Crack Propagation Behavior of Y-TZP Ceramics

Y-TZP ceramics present the twofold advantage of a very fine grain size and a phase transformation leading to some reinforcement, making them useful in hip joint head prostheses. The subcritical crack growth of such a material that mainly drives the lifetime of the components has been evaluated for wide crack velocity ranges down to 10^-11 m/s by using both double torsion and static fatigue tests. It appeared that subcritical crack growth was activated by H₂O. It is shown that two types of flaws should be considered: surface flaws caused by machining located in the compressive layer present at the surface, and so-called volume flaws. The subcritical crack growth and the toughness of both types of defects are different. The relevance of subcritical crack growth analysis for such surface cracks is discussed. The compressive layer at the surface leads to a threshold value below which no crack growth occurs.     

Dynamic Observation of Crack Propagation in Piezoelectric Multilayer Actuators

Crack propagation in multilayer piezoelectric actuators made of Pb((Ni_1/3Nb_2/3), Ti, Zr)O₃ ceramics with an interdigital electrode configuration was observed dynamically under an applied cyclic electric field using charge coupled device microscopy. The crack was observed only under a high electric field, and it healed under zero field. The crack was initiated at the internal electrode edge and propagated from the electrode edge in three directions. The electric-field-induced displacement was measured simultaneously with the propagation. The displacement of the multilayer actuator became gradually smaller and asymmetric with respect to the sign of the field with increased driving cycle.     

Cyclic Fatigue Crack Growth in Three-Point Bending PZT Ceramics under Electromechanical Loading

This paper investigates experimentally and analytically the cyclic fatigue crack growth in piezoelectric ceramics under electromechanical loading. Cyclic crack growth tests were conducted on lead zirconate titanate (PZT) ceramics subjected to dc electric fields, and a finite element analysis was used to calculate the maximum energy release rate for the permeable crack model. Based on bending experiments using single-edge precracked-beam specimens, cyclic fatigue crack growth rates are found to be sensitive to the maximum energy release rate and applied dc electric fields. Possible mechanisms for crack growth were discussed by scanning electron microscope examination of the fracture surface of the PZT ceramics.     

Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics

Crack deflection and the subsequent growth of delamination cracks can be a potent source of energy dissipation during the fracture of layered ceramics. In this study, multilayered ceramics that consist of silicon nitride (Si₃N₄) layers separated by boron nitride/silicon nitride (BN/Si₃N₄) interphases have been manufactured and tested. Flexural tests reveal that the crack path is dependent on the composition of the interphase between the Si₃N₄ layers. Experimental measurements of interfacial fracture resistance and frictional sliding resistance show that both quantities increase as the Si₃N₄ content in the interphase increases. However, contrary to existing theories, high energy-absorption capacity has not been realized in materials that exhibit crack deflection but also have moderately high interfacial fracture resistance. Significant energy absorption has been measured only in materials with very low interfacial fracture resistance values. A method of predicting the critical value of the interfacial fracture resistance necessary to ensure a high energy-absorption capacity is presented.     

Subcritical Crack Propagation in 3Y-TZP Ceramics: Static and Cyclic Fatigue

A detailed analysis of crack propagation in tetragonal zirconia polycrystals doped with 3 mol% of Y₂O₃ (3Y-TZP) ceramics is presented. Crack propagation tests have been conducted for crack velocities of 10^-12-10^-3 m/s in several environments, including air, water (in the temperature range of 3°-85°C), secondary vacuum (10^-5 mbar), and silicon oil. Analysis of the experimental results-three propagating regimes that are dependent on the environment and a marked threshold below which no propagation occurs-shows that stress corrosion by water molecules is the key mechanism for crack propagation. The effect of grain size on the crack velocity is quantified and analyzed in terms of transformation toughening. Experiments under cyclic loading have been conducted to quantify the effects of cyclic fatigue. Crack velocities are higher under cyclic loading than that predicted by stress corrosion alone, and the threshold is lower. Experiments that have been conducted at two different frequencies (0.1 and 1 Hz) and static-fatigue/cyclic-fatigue sequences show that both stress corrosion by water and pure cyclic-fatigue effects are operative under alternative stresses.     

Refined Model of Electromigration of Ag/Pd Electrodes in Multilayer PZT Ceramics Under Extreme Humidity

The phenomenon of silver electromigration has been examined in multilayer structures comprised of a low-firing PZT-based ceramic with inner Ag_{1− x} Pd _x electrodes (0< x <0.3). Under high humidity and dc bias, the time-to-failure was found to increase significantly with Pd content for x >0.1. Failed samples exhibited metallic-like transport properties including Ohmic behavior and a low temperature coefficient of resistance. Subsequently, electron microscopy of cross-sectioned samples revealed high concentrations of Ag in grain boundaries confirming Ag migration as the failure mechanism. Furthermore, it was found that the failure rate did not improve substantially by replacing a pure Ag anode with a higher Pd composition. This observation appears to conflict with previous conceptual models in which Ag migration originates mainly from the anode. It is suggested that Ag diffusion during sintering results in a background concentration of Ag within the ceramic which later contributes to the electrolytic flux of Ag⁺ ions under high humidity. This effect may be more apparent in low temperature sintered materials where residual grain boundary phases become vulnerable pathways for moisture penetration.     

Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics

A fracture-mechanical theory is presented for crack propagation in brittle ceramics subjected to thermal shock. The criteria of crack stability are derived for a brittle solid uniformly cooled with triaxially constrained external boundaries. Thermal stress crack instability occurs between two values of critical crack length. For short initial crack length, crack propagation occurs kinetically, with the total area of crack propagation proportional to the factor S_t² (1-2 v )/EG, where S_t is tensile strength, v is Poisson's ratio, E is Young's modulus, and G is surface fracture energy. Under these conditions the newly formed crack is subcritical and requires a finite increase in temperature difference before propagation will proceed. For long initial crack length, crack propagation occurs in a quasi-static manner and can be minimized by maximizing the thermal stress crack stability parameter R_st= [G/α² E ]^1/2, where α is the coefficient of thermal expansion. For heterogeneous brittle solids, such as porous refractories, the concept of an "effective flaw length" is introduced and illustrated on the basis of experimental data in the literature. The relative change in strength of a brittle solid as a function of increasing severity of thermal shock is estimated. Good qualitative agreement with literature data is found.     

凝胶注模成型法制备PZT压电陶瓷

本文研究了凝胶注模成型法制备PZT陶瓷工艺.分别单因素分析了pH值、搅拌时间、分散剂的用量及固相体积含量对PZT陶瓷浆料粘度的影响,并对该工艺不同固相含量的陶瓷样品以及传统模压成型工艺制备的样品进行了电学性能测定.实验表明当pH值等于12,搅拌时间为15 min,分散剂的质量分数为0.1％时,浆料的流变性最佳,当固相含量为52.5vol％时,粘度为249 mPa·s,陶瓷样品的电学性能最佳,此样品可以达到传统模压成型法制备的PZT陶瓷所具有的电学性能.     

Cyclic Fatigue Crack Propagation Behavior of 9Ce-TZP Ceramics with Different Grain Size

Cyclic fatigue crack growth behavior has been investigated in 9 mol% Ce-TZP ceramics with grain sizes varying from 1.1 to 3.0 μm. To ascertain the interaction between crack resistance curve behavior and cyclic fatigue crack growth, cyclic fatigue tests were conducted with short double-cantilever-beam specimens in two conditions: (a) with a sharp precrack without preexisting t - m transformation and (b) with a sharp crack after R -curve measurements, i.e., with preformed t - m transformation in the crack region. Fatigue crack propagation occurs at applied stress intensity factor values as low as about 40% of the K _I,∞ values measured in the R -curves. The size and shape of the t - m transformation zones are found to be different for specimens obtained in monotonic loading R -curve measurements and in cyclic fatigue tests. For the specimens without preexisting t - m transformation the overall crack growth behavior can be described by the Paris power law relation: d a /d N = AδK^m _I with m values of 15 for the 1.1-μm grain size and between 8 and 9 for the material with larger grain sizes. For the specimens with the preformed transformation zone, a "V"shape d a /d N versus Δ K _I relation is obtained. Explanations for these different results in the two conditions are discussed in terms of crack tip shielding effects.     

抗还原型PZT压电陶瓷的制备与性能

采用固相法制备出可在低氧压和还原性气氛中烧结的压电陶瓷材料。材料最佳组成为:Pb0.95Sr0.05(Zr0.54Ti0.46)O3+0.03%(质量分数)CuO+0.05%Nd2O3+1.00%Sb2O3。通过施主和受主共掺杂,既抑制了烧结过程中氧空位扩散,又避免了Ti4+与自由电子结合转变成为Ti3+,使陶瓷保持了压电性能。结果表明:添加半径合适的稀土元素,是使陶瓷具有抗还原性能的关键之一。当烧结温度为1050℃时,陶瓷压电应变常数d33=294 pC/N,平面机电耦合系数kp=43.56%,相对介电常数εT33/ε0=1 333,介电损耗tanδ=0.019 7。该材料可应用于与Ni、Cu等贱金属低温共烧的叠层压电器件中,能够大大降低器件的成本。     

Acoustic Characterization of Poling Effects in PZT Ceramics

Acoustic velocity and attenuation were measured in poled and unpoled lead zirconate titanate (PZT) ceramics prepared by sintering and hot-pressing under-different conditions. Hot-pressed PZT was found to have attenuation values approximately 1 order of magnitude smaller than simered PZT. For both materials, poling caused a decrease in attenuation. Depolarization and phase transition phenomena were also observed at elevated temperatures using a novel laser-ultrasound technique in combination with conventional pulse-echo measurements.     

