A nanoscale approach for the characterization of amorphous carbon-based lubricant coatings

The development of advanced materials providing new functionalities and improved performance is strongly sustained on the understanding of their structure and chemical composition at the nanoscale. For tribological applications, amorphous carbonaceous films (a-C, a-C:H, DLC, CNx) are a noteworthy example of thin films whose lubricant properties can be varied over a wide range of structures and compositions. Therefore, an appropriated knowledge about their particular structure is necessary to understand the physico-chemical phenomena appearing at the contact surfaces in relative motion and its influence on the friction behavior. However, the characterization of these coatings is complicated, mainly due to the absence of a reference compound, to the lack of long-range order, and to the poor knowledge about their bonding structure. In this paper, we report different examples of nanometer-scale characterization on amorphous carbon-based thin films (CNx, Si-CNx and TiC/a-C) with different experimental tools (e.g. energy filtered transmission electron microscopy (EFTEM); electron energy-loss spectroscopy (EELS); nuclear magnetic resonance (NMR); friction force microscopy (FFM)) that provide information about fine structural aspects and local atomic environment with high lateral resolution. The employment of surface-sensitive techniques as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) is also presented as a very useful route to investigate the nature of the processes induced in the counterfaces and the interpretation of the tribological response.