Unstabilized ZrO2 Research Articles

Structural and Mechanical Properties of Alumina-Zirconia (ZTA) Composites with Unstabilized Zirconia Modulation

Zirconia toughened alumina (ZTA) ceramics are very promising materials for structural and biomedical applications due to their high hardness, fracture toughness, strength, corrosion and abrasion resistance and excellent biocompatibility. The effect of unstabilized ZrO2 on the density, fracture toughness, microhardness, flexural strength and microstructure of some Zirconia-toughened alumina (ZTA) samples was investigated in this work. The volume percentage of unstabilized ZrO2 was varied from 0% - 20% whereas sintering time and sintering temperature were kept constant at 2 hours and 1580°C. The samples were fabricated from nanometer-sized (α-Al2O3: 150 nm, monoclinic ZrO2: 30 - 60 nm) powder raw materials by the conventional mechanical mixing process. Using a small amount of sintering aid (0.2 wt% MgO) almost 99.2% of theoretical density, 8.54 MPam? fracture toughness, 17.35 GPa Vickers microhardness and 495.67 MPa flexural strength were found. It was observed that the maximum flexural strength and fracture toughness was obtained for 10 vol% monoclinic ZrO2 but maximum Vickers microhardness was achieved for 5 vol% ZrO2 although the maximum density was found for 20 vol% ZrO2. It is assumed that this was happened due to addition of denser component, phase transformation of monoclinic ZrO2 and the changes of grain size of α-Al2O3 and ZrO2.

Effect of ZrO2 types on ZrSiO4 formation

The effect of various types of ZrO2 on ZrSiO4 formation was evaluated by the calcination of three types of nanosized ZrO2 (unstabilized ZrO2, 3Y–ZrO2, and 8Y–ZrO2) and amorphous SiO2 powders. Reaction was conducted in vacuum and air atmosphere. Results indicated that under a vacuum atmosphere, the yield of ZrSiO4 during synthetic reaction markedly varied when different types of ZrO2 were used. When 3Y–ZrO2 was used, the reaction reached equilibrium after only 6 h. This occurrence was facilitated by the stabilized ZrO2 tetragonal structure. ZrSiO4 synthetic mechanism by which SiO2 diffuses into a t-ZrO2 crystal structure and then ZrSiO4 nucleates and grows along on t-ZrO2 (101) planes was confirmed. That is, t-ZrO2 acts as a host for ZrSiO4 formation. However, the investigation under air atmosphere demonstrated that ZrSiO4 could be synthesized using unstabilized ZrO2 as previously reported; meanwhile, ZrSiO4 formation was nearly suppressed in samples containing Y2O3-stabilized ZrO2. This suppression was attributable to the oxygen absorption of Y2O3-stabilized ZrO2, affecting its internal micro-chemical environment and impeding ZrSiO4 nucleation.

Three different alumina–zirconia composites: Sintering, microstructure and mechanical properties

Two commercial 3mol% yttria–partially stabilized zirconia powders, with 0.3wt% Al2O3 (Y–PSZA) and without Al2O3 (Y–PSZ), and a Zr (IV) precursor were used to produce alumina (Al2O3)–zirconia (ZrO2) slip cast composites. The influence of both the zirconia content and the reduction of zirconia particle size on the sintering behavior, microstructure development and mechanical properties were investigated. The increase in the zirconia content from 10.5 to 22vol% increased the hardness; whereas, above 22vol% ZrO2 the hardness decreased. A significant increase in the fracture toughness with increasing the ZrO2 content over 22vol% was obtained by the stress-induced phase transformation. The flaw size limited the strength below 22vol%; whereas, above 22vol% ZrO2 the strength was controlled by the stress-activated phase transformation. For 10.5vol% ZrO2, the smaller ZrO2 grains produced by using the Zr (IV) precursor were more effective in preventing the Al2O3 grain growth resulting in higher hardness. However, the tetragonal–monoclinic (t–m) transformation of some unstabilized ZrO2 grains during cooling reduced Young's modulus and fracture toughness.

