Do the brake discs have grooves and need to be replaced?

Why the brake pads make grooves on the disc starts with the structure and raw material composition of the brake pads:

The main body of the brake pad is generally divided into two parts: the friction layer and the steel back (or back plate). What we see from the outside of the tire is the steel back. The steel back has no friction. Its main function is to facilitate installation inside the brake caliper and improve braking. The strength of the sheets and the steel backs produced by different manufacturers are not very different and the technical content is not high.

The main difference between the quality of brake pads is the friction layer. The friction layer is a composite material, which is evenly mixed with organic fibers, inorganic fibers, mineral fillers, phenolic resins, etc., and is hot-pressed and formed in a high-temperature and high-pressure environment in a press.

Many brake pads today will divide the friction layer into two layers of material, the bottom layer material and the top layer material. The bottom layer material is between the steel backing and the top layer material. During the use of the brake pads, the top layer material will be worn away first, and then It will wear to the base material, and then wear to the steel backing. Generally, the brake pads should be replaced before the surface material wears out.

The brake pad itself causes scratches on the brake disc. The brake pad surface material mentioned above

In fact, the hard substance in brake pads is a kind of slag balls in mineral fibers. Glass fibers will also form small balls under high temperature during braking. The hardness is also very high. There are also slag balls in ceramic fibers. Many natural Mineral fillers may also contain impurities. Therefore, brake pad manufacturers will choose raw materials that contain very few or without such hard substances. Such as glass fiber, which is no longer used in discs. But like mineral fiber, as one of the main materials of brake pads, there is no better material to replace it.