buoy up vs sustain

buoy up and sustain both are verbs.

Word	Noun	Adjective	Verb	Adverb
buoy up	No	No	Yes	No
sustain	No	No	Yes	No

As verbs, sustain is a hypernym of buoy up; that is, sustain is a word with a broader meaning than buoy up:

Other hypernyms of buoy up include hold, hold up, support.

buoy up (verb)	sustain (verb)
become more cheerful	undergo (as of injuries and illnesses)
keep afloat	establish or strengthen as with new evidence or facts
make more cheerful	admit as valid
	supply with necessities and support
	provide with nourishment
	be the physical support of; carry the weight of
	lengthen or extend in duration or space