Effect of sp2/sp3 bonding ratio and nitrogen content on friction properties of hydrogen-free DLC coatings

► DLC coatings with different sp3 ratio and different nitrogen content were prepared. ► Their friction properties were evaluated under a lubricated condition. ► Low sp3 ratio or high nitrogen content of DLC causes high friction coefficients. ► The addition of nitrogen on DLC reduces sp3 ratio and number of dangling bonds. ► Higher friction arises by less number of dangling bonds as adsorption sites on DLC.