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Damage characteristics of lead zirconate titanate piezoelectric ceramic during cyclic loading Mitsuhiro Okayasu*, Nozomi Odagiri, Mamoru Mizuno Department of Machine Intelligence and Systems Engineering, Akita Prefectural University, 84-4 Ebinokuchi, Tuchiya-aza, Yurihonjo-city, Akita 015-0055, Japan a r t i c l ei n f o Article history: Received 1 January 2009 Received in revised form 31 March 2009 Accepted 1 April 2009 Available online 10 April 2009 Keywords: PZT ceramic Fatigue test Domain switching Material damage Electrical property a b s t r a c t The effects of the damage characteristics on the material properties of a piezoelectric ceramic are exam- ined during cyclic loading. The material being examined is a lead zirconate titanate piezoelectric ceramic (PZT). The electrical properties, such as the electromechanical coupling coeffi cient (k33), are changed dur- ing cyclic loading. The k33 coeffi cient decreases rapidly to a low level as the sample is loaded cyclically with a high applied stress, while the k33value decreases slowly or does not change when loaded with a low applied stress. Such a change of material degradation is infl uenced by the severity of the material damage in the PZT ceramic. The material damage in the PZT occurs, and this occurrence is related to a lightning-like phenomenon (a bright fl ash with a click). Details of the damage characteristics in PZT cera- mic are discussed in the present work. ? 2009 Elsevier Ltd. All rights reserved. 1. Introduction The signifi cance of piezoelectric ceramics is the transfer of an in- duced voltage difference that appears across two of the surfaces of the ceramic as the shape of the ceramic is subjected to high alter- nating stresses. Using this unique material characteristic, these ceramics have been utilized in various engineering applications including memory devices, precision positioning, electro-mechani- cal actuators, power transducers and vibration sensors. To employ the PZT ceramic for a long period of time, it is necessary to under- stand the material response to the application. The effi ciency of the piezoelectric property in the ceramic can be changed if an over- load is applied, due to material damage in the ceramic. There have beenseveralpossiblekindsof damageinPZTsreportedin published papers, e.g., microcrack, grain sliding and domain switching (polar- ization). The material damage occurs when the electric fatigue crack initiates from a porous region of the PZT 1. The damage in PZT ceramics can also be detected if the electrogeneration is trapped at a defect in the sample, which leads to the change of the electric domain orientation 2. Domain switching can occur with a high applied stress and the consequent elastic strain 3,4. Because of domain switching in PZT ceramics, the material proper- ties, e.g., piezoelectric constant, can be altered 5. Recently, Shindo et al. have examined the damage characteristics in PZT ceramic numerically and theoretically. It appeared from their work that the localized switching near the crack tip signifi cantly affects the fracture mechanical parameters, such as stress intensity factor and energy release rate. In addition, these parameters can be chan- ged by the crack growth length 6. It appeared from the above literature survey that there are sev- eral damage characteristics in PZT ceramics, and these can change the material properties. However, details on how to induce damage characteristics in the material properties have not been clarifi ed. One reason is the technical diffi culty of revealing the microscopic defects in PZT during the loading process 7. Information concern- ing damage characteristics in PZT is indispensable for understand- ing their material properties. The main purpose of this paper is, therefore, to investigate the effects of material damage on the elec- trical and mechanical properties during the loading process. In addition, an attempt is made to reveal directly the damage charac- teristics in the PZT ceramic via unique experimental techniques. 2. Material and experimental procedures 2.1. Specimen preparation The material selected for the present work was a commercial bulk lead zirconium titanium oxide ceramic (PZT), produced by Fuji Ceramics Co. in Japan. The nominal grain size of this ceramic is about 5lm in diameter. Silver based electrodes 10lm thick were plated on to the specimen surfaces by the following process: silver-metal powder with glass frit was coated on to the PZT surface; then the coated metal was fi red in air at 973 K for a few hours 8. After the electrode attachment, the sample was polar- ized between the two electroplates. Two types of specimen were 0142-1123/$ - see front matter ? 