diary vs written material

diary and written material both are nouns.

Word	Noun	Adjective	Verb	Adverb
diary	Yes	No	No	No
written material	Yes	No	No	No

As nouns, written material is a hypernym of diary; that is, written material is a word with a broader meaning than diary:

diary: a daily written record of (usually personal) experiences and observations
written material: the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)

Other hypernyms of diary include piece of writing, writing.

diary (noun)	written material (noun)
a personal journal (as a physical object)	the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)
a daily written record of (usually personal) experiences and observations