Yes, but in the abscence of contaminants (primarily road grit) pad wear is a very slow process - pads do a lot of braking with only a tiny loss of material, until grit from the road is introduced in damp/wet conditions.
It does take energy to remove pad material, but the method of removal results in an enormous variation in energy required; for example, in clean dry conditions, provided that the pad doesn't overheat, then the rate of wear is tiny i.e. the energy required to remove material is very large. In other conditions such as overheating or abrasive contamination, the energy required to remove material is very low and so the wear rate is high.Hexsense wrote: ↑Fri Jan 07, 2022 5:31 amFirst, it takes energy to shear off the pad compound. And if any part of the pad compound vaporize (gas out), that's even more energy not turning into heat. Pad gassing out is a real thing, the main reason rotors have cut out channel to let the air out rather than pushing apart against brake pads and reduce the braking force, isn't it?
I don't agree with the "pad gassing" bit - pad/rotor friction can (and does) reduce precipitously at a certain temperature, but there isn't a layer of gas between pad and rotor; there would be effectively zero friction if that really occurred. A nice example here is the carbon-carbon rotors used for performance applications in motorsport or aerospace - no holes or channels on the rotor surface, and at high temperatures some of the wear debris is....carbon dioxide.
For the rotors used in bicycle brakes, cutouts/channels do provide a means for grit to "escape", but also allow a higher stiffness-to-weight ratio for the friction ring - for a given weight, the rotor can be thicker which greatly increases its stiffness if done carefully (buckling stiffness is critical) - and also a higher surface area to weight ratio which helps with cooling.
As youngs_modulus has pointed out, the fundamental principle of braking by friction is conversion of kinetic energy to heat (thermal energy). Pad/rotor/caliper must be designed together to achieve this satisfactorily, but road bike disc brakes are still in the development phase I think, as requirements for low weight and exposure to contaminants (including abrasive ones) make this difficult to solve.