Orientation Relationship between the TRIP Steel Substrate and the ZrO2 thin Film

Distinct orientation relationships between face-centred cubic austenite (γ-Fe) and cubic c-ZrO2 were found and assigned using high-resolution transmission electron microscopy (HRTEM) that was complemented by the Fast Fourier Transform (FFT) of the HRTEM micrographs. These orientation relationships were described as [110] γ-Fe || [111] c-ZrO2 and () γ-Fe || () c-ZrO2. They imply the heteroepitaxial growth of the counterparts that improved substantially the adhesion between γ-Fe and c-ZrO2. The analysed specimens were prepared by magnetron sputtering from an unstabilized ZrO2 ceramic target in argon atmosphere on TRIP steel substrate. The microstructure of the thin zirconia films was analysed using glancing-angle x-ray diffraction (GAXRD), scanning electron microscope (SEM) and TEM.

Formation of Unstabilized and Yttria Stabilized ZrO2 Fibers from a Suspension of Monodispersed ZrO2

Unstabilized and yttria stabilized ZrO2 fibers were prepared from a monodispersed ZrO2 particles derived from hydrolysis of Zr(OBu)4. Unstabilized ZrO2 fibers with 71 to 151 μm of width and yttria stabilized ZrO2 fibers with 96 to 214 μm of width, were formed by drying the ethanol suspension of monodispersed ZrO2 particles with 116 nm in mean diameter. The morphology of the ZrO2 fibers were plate like and consisted of densely packed ZrO2 nano particles. The preparation conditions affected the fiber width. This was due to the lower concentration of ZrO2 particles in suspension, or the higher drying temperatures, which gave the narrower fiber width. The mean particle size of the 5 mol% Y2O3 doped ZrO2 fiber heat treatment at 1473 K was 0.14 μm and the fiber showed 11.0 GPa on the Vickers Hardness Test.

Ultrasonic through-transmission method of evaluating the modulus of elasticity of Al2O3–ZrO2 composite

The elastic properties of Al2O3–ZrO2 composite were determined from ultrasonic velocity measurements, and were found to be dependent upon the amount of ZrO2 phase, the compacting pressure of the green ceramic and sintering time. The velocity in the Al2O3–ZrO2 composite increased to a maximum for about 3 wt% unstabilized ZrO2 dispersed in Al2O3. The velocity decreased monotonically thereafter. The increase in moduli, as shown by an increase in velocity, has been attributed to phase transformation of the unstabilized ZrO2 from tetragonal to a monoclinic phase, which presumably leads to a toughening and strengthening effect, and also due to the action of ZrO2 in stopping grain growth of Al2O3 during densification. The excessive shear strain, induced by the tetragonal→monoclinic transformation phase, with greater than 50 wt% ZrO2 content, caused microcracks to appear in the composite. This reduced the elastic moduli of the composite. It was found that the composition dependence of the elastic moduli lie outside the theoretical bound of Voigt and Reuss for the elastic moduli of two-phase materials, and that by increasing the compacting pressure, an improvement in the elastic moduli of the sintered composite occurred irrespective of ZrO2 content. The thermal expansion of the composites showed no appreciable change with addition of zirconia up to 5 wt% ZrO2. However, dimensional changes due to phase transformation particularly with high zirconia content have been established.

Influence of sintering conditions on mechanical and thermal properties of cordierite-ZrO2 composites

Abstract Bend strength, fracture toughness and thermal diffusivity of cordierite composites with 30 vol.% of either unstabilized or Y2O3-stabilized ZrO2 sintered at 1400°C have been determined as a function of sintering time. Depending on sintering time, the phase composition varies from cordierite + monoclinic ZrO2 to spinel + zircon. Cordierite with tetragonal ZrO2 sintered for 1 h exhibits a fracture toughness of ∼4·1 M Pa m , while composites consisting essentially of spinel and zircon with both inter- and intragranular monoclinic ZrO2 particles show the highest fracture toughness ( ∼4·7 M Pa m ). These composites are obtained when cordierite is sintered with unstabilized ZrO2 at 1400°C for >4h. They also exhibit the highest thermal diffusivity (48 × 10−3 cm2/s).

