DECORATIVE HARD COATINGS - INTERDEPENDENCE OF OPTICAL, STOICHIOMETRICAND STRUCTURAL-PROPERTIES

Transition metal compounds (nitrides, carbides etc.) find increasing u se as decorative coatings combining intensive colours, high wear resis tance and good corrosion resistance. Some of the hard coatings (e.g. T iN, TiC, TiCN and TiAlCN) have different colours. While the golden TiN and the dark-grey TiC are limited in their colour variation, the addi tion of a second metal or a second metalloid widens the spectrum of de corative colours, e.g. to dark-blue with (Ti, Al)N. Within the scope o f the search for new colours a number of nitrides binary IVa metal-bas ed alloys such as (Ti, V)N, (Zr, V)N, (Zr, Al)N and (Zr, Cr)N were dep osited by reactive magnetron sputtering. For the description of colour the Commission Internationale de l'Eclairage L-a*-b* system was used . By means of goniospectroscopy, clear dependences of L, a* and b* on the angle of detection could be demonstrated for Ti-based coatings in the visible wavelength range. Spectroscopic ellipsometry was introduc ed to determine material-related parameters (complex refractive index N = n + ik) with a high sensitivity (changes in stoichiometry for TixN 1-x layers) and to separate interference effects (shown for (Zr, Cr)N) as well. In general, both stoichiometry and structure can influence t he colour of a coating. The correlation between colour, stoichiometry and structure was determined by surface-analytical methods (Auger elec tron spectroscopy, small-spot X-ray photoelectron spectroscopy, glow d ischarge optical spectroscopy, and X-ray diffraction). Proper selectio n of the process parameters (target composition, N2 flow rate etc.) re sults in coatings with different colours exceeding the conventional sp ectrum of decorative coatings. As an example of structural influences on colour, (Zr, V)N coatings are discussed.