frontal vs head-on

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

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

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

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

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

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