ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

Toughening Behavior Involving Multiple Mechanisms: Whisker Reinforcement and Zirconia Toughening

Since the contribution of transformation toughening increases with the loca crack resistance (which is proportional to the toughness of the matrix), ZrO₂ particles were added to a toughened, whisker-reinforced ceramic matrix. Analysis revealed that the combination of these multiple toughening agents should result in ceramic composites tougher than (1) that achieved by either mechanism by itself or (2) the sum of the two processes. The toughness of mullite could be increased 1.8-and 2.4-fold with a 20 vol% addition of ZrO₂ particles or SiC whiskers, respectively. However, when 20 vol% of both ZrO₂ particles and SiC whiskers were added, the toughness was increased at least 3-fold with monoclinic m -ZrO₂ and by >5-fold with tetragonal t -ZrO₂. The differences in the toughening achieved when t -ZrO₂ vs m -ZrO₂ is present in the SiC-whisker-reinforced mullite are attributed to differences in their interdependencies upon the whisker reinforcement.     

Slow Crack Growth Behavior in Transformation-Toughened Al2O3-ZrO2(Y2O3) Ceramics

Significant increases in the critical fracture toughness (K _IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO₂ particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO₂ particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO₂ phase transformation by tailoring the Y₂O₃ stabilizer content not only increases the critical fracture toughness (K_IC) but also the K _Ia to initiate slow crack growth. For example, crack velocities ( v )≥10^–9 m/s are obtained only at K _Ia≥5 MPa.m^1/2 in transformation-toughened ( K _IC=8.5 MPa.m^1/2) composites vs K _Ia≥2.7 MPa.m^1/2 for comparable velocities in composites where the transformation does not occur ( K _IC=4.5 MPa.m^1/2). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO₂ particles which reside along the intergranular crack path.     

Fracture Toughness of Al2O3 with an Unstabilized ZrO2Dispersed Phase

The fracture toughness of Al₂O₃ is considerably increased by the incorporation of fine monoclinic ZrO₂ particles. Hot-pressed composites containing 15 vol % ZrO₂ yield K_lcvalues of ∼ 10 MN/m^3/2, twice that of the A1₂O₃ matrix. It is hypothesized that this increase results from a high density of small matrix microcracks absorbing energy by slow propagation. The microcracks are formed by the expansion of ZrO₂during the tetragonal → monoclinic transformation. Since extremely high tensile stresses develop in the matrix, very small ZrO2 particles can act as crack formers, thus limiting the critical flaw size to small values.     

Slow Growth of Microcracks: Evidence for One Type of ZrO2 Toughening

Alumina containing 15 vol% monoclinic ZrO₂ dispersed at the grain boundaries exhibited very high room-temperature fracture toughness (∼11 MPa·m^1/2) on cooling from 1275°C when microcrack precursors nucleated at Ts. With increasing time (up to ∼12 h) at room temperature, K_Ic and Young's modulus decreased when dilational and thermal-expansion strains subcritically propagated inter granular microcracks. Thus, transformation toughening of ceramics with inter crystalline ZrO₂ dispersions is to a great extent caused by microcrack nucleation and extension.     

Experimental Evaluation of Toughening Mechanisms in Alumina-Zirconia Composites

The contributions of transformation toughening and microcrack toughening in an 85Al₂O₃15ZrO₂ (vol%) composite were quantitatively evaluated. Three types of hot-pressed samples with similar size (∼0.5 µm) and size distribution of ZrO₂ grains, but with different contents (vol%) of monoclinic- ( m -) ZrO₂ (0 (AZS), 45 (AZT), and 67 (AZM)) were prepared using Y₂O₃ and MoO₂ dopants. Therefore, the possible effect of ZrO₂ grain size on each toughening mechanism was eliminated. When the measured fracture toughnesses of m -ZrO₂ volume fractions before and after fracture were compared, the transformation-toughening constant was estimated as 3.0 MPam^1/2 and the microcrack-toughening constant 0.2 MPam^1/2 for a 100% monoclinic transformation of ZrO₂ grains. This result indicates that transformation toughening was the dominant toughening mechanism in the studied composite.     

