Carbon nitride (CNx) coating has been found to exhibit an ultra-low friction coefficient (less than 0.01) when sliding against a silicon nitride (Si3N4) ball in a dry nitrogen gas (N-2) atmosphere; however, no direct surface analysis of the topmost layer of the ultra-low friction coating has been conducted. In the present study, we examine the wear tracks formed on a CNx coating after a sliding test in both dry N2 and air atmospheres, using scanning transmission electron microscopy combined with electron energy-loss spectroscopy, to elucidate the structure of the surface transition layer quantitatively on the nanometer scale. After the sliding test in dry N-2, a measurement taken from the surface towards the material bulk indicates that the plasmon peak has shifted. We discuss the plasmon peak shift in terms of the density and chemical bonding nature, as a measure of the microscopic structural change. (C) 2016 Elsevier B.V. All rights reserved.