In this study, six types of DLC coatings were prepared, featuring different carbon structures (including amorphous ta-C coatings and GNC coatings with nanocrystallites) and different doped Ta amounts, to investigate friction characteristics. The results of friction tests with MoDTC-added lubricant revealed a consistent trend: DLC coatings with a higher ID/IG ratio exhibited lower friction coefficients. In addition, in situ observations using reflectance spectroscopy highlighted that the tribofilm formed on DLC coatings with a higher ID/IG ratio maintained a higher \({{\text{MoS}}}_{2}/({{\text{MoS}}}_{2}+{{\text{MoO}}}_{3})\) ratio, which exhibited a strong correlation with the friction coefficient. Measurements of a work function of each DLC coating indicated that those with a higher ID/IG ratio had a higher work function, suggesting the inclusion of a larger amount of graphite structure defects. These active defects in the graphite structure were deemed responsible for enhancing the friction reduction effect of MoDTC. The outcomes of this study propose a material design approach for DLC coatings that amplifies the effectiveness of lubricant additives in friction reduction. Graphical abstract