Tensile Strength of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Tensile strengths of 2.0 to 5.0 mol% Y₂O₃-stabilized ZrO₂ polycrystals were described using the newly developed tensile testing method. The tensile test was conducted by attaching three strain gauges on both sides of a rectangular bar that was 10 mm by 1 mm by 200 mm. The tensile strength of tetragonal ZrO₂ polycrystals (TZP) containing 2.0 mol% Y₂O₃ showed 745 MPa, whereas the bend strength of this material was 1630 MPa. Inelastic behavior of the stress-strain curve was observed at critical stresses and strains of 500 to 700 MPa and 0.25% to 0.35%, respectively. Although deviation from proportionality was observed to be small, it increased with the increase of temperature from −100° to 200°C.     

Strength Analysis of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Tensile strength of Y₂O₃-stabilized ZrO₂ polycrystals (Y-TZP) was measured by a newly developed tensile testing method with a rectangular bar. The tensile strength of Y-TZP was lower than that of the three-point bend strength, and the shape of the tensile strength distribution was quite different from that of the three-point bend strength distribution. It was difficult to predict the distribution curve of the tensile strength using the data of the three-point bend strength by one-modal Weibull distribution. The distribution of the tensile strength was analyzed by two- or three-model Weibull distribution coupled with an analysis of fracture origins. The distribution curve of the three-point bend strength which was estimated by multimodal Weibull distribution agreed favorably with that of the measured three-point bend strength values. A two-modal Weibull distribution function was formulated approximately from the distributions of the tensile and three-point bend strengths, and the estimated two-modal Weibull distribution function for the four-point bend strength agreed well with the measured four-point bend strength.     

Slip Casting of Yttria-Stabilized Tetragonal Zirconia Polycrystals

The rheological and casting parameters of 3-mol%-yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powder obtained by a wet-chemical coprecipitation route have been studied. Colloidal stability has been studied through zeta potential measurements. An organic surfactant has been used as deflocculant. Viscosity, casting rate, and green densities have been determined for suspensions with 28.2, 33.6, and 40.3 vol% solids content. Relative density, grain size, and t -ZrO₂ evolution versus temperature and soaking time are also reported.     

Dynamic Grain Growth During Superplastic Deformation of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Both static and dynamic grain growth were studied during superplastic deformation of fine-grained yttria-stabilized tetragonal zirconia. It was found that significant grain growth does not take place below 1300°C. Both static and dynamic growth were found to obey a similar equation of the form D³−D³₀=kt, where D and D₀ are the instantaneous and initial grain sizes, respectively, t is the annealing time, and k is the kinetic constant for either static or dynamic grain growth. The activation energies were approximately 580 and 520 kJ/mol for static and dynamic grain growth, respectively.     

Spherical and Vickers Indentation Damage in Yttria-Stabilized Tetragonal Zirconia Polycrystals

Damage induced in an yttria-stabilized tetragonal zirconia polycrystal by spherical and Vickers indentations was investigated. Scanning acoustic microscopy revealed that, as indentation stress increased, the spherical indentation gradually developed subsurface damage, until it experienced a transition to a fully plastic state, characterized by a highly anisotropic variation in the leaky Rayleigh wave velocity, v _R, and very similar to that for Vickers indentation. The transition was a result of the formation of a microcracked core beneath the contact. Indenter geometry had an appreciable effect only within the core; the distribution of microcracks differed depending on the indenter used, as confirmed by direct observations using a scanning electron microscope. In contrast, the residual stresses in the elastic-plastic zone were insensitive to indenter geometry. The resulting plastic zone was not hemispherical but rather cylindrical, irrespective of indenter geometry.     

