PZT陶瓷本构行为与断裂性能的相关性研究.pdf

第 38 卷 第 7 期 无 机 材 料 学 报 Vol.38 No.7 2023 年 7 月 Journal of Inorganic Materials Jul.,2023 Received date:2022-10-31;Revised date:2022-12-23;Published online:2023-03-09 Foundation item:National Natural Science Foundation of China(11972262);Guangdong Provincial Key Laboratory Program(2021B1212040001)Biography:WANG Xueyao(1996),female,PhD candidate.E-mail: 王雪瑶(1996),女,博士研究生.E-mail: Corresponding author:LI Yingwei,associate professor.E-mail:;LIANG Ruihong,professor.E-mail: 李应卫,副教授.E-mail:;梁瑞虹,研究员.E-mail: Article ID:1000-324X(2023)07-0839-06 DOI:10.15541/jim20220638 Correlation between Constitutive Behavior and Fracture Performance of PZT Ceramics WANG Xueyao1,WANG Wugang2,LI Yingwei1,PENG Qi1,LIANG Ruihong2(1.School of Civil Engineering,Wuhan University,Wuhan 430072,China;2.Shanghai Institute of Ceramics,Chinese Academy of Sciences,Shanghai 200050,China)Abstract:The fracture properties of ferroelectrics directly determine their processability and reliability of devices made of them.However,both experimentally and theoretically reported fracture toughness of piezoelectric ceramic materials remains nearly the same as that reported 30 years ago,limiting the application of piezoelectric devices in situation where high reliability is required.Here,we try to reveal the parameters that could be used to optimize the fracture performance of ferroelectrics.Specifically,stress-strain curves,intrinsic fracture toughness and long-crack fracture toughness of three typical PZT ceramics were measured by uniaxial compression method,crack-tip opening displacement(COD)technique and single-side V-notch beam(SEVNB)technique,respectively.It is shown that the intrinsic fracture toughness is positively correlated with the Youngs modulus of the material,which suggests that improving the Youngs modulus of ferroelectrics is an effective way to improve their intrinsic fracture toughness.The long-crack fracture toughness is related to the intrinsic toughness and extrinsic ferroelastic domain switching/phase transformation toughening,which also suggests that optimizing the ferroelastic switching behavior of piezoelectric ceramics can improve their extrinsic effect.Compared to the hard doped PZT,the soft doped PZT has low coercive stress,high remanent strain and high shielding toughness.The fracture patterns observed in different PZT materials are related to the different ferroelastic switching behavior of the materials.Soft PZT ceramics exhibit intergranular fracture,while hard PZT with weak ferroelastic switching behavior exhibits transgranular fracture.In conclusion,fracture toughness of ferroelectrics is enhanced by optimizing Youngs modulus and toughening of ferroelastic switching.Key words:lead titanate zirconate;crack-tip fracture toughness;ferroelastic domain switching;single-edge V-notch beam;fracture mode Piezoelectric ceramics have been widely used in modern industries working as actuators,transducers,etc.,due to their unique electromechanical coupling performance,ultra-fast response and small size1-2.After several decades of intensive investigations,the functional property of piezoelectric ceramic materials have been significantly improved,and some new piezoelectric ceramic systems were also developed3-4.In comparison,the mechanical property of piezoelectric ceramic materials,e.g.,fracture toughness and strength5,are also very important,as the mechanical performance directly determines their processability,and the reliability of piezoelectric devices6-7.However,although tremendous investigations related to the fracture behavior of piezoelectric ceramic materials have been conducted by researchers in the last 30 years,both experimentally and theoretically8-9,the reported fracture toughness IcK of piezoelectric ceramic materials remains near the same as that reported 30 years ago,in range of 0.5 1.1 MPam1/2 for unpoled PZT ceramics10-14.To a certain extent,this situation has limited the application of piezoelectric devices in situation where high reliability is required.At the moment,most of the fracture behaviors of piezoelectric ceramic materials observed in experiments or problems encountered in the industries could be well 840 无 机 材 料 学 报 第 38 卷 explained,e.g.,the crack length-dependent fracture toughness(R-curve)15-16 and the fracture toughness anisotropy of the materials after texturing10-11,17-18.As reviewed by Schneider17,Kuna19,Webber20,et al.,only a small number of problems are not well explained yet,for instance,the strange fracture surface modesome piezoelectric ceramic materials undergo intergranular fracture,while some are dominated by transgranular fracture21,and the different fracture mode in terms of hard and soft PZT22.Therefore,the next step of investigation on the fracture behavior of piezoelectric ceramic materials should focuse on proposing ideas or schemes that could be used to optimize the fracture behavior of ferroelectric ceramic materials.However,different from metals and other ceramics