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Damage characteristics of lead zirconate titanate piezoelectric ceramic duringcyclic loadingMitsuhiro Okayasu*, Nozomi Odagiri, Mamoru MizunoDepartment of Machine Intelligence and Systems Engineering, Akita Prefectural University, 84-4 Ebinokuchi, Tuchiya-aza, Yurihonjo-city, Akita 015-0055, Japana r t i c l ei n f oArticle history:Received 1 January 2009Received in revised form 31 March 2009Accepted 1 April 2009Available online 10 April 2009Keywords:PZT ceramicFatigue testDomain switchingMaterial damageElectrical propertya b s t r a c tThe 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 coefficient (k33), are changed dur-ing cyclic loading. The k33coefficient decreases rapidly to a low level as the sample is loaded cyclicallywith a high applied stress, while the k33value decreases slowly or does not change when loaded witha low applied stress. Such a change of material degradation is influenced by the severity of the materialdamage in the PZT ceramic. The material damage in the PZT occurs, and this occurrence is related to alightning-like phenomenon (a bright flash 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. IntroductionThe significance of piezoelectric ceramics is the transfer of an in-duced voltage difference that appears across two of the surfaces ofthe ceramic as the shape of the ceramic is subjected to high alter-nating stresses. Using this unique material characteristic, theseceramics have been utilized in various engineering applicationsincluding memory devices, precision positioning, electro-mechani-cal actuators, power transducers and vibration sensors. To employthe PZT ceramic for a long period of time, it is necessary to under-stand the material response to the application. The efficiency ofthe piezoelectric property in the ceramic can be changed if an over-load is applied, due to material damage in the ceramic. There havebeenseveralpossiblekindsof damageinPZTsreportedin publishedpapers, e.g., microcrack, grain sliding and domain switching (polar-ization). The material damage occurs when the electric fatiguecrack initiates from a porous region of the PZT 1. The damage inPZT ceramics can also be detected if the electrogeneration istrapped at a defect in the sample, which leads to the change ofthe electric domain orientation 2. Domain switching can occurwith 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, Shindoet al. have examined the damage characteristics in PZT ceramicnumerically and theoretically. It appeared from their work thatthe localized switching near the crack tip significantly affects thefracture mechanical parameters, such as stress intensity factorand 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 changethe material properties. However, details on how to induce damagecharacteristics in the material properties have not been clarified.One reason is the technical difficulty of revealing the microscopicdefects 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. Inaddition, an attempt is made to reveal directly the damage charac-teristics in the PZT ceramic via unique experimental techniques.2. Material and experimental procedures2.1. Specimen preparationThe material selected for the present work was a commercialbulk lead zirconium titanium oxide ceramic (PZT), produced byFuji Ceramics Co. in Japan. The nominal grain size of this ceramicis about 5lm in diameter. Silver based electrodes 10lm thickwere plated on to the specimen surfaces by the following process:silver-metal powder with glass frit was coated on to the PZTsurface; then the coated metal was fired in air at 973 K for a fewhours 8. After the electrode attachment, the sample was polar-ized between the two electroplates. Two types of specimen were0142-1123/$ - 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) 14341441Contents lists available at ScienceDirectInternational Journal of Fatiguejournal homepage: /locate/ijfatigueemployed in the present work, as illustrated in Fig. 1: (a) a roundrod; (b) a rectangular bar. All specimens were obtained from thesame manufactured lot. The compressive strength of the roundbar was about 750 MPa, and the bending strength of the rectangu-lar rod was about 80 MPa.2.2. Fatigue and bending testsLow cycle fatigue and bending tests were carried out using ascrew driven type universal testing machine with 10 kN capacity.Using the round rod specimen, a compressioncompression fatiguetest was conducted at an R ratio of 0.05 and frequency of 0.05 Hz7,8. The maximum cyclic load,rmax, was determined on the basisof the compressive strength (rB) of this ceramic, wherermaxis de-signed to be less than 67% ofrB5,7. Using the three point bendingspecimen, a bending test was executed at elevated temperaturewith a loading speed of 1 mm/min to final fracture. A muffle fur-nace with an accuracy of better than 0.1 K was employed for thehigh temperature bending tests. The furnace was designed origi-nally to be fitted into the testing machine. At all times during thetest, the actual temperature of the specimens was controlled. Theelectrical properties of this PZT ceramic, e.g., electromechanicalcoupling coefficient, k33, and piezoelectric