative permittivity values and large piezoelectric effects are necessary for such applications. Extraordinary high values are found at the morphotropic phase boundary (MPB). The MPB describes the area where the two ferroelectric modi®cations coexist [1]. However, the piezoelectric properties of PZT are not only determined by the relative amount of these two phases, but also by the type and concentration of dopants and the microstructure [2]. Apart from the changes of the electrical properties, the additives also in¯uence the microstructure formation and the mechanical properties. For diverse industrial use of PZT ceramics, it is essential to develop and characterize a tailored microstructure. Grain size, density and the development of the domain structure are extremely sensitive on even small variations of the composition. The interest on the mechanical behavior of $ Dedicated to Professor S. Somiya on the occasion of his 70th birthday * Corresponding author. Tel. +49-711-6861-234; fax: +49-711-6861-131 E-mail address: [email protected] (G. Thurn)
these ceramics raised in recent time as compared to the electrical properties. Whereas until now ultrasonic transducers were exposed to the highest mechanical loading, new applications of piezoelectric powered pressure heads [3] or fuel injection systems of the automotive technology [4,5] surpass these values by far. As a result of the higher mechanical loading, material inhomogeneities are generally responsible for failures like crack growth and fracture [6]. Little knowledge of the mechanical properties of PZT prevents estimations on the materials reliability which hinders the technical use of these ceramics. To increase the reliability of PZT devices, calculation of the mechanical stress inside the device during operation is important. The large signal stress±strain relationship under compressive loading was investigated by SchaÈufele and HaÈrdtl [7] and considerable ferroelastic deformation was found. Ferroelastic deformation is time-dependent and related to domain switching. This was shown with X-ray diffraction by Metha and Virkar [8]. Ferroelastic deformation is a toughening mechanism during slow crack propagation in ferroelectric ceramics. It is thought to cause a process zone in front of the crack tip. Such a toughening mechanism can act only below Curie-temperature. Therefore, the fracture toughness of ferroelectric ceramics is temperaturedependent [9]. Anisotropic fracture toughness was observed
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 0 8 - X
R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
in poled ferroelectric ceramics [10] and it was suggested by Zhang and Raj [11] that the fracture toughness depends on the volume fraction of domains which are aligned favorably in front of the crack tip. The development of a process zone should lead to R-curve behavior. Such behavior was measured by Meschke et al. for ferroelectric BaTiO3 [12]. Though many details of the mechanical behavior of ferroelectric ceramics were previously investigated, the interrelation between fracture toughness, R-curve behavior, bending strength and ferroelastic behavior is still not understood. It was the objective of this study to present experimental data on the mechanical properties of different PZT ceramics considering all the previous effects for the ®rst time. The in¯uence of domain switching processes on the mechanical properties is discussed. The investigations on the mechanical properties are focused on PZT ceramics with tailored microstructure. The compositions are close to the MPB that is relevant for applications which are of industrial interest. The materials have been fabricated using 3 and 5 valence ions as dopants. 2. Experimental Soft PZT ceramics were prepared by the mixed-oxide process using 3 valence (Sm2O3, Bi2O3) and 5 valence (Ta2O5, Nb2O5) ions as dopants. Commercial powders in the form of oxides with a high purity and small particle size were used as starting materials and compositions close to the MPB with Zr/Ti ratios between 45/55 and 58/42 were produced. The starting oxides were attrition milled in isopropanol for 3 h using Y2O3 stabilized ZrO2 grinding balls. The slurry was separated from the grinding media by means of a sieve chain and the solvent distilled off by a rotary evaporator. After the powder was completely dried, it was sieved (160 mm) and calcinated at 7008C for 2 h. The pre-reacted powders were then cold isostatic pressed in cylindrical rubber moulds at 650 MPa and sintered in Al2O3 crucibles for 2 h at 12008C in an oxygen atmosphere. A powder bed of PbZrO3 + 8 mol% ZrO2 was used in order to keep a constant PbO vapor pressure. The density of sintered specimens was determined by the Archimedes method. For the analysis of the phase content, cylindrical samples were cut in pieces. The samples for the X-ray investigations were ground in a mortar and annealed for 4 h at 5008C before being analyzed (Siemens D5000, CuKa-radiation) to prevent texture effects due to the ferroelastic properties of PZT. Quantitative phase ratios were calculated from the [002] and [200] peaks of the tetragonal phase and the [200] peaks of the rhombohedral phase through the Lorentz ®t function to determine the MPB. For optical microscopy and SEM analysis, samples were embedded in a conductive arti®cial resin and polished with semi-automatic grinding machines to a 0.5 mm smoothness. The pictures taken by optical microscopy were used to
25