such as alumina ceramics,revealing the parameters that have influence on the fracture behavior of ferroelectric ceramic materials is a difficult task.This is because,on the one hand,the measured fracture toughness of piezoelectric ceramic materials is technique dependeddifferent charactering techniques normally lead to different fracture toughness values18,23-24.On the other hand,piezoelectric ceramic materials with different ferroelastic domain switching behavior have different toughening performance10,13,20.Hence,to reveal the parameters that could be referred for designing piezoelectric ceramic materials with better reliability,the constitutive behavior and the fracture performance should be both characterized.Moreover,materials with different performance but similar compositions should be characterized in comparison,which we tried to accomplish here.In this work,three types of commercially available PZT ceramics(PZT-4,-5,and-8)with different ferroelastic domain switching behaviors were chosen as model materials.The stress-strain curves of ceramics were firstly characterized during a uniaxial compression process to determine the parameters that have influence on the fracture behavior of ceramics.Then the intrinsic fracture toughness(also known as crack-tip fracture toughness)of ceramics,I0K,was determined by the crack opening displacement(COD)technique.The long-crack fracture toughness of ceramics was also measured by the single-edge V-notch beam(SEVNB)technique.The scanning electron microscopy(SEM)was used to characterize the fracture mode of the crack surface.Based on the measured data,the different fracture performances of the three types of ceramics were discussed in comparison,by combining the measured fracture toughness,the constitutive behavior,and the intrinsic fracture toughness of these materials.1 Experimental 1.1 Material description The material compositions of the used PZT-4,-5,and-8 are Pb0.95Sr0.05(Sb0.33Nb0.77)0.1Zr0.45Ti0.45O3-1%Fe,Pb0.98Sr0.02(Zn0.33Nb0.77)0.1Zr0.45Ti0.45O3,and PbZr0.53Ti0.47O3-0.6%Fe(in mol).They were fabricated by a standard solid-state synthesis process(S1).Their grain size and density are tabulated in Table S1.1.2 Stress-strain curve measurement The testing setup for characterizing the stress-strain behavior of the materials during uniaxial compression can be found in our previously published work11 and S2.1.3 Fracture toughness testing Fig.1 reveals an optical microscope image of a V-notch.The long-crack fracture toughness IvnbK of the specimen was calculated by23:0.5*12Ivnb1.51.5()32(1)F SSKYBW(1)where,F is the fracture stress,1S is the outer span,2S is the inner span,B is the specimen thickness,W is the specimen depth and/a W,a is the average V-notch depth,*Y is a dimensionless correction factor based on the ratio of/a W.The details of the test setup and the geometric dimensions of samples can be found in S3.1.4 COD measurement The crack-tip opening displacement(COD)technique was used to characterize I0K.If no additional transverse or conical cracks appear and the residual stress field relaxes due to residual stress after the Vickers indentation is performed,the crack remains open.The details of measurement and computation are shown in S4.Due to the toughening effect of the ferroelectric domain,the measured values gradually become larger than the estimated values,and the successive data points vary from the fitted straight line(Fig.2(a).In addition to the preceding computation method mentioned above,the crack-tip fracture toughness can also be obtained by Irwin fitting25.That is,only 0A(the coefficient based on the ratio of a/b)is used.This approach has the advantage of not requiring half-diagonal Fig.1 Optical microscope image of a V-notch 第 7 期 WANG Xueyao,et al:Correlation between Constitutive Behavior and Fracture Performance of PZT Ceramics 841 Fig.2 Measured COD plotted versus the calculated COD(a)PZT-4;(b)PZT-5;(c)PZT-8 length data for crack lengths and indentations.However,this evaluation is limited to the near-tip crack opening displacement field25.Fig.S1 shows the measured crack opening displacement COD as a function of the distance x from the crack tip and the least-squares fitting curves with xmax=10 m.2 Results and discussions 2.1 Stress-strain curves of the tested materials Fig.3 depicts the measured stress-strain curves during mechanical compression.At first,all of the materials reveal a linear stress-strain behavior,and then follow a significant nonlinear regime due to ferroelastic switching(in addition to non-180 ferroelastic domain switching,ferroelastic phase transformation can also induce nonlinear deformation for ferroelectrics,and both mechanisms can be called ferroelastic switching26),during which the strain accumulates quickly with the increase of the applied stress.With further increase of compressive stress,the stress-strain curves gradually change from nonlinearity to linearity,due to the decrease of the volume of domains that could be switched