head-on vs frontal

head-on is not a noun while frontal is a noun.

Word	Noun	Adjective	Verb	Adverb
head-on	No	Yes	No	Yes
frontal	Yes	Yes	No	No

As adjectives, head-on and frontal are synonyms defined as:

head-on (noun)	frontal (noun)
	an adornment worn on the forehead
	a drapery that covers the front of an altar
	the face or front of a building

head-on (adjective)	frontal (adjective)
meeting front to front	of or relating to the front of an advancing mass of air
characterized by direct opposition	meeting front to front
	belonging to the front part
	of or adjacent to the forehead or frontal bone

head-on (adverb)	frontal (adverb)
with the front foremost
in direct opposition; directly