determine the porosity by quantitative image analysis. The samples for SEM were etched to make the grain structure visible, and then sputtered with gold to prevent an electrical charge build-up. The grain size was determined from the resulting pictures, again by quantitative image analysis. For the measurement of Young's modulus and the fracture resistance, cylindrical samples were ground, polished to 1 mm smoothness and then annealed at 4508C for 8 h. Young's modulus was determined by the impulse-echomethod. Stress±strain relationship was investigated in four-point bending experiments. The fracture toughness of poled and unpoled sample was investigated by the indentation method. The shape of the cracks under the indent was determined with a microscope and the KIc-values were calculated from the crack lengths. The dimensions of the specimen were 4 mm 4 mm 10 mm. The top and bottom sides were sputtered with Ê of platinum and poled with 2.5 kV/mm in silicon 1000 A oil. One of the unsputtered faces was ceramographically prepared for the indentations. The R-curve behavior was investigated using compacttension specimen [13]. Crack propagation during the mechanical loading was observed in-situ using a long distance microscope. The bending strength was determined by the four-point bending method at different temperatures using a ®xture with inner and outer spans of 20 and 40 mm, respectively. The dimension of the specimen was 3 mm 4 mm 45 mm. A statistical method was used to quantify the bending strength distribution as the strength of ceramics is scattered by faults and inhomogeneities in the material. Weibull moduli were calculated from this by the maximum likelihood method. 3. Results and discussion The investigation revealed that the microstructural development is not affected by the different types of additives used. Ions of different valence substitute for different cation sites in the perovskite crystal structure. However, vacancies in the Pb-lattice are being formed in all cases to assure charge equilibrium. Therefore, it is claimed that Pb-vacancies dominate the microstructural development of PZT ceramics. Supplementary, Pb±O-polarization chains of the perovskite lattice are interrupted in comparison to undoped PZT. This results in a decrease of both the amount of polarization and the Curie-temperature. Variation of the Zr/Ti-ratio revealed no in¯uence neither on the density nor on the grain size. The highest porosity and the largest grain size was detected for materials with 0,1 mol% additive. In contrast, the lowest porosity and smallest grain size was evaluated for materials with 2 mol% additive. The latter samples were sintered to near theoretical density and exhibit an average grain size of
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R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
Fig. 1. SEM image of 2 mol% Ta2O5 doped PZT.
2 mm. Fig. 1 depicts a characteristic SEM image of a 2 mol% Ta2O5 doped ceramic. The MPB of a PZT with 2 mol% additive is shifted towards higher PbZrO3 content. This indicates a stabilization of the tetragonal phase independent of the kind of additives investigated. The quantitative phase distribution of the tetragonal and rhombohedral phase in Fig. 2 is typical for all utilized additives compared to undoped PZT. The MPB is not situated in the middle of the phase region of coexistence, but slightly shifted towards the rhombohedral phase. The asymmetric behavior can be explained through the different coordination of Zr4+, Ti4+ and Pb2+ in the tetragonal and rhombohedral phase. In the tetragonal phase, each O2ÿ-ion is coordinated tetrahedrally by two Pb2+ and Ti4+-ions, respectively [14]. Due to the ion radius of Zr4+,
these ions cannot be incorporated into the tetrahedron. Therefore, the energetic stability of the tetragonal lattice decreases by increasing the amount of PbZrO3. If the amount of PbZrO3 increases to a critical concentration, which corresponds to the MPB, any further increase results in an instabilization of the tetragonal phase. As a consequence only the rhombohedral phase will develop subsequently. Undoped and Ta2O5 doped PZT ceramics were utilized to investigate the mechanical properties. The temperaturedependent bending strength and Weibull moduli were evaluated. The highest values of bending strength could be detected for the paraelectric cubic phase above the Curietemperature (Fig. 3). The decrease of the bending strength at the Curie-temperature can be correlated with internal stresses which develop according to the phase transformation. Decreasing the temperature further, the bending strength increases again. In contrast, the Weibull moduli were temperature independent. The pure phase reveal a higher bending strength and Weibull modulus compared to the composition at the MPB. As a consequence the latter samples were fractographically investigated using SEM. Each determined bending strength could be correlated with an inhomogeneity which caused fracture. The detected inhomogenities are shown to be mainly pores. Hexagonal crystals of PbO were observed inside these pores. It is thought that the hexagonal crystals in pores near the surface can be traced back to PbO evaporation from the powder bed used for densi®cation. Fig. 4 shows a pore with hexagonal crystals of PbO which caused fracture. In agreement with the results of the bending strength and Weibull moduli, minimum values of the Vickers hardness
Fig. 2. Amount of tetragonal and rhombohedral phases in dependence of the PbZrO3 concentration of undoped and 2 mol% Sm2O3 doped PZT.
