MICROSTRUCTURAL EXAMINATION OF SOLAR-BEAM-IRRADIATED TIN HARD COATINGS

Microstructural examination and characterization of hard coatings may enable engineers to improve coating deposition processes and bring tri bologists a step closer to the understanding of tribological mechanism s responsible for the failure of hard coatings. Since a thin oxide fil m on top of hard coatings could act as a solid lubricant and improve t he wear and fretting behaviour of these coatings, the examination of s uch films is of special interest. The oxide films were produced on phy sical-vapour-deposited TiN coatings a few micrometres thick by irradia tion with concentrated solar energy in ambient atmosphere at 800-degre es-C and for holding times between 10 and 350 s. For the analysis and characterization of the oxidized coating microstructure, micro-Raman s pectroscopy and atomic force microscopy have been performed on oxidize d surfaces. Light optical and scanning electron microscopy were done,o n polished low angle and fractured coating cross-sections, while trans mission electron microscopy has been performed on thin foils showing t he coatings either parallel or cross-sectional to the surface. The dar k grey oxide films found on TiN have been identified as rutile TiO2. T hese layers appeared with a uniform thickness but contain a non-unifor m porosity over the oxide film thickness. Close to the TiN interface g enerally a higher porosity has been observed in the rutile. No sharp i nterface between TiN and rutile could be found. The TiN-rutile interfa ce is characterized by oxide formation along the TiN grain boundaries.