Indentation Studies on Y2O3-Stabilized ZrO2: II, Toughness Determination from Stable Growth of Indentation-Induced Cracks

Stable indentation cracks were grown in four-point bend tests to study the fracture toughness of two Y₂O₃-stabilized ZrO₂ ceramics containing 3 and 4 mol% Y₂O₃. By combining microscopic in situ stable crack growth observations at discrete stresses with crack profile measurements, the dependence of toughness on crack extension was determined from crack extension plots, which graphically separate the crack driving residual stress intensity and applied stress intensity factors. Both materials exhibit steeply rising R -curves, with a plateau toughness of 4.5 and 3.1 Mpa·m^1/2 for the 3- and 4-mol% materials, respectively. The magnitude of the plateau toughness reflects the fraction of tetragonal grains contributing to transformation toughening.     

Zirconia Particle Coarsening and the Effects of Zirconia Additions on the Mechanical Properties of Certain Commercial Aluminas

The effects of ZrO₂ additions on the mechanical properties of four commercial aluminas have been studied, and the changes in intergranular and intragranular ZrO₂ particle sizes during aging have been determined for one of the aluminas. In this ceramic, the intercranular and intragranular ZrO₂ particles coarsen obeying t ^1/2 and t ^1/3 kinetics, respectively. Changes in strength of the aluminas with additions of ZrO₂ are shown to be related to changes in the Al₂O₃ matrix microstructure. Differences in toughness of the resulting composites are explained in terms of the different size and morphology of the intergranular ZrO₂ particles and the operation of several toughening mechanisms, with transformation toughening a relatively minor contributor.     

Microhardness and Fracture Toughness Anisotropy in Cubic Zirconium Oxide Single Crystals

Vickers and Knoop indentation tests have been used to study the fracture and deformation characteristics of 9.4-mol%-Y₂O₃-stabilized ZrO₂ single crystals. K_c is anisotropic, with values of 1.9 and 1.1 MPa·m^1/2 for radial cracks propagating along (100) and (110), respectively. The toughness for these two orientations was also determined using the single-edge notched-beam geometry, and yielded values of 1.9 and 1.5 MPa·m^1/2.     

Stress-Induced Transformation During Subcritical Crack Growth in Partially Stabilized Zirconia

Fracture surfaces of a commercial partially stabilized ZrO₂ that had undergone subcritical crack growth in H₂0 were characterized by scanning and transmission electron microscopy. At high stress intensities (≥4.6 MPa·m^−1/2), fracture was primarily trans granular, and coherent tetragonal ZrO₂ precipitates had undergone a martensitic transformation to monoclinic symmetry. At lower stress intensities, where power-law crack growth occurred, fracture suflaces were primarily inter-granular, and the tetragonal ZrO₂ had transformed to a new ZrO₂ polymorph with orthorhombic symmetry.     

Mechanical Properties of Hot-Pressed TiB2-ZrO2 Composites

The use of monoclinic ZrO₂ as an additive improves the mechanical properties of TiB₂-based composites without the use of stabilizers. In particular, TiB₂-30% ZrO₂ compacts exhibited a transverse rupture strength of 800 MN/m², few pores, and a K_{I c} of 5 MPa·m^1/2. The high strength and toughness are thought to result mainly from the presence of partially stabilized tetragonal ZrO₂ and from solid solution of (TiZr)B₂ formed in sintering.     

Transformation and Microcrack Toughening as Complementary Processes in ZrO2-Toughened Al2O3

Both tetragonal ( t ) and monoclinic ( m ) ZrO₂ particles in ZrO₂-toughened Al₂O₃ can give rise to toughening. In the stress field of propagating cracks, the t -ZrO₂ particles can undergo the stress-induced t → m transformation, and the residual stresses around already-transformed m -ZrO₂ particles can cause microcracking. The t -ZrO₂ particles transformed in crack tip stress fields do not, however, also cause appreciable microcracking. The toughening increments via these distinct mechanisms are comparable. It appears that optimally fabricated Zr0₂-toughened Al₂O₃'s should contain a mixture of t - and m -ZrO₂.     