R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
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Fig. 3. Bending strength of 2 mol% Ta2O5 doped PZT.
and Young's modulus could be detected for both doped and undoped PZT at the MPB. SEM investigations revealed for Vickers indents the development of elliptical cracks, i.e. Palmquist cracks. Using the formalism for Palmquist cracks [15], the fracture toughness was determined from the crack length of the Vickers indents. The lowest values were found at the MPB. Qualitatively, the decrease of the fracture toughness is in accordance with the decrease of the room temperature bending strength. Poling of the material results in the development of an anisotropic fracture toughness. Fig. 5 reveals that parallel to the poling direction higher fracture toughness values can be detected as compared to
Fig. 4. Origin of fracture showing a pore with hexagonal crystals. EDX analysis revealed the crystals to consist of lead oxide. Mean grain size of the matrix was determined to be 2 mm.
unpoled specimen. Energy dissipating domain switching processes of 908 domains in front of the crack tip were identi®ed to be responsible for the increase of the fracture toughness. In contrast, lower fracture toughness values as compared to unpoled specimen were found normal to the poling direction. The R-curve behavior was determined using the compacttension assessment with a de®ned loading speed of 4 10ÿ5 mm/sec. For all samples a raising fracture toughness with increasing crack length could be detected until a crack length of 1 mm. SEM investigation revealed that micro crack formation in front of the crack tip does not contribute to the measured R-curve behavior. Although intercrystalline crack propagation can be observed (Fig. 6), the R-curve behavior can not be explained by crack border interaction in the crack wake as the mean grain size of 2 mm is too small. Therefore, it is claimed that domain switching processes are responsible for a ferroelastic behavior of the PZT ceramics which generates a process zone in front of the crack tip. This behavior was investigated in four-point bending experiments. The stress±strain curves in Fig. 7 show that samples with the composition of the MPB exhibit the highest ferroelastic strain. After unloading the highest remnant strain of all materials investigated was observed for these compositions. Based on this result, it is concluded that samples at the MPB consist of the highest amount of domains, which can mechanically induced be reoriented. Xray investigation under applied mechanical stress revealed that the ferroelastic strain is caused by switching processes of 908 domains [16]. The amount of the ferroelastic strain decreases with increasing loading rate. Therefore, the stiff-
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R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
Fig. 5. Influence of the PbZrO3 content on the fracture toughness (indentation method).
