High-temperature wear — Which oxide ceramic is generally recognized for the highest hardness and is therefore used where very high wear resistance at elevated temperature is required?

Introduction:
Oxide ceramics are chosen for applications combining high temperature capability, corrosion resistance, and wear resistance. Among common oxides, alumina stands out for its excellent hardness, making it a material of choice for grinding media, seal faces, nozzles, and wear linings in hot, abrasive environments.

Given Data / Assumptions:

• Comparison limited to common structural oxides: Al₂O₃, ZrO₂, MgO, BeO, SiO₂.
• Metric of interest: hardness and attendant wear resistance at temperature.
• Ignoring non-oxide ultra-hard materials (e.g., SiC, B₄C) for this question.

Concept / Approach:
Alumina exhibits high hardness (Vickers/Knoop), good hot hardness retention, and chemical stability, supporting excellent wear resistance. Zirconia offers high fracture toughness (transformational toughening) but is generally softer than alumina and can experience phase stability issues at high temperatures if not stabilized. Magnesia and beryllia provide excellent refractoriness/thermal conductivity respectively but lower hardness than alumina for abrasive wear roles. Silica is relatively soft and not favored for severe wear.

Step-by-Step Solution:
Rank common oxides by hardness: Al₂O₃ typically highest among the listed choices.Link hardness to wear: harder materials resist micro-abrasion better at temperature.Select alumina as the oxide used where very high wear resistance is required.

Verification / Alternative check:
Materials handbooks list alumina with superior hardness compared to stabilized zirconia and magnesia; industrial wear components frequently specify high-purity alumina grades (e.g., 92–99.9%) for maximum life.

Why Other Options Are Wrong:

• Zirconia: tougher but generally less hard than alumina.
• Magnesia: refractory but softer for abrasive service.
• Beryllia: excellent thermal properties but not the hardest; also toxicity concerns.
• Silica: comparatively soft.

Common Pitfalls:
Equating fracture toughness with hardness; overlooking that wear modes vary (erosion, abrasion, adhesion) and that hardness dominance is strongest in abrasive regimes.

Final Answer:
Alumina (Al₂O₃)