POWER AND PRESSURE EFFECT ON HiPIMS TUNGSTEN COATINGS

High power impulse magnetron sputtering (HiPIMS) technique was used to grow tungsten (W) films on titanium substrates. Pulse length (50 ?s), frequency (300 Hz) and substrate bias (-50 VDC) were maintained constant while argon pressure was changed in the range 2x10-3 - 1.6x10-2 mbar. The effect of inert gas pressure on the cathode/anode V-I characteristic and film microstrucure was investigated. Finally a comparison of coatings properties at different target average power (600 and 1200 W) was made too. The target current peak changes with pressure and average power. This corresponds to a modification of ions to neutrals ratio in the plasma. A higher peak current (> average power and/or < Ar pressure) can be associated with an improved metal atoms ionization rate. Another striking feature is the current peak steepness. It increases when reducing pressure and increasing the average target power: this behavior indicates a better argon gas ionization efficiency. Morphology and chemical composition of the samples were analyzed using SEM with energy dispersive spectroscopy (EDS) for elemental analysis. Tungsten coating thickness has been measured through a Calotest (CSM). Microstructure was verified via XRD, crystallographic parameters have been evaluated using MAUD (Material Analysis Using Diffraction), a software based on the Rietveld refinement method. Scratch tests have been performed to evaluate film-substrate adhesion. Film thickness increases with average target power; this is much more evident at low pressure. When compared to W films deposited by dc magnetron sputtering, the HiPIMS ones show a decreased grain size. Moreover HiPIMS process promotes the growth of the alpha-W phase.