__________ is the hardest oxide and is hence used where high wear resistance at high temperature is required.

ANS:C - Alumina

Alumina refers to aluminum oxide (Al2O3), a compound composed of aluminum and oxygen. It is one of the most widely used ceramic materials due to its excellent combination of properties, including:

1. High Hardness: Alumina is a hard material, ranking just below diamond and cubic boron nitride in terms of hardness. This property makes it suitable for applications requiring wear resistance, such as cutting tools and abrasion-resistant components.
2. High Strength: Alumina exhibits high mechanical strength, allowing it to withstand mechanical loads and stresses. This property makes it useful in structural applications where strength and rigidity are essential.
3. Chemical Inertness: Alumina is chemically inert and resistant to corrosion by most acids and alkalis. This property makes it suitable for use in harsh chemical environments, such as in the chemical processing industry.
4. High Thermal Stability: Alumina retains its mechanical properties at high temperatures, making it suitable for use in high-temperature applications such as refractory materials, furnace linings, and thermal insulators.
5. Electrical Insulation: Alumina is an excellent electrical insulator, making it useful in electrical and electronic applications where insulation is required.

Alumina is available in various forms, including ceramics, powders, fibers, and coatings, and it finds widespread use in industries such as aerospace, automotive, electronics, metallurgy, and medical devices. Its versatility and combination of properties make it a highly valuable material in various engineering applications.