[dtronvig]

Well, when you treat materials to make them harder they generally become more brittle, more inclined to break under tension than to stretch. If the whole thickness of the material is hard/brittle it'll all break in a hurry, like glass, or steel that's hardened all the way through. If you're really careful to harden just the outside and to leave the inside somewhat malleable, like in a good sword, for example, the hardened surface will nick, but the whole thing won't shatter.

So I can buy that a hardened version of polycarbonate would be brittle, able to take high stress up to a point, but then shattering, because it can't bend.

Thanks for all the suggestions,
Drew