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si3N4-ZrO2 composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO2 and ZrO2 stabilized with 3 mol% Y2O3. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al2O3 and 0–6 wt% Y2O3. Composites prepared at 1550 °C contained substantial amounts of unreacted α-Si3N4, and full density was achieved only when ⩾ 1 wt% Al2O3 or ⩾ 4 wt % Y2O3 had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO2 starting powder was stabilized by Y2O3, fully dense Si3N4-ZrO2 composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm−2 and 4.5 MPam1/2, respectively. Examples of such materials were either Si3N4 formed with an addition of 2–6 wt% Y2O3 or Si3N4-ZrO2 composites with a simultaneous addition of 2–6 wt%Y2O3 and 2–4 wt% Al2O3.

Measurement of Residual Stresses in Oxide-ZrO2 Three-Layer Composites

Al2O3–ZrO2 and MgO–ZrO2 layered composites were fabricated in such a way that the outer layers of bar-shaped specimens consisted of the oxide (Al2O3 or MgO) and unstabilized ZrO2, while the bulk consisted of the oxide and partially stabilized ZrO2. During cool-down from the sintering or the annealing temperature, the outer layers expanded because of the tetragonal → monoclinic transition in ZrO2, thereby creating compressive stresses in the outer layers and tensile stresses in the bulk. Residual stresses were determined using a strain gage technique in which a strain gage was mounted on one face while the opposing face was incrementally ground off. Measurement of the strain as a function of thickness permitted the evaluation of residual stresses using pertinent equations from simple beam theory.

ChemInform Abstract: Strength Improvement in Transformation‐Toughened Alumina by Selective Phase Transformation.

AbstractAl2O3‐15 vol.% ZrO2 bar‐shaped ceramic specimens are prepared in the green state in such a manner that the outer regions consist of Al2O3 and unstabilized ZrO2, whereas the inner region contains ZrO2 partially stabilized with Y2O3.

Hot Isostatic Pressing of Al2O3-ZrO2Composites

This paper reports the preliminary results of fabricating Al2O3-ZrO2composites by Hot Isostatic Pressing (HIPing). The idea was to produce toughened (by addition of ZrO2) and completely dense Al2O3—ZrO2 composites. 15 mol % of ultrafine unstabilized ZrO2 was milled with Al2O3. The as-mixed powder was cold isostatically pressed (CIPed), sintered at 1400°C for 1 hr and HIPed at 1450°C for 0.5 hr. The samples reached their theoretical density values. The results were compared to HIP diagrams described in the literature. It could be seen that in spite of some simplifying assumptions, theoretical predictions matched with the experimental results. Boundary diffusion was interpreted to be the dominant mechanism.

Strength Improvement in Transformation‐Toughened Alumina by Selective Phase Transformation

A12O3‐15 vol% ZrO2 bar‐shaped ceramic specimens were fabricated in the green state in such a way that the near surface regions consisted of A12O3 and unstabilized ZrO2 while the bulk consisted of A12O3 and partially stabilized ZrO2. After sintering, specimens had macroscopic residual compressive surface stresses and balancing interior tensile stresses due to the tetragonal‐to‐monoclinic phase transformation in the outer layers which occurs during cooling. The depth of the surface region was controlled during green forming. Residual surface compressive stresses at room temperature varied between 100 and 400 MPa depending on the outer‐layer thickness. The increased strengths of the three‐layer specimens were obtained in the as‐fired unground condition, demonstrating that the stresses introduced are the result of transformation of tetragonal zirconia into monoclinic polymorph which occurred upon cooling from the sintering temperature. Specimens with residual surface compressive stresses were 200 MPa stronger at 750°C than monolithic specimens, demonstrating the viability of this approach for improving elevated‐temperature mechanical properties.

Effect of ZrO2 inclusions on fracture properties of MgCr204

The effects of unstabilized ZrO2 inclusions on the strength, fracture surface energy and thermal-shock resistance of MgCr2O4 have been evaluated. The fracture surface energy for MgCr2O4-ZrO2 composites was observed to depend on the agglomerate particle size distribution, and volume fraction of the ZrO2 inclusions. Large, nonuniformly distributed ZrO2 inclusions tended to produce a relatively small increase in the fracture surface energy of MgCr2O4. The fracture surface energy increased with increasing ZrO2 content to a maximum value of 24.5J m−2 at 16.5 vol % ZrO2, and decreased as the ZrO2 content increased further. It is proposed that this four-fold increase in fracture surface energy results from the absorption of energy due to microcrack formation in the MgCr2O4 matrix, which results primarily from the tensile stresses due to the tetragonal → monoclinic phase transformation of ZrO2 and the associated volume expansion. The improvement in mechanical properties, specifically the four-fold increase in fracture surface energy, resulted in a substantial increase in thermal-shock resistance of MgCr2O4-ZrO2 composites as indicated by the results of thermal-shock experiments.

