The mechanism in partially stabilized zirconia by which stress at a crack tip transforms tetragonal to monoclinic, and the accompanying volume expansion closes the crack and raises toughness.
Japanese name: 変態強化
Transformation toughening appears in 2 practice questions across 2 topics, including Ceramics and glasses, Biomaterials.
Field: Ceramics and glasses
Zirconia (ZrO₂) is cubic, tetragonal and monoclinic in order from high temperature, and the tetragonal-to-monoclinic transformation expands its volume by about 4%. In partially stabilized zirconia, metastable tetragonal grains are retained at room temperature by the constraint of their surroundings. As a crack advances, the tensile stress at its tip transforms grains in a band around the crack (the transformation zone) to monoclinic, and their expansion puts the crack faces under compression and resists opening. Because the zone widens as the crack grows, the material shows R-curve behavior, with resistance increasing as the crack extends.
To retain the tetragonal phase, stabilizers such as Y₂O₃, MgO or CeO₂ are added. Fine-grained tetragonal polycrystals with about 3 mol% Y₂O₃ (3Y-TZP) have a good balance of strength and toughness and are the most widely used. Mg-PSZ, with MgO added, has tetragonal precipitates dispersed in a cubic matrix. If the grains are too large they transform spontaneously to monoclinic during cooling, and if too small they do not transform even under stress, so in Y-TZP the grain size is kept within a suitable submicrometer range.
Transformation toughening exploits the energy difference between the tetragonal and monoclinic phases, so its effect diminishes as temperature rises and the tetragonal phase becomes more stable. As a high-temperature structural material, the toughness advantage therefore tends to be lost. In moist environments, low-temperature degradation also occurs, in which the transformation to monoclinic proceeds slowly from the surface even near body temperature. Note also that what it raises is mainly fracture toughness rather than strength; the strength levels off at the stress that starts the transformation, so raising toughness does not keep raising strength.
Partially stabilized zirconia has a toughness about twice that of alumina and is used for dental crowns, femoral heads of artificial hip joints, knives, grinding balls and parts of optical fiber connectors. Zirconia-toughened alumina (ZTA), with zirconia particles dispersed in alumina, uses the same mechanism. How much monoclinic phase has formed on a fracture surface can be found from the intensity ratios of XRD peaks. TRIP steel, in which retained austenite transforms to martensite during deformation and increases ductility, is similar in that it also makes use of a phase transformation.
Answer: C. Metastable tetragonal grains transform to monoclinic at the crack tip, and their volume expansion compresses the crack
In partially stabilized zirconia, tetragonal particles that would normally become monoclinic at low temperature are held metastably by the constraint of their surroundings. As a crack advances, the tensile stress at its tip transforms the tetragonal phase martensitically to monoclinic, with a volume expansion of about 4%. This expansion creates compressive stress around the crack that acts to close it, so extra energy is needed to propagate the crack. This is called transformation toughening (stress-induced transformation toughening). The choice with the transformation direction and volume change reversed is wrong. Blunting by viscous flow is a high-temperature phenomenon in materials containing a glassy phase.
Answer: A. In a moist environment, tetragonal grains transform to monoclinic from the surface with a volume expansion, roughening the surface and lowering the strength
Y-TZP gets its high toughness from transformation toughening, in which metastable tetragonal grains transform to monoclinic and expand under the stress at a crack tip. In a moist environment, however, the tetragonal → monoclinic transformation proceeds slowly from the surface even at temperatures as low as body temperature. The volume expansion of about 4% creates microcracks and grain pull-out at the surface, roughening it and lowering the strength and wear resistance. This is low-temperature degradation (hydrothermal degradation); manufacturing conditions accelerated the transformation and fractures of femoral heads became a problem. For this reason, alumina–zirconia composites and similar materials have been developed. Transformation to the cubic phase does not occur at low temperature and does not strengthen transformation toughening. Pitting is corrosion in which the passive film of a metal breaks down locally; it is not a degradation mechanism of oxide ceramics.
Mainly it raises fracture toughness, the resistance to crack growth. In Japanese it is often called by a name meaning transformation strengthening, but strength levels off at the stress that starts the transformation, so it does not rise in step with toughness.
No. Fully stabilized zirconia with a large amount of stabilizer is cubic down to room temperature and has no tetragonal phase to transform. Material with about 8 mol% Y₂O₃ is used for its oxide-ion conduction rather than toughness, as the electrolyte of fuel cells and sensors.
It varies with composition, microstructure and measurement method, but values of roughly 5–10 MPa·√m are reported for partially stabilized zirconia. This is higher than about 3–5 MPa·√m for alumina, making it one of the tougher ceramics.
This is called transformation toughening (stress-induced transformation toughening).
What is the main reason partially stabilized zirconia has a…Y-TZP gets its high toughness from transformation toughening, in which metastable tetragonal grains transform to monoclinic and expand under the stress at a crack tip.
Which statement correctly describes the low-temperature deg…Fracture toughness Pitting corrosion Passivity Perovskite structure Piezoelectric effect Sintering Weibull modulus