The thick velvet in semi-metallic brake pads is mainly used to reinforce steel fibers. From the appearance (fine fibers and particles), it is easy to distinguish asbestos-type and asbestos-free organic brake pads. In addition, it also has certain magnetic properties. 

Steel wool has high strength and high thermal conductivity, which makes semi-metallic brake pads and traditional asbestos brake pads have different braking characteristics. "Semi-metallic" brake discs have a high metal content and high strength. At the same time, high metal content also changes the friction characteristics of the brake, which usually means that semi-metallic brake pads require higher braking pressure to achieve the same braking effect. Especially at low temperatures, the high metal content also means that the brake discs lead to greater disc or drum wear and thus louder noise.
The main advantages of semi-metallic brake pads are temperature control ability and high braking temperature. Compared with the poor heat transfer performance of asbestos type and the poor cooling capacity of brake discs and brake drums, it can help brake discs and brake pads. The drum releases heat from its surface, and the heat is transferred to the caliper and its components. Of course, if this heat is not handled properly, there will be problems. After heating, the temperature of the brake fluid will increase. If the temperature reaches a certain level, the brakes will shrink and the brake fluid will boil. This heat also affects the calipers, piston seals and return springs, which will accelerate the aging of the components, which is why calipers need to be reassembled and metal parts replaced during brake maintenance.