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2009.04.002 * Corresponding author. Tel.: +81 184 27 2211. E-mail address: okayasuakita-pu.ac.jp (M. Okayasu). International Journal of Fatigue 31 (2009) 14341441 Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: employed in the present work, as illustrated in Fig. 1: (a) a round rod; (b) a rectangular bar. All specimens were obtained from the same manufactured lot. The compressive strength of the round bar was about 750 MPa, and the bending strength of the rectangu- lar rod was about 80 MPa. 2.2. Fatigue and bending tests Low cycle fatigue and bending tests were carried out using a screw driven type universal testing machine with 10 kN capacity. Using the round rod specimen, a compressioncompression fatigue test was conducted at an R ratio of 0.05 and frequency of 0.05 Hz 7,8. The maximum cyclic load,rmax, was determined on the basis of the compressive strength (rB) of this ceramic, wherermaxis de- signed to be less than 67% ofrB5,7. Using the three point bending specimen, a bending test was executed at elevated temperature with a loading speed of 1 mm/min to fi nal fracture. A muffl e fur- nace with an accuracy of better than 0.1 K was employed for the high temperature bending tests. The furnace was designed origi- nally to be fi tted into the testing machine. At all times during the test, the actual temperature of the specimens was controlled. The electrical properties of this PZT ceramic, e.g., electromechanical coupling coeffi cient, k33, and piezoelectric constant, d33, were examined during the cyclic loading. In this approach, anti reso- nance frequency fa, resonance frequency frand electrostatic capac- ity CTare measured during the tests using an impedance analyzer in advance. In this measurement, the parameters are examined as the applied load is removed to zero. With faand frvalues, k33can be obtained by the following equation 5: k33 ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi 1 a fr fa?fr b s 1 where a and b are the coeffi cient depending on vibration mode. On the other hand, piezoelectric constant, d33, can be described as: d33 k33 ffi ffi ffi ffi ffi ffiffi e33 cE 33 s 2 wheree33and cE 33 are dielectric constant and elastic coeffi cient, respectively, and those are assessed by the following formulas: e33 CTt A 2a cE 33 2lfr 2q for round rod sample2b where t is the distance between the two electrodes, and A is the area of electrode. l andqrepresent the length of the round rod and the density of PZT, respectively. Details of the method for the above cal- culations can be found in Ref. 5. 3. Experimental results 3.1. Material properties during fatigue test Fig. 2 shows the variation of the piezoelectric constant (d33) as a function of the cycle number for the round rod specimens loaded cyclically at various compression loads (rmax50500 MPa). It should be noted fi rst that the number in the legend indicates the maximum applied stress (rmax) and the cycle number to the frac- ture (Nf). The samples forrmax50 MPa and 100 MPa were loaded cyclically about 50,000 cycles and the cyclic load stopped, where those samples were not fractured completely. A high value for d33(3.1 ? 10?10m/V) is obtained in the sample during the cyclic loading at the low applied stress ofrmax50 MPa, whereas a low d33level, less than 1.3 ? 10?10m/V, is found for applied stress of more thanrmax200 MPa. In contrast, the d33value decreases inter- mittently with increasing cycle number in the 100 MPa sample, and its value settles after 20 cycles to a similar level to that found for the samples tested at more thanrmax200 MPa. To further investigate the change of the electrical properties during the fati- gue test, the electromechanical coupling coeffi cient (k33) vs. cycle number was investigated for the samples cycled atrmax50 MPa, 100 MPa and 200 MPa. The results obtained are shown in Fig. 3. As with the experimental results of Fig. 2, high and low values of the k33 coeffi cient are obtained for the 50 MPa and 200 MPa sam- ples, respectively. Also, the k33value decreases with increasing cy- cle number