Image processing of worn and unworn protective coatings of TiAlN and TiN on 100Cr6 steel

A model system, consisting of a titanium aluminum nitride (TiAlN) coating on top of an ‘optical’ titanium nitride (TiN) signal layer deposited on 100Cr6 steel substrates, was exposed to an extremely abrasive wear process. The TiAlN top-coatings, of thicknesses of up to 3 µm, were removed by a reciprocating wear process in a linear tribo-meter with up to 105 repetitive cycles, leaving the embedded TiN signal layers uncovered at the bottom the wear scars. The worn surfaces were characterized by subsequent image processing. A color detection, by a simple optical imaging system, of the wear scar with the exposed TiN layer showed a significant increase of ~40% of the relative color values from the TiAlN top layers to the embedded TiN signal layers. A similar reflectance detection experiment with a red laser optical system showed a comparatively significant signal increase of ~30% from the TiAlN top-coating to the TiN signal layer. The two different methods, image processing and laser reflectance measurements, lead thus to identical results, showing that image processing by means of color detection or monitoring and laser reflectance are potential techniques for intelligent determination of residual thickness of realistic tribological coatings of tools prior to complete wear.