constant, d33, wereexamined 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 analyzerin advance. In this measurement, the parameters are examined asthe applied load is removed to zero. With faand frvalues, k33can beobtained by the following equation 5:k33ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1afrfa?fr bs1where a and b are the coefficient depending on vibration mode. Onthe other hand, piezoelectric constant, d33, can be described as:d33 k33ffiffiffiffiffiffiffie33cE33s2wheree33and cE33are dielectric constant and elastic coefficient,respectively, and those are assessed by the following formulas:e33CTtA2acE33 2lfr2qfor round rod sample2bwhere t is the distance between the two electrodes, and A is the areaof electrode. l andqrepresent the length of the round rod and thedensity of PZT, respectively. Details of the method for the above cal-culations can be found in Ref. 5.3. Experimental results3.1. Material properties during fatigue testFig. 2 shows the variation of the piezoelectric constant (d33) as afunction of the cycle number for the round rod specimens loadedcyclically at various compression loads (rmax50500 MPa). Itshould be noted first that the number in the legend indicates themaximum applied stress (rmax) and the cycle number to the frac-ture (Nf). The samples forrmax50 MPa and 100 MPa were loadedcyclically about 50,000 cycles and the cyclic load stopped, wherethose samples were not fractured completely. A high value ford33(3.1 ? 10?10m/V) is obtained in the sample during the cyclicloading at the low applied stress ofrmax50 MPa, whereas a lowd33level, less than 1.3 ? 10?10m/V, is found for applied stress ofmore 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 foundfor the samples tested at more thanrmax200 MPa. To furtherinvestigate the change of the electrical properties during the fati-gue test, the electromechanical coupling coefficient (k33) vs. cyclenumber 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 ofthe k33coefficient 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 convincingevidence that the material properties of the PZT ceramic are al-tered by the loading condition. The change of the material propertyin this case might be affected by the material damage in the sampleduring the cyclic loading. To clarify this, a direct observation of thesample was conducted. Fig. 4 displays the SEM images of the spec-imen surface (round rod) before and after five cycles atrmax450 MPa. Note that both pictures were obtained from the samelocation. From Fig. 4b, the damage (or collapse) of the sample40mm 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+005.0E-111.0E-101.5E-102.0E-102.5E-103.0E-103.5E-104.0E-10020406080100120Number of cyclesPiezoelectric constant, m/V500(89)450(68)350(99)300(410)200(3,709)100(45,000)50max (MPa), Nf (cycle to fracture) 50MPa 100MPa 200MPa Fig. 2. Variation of piezoelectric constant d33as a function of the cycle number forthe specimen loaded at various applied stresses.M. Okayasu et al./International Journal of Fatigue 31 (2009) 143414411435surface is observed. It is considered from this result that the mate-rial damage in the PZT ceramic occurs during the fatigue test, andthis may affect the electrical properties.4. Discussion4.1. Material damage vs. electrical propertiesTo examine the relationship between the material damage andthe material degradation in Figs. 2 and 3, a further set of tests wascarried out. There are several damage characteristics in the PZTincluding microcrack (grain sliding) and domain switching 5,7.An attempt was made to examine the electrical properties of thespecimens after receiving artificial microcrack damage. Instead ofa 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. 5Fig. 4. SEM images of the PZT sample before and after cyclic loading at 450 MPa for five cycles.0.01101001000Number of cyclesElectromechanical coupling coefficient 200MPa100MPa50MPa(max ) 50MPa 100MPa 200MPa Fig. 3. Variation of electromechanical coupling coefficient k33as a function of cyclenumber 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) 14341441shows a schematic illustration of the samples showing the artificialdamage. Two types of machine slits were created; (Case I) on theedge 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, theelectrode was removed by a file except for the area just aroundthe electric wire attached to the sample surfaces. On the otherhand, the machined slit was being made deeper at every step inCase II. It should be pointed out that compared to the actual mate-rial damage (microcrack and grain sliding), the size of the machineslit is much larger. However, the machined slit has been used todeliberately induce a greater degree of damage in order to studythe 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 coefficient,k33, measured after each step for Cases I and II, respectively. It isseen that the value of k33did not change in Case I even thoughthe number of machined slits increased. On the other hand, a slightreduction in the k33coefficient can be seen machined slit wasdeepened in Case II, although the rate of reduction of k33is muchsmaller than that obtained in Fig. 3. This result might suggest thatmaterial