Which type of damage on compressor blades is specifically noted for occurring over time?

The type of damage on compressor blades that is specifically noted for occurring over time is gouges. This type of damage is typically a result of sustained operational wear and tear, as well as the cumulative effects of operating conditions, such as foreign object damage or material fatigue. Gouges occur due to long-term exposure to the high-stress environment within the compressor, where particles can make contact with the blades, leading to material being removed in a shaped recess or notch.

In contrast, scratches are usually indicative of a more immediate abrasive action and can often result from incidental contact rather than prolonged operational stress. Burns tend to be linked with thermal issues or excessive heat during operation, while burrs arise from mechanical processes such as machining or cutting, rather than from the operational degradation of the blades over time. Thus, gouging is the damage type that clearly correlates with the gradual impact of time and usage.