for the 100 MPa sample. This result is convincing evidence that the material properties of the PZT ceramic are al- tered by the loading condition. The change of the material property in this case might be affected by the material damage in the sample during the cyclic loading. To clarify this, a direct observation of the sample was conducted. Fig. 4 displays the SEM images of the spec- imen surface (round rod) before and after fi ve cycles atrmax 450 MPa. Note that both pictures were obtained from the same location. From Fig. 4b, the damage (or collapse) of the sample 40mm 30mm 3mm 3mm 3mm 7.5mm (a) (b) Electrode Electrode Fig. 1. Dimensions of the tested specimens: (a) a round rod; (b) a rectangular bar specimen. 0.0E+00 5.0E-11 1.0E-10 1.5E-10 2.0E-10 2.5E-10 3.0E-10 3.5E-10 4.0E-10 020406080100120 Number of cycles Piezoelectric constant, m/V 500(89) 450(68) 350(99) 300(410) 200(3,709) 100(45,000) 50 max (MPa), Nf (cycle to fracture) 50MPa 100MPa 200MPa Fig. 2. Variation of piezoelectric constant d33as a function of the cycle number for the specimen loaded at various applied stresses. M. Okayasu et al./International Journal of Fatigue 31 (2009) 143414411435 surface is observed. It is considered from this result that the mate- rial damage in the PZT ceramic occurs during the fatigue test, and this may affect the electrical properties. 4. Discussion 4.1. Material damage vs. electrical properties To examine the relationship between the material damage and the material degradation in Figs. 2 and 3, a further set of tests was carried out. There are several damage characteristics in the PZT including microcrack (grain sliding) and domain switching 5,7. An attempt was made to examine the electrical properties of the specimens after receiving artifi cial microcrack damage. Instead of a microcrack, a machined slit was created in the round rod speci- men by a thin diamond cutting saw, e.g., 0.35 mm thick. Fig. 5 Fig. 4. SEM images of the PZT sample before and after cyclic loading at 450 MPa for fi ve cycles. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1101001000 Number of cycles Electromechanical coupling coefficient 200MPa 100MPa 50MPa (max ) 50MPa 100MPa 200MPa Fig. 3. Variation of electromechanical coupling coeffi cient k33as a function of cycle number for several specimens loaded at 50 MPa, 100 MPa and 200 MPa. Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Step 1 Step 2 Step 3 Depth: 0.7mm 1.4mm 2.1mm 1.0mm Removed electrode (both faces) Electrical wire 1.0mm Case I (a) Case II (b) Machine slit Machine slit 3mm Removed electrode (a face) Fig. 5. Schematic illustration showing the specimen materials with mechanical damage created by machine slits. 1436M. Okayasu et al./International Journal of Fatigue 31 (2009) 14341441 shows a schematic illustration of the samples showing the artifi cial damage. Two types of machine slits were created; (Case I) on the edge of the sample and (Case II) in the middle of the specimen. In Case I, several slits of the same size were machined in the sam- ple, one after another, denoted as Steps 16. In Steps 7 and 8, the electrode was removed by a fi le except for the area just around the electric wire attached to the sample surfaces. On the other hand, the machined slit was being made deeper at every step in Case II. It should be pointed out that compared to the actual mate- rial damage (microcrack and grain sliding), the size of the machine slit is much larger. However, the machined slit has been used to deliberately induce a greater degree of damage in order to study the consequent behavior 9. The electrical properties of the spec- imens with the machined slit were examined after every cut. Fig. 6a and b shows the electromechanical coupling coeffi cient, k33, measured after each step for Cases I and II, respectively. It is seen that the value of k33did not change in Case I even though the number of machined slits increased. On the other hand, a slight reduction in the k33 coeffi cient can be seen machined slit was deepened in Case II, although the rate of reduction of k33is much smaller than that obtained in Fig. 3. This result might suggest that material damage, such as crack and grain sliding, does not play an important role in dictating the response of the electrical properties of the PZT ceramic (Figs. 2 and 3). 