damage, such as crack and grain sliding, does not play animportant role in dictating the response of the electrical propertiesof the PZT ceramic (Figs. 2 and 3).4.2. Damage characteristics during the loading processTo understand the reasons for the reduction in electrical prop-erties as shown in Figs. 2 and 3, the observation of the specimenmaterial during the static compressive loading to fracture was con-ducted using a video camera. From these observations, it wasfound that an electrical activity in the PZT occurs several times, re-lated to a lightning-like phenomenon and consisting of a brightflash with a click sound. Representative pictures of the specimenobtained in the loading process are displayed in Fig. 7: (a) beforeloading, (b) electrogenesis and (c) fracture. The intensity of theclick occurring during the loading can be identified in the soundwave in Fig. 8. As seen in Fig. 8a, the click is detected eight timesbefore the final fracture in this case. The enlarged wave for a clicksound is indicated in Fig. 8b. The point of the electrogenesis isfurther indicated on the compressive stress vs. displacement rela-tions (Fig. 9). As seen, a large number of the electrogenerativeevents are observed at the beginning of the loading process, espe-cially below 200 MPa. Because of the observation of the lightningphenomenon, it may be that the generation of electric charge isattributed to a part of the failure (or damage) in the PZT ceramic2.In order to examine the effect of the electrogenic phenomenonon the material property in the PZT ceramic, the experimental dataof Fig. 9 is correlated with the electromechanical coupling coeffi-cient (k33) vs. compressive stress. The results are shown inFig. 10. It is seen that the k33coefficient decreases nonlinearly withincrease 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 suggestthat 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 domainswitching in the PZT ceramic 2,10. To verify this clearly, anotherapproach was conducted. In the previous studies, it was reportedthat the domain switching can play a prominent role in the tough-ness and fatigue properties of the piezoelectric ceramics 3, wherethe poled PZT ceramics have a high fatigue strength compared tothe unpoled sample 4. This would be due to the change of latticestructure or an anisotropy effect 7,11. On the basis of previous re-ports, a study was performed to examine the variation of themicrohardness during the cyclic loading. The cyclic loading wascarried out with the maximum stress of about 60 MPa. The0.01.0012345678Electromechanical coupling coefficient, k33(a) Case I (b) Case II StepDepth of slit, mm Fig. 6. Variation of electromechanical coupling coefficient k33obtained in Cases Iand 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) 143414411437obtained data is shown in Fig. 11. It is seen that the microhardnesslevel is apparently increase with increasing the cycle number.Hence, the result obtained would suggest the change of domainorientation occurred during the cyclic loading.Further experimental approach was carried out, where themechanical properties were examined as a function of cyclic load-ing; and these were then compared to the electrical properties ofthe PZT ceramic (Fig. 3). The experimental results presented inFig. 12 demonstrate the variation of both the flexural modulus(EB) and the k33coefficient as a function of cycle number for sam-ples tested at Pmax50 MPa, 100 MPa and 200 MPa. It should bepointed out first that the flexural modulus obtained in Fig. 12was determined from the compressive stressdeflection curves8. In this case, the flexural modulus was used as a parameter inthis assessment, because the EBmodulus is very sensitive to mate-rial damage 8. The k33coefficient obtained in Fig. 3 is expressedas its negative function, ?k33, in order to compare it easily withthe EBmodulus. As in Fig. 12, a different trend of EBvariation is00100200300400500600700Applied compressive stress, MPaElectromechanical coupling coefficient Fracture Electrogeneration events Fig. 10. Relationship between the electromechanical coupling coefficient k33andapplied compressive stress at the electrogeneration events in the specimen.3003203403603800101,000100,000Number of cyclesVickers hardnessFig. 11. Variation of the microhardness of the PZT ceramic as a function of the cyclenumber.0 0.01 0.02 0.03 0.04 Time, sec 0 3.0 -3.0 -6.0 6.0 Sound intensity, V (b) 0 10 20 30 40 50 60 Time, sec 0 -3.1 2.8 5.8 -6.1 Sound intensity, V Click noise from specimen Fracture (a) Noise from testing machine Fig. 8. Variation of the sound intensity during the loading test, showing click noisefrom the sample.010020030040050060070080001Displacement, mmApplied compressive stress, MPa Fig. 9. Electrogeneration events during the loading process.1438M. Okayasu et al./International Journal of Fatigue 31 (2009) 14341441observed. Low and high levels of the flexural modulus were ob-tained at 50 MPa and 200 MPa, respectively; the EBmodulus forthe 200 MPa sample, 13.5 kN/mm, is about twice as high than thatfor the 50 MPa sample. On the other hand, EBfor the 100 MPa sam-ple varies linearly with increasing cycle number. Interestingly, thevariation