ness of the materials increases with increasing loading rate. This result indicates that domain switching processes are rate controlled. Fig. 8 illustrates the loading rate dependence of the specimens' stiffness. The higher the loading rate, the higher the measured modules of elasticity. It is possible to explain this behavior as the response of domain of switching is in the order of seconds. For the high loading rates domain are not able to switch, resulting in a high modules of elasticity. The temperature dependent bending strength was determined with a loading rate of 0.1 and 5 mm/sec for elevated and room temperature, respectively. Therefore, domain switching processes can be excluded for these investigations. In contrast, very low crack propagation rates from
4 10ÿ3 to 8 10ÿ3 mm/sec were measured during the experiments with compact-tension samples to determine the R-curve behavior of the PZT ceramics. Hence, it is obvious that domain switching processes occur during the mechanical loading. Due to the stress concentration at the crack tip a process zone develops due to domain switching processes. A process zone is thought to increases the fracture toughness. Considering the highest ferroelastic strain of samples at the MPB, fracture toughness should be the highest there. However, this contradicts the experimental results. Recalling that two competing energetic mechanisms contribute to crack propagation the apparent contradiction can be explained. Crack propagation generates new surfaces for which surface energy has to be delivered. This energy acts as a crack resistance energy and consumes the released distortion energy of the crystal lattice. It is believed that the coexistence of both ferroelectric phases at the MPB produces high internal stresses in the material. Relaxation of these internal stresses by crack propagation delivers an additional energy contribution which favors crack propagation. This energy contribution is thought to be responsible for the low fracture toughness of samples at the MPB. 4. Conclusions
Fig. 6. Crack path of a Ta2O5 doped PZT ceramic.
PZT ceramics with optimized microstructure were produced. No signi®cant difference in the microstructure development (grain size, porosity, displacement of the MPB) could be found between the 3 and 5 valence additives when varying the additive content and the Zr/Ti ratio. For the ®rst time a systematic investigation on the mechanical properties
R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
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Fig. 7. Initial stress±strain curves of three different compositions (tetragonal, rhombohedral, MPB).
Fig. 8. The Young's modules determined by stress±strain experiments with different loading rates and by the impulse-echo-method.
was carried for these materials. The objective was to understand the in¯uence of domain switching processes on the mechanical properties. The domain switching processes were shown to be dependent on the loading rate. for low crack velocities a process zone develops in front of the crack tip and domains are thought to be the main constituents of the process zone. It was found that the mechanical properties, i.e. the bending strength and the fracture toughness of the pure phases reach higher values than compositions at the
MPB. The minimal values at the MPB were explained by energy considerations. References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971. [2] H. Thomann, Piezoelektrische Mechanismen in Bleizirkonat±Titanat, Zeitschrift fuÈr angewandte Physik 20 (1966) 554±559.
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[3] K. Uchino, Ferroelectric ceramics, in: R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Materials Science and Technology: A Comprehensive Treatment, vol. 11, VCH Publishers, Weinheim, 1994, 637±677. [4] J. Koch, Valvo Unternehmensbereich Bauelemente der Philips GmbH, Piezoxide (PXE) Eigenschaften und Anwendungen, Dr. Alfred HuÈthig Verlag GmbH, Heidelberg, 1988. [5] A. Wolff, D. Cramer, H. Hellebrand, I. Probst, K. Lubitz, Optical two channel elongation measurement of PZT piezoelectric multilayer stack actuators, in: Proc. IEEE-ISAF 94, 1994. [6] S.W. Freiman, Fracture behaviour of electronic ceramics, Ferroelectrics 102 (1990) 381. [7] A.B. SchaÈufele, K.H. HaÈrdtl, Ferroelastic properties of lead zirconate titanate ceramics, J. Am. Ceram. Soc. 79 (1996) 2637±2640. [8] K. Mehta, A.V. Virkar, Fracture mechanism in ferroelectric± ferroelastic lead zirconate titanate (Zr : Ti = 0.54 : 0.46) ceramics, J. Am. Ceram. Soc. 73 (1990) 567±574. [9] R.F. Cook, S.W. Freiman, B.R. Lawn, Fracture of ferroelectric ceramics, Ferroelectrics 50 (1983) 267±272.
[10] T. Yamamoto, H. Igarashi, K. Okazaki, Internal stress anisotropies induced by electric field in lanthanum modified PbTiO3 ceramics, Ferroelectrics 50 (1983) 273±278. [11] Z. Zhang, R. Raj, Influence of grain size on ferroelastic toughening and piezoelectric behavior of lead zirconate titanate, J. Am. Ceram. Soc. 78 (1995) 3363±3368. [12] F. Meschke, A. Kolleck, G.A. Schneider, R-curve behavior of BaTiO3 due to stress-induced ferroelastic domain switching, J. Europ. Ceram. Soc. 17 (1997) 1143±1149. [13] D. Broek, Elementary Engineering Fracture Mechanics, Kluwer Academic Publishers, Dordrecht, 1986. [14] H. Thomann, A covalency model of ferroic phase transitions in perovskites, Ferroelectrics 73 (1987) 183±199. [15] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of KIc of brittle solids by the indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13±16. [16] R.A. Pferner, Mechanische Eigenschaften von PZT-Keramiken mit definiertem GefuÈge, Dissertation, UniversitaÈt Stuttgart, 1997.