4.2. Damage characteristics during the loading process To understand the reasons for the reduction in electrical prop- erties as shown in Figs. 2 and 3, the observation of the specimen material during the static compressive loading to fracture was con- ducted using a video camera. From these observations, it was found that an electrical activity in the PZT occurs several times, re- lated to a lightning-like phenomenon and consisting of a bright fl ash with a click sound. Representative pictures of the specimen obtained in the loading process are displayed in Fig. 7: (a) before loading, (b) electrogenesis and (c) fracture. The intensity of the click occurring during the loading can be identifi ed in the sound wave in Fig. 8. As seen in Fig. 8a, the click is detected eight times before the fi nal fracture in this case. The enlarged wave for a click sound is indicated in Fig. 8b. The point of the electrogenesis is further indicated on the compressive stress vs. displacement rela- tions (Fig. 9). As seen, a large number of the electrogenerative events are observed at the beginning of the loading process, espe- cially below 200 MPa. Because of the observation of the lightning phenomenon, it may be that the generation of electric charge is attributed to a part of the failure (or damage) in the PZT ceramic 2. In order to examine the effect of the electrogenic phenomenon on the material property in the PZT ceramic, the experimental data of Fig. 9 is correlated with the electromechanical coupling coeffi - cient (k33) vs. compressive stress. The results are shown in Fig. 10. It is seen that the k33 coeffi cient decreases nonlinearly with increase of applied stress, and its value settles out when the load- ing exceeds 200 MPa. Because many data points for the electrogen- eration are plotted in the region below 200 MPa, it might suggest that the electrogeneration is associated with the material degrada- tion in the PZT ceramic. Moreover, due to the material degradation, the electrogenesis might be attributed to the occurrence of domain switching in the PZT ceramic 2,10. To verify this clearly, another approach was conducted. In the previous studies, it was reported that the domain switching can play a prominent role in the tough- ness and fatigue properties of the piezoelectric ceramics 3, where the poled PZT ceramics have a high fatigue strength compared to the unpoled sample 4. This would be due to the change of lattice structure or an anisotropy effect 7,11. On the basis of previous re- ports, a study was performed to examine the variation of the microhardness during the cyclic loading. The cyclic loading was carried out with the maximum stress of about 60 MPa. The 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 012345678 Electromechanical coupling coefficient, k33 (a) Case I (b) Case II StepDepth of slit, mm Fig. 6. Variation of electromechanical coupling coeffi cient k33obtained in Cases I and II; see also Fig. 5. Base PZT specimen (Round bar) Loading direction (a) Before loading (b) Electrogeneration (c) Fracture instantly 2mm Fig. 7. Pictures of the specimen: (a) before loading; (b) electrogeneration and (c) fracture. M. Okayasu et al./International Journal of Fatigue 31 (2009) 143414411437 obtained data is shown in Fig. 11. It is seen that the microhardness level is apparently increase with increasing the cycle number. Hence, the result obtained would suggest the change of domain orientation occurred during the cyclic loading. Further experimental approach was carried out, where the mechanical properties were examined as a function of cyclic load- ing; and these were then compared to the electrical properties of the PZT ceramic (Fig. 3). The experimental results presented in Fig. 12 demonstrate the variation of both the fl exural modulus (EB) and the k33 coeffi cient as a function of cycle number for sam- ples tested at Pmax50 MPa, 100 MPa and 200 MPa. It should be pointed out fi rst that the fl exural modulus obtained in Fig. 12 was determined from the compressive stressdefl ection curves 8. In this case, the fl exural modulus was used as a parameter in this assessment, because the EBmodulus is very sensitive to mate- rial damage 8. The k33 coeffi cient obtained in Fig. 3 is expressed as its negative function, ?k33, in order to compare it easily with the EBmodulus. As in Fig. 12, a different trend of EBvariation is 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0100200300400500600700 Applied compressive stress, MPa Electromechanical coupling coefficient Fracture Elect