I think you mean to say that ceramic is not as hard as porcelain. Porcelain, or vitrified ceramic, is usually much harder than unvitrified ceramic, but is more brittle as well. So porcelain isn't very tough necessarily when it comes to fracturing.
Fracture issues have been problematic in the design of ceramic armor as well. The ceramic needed to be hard enough to stop incoming rounds, not completely shatter when impacted, and not break apart under normal wear and tear of handling and wearing. So upon impact with a ceramic piece of armor, the ceramic is formulated to stop the bullet, deform and absorb the impact and catch bullet spall, and do so without shattering the entire plate, thereby allowing the plate to be viable for multiple impacts. So the fractures and microfactures created on impact are prevented from spreading and as such, keeping the ceramic largely intact except for the directly impacted location.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011