Plasticity-Induced Fatigue Damage in Ceria-Stabilized Tetragonal Zirconia Polycrystals

Current studies on the fatigue lifetime of ceramics are mostly focused on the relation between the stress amplitude (or maximum Stress) and cycles to failure. For a more compliant and plastic ceramic which has a pronounced nonlinear stress–strain relation, the role of plastic strain in the fatigue damage is investigated for the first time in this study using a 12 mol% Ce-TZP. By testing at different temperatures, we were able to vary the amount of transformation plasticity with the same microstructure. The Coffin–Manson relationship, which suggests that fatigue lifetime in the low cycle fatigue regime is best correlated with the plastic strain range, was confirmed for the tough ceramic. Fatigue damage is found to be a bulk process which continuously degrades flaw tolerance by microcracking. Evidence for the latter mechanism was also provided by uniaxial cyclic tension–compression stress–strain response and by TEM examination. Despite such damage, the possibility of plasticity-induced surface-crack nucleation in fatiguing ceramics, unlike in metals, appears unimportant.     

Improved Low-Temperature Environmental Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystals by Surface Encapsulation

An encapsulating layer was deposited on the surface of tetragonal zirconia polycrystals doped with 3 mol% of yttria (3Y-TZP), to prevent low-temperature environmental degradation (aging) of the material. The layer, which was composed of silica and zircon, was formed on the surface by exposing the specimens next to a bed of silicon carbide powder in a flowing hydrogen atmosphere that contained ∼0.1% water vapor at 1450°C. The layer was ∼0.5 µm thick and is expected to be under strong residual compressive stress. This encapsulation process remarkably improved the low-temperature degradation of the material. The strength of the specimens also was improved by this process.     

Effect of Cation Doping on the Superplastic Flow in Yttria-Stabilized Tetragonal Zirconia Polycrystals

The superplastic characteristics of various cation-doped yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were examined. For 1 mol% cation doping the true stress of Y-TZP is very dependent on the ionic radii of the doped cations; for instance, smaller cation radii give rise to lower true stress when compared with the other compositions for the same grain size, strain rate, and testing temperature. The altered true stress level must be due to the change in diffusivity of the accommodation process for grain boundary sliding caused by the addition of cations in ZrO₂. The strain to failure of the doped zirconia is affected by both ionic radius and valence of the dopant cations.     

Fatigue of Yttria-Stabilized Zirconia: II, Crack Propagation, Fatigue Striations, and Short-Crack Behavior

Fatigue crack propagation in 3Y-TZP was investigated using controlled surface flaws. A unique growth law strongly dependent on the maximum stress intensity factor and quadratically dependent on the amplitude of the range of stress intensity factor was established. This growth law was found to apply for both surface flaws and internal flaws and could be used to predict fatigue lifetime. The presence of residual stress altered the growth mechanics so that an inverse growth rate dependence on the applied stress, reminiscent of the so-called "short-crack behavior," was manifested. Fatigue striations resulting from alternate overload fracture and fatigue fracture during stress cycling were observed. The appearance of striations varied with the R ratio and was very sensitive to the loading condition and crack geometry.     

Subeutectoid Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystal and Ceria-Doped Yttria-Stabilized Tetragonal Zirconia Polycrystal Ceramics

Yttria-doped tetragonal zirconia containing 2 and 3 mol% Y₂O₃ (Y-TZP), and CeO₂-doped Y-TZP containing 0 to 12 mol% CeO₂ were sintered at 1350°C in a tetragonal single-phase field for 2 h in air, and the degradation behavior at low temperature in air and in hot water was observed. X-ray photoelectron spectroscopy studies on the surface of hydrothermally treated samples show evidence for the formation of a YO(OH) species, along with the simultaneous formation of purely tetragonal zirconia nuclei that retain their coherence in the Y-TZP matrix. Above a critical size, the tetragonal nuclei spontaneously transform to a monoclinic structure, giving rise to macro- and microcracking. The strong tetragonal grains degrade to produce a spalling phenomenon that facilitates further degradation. Y-TZP ceramics alloyed with adequate amounts of CeO₂ show no tetragonal-to-monoclinic transformation after hydrothermal treatment.     

