Spectrophotometric study of oxide growth on are evaporated TiN and ZrN coatings during hot air oxidation tests

Spectral reflectance, roughness and microhardness measurements have been used to study the growth of the oxide layers formed on are evaporated TiN and ZrN coatings during hot air oxidation tests. The coatings were deposited onto polished high speed steel which were then repeatedly exposed to hot air in oven at three different temperatures, namely 350, 430 and 500 degrees C. The duration of each single exposure was 60 or 120 min, depending on test temperature; the temperature value was maintained within +/-5 degrees C. After each exposure spectral reflectance, roughness and microhardness of the coatings were measured. The oxide thicknesses were evaluated from fittings of calculated spectral reflectances to the measured ones and with Nuclear Reaction Analysis techniques, obtaining consistent results. Reflectance spectra turned out to be very sensitive to the oxide thickness, allowing to determine it with reasonable accuracy. Since this method is non destructive, simple and quick it could be attractive for use in industrial environments. In addition we proved that roughness, microhardness and adhesion (scratch testing) of TiN are not modified when it is oxidized in air at 350 degrees C up to 6 h.