Transformation Toughening in Large-Grain-Size CeO2-Doped ZrO2 Polycrystals

The fracture and transformation behavior of tetragonal polycrystalline ZrO₂ alloys containing 18 mol% CeO₂ (Ce-TZP) was investigated. In the absence of applied stress the tetragonal phase was found to be stable in large-grained (>30 μm) samples at room temperature. The monoclinic phase was detected in regions of high residual stress near hardness indentations although no evidence of a wake of monoclinic phase along the fracture surface was observed. The fracture toughness increased from 4 to 7 MPa · m^1/2 as density and/or grain size increased. It is proposed that the relatively high toughness of these materials is due to the occurrence of stress-driven tetragonal-to-monoclinic transformation near the crack tip, which reverses when the crack has passed.     

Temperature Dependence of Strength and Fracture Toughness of ZrO2 Single Crystals

Results of strength and fracture–toughness tests on single crystals of partially stabilized ZrO₂ at temperatures above and below the tetragonal–monoclinic transformation temperature are reported. The temperature dependence of toughening mechanisms is discussed with respect to theories for phase–transformation toughening and other toughening mechanisms.     

Preparation, Hardness, and Fracture Toughness of Tantalum Oxynitride Ceramics

Tantalum oxynitride powder with a baddeleyite crystal structure was synthesized and densified by hot pressing in Ar and under high pressure using a belt-type high-pressure apparatus. The tantalum oxynitride powder could not be densified completely under hot-pressing conditions at 1400°C. The use of high pressure resulted in dense materials. The samples showed a hardness of 16–17 GPa and a fracture toughness of 3–4 MPa·m^1/2. The hardness is higher compared with that of ZrO₂ and HfO₂ ceramics. The fracture toughness corresponds to the value of fully stabilized ZrO₂ due to the absence of any transformation toughening mechanism.     

Microstructure and Mechanical Properties of ZrB2-Based Composites Reinforced and Toughened by Zirconia

The present work reported the effect of addition of ZrO₂ on the microstructure and mechanical properties of ZrB₂-based ceramic composites by means of hot-pressed sintering. Observation of microstructure and systematic testing results of mechanical properties were carried out. Through X-ray diffraction analysis and calculation of the volume fraction of ZrO₂ phase transformability, the toughening mechanism of the present composites was explored. The phase transformation toughening by ZrO₂ additive played an important role in improving the fracture toughness of ZrB₂-based composites.     

Effect of Nucleation on Transformation Toughening

A simple model is presented to account for the effect of martensitic nucleation in ZrO₂ particles on theoretical predictions of toughness enhancement during transformation toughening. The model allows for shear and volumetric strain contributions during nucleation, whereas only volumetric strain contributes to toughness enhancement. It is predicted that toughness enhancement results for both stationary and growing cracks. The predicted toughness enhancement exceeds the theoretical prediction for the case when only volumetric strain is allowed to operate. The important material property is identified to be the ratio of volumetric to shear strain during nucleation of the martensitic product in the transformation from tetragonal to monoclinic symmetry of the ZrO₂ particles.     

Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

Mechanical Properties and Microstructure of Ca2SiO4–CaZrO3 Composites

Three types of dicalcium silicate (Ca₂SiO₄–calcium zirconate (CaZrO₃) composites were fabricated and their microstructures correlated with their mechanical properties. In the first type, Ca₂SiO₄ was added as a minor phase. The second type consisted of a 50 vol% Ca₂SiO₄-50 vol% CaZrO₃ mixture, while in the third type, CaZrO₃ constituted the minor phase. Pure CaZrO₃ was also studied as a control and found to have a toughness which depended on its grain size. In composites with Ca₂SiO₄ as the minor phase, a toughness increase was observed and found to be a function of matrix grain size. The composite with the second type of microstructure had the highest toughness of about 4.0 Mpa. m^1/2, which was about double that of the monolithic CaZrO₃. No evidence was found for transformation toughening by the orthorhombic (β) to monoclinic (γ) transformation in Ca₂SiO₄. The main toughening mechanisms identified were crack deflection and crack branching. Microstructural observations indicated the existence of weak grain boundaries in CaZrO₃ agglomerates as well as weak interfaces between the two phases.     

Infrared Absorption Spectroscopy in Zirconia Research

Infrared absorption spectra are shown for monoclinic ZrO₂ and for cubic stabilized ZrO₂. Nine bands are reported in monoclinic ZrO₂ in the region 800 to 200 cm^−l, whereas only one broad band is observed in cubic ZrO₂ over the same frequency range.