I agree that it serves its purpose of protecting the data layer of the CD. But again, CDs and their readers were designed in a way that typical surface scratches and other imperfections would not inhibit reading. And so polycarbonate was an acceptable choice. The same approach applies to eye glass lenses; a scratch on a lens is typically not in focus and therefore has little to no effect on vision.
So, TL;DR: Polycarbonate the material is not scratch resistant. CDs are scratch resistant, but that is due to their optical design and in spite of the polycarbonate.
You find it's good relative to other plastics at preventing penetration which is also why it's good at preventing deep scratches. So, yes if you want a crystal clear optical element then it's not scratch resistant and look for something high on the Mos scale. But, if you want a coating that protects from deeper scratches it's quite good. In that context it's good material to protect CD's from scratches that matter.
Which again is why I find the word resistance tricky because it's always resistant in some context.
Right. But that this started explicitly about calling the polycarbonate -- that clear layer -- as scratch resistant. And this is just flat out wrong:
"Polycarbonate is a durable material. Although it has high impact-resistance, it has low scratch-resistance."
https://en.wikipedia.org/wiki/Polycarbonate#Properties_and_p...
I agree that it serves its purpose of protecting the data layer of the CD. But again, CDs and their readers were designed in a way that typical surface scratches and other imperfections would not inhibit reading. And so polycarbonate was an acceptable choice. The same approach applies to eye glass lenses; a scratch on a lens is typically not in focus and therefore has little to no effect on vision.
So, TL;DR: Polycarbonate the material is not scratch resistant. CDs are scratch resistant, but that is due to their optical design and in spite of the polycarbonate.