piece of writing vs written material

piece of writing and written material both are nouns.

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

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

piece of writing and 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 synonyms of piece of writing include writing.

piece of writing (noun)	written material (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)