In-Situ Nanotribological Properties of Ultrananocrystalline Diamond Films Investigated with Ambient Pressure Atomic Force Microscopy

The relationship between nanoscale friction and the surrounding environment has long been a critical issue in the field of nanotribology. Here, we utilized ambient pressure–atomic force microscopy to investigate the effect of environmental gas on nanoscale friction of ultrananocrystalline diamond (UNCD) films. The frictional forces were measured in an atomic force microscopy (AFM) chamber in the environmental range from an ultrahigh vacuum to near ambient pressure in the presence of oxygen, nitrogen, and water. We observed that friction increased with the pressure of the oxygen responsible for the oxidation of the surface of the UNCD, while that in nitrogen gas remained unchanged. Interestingly, friction decreased in water, due to the tribochemical reaction caused by surface passivation. When two diamond materials come into contact under water conditions, the water molecules are dissociated because of normal pressure between the AFM tip and diamond surface, and the dissociative water molecule adsorption passivates the surfaces of the diamond-coated tip and UNCD, resulting in a reduction of friction force. The chemical state of the UNCD surface in various environmental conditions was confirmed using near ambient pressure X-ray photoelectron spectroscopy. This result elucidates the role of vapor-phase oxygen and water in the tribological properties of carbon-based materials.