That's an excellent technical question.

When discussing high-temperature ceramic bearings (typically made of Silicon Nitride, Si3N4), the term "hardness retention rate" refers to the material's ability to maintain its hardness as the ambient temperature rises.

Unlike steel, ceramic does not undergo a phase transformation or tempering softening. Therefore, for Silicon Nitride (Si3N4) —the industry standard for hybrid and full-ceramic bearings—the hardness retention rate is exceptionally high.

Here is the specific data:

1. The Hardness Retention Rate

Silicon Nitride retains approximately 90% to 95% of its room temperature hardness up to 1000°C (1832°F) .

• At Room Temperature (20°C): Hardness is typically 15–17 GPa (Vickers hardness, HV 1500–1700).

• At 1000°C: Hardness remains around 14–15 GPa.

In contrast, high-performance bearing steel (like 52100 or M50) begins to lose hardness rapidly above 150°C to 300°C , typically retaining less than 50% of its hardness by 500°C.

2. Why the Retention is So High

The retention rate is high due to the covalent bonding structure of silicon nitride. It does not exhibit "tempering" like steel. Steel loses hardness because its martensitic structure breaks down into softer ferrite and cementite.

Ceramics maintain their structure until sintering aids (oxides added during manufacturing) begin to soften the grain boundary phase, typically around 1100°C to 1300°C . The silicon nitride grains themselves remain hard until near the melting point (1900°C).

3. Important Distinction: Hybrid vs. Full Ceramic

You asked about "ceramic bearings," but the retention rate differs slightly depending on the type:

• Full Ceramic Bearings (Si3N4 rings + Si3N4 balls): The hardness retention is excellent (90%+) up to 800°C–1000°C. However, the load capacity drops significantly above 800°C due to the glass phase softening at the grain boundaries.

• Hybrid Bearings (Steel rings + Si3N4 balls): The ceramic balls retain their hardness near 100%, but the bearing is limited by the steel rings. Above 300°C, the steel rings soften, causing the overall bearing "hardness retention" (system performance) to collapse, regardless of the ceramic balls.

4. Summary Table

Conclusion

If you are asking strictly about the ceramic rolling elements (balls or rollers), the hardness retention rate is roughly 90% to 95% up to 1000°C.

If you are asking about the bearing assembly (full ceramic), the functional limit is typically defined not by hardness retention, but by the load capacity degradation (due to grain boundary softening) which usually limits full ceramic bearings to applications below 800°C in oxidative environments.