The recognition of the potential for enhanced fracture toughness that can be derived from controlled, stress-activated tetragonal (t) to monoclinic (m) transformation in ZrO2-based ceramics ushered in a new era in the development of the mechanical properties of engineering ceramics and provided a major impetus for broader-ranging research into the toughening mechanisms available to enhance the fracture properties of brittle-matrix materials. ZrO2-based systems have remained a major focal point for research as developments in understanding of the crystallography of the t→m transformation have led to more-complete descriptions of the origins of transformation toughening and definition of the features required of a transformation-toughening system. In parallel, there have been significant advances in the design and control of microstructure required to optimize mechanical properties in materials developed commercially. This review concentrates on the science of the t→m transformation in ZrO2 and its application in the modeling of transformation-toughening behavior, while also summarizing the microstructural control needed to use the benefits in ZrO2-toughened ceramics.

Transformation Toughening in Zirconia‐Containing Ceramics

Nacre (mother-of-pearl) from mollusc shells is a biologically formed lamellar ceramic. The inelastic deformation of this material has been experimentally examined, with a focus on understanding the underlying mechanisms. Slip along the lamellae tablet interface has been ascertained by testing in compression with the boundaries oriented at 45° to the loading axis. The steady-state shear resistance τss has been determined and inelastic strain shown to be as high as 8%. The inelastic deformation was realized by massive interlamellae shearing. Testing in tension parallel to the tablets indicates inelastic strain of about 1%, occurring at a steady-state stress, σsss ≈ 110 MPa. The strain was associated with the formation of multiple dilatation bands at the intertablet boundaries accompanied by interlamellae sliding. Nano-asperities on the aragonite tablets and their interposing topology provide the resistance to interfacial sliding and establish the level of the stress needed to attain the inelastic strain. Detailed mechanisms and their significance for the design of robust ceramics are discussed.

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Deformation mechanisms in nacre

The martensitic transformation in ceramics — its role in transformation toughening

Laminar composites, containing layers of Ce-ZrO2 and either Al2O3 or a mixture of Al2O3 and Ce-ZrO2, have been fabricated using a colloidal method that allowed formation of layers with thicknesses as small as 10 μm. Strong interactions between these layers and the martensitic transformation zones surrounding cracks and indentations have been observed. In both cases, the transformation zones spread along the region adjacent to the layer, resulting in an increased fracture toughness. The enhanced fracture toughness was observed for cracks growing parallel to the layers as well as for those that were oriented normal to the layers.

Enhanced Fracture Toughness in Layered Microcomposites of Ce‐ZrO2 and Al2O3

Toughening mechanisms in ceramic materials which depend on zone and contact screening are presented and their influence on the macroscopic toughness behaviour is shown with examples.

Toughening mechanisms for ceramic materials

Transformation zone shape, size, and crack-growth-resistance (R-curve) behavior were studied in precracked and annealed single-edge notch bend specimens of commercial-grade ceriapartially-stabilized zirconia polycrystals as a function of applied load. Well-defined transformation zones with a characteristic elongated shape in the plane of the crack were observed. It is shown that the observed zone shape is significantly different from the shape predicted by a combined shear/dilatation yield criterion and the stress field of the crack prior to the transformation. The length of the transformation zone directly ahead of the crack tip is in better agreement with the prediction of the Dugdale plastic strip zone model. The fracture toughness increment showed the characteristic square root dependence on the transformation zone width, but the magnitude of the toughness increment was not consistent with the predictions of the theoretical models of transformation toughening.

Transformation Zone Shape, Size, and Crack-Growth-Resistance [R-Curve] Behavior of Ceria-Partially-Stabilized Zirconia Polycrystals

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO{sub 2}-12 mol% CeO{sub 2}-10 wt% Al{sub 2}O{sub 3} ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs. The fatigue crack growth behavior was strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates.

Fatigue Crack Propagation in Ceria‐Partially‐Stabilized Zirconia (Ce‐TZP)‐Alumina Composites

In this paper crack tip shielding is evaluated for observed transformation zones in Ce-TZP/Al[sub 2]O[sub 3] composites, in which the transformation zone sizes were changed significantly by varying the sintering temperature to control the transformation yield stress. The calculated shielding effects are consistent with an observed insensitivity of crack resistance curves to transformation zone size; smaller zones in materials with higher yield stress were associated with larger ratios of wake length to zone width and correspondingly higher normalized shielding stress intensity factors. Shielding due to the dilatational component of the transformation strain accounted for most of the toughening observed in these materials.

Transformation Zone Shape Effects on Crack Shielding in Ceria‐Partially‐Stabilized Zirconia (Ce‐TZP)–Alumina Composites

Stress-strain behaviors in three-point bending, transformation zones in single-edge-notch-bend specimens, and transformation toughening were studied in two types of Ce-TZP/Al{sub 2}O{sub 3} composites. A commercial grade exhibited yield-point behavior triggered by autocatalytic transformation and elongated zones. A new grade of Ce-TZP/Al{sub 2}O{sub 3} composite showed monotonic stress-strain behavior and a zone shape close to theoretical prediction based on a shear-dilation yield criterion. The effects of zone sizes and shapes on fracture toughness of the two ceramic composites are shown to be qualitatively consistent with the predictions of transformation-zone shielding theories.

Cheng Sheng Yu

Papers

Transformation Zone Shape, Size, and Crack-Growth-Resistance [R-Curve] Behavior of Ceria-Partially-Stabilized Zirconia Polycrystals

Fatigue Crack Propagation in Ceria‐Partially‐Stabilized Zirconia (Ce‐TZP)‐Alumina Composites

Transformation Zone Shape Effects on Crack Shielding in Ceria‐Partially‐Stabilized Zirconia (Ce‐TZP)–Alumina Composites

Role of Autocatalytic Transformation in Zone Shape and Toughening of Ceria–Tetragonal-Zirconia–Alumina (Ce-TZP/Al2O3) Composites