Mechanical properties of PZT ceramics with tailored microstructure
$
R.A. Pferner, G. Thurn*, F. Aldinger
Max-Planck-Institut fuÈr Metallforschung and UniversitaÈt Stuttgart, Institut fuÈr Nichtmetallische Anorganische Materialien, Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, 70569 Stuttgart, Germany
Abstract PZT ceramics with tailored microstructure were fabricated using 3 and 5 valence ions as dopants. It was shown that the type of additive does not in¯uence the microstructural development. Vacancies in the Pb-lattice are being formed for all dopants to assure charge equilibrium. Thus, Pb-vacancies are identi®ed to dominate the microstructural development of PZT ceramics. Samples with 2 mol% additive which were sintered to near theoretical density exhibit a homogeneous microstructure with an average grain size of 2 mm. The morphotropic phase boundary (MPB) of doped PZT is shifted towards higher PbZrO3 content as compared to undoped PZT. This effect is explained in terms of crystallographic considerations. The investigations on the mechanical properties are focused on the area of the MPB that is relevant for industrial applications due to the extraordinary high electrical properties. For the ®rst time an entire evaluation of the mechanical properties have been conducted considering the fracture toughness of poled and unpoled samples, R-curve behavior, temperature dependent bending strength and ferroelastic behavior. From these measurements the in¯uence of domain switching processes on the mechanical properties are deduced and a correlation between mechanical, ferroelastic and ferroelectric behavior and the microstructure is supposed. # 1999 Elsevier Science S.A. All rights reserved. Keywords: PZT ceramics; Microstructure; Fracture toughness; Bending strength; Modulus of elasticity; Domain switching processes
1. Introduction PZT ceramics have been used as acoustic transducers, pressure sensors and large displacement actuators. High relative permittivity values and large piezoelectric effects are necessary for such applications. Extraordinary high values are found at the morphotropic phase boundary (MPB). The MPB describes the area where the two ferroelectric modi®cations coexist [1]. However, the piezoelectric properties of PZT are not only determined by the relative amount of these two phases, but also by the type and concentration of dopants and the microstructure [2]. Apart from the changes of the electrical properties, the additives also in¯uence the microstructure formation and the mechanical properties. For diverse industrial use of PZT ceramics, it is essential to develop and characterize a tailored microstructure. Grain size, density and the development of the domain structure are extremely sensitive on even small variations of the composition. The interest on the mechanical behavior of $ Dedicated to Professor S. Somiya on the occasion of his 70th birthday * Corresponding author. Tel. +49-711-6861-234; fax: +49-711-6861-131 E-mail address: [email protected] (G. Thurn)
these ceramics raised in recent time as compared to the electrical properties. Whereas until now ultrasonic transducers were exposed to the highest mechanical loading, new applications of piezoelectric powered pressure heads [3] or fuel injection systems of the automotive technology [4,5] surpass these values by far. As a result of the higher mechanical loading, material inhomogeneities are generally responsible for failures like crack growth and fracture [6]. Little knowledge of the mechanical properties of PZT prevents estimations on the materials reliability which hinders the technical use of these ceramics. To increase the reliability of PZT devices, calculation of the mechanical stress inside the device during operation is important. The large signal stress±strain relationship under compressive loading was investigated by SchaÈufele and HaÈrdtl [7] and considerable ferroelastic deformation was found. Ferroelastic deformation is time-dependent and related to domain switching. This was shown with X-ray diffraction by Metha and Virkar [8]. Ferroelastic deformation is a toughening mechanism during slow crack propagation in ferroelectric ceramics. It is thought to cause a process zone in front of the crack tip. Such a toughening mechanism can act only below Curie-temperature. Therefore, the fracture toughness of ferroelectric ceramics is temperaturedependent [9]. Anisotropic fracture toughness was observed
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 0 8 - X
R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
