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 tribologists a step closer to the understanding of tribological mechanisms responsible for the failure of hard coatings. Since a thin oxide film on top of hard coatings could act as a solid lubricant and improve the wear and fretting behaviour of these coatings, the examination of such films is of special interest. The oxide films were produced on physical-vapour-deposited TiN coatings a few micrometres thick by irradiation with concentrated solar energy in ambient atmosphere at 800-degrees-C and for holding times between 10 and 350 s. For the analysis and characterization of the oxidized coating microstructure, micro-Raman spectroscopy and atomic force microscopy have been performed on oxidized surfaces. Light optical and scanning electron microscopy were done,on polished low angle and fractured coating cross-sections, while transmission electron microscopy has been performed on thin foils showing the coatings either parallel or cross-sectional to the surface. The dark grey oxide films found on TiN have been identified as rutile TiO2. These layers appeared with a uniform thickness but contain a non-uniform porosity over the oxide film thickness. Close to the TiN interface generally a higher porosity has been observed in the rutile. No sharp interface between TiN and rutile could be found. The TiN-rutile interface is characterized by oxide formation along the TiN grain boundaries.