From a materials engineering standpoint, cockpit windshields, typically weighing between 25-40 pounds, depending on the type of windshield build, are typically constructed of several layers:
(1) A glass face-plate, roughly 1/10" thick;
(2) A Polyvinyl Butryal (PVB) layer, about 1/8" thick;
(3) A stretched acrylic layer, approximately 1" thick;
(4) An additional PVB layer, nearly 1/10" thick;
(5) An additional stretched acrylic layer, approximately 1" thick. Source
But even then, it is not unbreakable. We have had several cases of broken windshields, as mentioned in above source. Also, the co-pilot in Varig Flight 254 crash was wounded from a tree, which struck his shoulder after crash landing.
So, why are aircraft windshields not made from unbreakable Glass Ceramics ?