ICMCTF2007 Session H2: High Power Impulse Magnetron Sputtering (HIPIMS)

High-Power Impulse Magnetron Sputtering (HiPIMS) is an interesting deposition process as it represents a relatively easy way to increase vapor ionization and thus to promote energetic condensation of the films.

Parameters influencing the plasma and, in fine, thin film synthesis, are numerous. Among them, pulse duration plays a key role in the discharge behaviour as it acts on both deposition and ionization rates. The decrease of deposition rate is the major drawback of the technique as it is only a fraction of the deposition rate measured in DC magnetron discharges. For short pulses (a few microsec.) the discrepancy between the deposition rate in HiPIMS and DC discharges tends to be less pronounced whereas ionization rate drops. On the contrary, as discharge current increases with the pulse duration (tens of microsec.), sputtering wind becomes more marked and self-sputtering can occur. The deposition rate decreases and ionization rate rises.  In reactive discharges (Ar-O₂), pulse duration will also play on the plasma composition and especially on the density of strongly reactive species such as O atoms.

The transport of metal ions can also be discussed as it plays on the deposition rate. Results showing an increased collection speed of the metallic ions to the substrate in a hybrid discharge process - when an inductively coupled secondary plasma is superimposed to the HiPIM - are presented.

Finally, the influence of the substrate electrical properties on the titanium oxide thin film properties is also important. Grounded conductive substrate (steel) leads to the formation of high-temperature stable phase of rutile while anatase is found on glass.

Ion etching pre-treatments prior to PVD are commonly used to improve adhesion and corrosion performance by engineering the substrate/coating interface. This work compares the corrosion performance of CrN/NbN superlattice coatings grown by unbalanced magnetron sputtering and pretreated with high energy (1 keV) Nb ions produced by high power impulse magnetron sputtering (HIPIMS), HIPIMS with subsequent Nb HIPIMS interlayer and steered cathodic arc (CA) metal ion etching with CA interlayer.

The discharge during the pre-treatment step was analysed by optical emission spectroscopy revealing a HIPIMS plasma dominated by singly charged Nb¹⁺ ions and small fraction of Nb²⁺. CA plasma contained high fractions of 2+ and 3+ charged Nb ions. The average substrate currents for HIPIMS and CA pretreatment were ~3 mAcm^-2, with peak value of 300 mAcm^-2 during the HIPIMS pulse.

HIPIMS pre-treatment promoted the formation of a clean and droplet-free interface and growth-defect free and dense coatings. Localized epitaxial growth of the films, as examined by cross-sectional transmission electron microscopy, resulted in significantly improved coating adhesion (L_C = 56 N) compared to CA pre-treatment (L_C = 45 N).

Potentiodynamic polarization tests showed superior corrosion performance for the HIPIMS pre-treated coatings over CA providing passivation up to +1000 mV for SS and +660 mV for M2 substrates. The addition of HIPIMS Nb interlayer significantly reduced corrosion current densities to 4 x 10^-5 Acm^-2. The CA pretreatment did not exhibit corrosion passivation on either SS or HSS. The PVD CrN/NbN nanoscale multilayer coatings outperformed 20 µm thick commercial electroplated hard chrome.