Originally Posted by Threewheelbonnie

My area of expertise is the electronics and testing side of brake systems (and on trucks at that), but I'll have a go at answering.

The near perfect brake material is cast iron. The cast surface is rough so gives higher friction. The porosity and structure gives contaminants somewhere to go and it wears to leave a similar surface. It has few issues with heat. It is also cheap and easy to produce. The downside is that if not used it is cosmetically poor, it's heavy and as a structural material likes simple shapes.

Motorcycle fashion makes the last three serious. If manufacturer A offers a shiney, foil thin, flower shaped, collander style design and manufacturer B bolts on a rusty man hole cover guess who sells most. Everyone except racers would be better off with B but want A for the bling value of looking like the racers.

The manufacturers therefore had to find a middle ground. Surface coatings varied from annoying oil sprays which the dealer hopefully remembered to clean off through to lethal attempts to chrome and zinc plate. The former lubricant burns off by the time the bike is run in, the latter remains lethal. The coatings ruin the micro-structure the brake material wants to work against. Plan B was sticking with a homogenous material but adding anti-corrosion agents. The lower the grade of stainless the more like steel/iron it is, the more chrome/zinc/copper/iridium/unobtanium etc. you add the more it acts like it's chrome lubed.

The balance on cars and bikes is to use a low to mid grade disc specific stainless and make the disc bigger on the diameter. This ticks all the fashion boxes and the performance is still close enough not to attract attention. You get basic cast iron or machined steel disks disappearing in the 1980's and the current materials fully established by about 2000. There are 20-plus years worth of vehicles out there from the changeover period with odd sizes and varying materials, so for some the manufacturers simply can't afford to keep the semi-experimental OE materials going. If you are lucky they switched to plain steel and you live with the rust, if you are unlucky someone finds a failed experiment in adding chromed teflon or old scrap as we had in this case and flogs you a set.

Trucks BTW went to discs in 1995-2010 but are still cast iron, they get enough use to clear the rubbing surface the the rest is painted.

Andy