of EBfor all the samples shown in Fig. 12ac is very sim-ilar to that of the ?k33coefficient. The relationship between the k33and EBvalues is further indicated in Fig. 13. As can be seen, theflexural modulus is linearly related to the k33coefficient withR2= 0.93. We conclude from these results that the mechanicalstrengths during cyclic loading are associated directly with theelectrical properties, and the change of their properties can beinfluenced by the polarization switching.4.3. Damage characteristics during the heating processTo provide further support for the above results of the materialdamage effects on the material degradation, the material damage,i.e., the lightning phenomenon, was examined during the heatingto the rectangular bar PZT ceramic. This approach was executedbecause the temperature affects the electromechanical responseof piezoelectric materials 12. In this case, the electrical andmechanical properties were examined at elevated temperaturesfrom 293 K to above the Curie temperature of 573 K. Like theexperimental result of Fig. 9, the electrogeneration occurred inthe sample while heating up to 600 K. Fig. 14 exhibits the electro-generation events observed during the heating process and showsthat the electrogenic phenomenon is observed mostly in the hightemperature region, just before the Curie temperature. The dataobtained was further analyzed. Fig. 15 presents the sample tem-perature vs. electromechanical coupling coefficient (k31) at the0.01.0051015Flexural modulus, kN/mmElectromechanical coupling coefficientmax100MPa max200MPa k33 = - 0.053 EB + 0.87 Fig. 13. Relationship between the flexural modulus EBand electromechanicalcoupling coefficient k33.0100200300400500600700050100150200250300Time, secTemperature, KCurie temperature Fig. 14. Electrogeneration events during the heating process.0246810121416110100100010000Number of cyclesFlexural modulus, kN/mm-1.6-1.4-1.2-1.0-0.8-0.6-0.4Electromechanical coupling coefficient -k33-k33 coefficient Flexural modulus (a) max 50MPa 024681012141618110100100010000Number of cyclesFlexural modulus, kN/mm-1.0-0.8-0.6-0.4-0.20.0Electromechanical coupling coefficient -k33-k33 coefficient Flexural modulus (b) max 100MPa 46810121416182022110100100010000Number of cyclesFlexural modulus, kN/mm-0.7-0.6-0.5-0.4-0.3-0.2-0.10.0Electromechanical coupling coefficient -k33(c) max 200MPa -k33 coefficient Flexural modulus Fig. 12. Variation of the flexural modulus EBand electromechanical couplingcoefficient ?k33as a function of the cycle number at: (a)rmax50 MPa; (b) 100 MPaand (c) 200 MPa.M. Okayasu et al./International Journal of Fatigue 31 (2009) 143414411439point of electrogeneration. As can be seen, the k31value decreasesrapidly with increase of sample temperature, particularly over450 K, when many electrogenic events occurred. We infer from thisresult that, as with Fig. 10, the reduction in the k31coefficient is af-fected by material damage. To confirm this, the variation of theflexural modulus as a function of sample temperature was exam-ined. Fig. 16 presents the results. In this case, the flexural moduluswas obtained from the bending load vs. deflection. The flexuralmodulus increases with increase of sample temperature, particu-larly over 450 K, where the electrogeneration occurs. Thus, it isproposed that an increase of the flexural modulus of the sampleis caused by the change of material properties, e.g., polarizationswitching.5. ConclusionsThe damage characteristics in the PZT ceramic during cyclicloading were studied. Based on the experimental results, the fol-lowing conclusions can be made:1. The piezoelectric constant, d33, obtained from cyclic loadingmeasurements, has a high value, about 2.1 ? 10?10m/V,when the sample is loaded at low cyclic stress (rmax50 MPa), but a low value, less than 7 ? 10?10m/V, whenthe sample loaded with more thanrmax200 MPa. On theother hand, d33decreases with increasing cycle number inthe sample loaded cyclically atrmax100 MPa. On the con-trary, the microhardness in the PZT ceramic increases withincreasing the cyclic number. The change of the electricaland mechanical properties is attributed to the differentmaterial properties.2. The electrical properties of the PZT ceramic, e.g., electrome-chanical coupling coefficient, are not significantly influencedby microscopic defects such as microcrack and grain slidingin our artificial defect tests. Because the mechanical strengthincreases with increasing the cyclic number, the domainstructure could affect the material properties. Electrogenicphenomenon occurs several times in the PZT ceramic duringloading and heating processes, where a bright flash togetherwith a click noise is detected. This phenomenon may beattributed to the domain switching in the PZT ceramic.AcknowledgementsThe authors wish to thank Mr. Takeo Tukanome at Rion Co., Ltd.for his technical support providing the sound wave data. Theauthors would also like to express the appreciation to Prof. S. Ta-kane and Prof. K. Watanabe at Akita Prefectural Univ. for their use-ful comments. One of the authors (M.O.) acknowledges financialsupport from the Japanese government (Young Scientists
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