[Terr_]

I'd approach it by looking at the definition of a "spark", and the things we can change to avoid tripping that definition.

For a visual spark, it has to be something thrown off which is the right size, material, and a temperature to glow visibly to the human eye under normal light.

For a spark that starts a fire, it has to be something thrown off which has a certain temperature by the time it lands or travels to some other surface.

In both cases, hot very fine particles will cool quickly just by hitting the ambient air. If the underlying surface (e.g. brake disk) gets glowy-hot, we wouldn't call that a spark--it's too big, and fixed.