Invited Paper SE+PS-MoA8
Mechanisms of Adhesion Enhancement by High Power Impulse Magnetron Sputtering

Monday, October 15, 2007, 4:20 pm, Room 617

Adhesion to steel and carbide substrates is one of the primary factors determining coating performance under environmental attack such as wear in cutting and automotive engine operations, errosion-corrosion, and high temperature oxidation. Technologies that improve adhesion aim to sputter-clean the substrate by high energy ion bombardment with energy >500 eV. These energies are sufficient for ions to be implanted into the bulk of the substrate to a depth of several monolayers (1-3 nm). Therefore the chemical composition of the bombarding flux can have a strong influence on the structure of the coating-substrate interface. Technologies that use gas ion bombardment typically incorporate Ar as interstitial or at vacancy sites generated in the steel or carbide lattice by the high energy of irradiation. The inert nature of Ar means that it does not form bonds with the surrounding atoms and thus greatly disturbs atomic ordering and increases stress. In the case of high power impulse magnetron sputtering (HIPIMS) plasmas operating at peak current of 2 Acm^-2, the ion bombardment flux contains high fractions of metal ions. For HIPIMS of Cr and Ti, the ratio Ar¹⁺ : Me¹⁺ = 1 : 1 was observed with energy-resolved mass spectroscopy, whilst the metal ion-to-neutral ratio was Me¹⁺ : Me⁰ = 1 : 1 as determined from atomic absorption spectroscopy. Scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) analysis of 304 stainless steel bombarded at 600 V by HIPIMS of Cr showed a layer of implanted Cr ions with depth of 5-8 nm, resulting from ballistic implantation as confirmed with TriDyn simulations. High-resolution TEM revealed that this region is highly crystalline with low defect density, probably due to the substitutional incorporation of Cr ions in the steel lattice. Incorporation and retention of Cr is improved by irradiation- and temperature- enhanced diffusion. As a result of the crystalline interface, the coating nucleated in local epitaxial growth mode which was maintained over several microns in lateral direction. A number of susbtrate-coating combinations demonstrated such epitaxy, for example for steel substrates: CrAlN, CrN, VN, TiAlN, and for γ-TiAl substrates - CrAlN, and CrN. This resulted in significant improvements to the adhesion and performance in wear and cutting of Ti and Al tests.