Significance of Internal Stresses for the Martensitic Transformation in Yttria-Stabilized Tetragonal Zirconia Polycrystals During Degradation

Various aspects of the tetragonal ( t ) to monoclinic ( m ) transformation during degradation have been studied experimentally and theoretically in yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), i.e., polycrystalline t -ZrO₂ containing Y₂O₃ in solution. Transmission electron microscopy (TEM) revealed that protruding grains at the surface of Y-TZP specimens do not transform under corrosive conditions (250°C, humid atmosphere) even after an annealing time of 168 h. Eigenstresses due to anistropic thermal expansion In and around protruding and bulk grains have been calculated for Y-TZP containing 2 and 3 mol% Y₂O₃. The prominent role of these stresses on subsequent transformation nucleation during degradation is shown to agree qualitatively with an established free energy concept. The lack of complete transformation of m -ZrO₂ is attributed to characteristics of the nucleation- and growth-controlled transformation process.     

Isothermal Sintering Effects on Phase Separation and Grain Growth in Yttria-Stabilized Tetragonal Zirconia Polycrystal

The isothermal sintering behavior in 3 mol% yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) was investigated to clarify phase-separation and grain-growth mechanisms during sintering. In the Y-TZP sintered at 1300°C for 2 h, the Y³⁺ ion distribution of grain interiors in Y-TZP was nearly homogeneous, but Y³⁺ ions segregated along grain boundaries within a width of about 10 nm. When the holding time increased from 2 to 50 h, the cubic-phase regions with high Y³⁺ ion concentrations were clearly formed in the grain interiors adjacent to the grain boundaries, though the average grain size hardly increased. This result shows that the cubic-phase regions were formed without grain growth, which can be explained by the grain-boundary segregation-induced phase transformation mechanism. In the Y-TZP sintered at 1500°C for 2 h, the cubic-phase regions were already formed, and both of the cubic-phase region and average grain size increased with increasing holding time. This grain-growth behavior can be interpreted by the third-power growth low derived based on the solute drag theory, which indicates that the cubic-phase regions do not effectively act as the pinning points.     

Fatigue Crack Propagation in Transformation-Toughened Zirconia Ceramic

Fatigue crack propagation under tension-tension loading is observed in a transformation-toughened partially stabilized zirconia (PSZ) ceramic containing 9 mol% MgO. Such subcritical crack growth behavior is demonstrated to be cyclically induced, based on a comparison with behavior under sustained loading (at the maximum load in the fatigue cycle) and at varying cyclic frequencies. Crack extension rates, which are measured as a function of the cyclic stress intensity range ΔK over the range 10^-10 to 10^-6 m/cycle, are found to be load ratio dependent and to show evidence of fatigue crack closure, similar to behavior in metals. Cyclic crack growth rates are observed at ΔK levels as low as 3 MPa m^1/2 and are typically many orders of magnitude faster than reported data on environmentally assisted, subcritical crack growth in PSZ under sustained-load conditions.     

Diffusion-Induced Grain-Boundary Migration in Ceria-Stabilized Tetragonal Zirconia Polycrystals

The microstructure and microchemistry of grain-boundary regions in (CeO₂+ La₂O₃)-stabilized tetragonal ZrO₂ polycrystals (Ce(La)-TZP) were studied by means of transmission electron microscopy (TEM). Evidence was found for the existence of crystalline and vitreous intergranular phases situated in small pockets at multiple grain junctions and in thin films along grain boundaries. In this ceramic system grain-boundary migration was observed in situ in the TEM in sample areas subjected to electron irradiation. Interfaces migrated away from their centers of curvature. Evidence was found for Ce de-alloying in the volume swept by the advancing boundaries. It is suggested that the coherency lattice strain brought about by a partial reduction of Ce, resulting in the diffusion of Ce³⁺ along grain boundaries to free surfaces, is the driving force for this phenomenon.     

Correlations between Strength and Acoustic Properties in Yttria-Stabilized Zirconia Polycrystals

The strength, bulk density, and acoustic properties of nine Y₂O₃-stabilized ZrO₂ polycrystals (Y-TZPs) were evaluated in the present study. The samples showed a strong correlation between strength and acoustic properties, a phenomenon that reflects the internal bulk structure, especially the Pore size distribution. Strength increased with a decrease in the attenuation coefficient and with an increase in the transverse wave velocity of the samples. On the other hand, the bulk density of the samples showed no clear relationship with strength. Bulk density reflects only the amount of pore volume in the test piece, whereas the acoustic properties are sensitive to defects or inhomogeneities in the microstructure. The attenuation coefficient and transverse wave velocity thus seem to be good parameters for estimating the strength of Y-TZPs.     

