sustain vs buoy up

sustain and buoy up both are verbs.

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

As verbs, buoy up is a hyponym of sustain; that is, buoy up is a word with a more specific, narrower meaning than sustain:

Other hyponyms of sustain include scaffold, block, carry, chock, buoy, pole, bracket, underpin, prop, prop up, shore, shore up, truss, brace.

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