in poled ferroelectric ceramics [10] and it was suggested by Zhang and Raj [11] that the fracture toughness depends on the volume fraction of domains which are aligned favorably in front of the crack tip. The development of a process zone should lead to R-curve behavior. Such behavior was measured by Meschke et al. for ferroelectric BaTiO3 [12]. Though many details of the mechanical behavior of ferroelectric ceramics were previously investigated, the interrelation between fracture toughness, R-curve behavior, bending strength and ferroelastic behavior is still not understood. It was the objective of this study to present experimental data on the mechanical properties of different PZT ceramics considering all the previous effects for the ®rst time. The in¯uence of domain switching processes on the mechanical properties is discussed. The investigations on the mechanical properties are focused on PZT ceramics with tailored microstructure. The compositions are close to the MPB that is relevant for applications which are of industrial interest. The materials have been fabricated using 3 and 5 valence ions as dopants. 2. Experimental Soft PZT ceramics were prepared by the mixed-oxide process using 3 valence (Sm2O3, Bi2O3) and 5 valence (Ta2O5, Nb2O5) ions as dopants. Commercial powders in the form of oxides with a high purity and small particle size were used as starting materials and compositions close to the MPB with Zr/Ti ratios between 45/55 and 58/42 were produced. The starting oxides were attrition milled in isopropanol for 3 h using Y2O3 stabilized ZrO2 grinding balls. The slurry was separated from the grinding media by means of a sieve chain and the solvent distilled off by a rotary evaporator. After the powder was completely dried, it was sieved (160 mm) and calcinated at 7008C for 2 h. The pre-reacted powders were then cold isostatic pressed in cylindrical rubber moulds at 650 MPa and sintered in Al2O3 crucibles for 2 h at 12008C in an oxygen atmosphere. A powder bed of PbZrO3 + 8 mol% ZrO2 was used in order to keep a constant PbO vapor pressure. The density of sintered specimens was determined by the Archimedes method. For the analysis of the phase content, cylindrical samples were cut in pieces. The samples for the X-ray investigations were ground in a mortar and annealed for 4 h at 5008C before being analyzed (Siemens D5000, CuKa-radiation) to prevent texture effects due to the ferroelastic properties of PZT. Quantitative phase ratios were calculated from the [002] and [200] peaks of the tetragonal phase and the [200] peaks of the rhombohedral phase through the Lorentz ®t function to determine the MPB. For optical microscopy and SEM analysis, samples were embedded in a conductive arti®cial resin and polished with semi-automatic grinding machines to a 0.5 mm smoothness. The pictures taken by optical microscopy were used to
25
determine the porosity by quantitative image analysis. The samples for SEM were etched to make the grain structure visible, and then sputtered with gold to prevent an electrical charge build-up. The grain size was determined from the resulting pictures, again by quantitative image analysis. For the measurement of Young's modulus and the fracture resistance, cylindrical samples were ground, polished to 1 mm smoothness and then annealed at 4508C for 8 h. Young's modulus was determined by the impulse-echomethod. Stress±strain relationship was investigated in four-point bending experiments. The fracture toughness of poled and unpoled sample was investigated by the indentation method. The shape of the cracks under the indent was determined with a microscope and the KIc-values were calculated from the crack lengths. The dimensions of the specimen were 4 mm 4 mm 10 mm. The top and bottom sides were sputtered with Ê of platinum and poled with 2.5 kV/mm in silicon 1000 A oil. One of the unsputtered faces was ceramographically prepared for the indentations. The R-curve behavior was investigated using compacttension specimen [13]. Crack propagation during the mechanical loading was observed in-situ using a long distance microscope. The bending strength was determined by the four-point bending method at different temperatures using a ®xture with inner and outer spans of 20 and 40 mm, respectively. The dimension of the specimen was 3 mm 4 mm 45 mm. A statistical method was used to quantify the bending strength distribution as the strength of ceramics is scattered by faults and inhomogeneities in the material. Weibull moduli were calculated from this by the maximum likelihood method. 3. Results and discussion The investigation revealed that the microstructural development is not affected by the different types of additives used. Ions of different valence substitute for different cation sites in the perovskite crystal structure. However, vacancies in the Pb-lattice are being formed in all cases to assure charge equilibrium. Therefore, it is claimed that Pb-vacancies dominate the microstructural development of PZT ceramics. Supplementary, Pb±O-polarization chains of the perovskite lattice are interrupted in comparison to undoped PZT. This results in a decrease of both the amount of polarization and the Curie-temperature. Variation of the Zr/Ti-ratio revealed no in¯uence neither on the density nor on the grain size. The highest porosity and the largest grain size was detected for materials with 0,1 mol% additive. In contrast, the lowest porosity and smallest grain size was evaluated for materials with 2 mol% additive. The latter samples were sintered to near theoretical density and exhibit an average grain size of
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R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
Fig. 1. SEM image of 2 mol% Ta2O5 doped PZT.
