Question

Zirconia ceramics exhibit:

• A. High electrical conductivity
• B. High hardness
• C. Low hardness
• D. Transformation toughening

Hint:

Transformation toughening is why Partially Stabilized Zirconia (PSZ) is often referred to as "ceramic steel" due to its high fracture toughness.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for a unique mechanical phenomenon exhibited by zirconia ($\text{ZrO}_2$) ceramics.

Step 2: Key Formula or Approach:
Zirconia undergoes phase changes with temperature and stress:
\[ \text{Tetragonal phase} \xrightarrow{\text{stress-induced}} \text{Monoclinic phase} \]
This transition is accompanied by a localized volume expansion of approximately $3\%$ to $5\%$.

Step 3: Detailed Explanation:

• Zirconia can be stabilized in its high-temperature tetragonal phase at room temperature by adding dopants such as yttria ($\text{Y}_2\text{O}_3$).

• When a crack propagates through this partially stabilized zirconia, the high tensile stress field near the crack tip triggers a phase transformation from the metastable tetragonal phase to the stable monoclinic phase.

• This phase change is accompanied by a local volume expansion.

• The volume expansion generates localized compressive stresses around the crack tip.

• These compressive forces squeeze the crack shut, opposing the tensile stress trying to open it, which effectively halts crack propagation.

• This unique mechanism is known as transformation toughening, and it makes zirconia one of the toughest ceramic materials available.

Step 4: Final Answer:
Zirconia ceramics exhibit transformation toughening.