Strengthening of Oxide Ceramics by Transformation‐Induced Stress

Alumina‐zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar‐shaped specimens consisted of three layers. The outer layers contained A12O3 and unstabilized ZrO2 while the central layer contained A12O3 and partially stabilized ZrO2 (with 2 mol% Y2O3). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al2O3‐ZrO2 specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength‐limiting flaws were voids located in the central layer near the interface separating the central and the outer layers.

Subsolidus, high temperature phase relations in the systems Al2O3-Cr2O3-ZrO2, MgO-Cr2O3-ZrO2 and MgO-Al2O3-ZrO2

In the temperature range 1600 to 1900° C, the system A2O3-Cr2O3-ZrO2 is characterized by the coexistence of ZrO2 (unstablilized) and an (Al, Cr)2O3 solid solution series. In the systems MgO-Cr2O3-ZrO2 and MgO-Al2O3-ZrO2 a nearly stoichiometric spinel coexists with both stabilized and unstabilized ZrO2. At temperatures above 1600°C a new ternary Mg-Al-Zr oxide becomes stable in the MgO-rich part of the MgO-Al2O3-ZrO2 system.

Subsolidus, high temperature phase relations in the systems Al2O3-Cr2O3-ZrO2, MgO-Cr2O3-ZrO2 and MgO-Al2O3-ZrO2

In the temperature range 1600 to 1900° C, the system A2O3-Cr2O3-ZrO2 is characterized by the coexistence of ZrO2 (unstablilized) and an (Al, Cr)2O3 solid solution series. In the systems MgO-Cr2O3-ZrO2 and MgO-Al2O3-ZrO2 a nearly stoichiometric spinel coexists with both stabilized and unstabilized ZrO2. At temperatures above 1600°C a new ternary Mg-Al-Zr oxide becomes stable in the MgO-rich part of the MgO-Al2O3-ZrO2 system.

Strength Improvement in Transformation Toughened Ceramics using Compressive Residual Surface Stresses

ABSTRACTA1 2 03–15 vol. % ZrO2 bar shaped composite specimens were fabricated by pressing three layers. The two outer layers consisted of Al2O3 and unstabilized ZrO2 (primarily in the monoclinic polymorph), and the inner layer consisted of Al2O3 and partially stabilized zirconia in the tetragonal polymorph. The transformation of ZrO2 from tetragonal to monoclinic, upon cooling from sintering temperature, led to the establishment of residual compressive stresses in the outer layers. Flexural tests at room temperature showed that residual stresses contributed to strength increasing from 450 to 825 MPa. The existence of these stresses was verified by measuring apparent fracture toughness, as well as using strain gages. Strength and toughness data were obtained at 500, 750, and 1000°C. X-ray diffraction was used to explain the elevated temperature data by monitoring the monoclinic to tetragonal transformation upon heating to 1000°C.

Reaction of zirconia with metals

Investigation was carried out of the resistance of stabilized ZrO2 to the action of molten Fe, Cr, Ni, Al, and steel grades St. 3 and 1Kh18N9T in vacuo and in a reducing medium. The degree of impregnation with the melts is lower for products fabricated from unstabilized ZrO2 than for the other investigated refractories. Among the solid solutions of ZrO2 with a stabilizer the metal resistance was highest for the products molded from the cubic solid solution ZrO3-Nd2O3. The nature of the reaction of Cr and steel grade 1Kh18N9T with ZrO2 stabilized with CeO2 and Nd2O3 consists of partial destabilization of the ZrO2 in the contact zone as a result of the formation of chromites of the rare-earth elements. No crystalline regeneration occurred in the reactions with the other molten metals and steel grade St. 3. The product of the reaction of these agents with ZrO2 stabilized with CaO, Nd2O3, or CeO2 was a vitreous phase rich in the oxide of the metal concerned.