ICMCTF2005 Session B1-2: Sputtering Coatings and Technologies

TiAlN coatings exhibit high hardness and oxidation resistance over TiN, and have attracted substantial research and industrial interests. However, their tribological properties show insignificant improvement due to their unimproved high friction coefficient. Recently, CrAlN coatings had been reported exhibiting even higher oxidation resistance than TiAlN. CrN coatings are known to be superior to TiN in corrosion and wear resistances, friction behavior, and toughness. Therefore, in comparison with TiAlN, better tribological properties of CrAlN coatings and their promising applicability in high speed machining could be expected.

In this work, a series of Cr_1-xAl_xN coatings with 0≤x≤0.4 were deposited on high speed steel (HSS) substrates by a reactive unbalanced magnetron sputtering technique from Cr and Al elemental targets in an Ar+N₂ mixture atmosphere. During coating deposition, a negative bias was applied onto the substrates, but no deliberate heating was applied. The structural, mechanical and tribological properties of the as-deposited CrAlN coatings were characterized by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), nanoindentation, and pin-on-disc tribometer experiments. For the pure CrN coating sample, all of the major XRD peaks arise from the HSS substrate. With the addition of aluminum, the CrAlN coatings gradually crystallize in the pseudo binary, rocksalt-type cubic structure, showing a (111) preferential orientation. With the increase of aluminum content, the hardness of the CrAlN coatings increases gradually from about 18 GPa up to a maximum value of 30 GPa, and then drops down slightly with increasing aluminum content further. The wear resistance of all the CrAlN coatings are much better than that of the pure chromium nitride coating deposited under similar conditions.

Ag-Pd decorative coatings are supposed to provide better tarnishing resistance as compared to pure Ag, while keeping the same optical and mechanical properties. Palladium has similar optical appearance as silver, and forms a thin protective oxide surface layer. For this reason, small incorporation of Pd in Ag could improve its tarnishing resistance.

Thin Ag-Pd films were deposited by magnetron co-sputtering from Pd and Ag targets on stainless steel substrates. The Ar working gas pressure, Ag and Pd target power, substrate temperature were varied in order to modify the chemical composition of the films and optimise their properties.

The optical properties of the films were evaluated by spectroscopic ellipsometry. The reflectivity in VIS of the Ag-Pd films decreases with increasing Pd content and the reflectivity edge becomes less pronounced. For the film with 30 wt.% Pd the reflectivity is 70% which is lower than that of pure Pd. High values of reflectivity (80-95% in VIS) are observed for films with Pd content between 2 and 6 wt.%. The reflectivity (at 633 nm) of Ag-Pd film with 5 wt.% Pd decreases with increasing substrate temperature from 90% (without substrate heating) to 74% (Ts=300°C). High working gas pressure results in a drastic decrease of the specular reflectivity of the films : the reflectivity (at 633 nm) of two films with 15 wt.% Pd deposited at high (4 Pa) and low (0,5 Pa) pressure are 65 and 90% respectively.

The tarnishing resistance was evaluated by sulfidation test. Different Ag-Pd films together with an Ag 925 reference were exposed to ammonium sulfide during 40 min. The color of the samples was measured before and after the sulfidation test by colorimetry (CIELAB). The first results indicate an improvement of tarnishing resistance for Ag-Pd films.

The paper is focused on the relationship between deposition conditions, chemical composition, film morphology and color, mechanical properties and tarnishing resistance of the coatings.