Is hard a common noun

Yes, the noun hard (a firm or paved solid beach for hauling boats out of the water) is a common noun.

The Word hard is more commonly an adjective, a Word that describes a noun or an adverb, a Word that modifies a verb or an adjective. Example uses:

Adjective: There were some hard questions on that test.

Adverb: The rain had frozen hard on the steps overnight.