Al203 works fine if it is a part of a small rigid frame - watch or small phone.
Examples I have owned over the years - Polyot (not the Shturmanskie, one of the later ones) - 25+ years ago and Motorola KRZR. If Al203 is framed or attached into a small rigid structure it is nearly indestructible. The KRZR I had 8 years ago is still alive, kicking and taking daily abuse (and regular drops) from my mom. She has been unable to break it after I handed it down After all this years there is only a single chip on the outer Al2O3 cover on the top of the clamshell :)
I do not see how it can work with a large sheet though. Even an iPhone 1-4 size phone will be too large. A thick enough Al2O3 coat will be unacceptably thick and and heavy. It will probably be too thick for a capacity screen too. IMHO the KRZR was the first and last application of AL2O3 in this area. The likelihood of another one is pretty slim.