2 mm. Fig. 1 depicts a characteristic SEM image of a 2 mol% Ta2O5 doped ceramic. The MPB of a PZT with 2 mol% additive is shifted towards higher PbZrO3 content. This indicates a stabilization of the tetragonal phase independent of the kind of additives investigated. The quantitative phase distribution of the tetragonal and rhombohedral phase in Fig. 2 is typical for all utilized additives compared to undoped PZT. The MPB is not situated in the middle of the phase region of coexistence, but slightly shifted towards the rhombohedral phase. The asymmetric behavior can be explained through the different coordination of Zr4+, Ti4+ and Pb2+ in the tetragonal and rhombohedral phase. In the tetragonal phase, each O2ÿ-ion is coordinated tetrahedrally by two Pb2+ and Ti4+-ions, respectively [14]. Due to the ion radius of Zr4+,
these ions cannot be incorporated into the tetrahedron. Therefore, the energetic stability of the tetragonal lattice decreases by increasing the amount of PbZrO3. If the amount of PbZrO3 increases to a critical concentration, which corresponds to the MPB, any further increase results in an instabilization of the tetragonal phase. As a consequence only the rhombohedral phase will develop subsequently. Undoped and Ta2O5 doped PZT ceramics were utilized to investigate the mechanical properties. The temperaturedependent bending strength and Weibull moduli were evaluated. The highest values of bending strength could be detected for the paraelectric cubic phase above the Curietemperature (Fig. 3). The decrease of the bending strength at the Curie-temperature can be correlated with internal stresses which develop according to the phase transformation. Decreasing the temperature further, the bending strength increases again. In contrast, the Weibull moduli were temperature independent. The pure phase reveal a higher bending strength and Weibull modulus compared to the composition at the MPB. As a consequence the latter samples were fractographically investigated using SEM. Each determined bending strength could be correlated with an inhomogeneity which caused fracture. The detected inhomogenities are shown to be mainly pores. Hexagonal crystals of PbO were observed inside these pores. It is thought that the hexagonal crystals in pores near the surface can be traced back to PbO evaporation from the powder bed used for densi®cation. Fig. 4 shows a pore with hexagonal crystals of PbO which caused fracture. In agreement with the results of the bending strength and Weibull moduli, minimum values of the Vickers hardness
Fig. 2. Amount of tetragonal and rhombohedral phases in dependence of the PbZrO3 concentration of undoped and 2 mol% Sm2O3 doped PZT.
R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
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Fig. 3. Bending strength of 2 mol% Ta2O5 doped PZT.
and Young's modulus could be detected for both doped and undoped PZT at the MPB. SEM investigations revealed for Vickers indents the development of elliptical cracks, i.e. Palmquist cracks. Using the formalism for Palmquist cracks [15], the fracture toughness was determined from the crack length of the Vickers indents. The lowest values were found at the MPB. Qualitatively, the decrease of the fracture toughness is in accordance with the decrease of the room temperature bending strength. Poling of the material results in the development of an anisotropic fracture toughness. Fig. 5 reveals that parallel to the poling direction higher fracture toughness values can be detected as compared to
Fig. 4. Origin of fracture showing a pore with hexagonal crystals. EDX analysis revealed the crystals to consist of lead oxide. Mean grain size of the matrix was determined to be 2 mm.
