The Effect of Carbon Structure of DLC Coatings on Friction Characteristics of MoDTC-Derived Tribofilm by Using an In Situ Reflectance Spectroscopy
Naoya Hashizume, Yusei Yamamoto, Cheng Chen, Takayuki TOKOROYAMA, Ruixi Zhang, Dongfeng Diao, Noritsugu Umehara
Abstract
Abstract 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 I D / I G ratio exhibited lower friction coefficients. In addition, in situ observations using reflectance spectroscopy highlighted that the tribofilm formed on DLC coatings with a higher I D / I G ratio maintained a higher $${{\text{MoS}}}_{2}/({{\text{MoS}}}_{2}+{{\text{MoO}}}_{3})$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mtext>MoS</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>/</mml:mo> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mtext>MoS</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>+</mml:mo> <mml:msub> <mml:mtext>MoO</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> 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 I D / I G 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