My experience with helmets is that the visor always quickly ends up covered in tiny scratches. The one I have now is a Scorpion brand helmet that I tried hard to baby (soaking the visor before cleaning it and such), to no avail.
In researching eye glasses, I came across information suggesting that a couple of the coatings out there are harder than glass. So in theory it should be possible to have a scratch resistant visor.
Are there any?