掺杂对PZT压电陶瓷的影响及研究进展

在介绍掺杂PZT压电陶瓷材料制备的基础上,总结出生产掺杂PZT材料的最佳掺杂条件,以及配方对压电陶瓷性能的影响,只要工艺合适,性能完全能再提高,对压电陶瓷材料的发展趋势进行了展望。     

压电报警器PZT压电陶瓷的制备研究

本文采用传统的固相法制备PZT二元系压电陶瓷。研究了掺杂不同含量为0.1%,0.15%,0.2%,0.25%,0.3%和0.35%的MnO_2和CeO_2对PZT压电陶瓷的结构、介电性能、压电性能和介电损耗的影响。并对其微观组织进行了研究。当锰的掺杂量为0.15%时,压电陶瓷的性能得到最佳的优化:tgδ=0.0095;kp=0.634p C/N;d33=611;ε=2523。铈的掺杂使陶瓷的烧结温度升高,当铈的掺杂量为0.15%时,压电陶瓷的性能也得到了最佳的优化:tgδ=0.017;kp=0.623;d33=563p C/N,ε=3310。在原配方材料的基础上压电常数和机电耦合系数都有所增加。这对压电报警器的声压的提高、体积的减小有着重要的意义。     

Multilayer Actuator Composed of PZN–PZT and PZN–PZT/Ag Fabricated by Co-Extrusion Process

A multilayer ceramic actuator composed of piezoelectrically active Pb(Zn_1/3Nb_2/3)_0.2–Pb(Zr_0.5Ti_0.5)O_0.8 (PZN–PZT) layers and electrically conducting PZN–PZT/Ag layers was fabricated by the co-extrusion process. For the piezoelectric layers, PZN–PZT, which is sinterable at a low temperature (900°C), was used. For the conducting layers, a PZN–PZT/Ag composite, made by mixing silver particles with the PZN–PZT matrix, was employed. For the co-extrusion process, piezoelectric and conducting feedrods were made by mixing the PZN–PZT and PZN–PZT/Ag, respectively, with a thermoplastic polymer. The initial feedrods, which were composed of five 3 mm-thick PZN–PZT layers, two 1.5 mm-thick PZN–PZT layers, and six 1 mm-thick PZN–PZT/Ag layers, were co-extruded through a 24 mm × 2 mm reduction die at 105°C to produce continuous multilayered green sheets. The sheets were stacked, warm pressed, and sintered at 900°C for 4 h after binder burnout. The sintered multilayer actuator showed distinct layers without any reaction products or cracks at the interface. The thicknesses of the piezoelectric and conducting layers were about 200 and 70 μm, respectively. The displacement of the multilayer actuator, composed of 40 piezoelectric layers (with a total height of 10.8 mm), was about 10 μm at an applied voltage of 500 V.     

Brittleness Dependence of Crack Branching in Ceramics

A linear correlation between the degree of overloading on the crack tip and brittleness was observed. The intercept, which was √2, is identified with a minimum condition of overloading where branching occurs at twice the fracture energy. The slope was identified with the degree of plastic deformation at the crack tip necessary to nucleate the branching crack. A total crack tip strain criterion for crack branching in ceramics was investigated. This criterion is supported by the fact that average total elastic strain at the boundary of the yield zone is approximately equal to the square of the slope of the curve.     

建陶制品裂纹釉的研制

本文简要介绍了裂纹釉的形成机理、组成及其影响因素,为在陶瓷制品上施以裂纹釉提供参考。     

Cyclic Fatigue Crack Growth in PZT Under Mechanical Loading

Crack growth under static and cyclic mechanical loading in lead zirconate titanate was studied using four point bend specimens in poled and unpoled states. Fatigue crack growth occurred at lower stress intensity factors than crack growth observed under static loading. The relation between crack velocity and applied stress intensity factor under static loading was affected by poling and followed a power-law relationship. Crack velocity vs. stress intensity amplitude under cyclic loading followed a Paris power-law relationship and was found to be unaffected by poling. A controlled unloading experiment revealed that the apparent stress intensity factor for crack extension decreased with increased unloading time but was essentially unaffected when the unloading cycle was less than five seconds, hence indicating the absence of an extrinsic fatigue mechanism.     

Acoustic and Elastic Properties of PZT Ceramics with Anisotropic Pores

It has been shown that anisotropic porosity in PZT reduces the d₃₁/d₃₃, ratio, raising the hydrostatic piezoelectric response of PZT significantly. In the present work, this behavior is related to Poisson's ratio by applying the Gibbs thermodynamic energy function, literature data for dense PZT ceramics and for materials with hexagonal crystal symmetry, and experimental results for PZT with anisotropic porosity. Acoustic as well as mechanical tests of PZT ceramics with anisotropic pores demonstrate the elastic anisotropies that cause the reduction in Poisson's ratio. The dependence of d₃₁/d₃₃ on Poisson's ratio is also detailed. In addition, a close relation between d₃₁/d₃₃ and the longitudinal acoustic velocity is demonstrated. Flexural strength tests indicate that anisotropic pores provide dh_n-enhanced PZT ceramics with significantly greater strength than isotropic pores.