strengthen vs secure

strengthen is not an adjective while secure is an adjective.

Word	Noun	Adjective	Verb	Adverb
strengthen	No	No	Yes	No
secure	No	Yes	Yes	No

As verbs, secure is a hyponym of strengthen; that is, secure is a word with a more specific, narrower meaning than strengthen:

strengthen (adjective)	secure (adjective)
	free from fear or doubt; easy in mind
	free from danger or risk
	not likely to fail or give way
	immune to attack; incapable of being tampered with
	financially safe

strengthen (verb)	secure (verb)
give a healthy elasticity to	make certain of
gain strength	cause to be firmly attached
make strong or stronger	fill or close tightly with or as if with a plug
	furnish with battens
	get by special effort
	assure payment of