by the applied stresses.Additionally,based on the obtained stress-strain curves,the coercive stress c,the remanent strain r,the Youngs modulus E and the Poissons ratio of the tested materials can be determined and tabulated in Table S5.2.2 Fracture toughness and fracture mode of the tested materials Fig.4 depicts the measured IvnbK and I0K for unpoled PZT ceramics with different compositions.The IvnbK of PZT-5 is approximately 1.10 MPam1/2,with a value similar to the results reported by others on PZT 15110,27-29,e.g.,by Schneider with a value of 1.0 MPam1/229.In comparison,the IvnbK for PZT-4 is 0.91 MPam1/2;for PZT-8,its IvnbK is 0.78 MPam1/2.As for I0K,the highest value is for PZT-4;the lowest value is for PZT-5;the intermediate value is for PZT-8.Additionally,for PZT-4,PZT-5 and PZT-8,the I0K values determined by Eq.(S4-1)-Eq(S4-2)are 0.63,0.52,and 0.56 MPam1/2,respectively;which are slightly lower than the values of 0.75,0.59,and 0.6 MPam1/2 obtained via Irwin parabola.This is because 1A(the coefficient based on the ratio of a/b)is positive for all the cracks evaluated in this work.Therefore,according to Eq.(S4-1),the I0K for the same()u x is always higher if only 0A is considered(Irwin parabola)25.Even the I0K values determined by Irwin parabola could be higher than the observed ICK value30.Thus,the three-term-approximation should be the first choice.Fig.S2 displays the photography of the fracture surface characterized by SEM.For PZT-5,the fracture morphology is intergranular fracture dominated(90%);Fig.3 Stress-strain curves of PZT obtained during mechanical compression(a)PZT-4;(b)PZT-5;(c)PZT-8 842 无 机 材 料 学 报 第 38 卷 Fig.4 Fracture toughness IvnbK and intrinsic fracture toughness I0K for PZT-4,-5,and-8 while for PZT-4 and PZT-8,most parts of the fracture morphology are transgranular(70%for PZT-4 and 95%for PZT-8).It is in accordance with the previous reports of Guillon,et al22.They found that the fracture mode of PZT piezoceramics is related to the dopants，PZT with soft additives is mainly intergranular while PZT with hard additives is rather trangranular.2.3 Discussions As the intrinsic toughness is controlled by the atomic interactions of the materials,and is generally believed to be proportional to their theoretical tensile strength.Additionally,the theoretical tensile strength of a material is thought to be one tenth of its Youngs modulus.For PZT-4,-5,and-8,1:0.7:0.8 for both initialE and I0K.This demonstrates that to improve the intrinsic fracture toughness of piezoelectric ceramics,we have to find strategy to improve their Youngs modulus.Additionally,from Fig.4,it can be seen that the measured values of IvnbK and I0K are controversial,e.g.,the largest IvnbK was observed in PZT-5,while it reveals the smallest I0K.According to the previous investigations23,such controversy can be rationalized by the toughening mechanism of ferroelastic switching.Under external load,stress concentrates near the crack tip with distributions dominated by the applK-stress field.Due to ferroelastic switching,this applK-stress field will trigger a process zone with dimensions related to the stress field distribution,the coercive stress c,and the intrinsic toughness I0K of the materials.For unpoled PZT ceramics,the domains with random orientations switch into the direction along the applied tensile stresses.This process zone,obviously,will shield the applK-stress field.With the growth of the crack,the process zone triggered by the applK-stress field becomes the crack wake.For PZT-4 and-5,the switched domain remained in the wake of crack,leading to the increase of the closure shielding stresses at the crack tip(Fig.5(a).To promote further increment of the crack,a larger applied stress is required.Therefore,under controlled load,the fracture toughness of piezoelectric ceramic materials normally reveals an R-curve behavior,and when the crack propagates a certain distance,a plateau value maxIRK can be achieved.Obviously,the value maxIRK is nearly equal to the sum of I0K and maxK(maxIRK means the largest fracture toughness of ferroelectrics under controlled load;maxK presents the maximum shielding toughness).That is maxmaxIRI0KKK.The calculated shielding toughness maxK(S7)for PZT-4,-5,and-8 was 0.17,0.24,and 0 MPam1/2,respectively.Then the maxIRK for PZT-4,-5,and-8 should be 0.80,0.76,and 0.56 MPam1/2,respectively,which are smaller than the test value of 0.91,1.10,and 0.78 MPam1/2.This phenomenon was possibly induced by other toughening mechanism that was not taken into consideration in Eq.(S7-1),for instance,the shielding effect of process zone near the crack tip,crack deflection toughening,the kink of the crack during propagation,and the crack-interface grain bridging.Taken PZT-8 as an example,the calculated maxK is zero based on Eq.(S7-1)as the switched domains switch back in the crack wake(Fig.5(b).However,the measured IvnbK is larger than I0K,with a value of 0.22 MPam1/2.Fig.5 Illustration of the toughening mechanism(a)PZT-4 and