Anisotropic Thermal Expansion of Tetragonal Zirconia Polycrystals

Thermal expansion coefficients (α _a and α _c ) in two crystallographic axes ( a and c ) of the tetragonal phase are measured at 25°–1200°C in ZrO₂–M₂O₃ (M = Sc, In, Yb) and in ZrO₂–YTaO₄. The difference between these two thermal expansion coefficients, α _c –α _a , decreases with M₂O₃ or YTaO₄ composition even though the tetragonality ( c/a ) behaves differently in these two systems. The locus of α _c =α _a represents a maximum tetragonality for the tetragonal phase, but not the phase boundary for the cubic phase. The relationships among thermal expansion, temperature, and composition are discussed.     

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.     

Aging Behavior of Ceria-Stabilized Tetragonal Zirconia Polycrystals

A commercial ceria-stabilized tetragonal zirconia polycrystal is shown not to degrade at low temperatures. Slight strength decrease is found for elevated-temperature treatments, which is not related to formation of the monoclinic phase. A possible mechanism to explain the behavior based upon the ferroelastic nature of the tetragonal phase is proposed.     

Method of Fabricating Ceria-Stabilized Tetragonal Zirconia Polycrystals

Most zirconia-based toughened ceramics need specialized processes to achieve their desired properties. In this paper, we report the fabrication of toughened ceria-stabilized tetragonal zirconia polycrystal by conventional processing, i.e., ball milling and cold-pressing followed by sintering. We believe that ball milling works here because a somewhat coarser particle size is actually beneficial in this case. Although the samples were not fully dense (they need not be), a composition of ZrO₂-12 mol% CeO₂ yielded a fracture toughness value of 14.1 MPa·m^1/2. This is comparable to values reported for materials processed by specialized techniques and can be rationalized in terms of R-curve behavior.     

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.     

Transformations during Grinding of Ceria-Stabilized Tetragonal Zirconia Polycrystals

Ce-TZP is reported to exhibit cyclic transformation during machine grinding. Previous conclusions were based on the results obtained by grinding Ce-TZP at a single speed and depth of cut. In the present work, grinding parameters were carefully chosen to induce varied stresses. Results show that Ce-TZP exhibits cyclic transformation, not always, but only under certain favorable grinding conditions. Results are supported by XRD, optical microscopy, and dynamometric observations. Three stages of transformation, namely, a mechanical-stress-induced t to m , a frictional-heat-driven m to t , and a thermal-quenching-stress-induced t to m , in sequence, are clearly identified.     

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.     

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.     

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.     

Grain-Size Dependence of Thermal-Shock Resistance of Yttria-Doped Tetragonal Zirconia Polycrystals

Thermal-shock fracture behavior of yttria-doped tetragonal zirconia polycrystals (Y-TZP) of various grain sizes was evaluated by the quenching method using water as the quenching solvent. The tetragonal-to-monoclinic phase transformation behavior of Y-TZP around cracks introduced by thermal stress was investigated by using Raman microprobe spectroscopy. The critical quenching temperature difference (Δ T _c ) of Y-TZP ceramics increased from 250° to 425°C with increasing grain size of zirconia from 0.4 to 3.0 μm, while the fracture strength decreased from 900 to 680 MPa. The improvement of Δ T _c of Y-TZP with increasing grain size of zirconia corresponded with the quantity of tetragonal-to-monoclinic phase transformation around cracks introduced by thermal stress.     

Strengthening of Ceria-Doped Tetragonal Zirconia Polycrystals by Reduction-Induced Phase Transformation

Phase-transformation-induced compressive surface stresses were introduced into ceria-doped tetragonal zirconia polycrystals by reduction of CeO₂. Four-point-bending strength of sintered ZrO₂ containing 12 mol% CeO₂ increased from 240 to 545 MPa after it was annealed at 1400°C for 2 h in nitrogen. The strength of the same material hot isostatically pressed in oxygen increased after it was annealed in nitrogen for 2 h at 1500°C from 430 to 595 MPa.     

