written material vs piece of writing

written material and piece of writing both are nouns.

Word	Noun	Adjective	Verb	Adverb
written material	Yes	No	No	No
piece of writing	Yes	No	No	No

As nouns, written material and piece of writing are synonyms defined as:

written material and piece of writing: 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 synonyms of written material include writing.

written material (noun)	piece of writing (noun)
the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)	the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)