unpoled specimen. Energy dissipating domain switching processes of 908 domains in front of the crack tip were identi®ed to be responsible for the increase of the fracture toughness. In contrast, lower fracture toughness values as compared to unpoled specimen were found normal to the poling direction. The R-curve behavior was determined using the compacttension assessment with a de®ned loading speed of 4 10ÿ5 mm/sec. For all samples a raising fracture toughness with increasing crack length could be detected until a crack length of 1 mm. SEM investigation revealed that micro crack formation in front of the crack tip does not contribute to the measured R-curve behavior. Although intercrystalline crack propagation can be observed (Fig. 6), the R-curve behavior can not be explained by crack border interaction in the crack wake as the mean grain size of 2 mm is too small. Therefore, it is claimed that domain switching processes are responsible for a ferroelastic behavior of the PZT ceramics which generates a process zone in front of the crack tip. This behavior was investigated in four-point bending experiments. The stress±strain curves in Fig. 7 show that samples with the composition of the MPB exhibit the highest ferroelastic strain. After unloading the highest remnant strain of all materials investigated was observed for these compositions. Based on this result, it is concluded that samples at the MPB consist of the highest amount of domains, which can mechanically induced be reoriented. Xray investigation under applied mechanical stress revealed that the ferroelastic strain is caused by switching processes of 908 domains [16]. The amount of the ferroelastic strain decreases with increasing loading rate. Therefore, the stiff-
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Fig. 5. Influence of the PbZrO3 content on the fracture toughness (indentation method).
ness of the materials increases with increasing loading rate. This result indicates that domain switching processes are rate controlled. Fig. 8 illustrates the loading rate dependence of the specimens' stiffness. The higher the loading rate, the higher the measured modules of elasticity. It is possible to explain this behavior as the response of domain of switching is in the order of seconds. For the high loading rates domain are not able to switch, resulting in a high modules of elasticity. The temperature dependent bending strength was determined with a loading rate of 0.1 and 5 mm/sec for elevated and room temperature, respectively. Therefore, domain switching processes can be excluded for these investigations. In contrast, very low crack propagation rates from
4 10ÿ3 to 8 10ÿ3 mm/sec were measured during the experiments with compact-tension samples to determine the R-curve behavior of the PZT ceramics. Hence, it is obvious that domain switching processes occur during the mechanical loading. Due to the stress concentration at the crack tip a process zone develops due to domain switching processes. A process zone is thought to increases the fracture toughness. Considering the highest ferroelastic strain of samples at the MPB, fracture toughness should be the highest there. However, this contradicts the experimental results. Recalling that two competing energetic mechanisms contribute to crack propagation the apparent contradiction can be explained. Crack propagation generates new surfaces for which surface energy has to be delivered. This energy acts as a crack resistance energy and consumes the released distortion energy of the crystal lattice. It is believed that the coexistence of both ferroelectric phases at the MPB produces high internal stresses in the material. Relaxation of these internal stresses by crack propagation delivers an additional energy contribution which favors crack propagation. This energy contribution is thought to be responsible for the low fracture toughness of samples at the MPB. 4. Conclusions
Fig. 6. Crack path of a Ta2O5 doped PZT ceramic.
PZT ceramics with optimized microstructure were produced. No signi®cant difference in the microstructure development (grain size, porosity, displacement of the MPB) could be found between the 3 and 5 valence additives when varying the additive content and the Zr/Ti ratio. For the ®rst time a systematic investigation on the mechanical properties
R.A. Pferner et al. / Materials Chemistry and Physics 61 (1999) 24±30
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Fig. 7. Initial stress±strain curves of three different compositions (tetragonal, rhombohedral, MPB).
Fig. 8. The Young's modules determined by stress±strain experiments with different loading rates and by the impulse-echo-method.
was carried for these materials. The objective was to understand the in¯uence of domain switching processes on the mechanical properties. The domain switching processes were shown to be dependent on the loading rate. for low crack velocities a process zone develops in front of the crack tip and domains are thought to be the main constituents of the process zone. It was found that the mechanical properties, i.e. the bending strength and the fracture toughness of the pure phases reach higher values than compositions at the
MPB. The minimal values at the MPB were explained by energy considerations. References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971. [2] H. Thomann, Piezoelektrische Mechanismen in Bleizirkonat±Titanat, Zeitschrift fuÈr angewandte Physik 20 (1966) 554±559.
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