Macrochanneled Tetragonal Zirconia Polycrystals Coated by a Calcium Phosphate Layer

A coextrusion process was used to fabricate three-directionally macrochanneled tetragonal zirconia polycrystals (TZP) with a calcium phosphate coating layer. The three-directionally connected structure was built by a unique alignment and a lamination of the TZP surrounded by calcium phosphate and carbon black filaments. After a thermal treatment (binder burnout and sintering), a 52 vol% array of 290 μm, three-directionally connected macrochannels, which were clad on the inside with bioactive calcium phosphate, had formed on the sintered TZP body. For a comparison, porous calcium phosphate with a similar structure was also fabricated. The compressive strength (96 MPa) of the three-directionally macrochanneled TZP with a bioactive calcium phosphate layer was much higher than that (24 MPa) of the three-directionally macrochanneled calcium phosphate.     

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.     

Structural Changes by Compressive Stresses of 2.0-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals

Reorientation of the tetragonal (002) peak in tetragonal ZrO₂ polycrystals (TZP) (so-called domain switching) was studied by XRD and residual stress measurement using TZP specimens containing 2.0 mol% Y₂O₃ that had undergone mechanical and thermal treatments and compressive stress. The observed domain switching was due to a preferred transformation of the tetragonal phase caused by compressive stress above 70 MPa leading to a remnant c-axis orientation normal to the compressive direction. Domain switching did not depend on thermal stress but arose directly from the tetragonal phase with little relation to monoclinic phase.     

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.     

Indirect Determination of the Piezospectroscopic Coefficients of Ceria-Stabilized Tetragonal Zirconia Polycrystals

A method is proposed for the indirect determination of the stress dependence (expressed as piezospectroscopic (PS) coefficients) of spectroscopic bands of ceramic materials/phases. This method is based on the intimate mixture (intimate at the microstructural, grain-size level) of two phases/materials when the stress in one is independently known; it is used to determine the PS coefficients of the most intense Raman bands in ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP). Different amounts of Ce-TZP were mixed with alumina and the composite pellets sintered; subsequently, the stress in alumina was determined through the PS coefficient of its R2 luminescence band and the stress in Ce-TZP derived from the static equilibrium condition. The frequency shifts of each Raman band of Ce-TZP have been plotted against the stress and the slopes provide the PS coefficients. The method has the advantage of not requiring any type of loading device (i.e., diamond anvil-cell, bending jig). Finally, the limits are also discussed, the most important one being the requirement of immiscibility of the two materials/phases.     

Experimental Assessment of Plasticity of Nanocrystalline 1.7 mol% Yttria Tetragonal Zirconia Polycrystals

High temperature mechanical behavior of nanocrystalline 1.7 mol% (3 wt%) yttria tetragonal zirconia polycrystals (nc-YTZP) was characterized by compression creep tests. The hot isostatically pressed nc-YTZP with mean grain size of 120 nm was subjected to grain growth to obtain grain sizes in the range of 120–310 nm. Direct measurements of the creep parameters were performed in the temperature range 1150°–1300°C and stress range 5–400 MPa. The strain rates at 1150°C ranged between 2 × 10⁻⁷ and 9 × 10⁻⁵ s⁻¹ when increasing the stress from 15 to 400 MPa. Values of the stress exponent, n =2.0±0.3, and the activation energy, Q =630±40 kJ/mol, were obtained for all test conditions. A value of the grain size exponent, p =1.5±0.3, was obtained at 1150°C in the stress range studied. Detailed microstructural observations revealed the absence of glassy phase at the grain boundaries. The creep parameters were compared with those from the literature, and the results were discussed in terms of the model recently developed by the authors, with a reasonable agreement.     

A New Technique for Monitoring Aging in Yttria–Tetragonal Zirconia Polycrystals

Conventional techniques used to monitor aging-induced phase relationships in zirconia ceramics are shown to be limited as to the amount of information that can be obtained without sectioning the sample. Measurement of the microwave dielectric loss tangent of progressively aged Y-TZP samples is shown to be a suitable technique to monitor the growth of the transformed monoclinic layer with time.     

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.