Surface breakage of smart phone is mainly through impact loading (sudden drop) from a appreciable height. Though graphene is the strongest material ever tested having intrinsic tensile strength of 130.5 GPa and a Young's modulus of 1 TPa , it has low fracture toughness in range of 4 MPa√m (https://www.nature.com/articles/ncomms4782) which makes them relatively brittle. This might be the technical limitation of usage of graphene for impact proofing of mobile phone.

there may not be a need to use graphene(s); rather we have an abundant knowledge of glass technology,along with ceramic materials development. sensibly directed research into these materials, as well as development of tough self repair inexpensive polymers may well be a faster and more economic approach.

Other than the fact that it may still break on impact, as stated in the answers above, it is also debated if there are health hazards similar to asbestos.

Outside of the realm of smart phones, Graphene has some serious difficulties in production.

I've found that this website is highly useful on getting Graphene information http://www.thegraphenecouncil.org and they have a article on commercialization http://www.thegraphenecouncil.org/?page=JonesGrapheneComm

For smartphones - new developments in "glass" and synthetic mineral (aka sapphire glass) are probably going to be more cost effective and develop materials that are better suited for consumer electronics.

James

Graphene would not be the material choice for protective coating requiring impact resistance and fracture toughness. But polymer materials or composite materials (combining inorganic/ceramic materials with polymers) can do. Research should focus on polymer and composite materials.

Ceramic coatings are much cheaper and much more robust than organic coatings, including graphene. Graphene is not a cure-all material. Its popularization has driven the academic sector to use it in several applications that researchers would not have other wise employed it and has shown poor performance. Surface coatings for anti-break or anti-scratch would be another application space where graphene would likely exhibit sub-par performance.