title: A shear localization mechanism for lubricity of amorphous carbon materials.
abstract: Amorphous carbon is one of the most lubricious materials known, but the mechanism is not well understood. It is counterintuitive that such a strong covalent solid could exhibit exceptional lubricity. A prevailing view is that lubricity of amorphous carbon results from chemical passivation of dangling bonds on surfaces. Here we show instead that lubricity arises from shear induced strain localization, which, instead of homogeneous deformation, dominates the shearing process. Shear localization is characterized by covalent bond reorientation, phase transformation and structural ordering preferentially in a localized region, namely tribolayer, resulting in shear weakening. We further demonstrate an anomalous pressure induced transition from stick-slip friction to continuous sliding with ultralow friction, due to gradual clustering and layering of graphitic sheets in the tribolayer. The proposed shear localization mechanism sheds light on the mechanism of superlubricity, and would enrich our understanding of lubrication mechanism of a wide variety of amorphous materials.
authors.affiliation: State Key Laboratory of Tribology, Tsinghua University, 100084 Beijing, China.
authors.forename: Tian-Bao
authors.initials: TB
authors.lastname: Ma
authors.affiliation: State Key Laboratory of Tribology, Tsinghua University, 100084 Beijing, China.
authors.forename: Lin-Feng
authors.initials: LF
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Tribology, Tsinghua University, 100084 Beijing, China.
authors.forename: Yuan-Zhong
authors.initials: YZ
authors.lastname: Hu
authors.affiliation: Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, 100081 Beijing, China.
authors.forename: Xin
authors.initials: X
authors.lastname: Li
authors.affiliation: State Key Laboratory of Tribology, Tsinghua University, 100084 Beijing, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Wang