Atsuo Kawana

High-temperature oxidation of ion-plated TiN and TiAlN films

The high-temperature oxidation of TiN, Ti 0.9 Al 0.1 N, and Ti 0.6 Al 0.4 N films which were deposited onto stainless steel substrates using an arc ion-plating apparatus was studied at temperatures ranging from 923 to 1173 K for 0.6 to 60 ks in air. The oxidation rate obtained from mass gain as a function of time was found to fit well to a parabolic time dependence. From their temperature dependence, the apparent activation energies of oxidation were determined. With increasing Al contents, the oxidation rate decreased, and the activation energies of oxidation reaction increased. Formed oxide layers were analyzed by XRD, SEM, and EPMA. With increased Al content in TiAlN films, the rate-determining step changes from oxygen ion diffusion in formed rutile to oxygen or aluminum ion diffusion in the formed Al 2 O 3 layer.

HIGH TEMPERATURE OXIDATION OF ION-PLATED CRN FILMS

The high-temperature oxidation of CrN films which were deposited onto stainless steel substrates using an arc ion plating apparatus was studied at temperatures ranging from 1023 to 1173 K for 0.6 to 480 ks in air. The oxidation rate obtained from mass gain as a function of time was found to fit well to a parabolic time dependence. Formed oxide layers were analyzed by XRD, SEM, and SAM. An activation energy of the oxidation of CrN was slightly lower than that of the self-diffusion coefficient of Cr ion in Cr 2 O 3 . It is concluded that the oxidation of CrN is controlled by the outward diffusion of Cr ions through the Cr 2 O 3 layer formed on each CrN grain.

HOT OXIDATION OF ARC ION-PLATED NITRIDE COATINGS

The thermal oxidation of TiN, CrN and TiAlN films were studied in air at temperature from 650 C to 900 C. These nitride films were deposited onto stainless steel substrates using an arc ion-plating method. Oxidation rates were calculated by the weight changes with times. Formed oxide layers were analyzed by XRD, SEM and AES. The oxidation rates were found to fit well to a parabolic time dependence. The high dependence of oxidation rates on film materials was evident from the Arrhenius plots of the calculated rate constant. The calculated activation energy of 32.5 kcal/mol for oxidation of TiN was less than that of 43 kcal/mol for oxygen diffusion, in TiO 2 . This difference suggested that the oxidation of TiN is enhanced by grain boundary diffusion of oxygen in formed oxide film. The activation energy for CrN was 60.1 kcal/mol, which was almost same value as that of diffusion of Cr ion in Cr 2 O 3 . The oxidation reaction of Ti 0.9 Al 0.1 N is much faster than that of Ti 0.6 Al 0.4 N and the activation energies were 51.5 kcal/mol and 112.6 kcal/mol respectively. The activation energy for Ti 0.6 Al 0.4 N was almost same as that of diffusion of Al ion in Al 2 O 3 .

Corrosion properties of ion plated TiN and CrN coatings

High temperature oxidation of TiN and CrN films was studied in air at temperature from 923 to 1173 K. Oxidation rates were calculated by the mass gain with reaction time. Formed oxide layers were analyzed by XRD, SEM, SAM, and EPMA. The oxidation of TiN films is controlled by the oxygen diffusion through the formed TiO2 layer. The oxidation of CrN, however, is controlled by the outward diffusion of Cr ion through Cr2O3 layer formed along the grain boundaries. Oxidation resistance of CrN was superior than that of TiN. The pin-hole densities of TiN and CrN films were evaluated as a function of film thickness by means of Ferroxyl test. The observed pin-hole densities of TiN and CrN were decreased with increasing film thickness. The sufficient film thickness for TiN coatings with pin-hole zero was estimated as more than 15μm for TiN. The pin-hole densities of CrN films were remarkably less than that of TiN films. Corrosion resistance in 10 % HCl solution was measured for the TiN and CrN coatings deposited under the various conditions. CrN coatings showed the high corrosion resistance.

Corrosion properties of TiN and CrN coated stainless steels in HCl solution

Corrosion resistance of TiN and CrN coated stainless steels in 10% HCl solution was measured by weight loss method. Corrosion resistance of coated stainless steels strongly depended on the pin-hole densities of TiN and CrN films. The observed pin-hole densities decreased with increasing film thickness. The pin-hole densities of CrN films were remarkably less than that of TiN films. CrN coated stainless steels showed higher corrosion resistance.