Underlying Mechanisms of Improved Cryogenic Tribological Performance of the Pure Graphite: Suppressed Thermal Atomic Motions and Enhanced Mechanical Properties

Abstract

The cryogenic tribological performance of the pure graphite was investigated through experiments and molecular dynamics simulations in this study. As a brittle material, the pure graphite cooled by liquid nitrogen exhibited unexpected improvements in friction and wear characteristics. Hardness, X-ray diffraction (XRD), and transmission electron microscopy (TEM) tests at low temperatures were conducted to explore the underlying mechanisms. The Leeb hardness of the graphite at low temperature (665.3 HL) was 13.4% higher than at ambient temperature (586.5 HL). The TEM observations at −167 °C were conducted using commercially available cryo-electron microscopy, and the graphite specimens were fabricated using the focused ion beam technique. The XRD measurements at −130 °C were performed using a tester equipped with a liquid nitrogen circulation cooling system, and the graphite samples used were the same as those in the hardness tests. These test results indicated that the mechanical properties improved and interlayer spacing decreased due to the suppression of thermal atomic motions at low temperatures. Furthermore, a model consisting of a graphite substrate and a spherical diamond indenter was developed to conduct molecular dynamics simulations, and the AIREBO potential was employed to characterize the graphite substrate. The simulation results revealed a reduction of approximately 51.2% in thermal motion at low temperature. The reduced fluctuation range resulted in enhanced atomic interactions and made the carbon bonds less susceptible to rupture when stressed mechanically during sliding, which were the underlying microscopic mechanisms of improved cryogenic tribological performance of the pure graphite.