toughness vs difficultness

toughness and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
toughness	Yes	No	No	No
difficultness	Yes	No	No	No

As nouns, difficultness is a hypernym of toughness; that is, difficultness is a word with a broader meaning than toughness:

toughness: impressive difficulty
difficultness: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of toughness include difficulty.

toughness (noun)	difficultness (noun)
the property of being big and strong	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
enduring strength and energy
impressive difficulty
the elasticity and hardness of a metal object; its ability to absorb considerable energy before cracking