DECORATIVE HARD COATINGS - ADVANCES IN OPTICAL CHARACTERIZATION TECHNIQUES

Transition metal compounds are finding increasing use in decorative coatings, combining intense colour, high wear resistance and good corrosion resistance. From the decorative point of view, colour and gloss are the most important features. The physiological colour impression is well described by the CIE-L*a*b colour space. Integrating sphere measurement (ISM) and goniospectroscopy (GS) derive colours from a measurement of the spectral reflectance factor R(lambda) relative to a white reference. Spectroscopic ellipsometry (SE) is a reference-free technique for measuring amplitude ratios and phase changes of polarized light upon specular reflectance. ISMs of (Zr,Y)N coatings are presented, which show considerable yellow and red values at intermediate lightnesses. The effect of the surface roughness is discussed for TiN coatings. The angular dependence of L*a*b* and the behaviour of R(lambda) are shown by means of GS for these TiN coatings in off-specular directions. These results of ISM and GS are compared with the SE data. The high sensitivity of SE to changes in stoichiometry as well as the capability of separating additional interference effects are demonstrated for (Zr,Cr)N and (Zr,Al)N coatings. However, it is shown especially in the case of rough surfaces that each technique is helpful in distinguishing features of physical and physiological interest.