"Even Hulk can't break Adamantium." I may be wrong.. But hasn't Hulk ripped Wolverine in half in Ultimate Wolvey vs Hulk?...
Yes. Also, There are different types of Adamantium and The Hulk has dent, broken and twisted some Adamantium around. Others have affected different qualities of Adamantium before, as well.
Note: Cap's Shield is still just Vibranium/Steel Alloy!