Question

Ceramic oxide used for thermal barrier coating in Turbine blades is

• A. Thoria
• B. Calcium oxide
• C. Magnesia
• D. Zirconia

Hint:

YSZ (Yttria-Stabilized Zirconia) is the gold standard for TBCs.
The yttria stabilizer inhibits the tetragonal-to-monoclinic martensitic transformation, which would otherwise destroy the coating during thermal cycling.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question asks about the specific ceramic oxide commonly used as a Thermal Barrier Coating (TBC) on high-pressure gas turbine blades to protect the metallic superalloy substrate from extreme gas temperatures.

Step 2: Key Formula or Approach:
Thermal barrier coatings require materials with a unique combination of physical properties:
1. Extremely low thermal conductivity to minimize heat transfer.
2. High thermal expansion coefficient (close to that of nickel-based superalloy substrates) to minimize thermal cyclic stresses.
3. Outstanding phase stability at temperatures exceeding \( 1200^{\circ}\text{C} \).

Step 3: Detailed Explanation:

• Zirconia as a TBC:
- Zirconia (\( \text{ZrO}_2 \)) has exceptionally low thermal conductivity (\( \sim 2\text{ W/m}\cdot\text{K} \)) and a high melting point (\( \sim 2700^{\circ}\text{C} \)).
- However, pure zirconia undergoes a destructive phase transition from tetragonal to monoclinic upon cooling, which is accompanied by a large volume expansion (\( \sim 4\% \)) that causes cracking.
- To prevent this, Yttria (\( \text{Y}_2\text{O}_3 \)) is added (typically \( 7-8\text{ wt}\% \)) to stabilize the high-temperature cubic/tetragonal phases down to room temperature. This material is known as Yttria-Stabilized Zirconia (YSZ).
- YSZ remains the standard material of choice for turbine blades due to its thermal shock resistance and matched thermal expansion with the underlying metallic bond coat.

• Other Ceramics: Thoria (\( \text{ThO}_2 \)) is radioactive and difficult to process; Calcium oxide (CaO) and Magnesia (MgO) are highly susceptible to hydration and chemical degradation.


Step 4: Final Answer:
The ceramic oxide used for thermal barrier coatings is zirconia (specifically Yttria-Stabilized Zirconia).
Therefore, the correct choice is option (D).