Long mineral fibres can form fibre nests, introducing voids into the fiction matrix. When the voids reach the external surface of the brake pad and enter into contact with the disc during the braking event, wear debris is generated and accumulated inside the voids. These voids act as a reservoir for wear debris. That has a number of consequences:
- Wear debris does not become immediately released powder (dust).
- Wear debris changes the surface of the friction material by forming spots of debris similar to the scondary plateaus but thicker. These thicker spots of debris increase the contact area between disc and friction material, increasing friction stability.
- The voids act as damping spots which reduce the occurence of noise.
This is called the reservoir effect.