The brake dust that we all see on automotive wheels is 92% Fe which simply put is in part iron caused by abrasion of the cast iron brake rotor by the pad and secondly fibers from the semi-metallic elements of the brake pad. In addition, brake dust also contains some of the metallic elements of the brake pad and carbon residue, which also comes from the brake pad.

99% of automotive brake pads made these days are made as semi-metallic meaning a bundle of steel fibers compressed and then fused with other additives to build a pad. Steel fiber can actually make up 30% of the brake pad so when it is pressed against the brake rotor it scratches the rotor and creates brake dust.

Non-metallic pads are only available as an upgrade in the aftermarket and very few manufacturers opt to use these more expensive materials simply for build cost purposes. These pads are known an organics and often outperform and outlast their semi-metallic counterparts as well as reducing rotor abrasion and dust caused in the braking action.

Image from the network