Ceramics and glasses
What is the main reason partially stabilized zirconia has a high fracture toughness?
- AThe tensile stress at the crack tip transforms monoclinic to tetragonal, and the roughly 4% volume contraction relaxes the stress
- BThe glassy phase left at the grain boundaries flows viscously at high temperature, blunting the crack tip and relieving the stress concentration
- CMetastable tetragonal grains transform to monoclinic at the crack tip, and their volume expansion compresses the crack
Show answer and explanation
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.