Influence of an Amorphous Second Phase on the Properties of Yttria-Stabilized Tetragonal Zirconia Polycrystals (Y-TZP)

Y-TZP ceramics with various amounts of glassy intergranular phase were fabricated in order to investigate the effect of this amorphous phase on the properties of TZP. The presence of a liquid silicate phase not only enhanced sintering but also controlled the evolution of distinctly different grain morphologies, dependent on the amount of liquid phase present. The influence of the glass phase on the transformability of grains is discussed with respect to the room-temperature fracture toughness and surface degradation at 250°C.     

Solid-Solution Effects of a Small Amount of Nickel Oxide Addition on Phase Stability and Mechanical Properties of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Stability and mechanical properties of the tetragonal phase were investigated for NiO-doped yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) systems. Only 0.3 mol% of NiO in solid solution could be added to the Y-TZP while maintaining the tetragonal phase. Fracture toughness improved remarkably on addition of a small amount of NiO. Raman spectroscopy analysis around cracks introduced by Vickers indentation revealed that the amount of monoclinic phase transformed from tetragonal phase was increased. It was confirmed that fracture toughness improvement was due not only to increased grain size, but also to Y-TZP destabilization by solid solution of NiO.     

Grain-Size Dependence of Sliding Wear in Tetragonal Zirconia Polycrystals

Using a pin-on-plate tribometer with the reciprocating motion of SiC against yttria-doped tetragonal zirconia polycrystal (Y-TZP) plates, the friction and wear of Y-TZP ceramics were investigated as a function of grain size in dry N₂ at room temperature. The results showed that the overall wear resistance increased as the grain size of Y-TZP ceramics decreased. For grain sizes ≤0.7 μm, the wear results revealed a Hall-Petch type of relationship ( d ^−1/2) between wear resistance and grain size. In this case, the main wear mechanisms were plastic deformation and microcracking. For grain sizes ≥0.9 μm, the wear resistance was proportional to the reciprocal of the grain diameter. In this regime, delamination and accompanying grain pullout were the main mechanisms. In this case, the phase transformation to monoclinic zirconia had a negative effect on the wear resistance of TZP ceramics. The coefficient of friction tended to be higher for fine-grained TZP-SiC couples than for coarse-grained TZP-SiC couples, whereas, for a specific regime of grain size, the coefficient of friction was almost independent of the grain size.     

Superplastic Flow of Fine-Grained Yttria-Stabilized Zirconia Polycrystals: Constitutive Equation and Deformation Mechanisms

Available deformation data for superplastic yttria-stabilized zirconia polycrystals with grain size <1 µm have been analyzed at temperatures between 1250° and 1450°C as a function of stress, grain size, and impurity content. The apparent stress exponent n for the higher-purity materials (residual impurity content <0.10 wt%) varies from 2 (region II) to greaterthan equal to3 (region I), and then toward 1 when the stress is decreased. The stress for transition between region II and region I decreases when the temperature and/or grain size is increased. The activation energy Q for flow in region II is 460 kJ/mol, which is approximately that for cation lattice diffusion. The grain-size exponent p decreases continuously and Q increases continuously with decreasing stress in region I. The constitutive equation for superplastic flow in region II is identical to that for metallic systems when lattice diffusion is the rate-controlling mechanism. The experimental results have been correlated with a single deformation process that incorporates a threshold stress, below which grain-boundary sliding does not contribute to strain. The threshold stress may result from yttrium segregation at grain boundaries and its interaction with grain-boundary dislocations. A single deformation regime with n = 2 exists for low-purity materials (impurity content >0.10 wt%) over the entire stress range. The strain-rate enhancement with respect to high-purity materials is related to the grain-boundary amorphous phase present in such materials.