Influence of hydrogen and tungsten concentration on the tribological properties of DLC/DLC contacts with ZDDP

► a-C coating provide lower boundary friction than the other types studied. ► H and W contents in DLCs do not affect friction greatly. ► H and W contents in DLCs have a marked effect on film-forming and wear properties. ► W-DLCs experience higher wear irrespective of tungsten content. ► a-C:Hs experience very low wear irrespective of hydrogen content.