[Back]

L. Zauner:
"Reactive HiPIMS deposition of Ti-Al-N: Influence of the deposition parameters on the cubic to hexagonal phase transition";
Supervisor: H. Riedl, P.H. Mayrhofer; Institute of Materials Science and Technology/E308-01, 2019; final examination: 2019-04-11.

High power impulse magnetron sputtering (HiPIMS) is often regarded as a key-technology for the deposition of future hard and multifunctional coating materials. Through the introduction of high amplitude impulses at relatively low duty cycles, peak power densities in the order of several kW∙cm-2 are generated, leading to a drastic increase in the amount of ionized species in the film-forming vapor. These highly dense plasmas in front of the target implement various consequences on the film growth kinetics and hence the coating properties, as well as on the discharge mechanisms of the target material itself (i.e. self-sputtering or gas-recycling). Moreover, applying reactive gas mixtures to the process, e.g. Ar/N2 for the deposition of nitride-based coatings, leads to further complexity within the discharge process. Nevertheless, several studies clearly highlighted outstanding coating properties as well as metastable phases accessible, when using reactive HiPIMS compared to conventional DC magnetron sputtering (DCMS). Since the majority of current research interests has been focused on the intricate plasma physics as well as the film growth using single phased target materials, industrial applications are left with a strong demand for a comprehensive extension on multinary coatings and hence target materials. Therefore, within this study we investigate the reactive HiPIMS deposition of Ti-Al-N coatings using Ti1−xAlx composite targets (x= 0.40 to 0.60) in mixed Ar/N2 atmospheres. The influence of HiPIMS parameters such as the pulse frequency (f) and duration (ton), but also nitrogen partial pressure, substrate bias, or target composition has been investigated methodically. The so obtained coating structures are analysed with respect to phase stability, chemical composition, mechanical properties (i.e. hardness and residual stress) and morphology applying nanoindentation, white-light interferometry, energy dispersive X-ray spectroscopy as well as X-ray diffraction measurements combined with electron imaging techniques (SEM and HR-TEM). Using a HiPIMS discharge (f= 500 Hz and ton= 75 µs) with an applied substrate bias of VS= -50 V, we are able to deposit polycrystalline Ti0.40Al0.60N thin films of high hardness (> 34GPa) and low residual stress (∼2 GPa compressive). In addition, the coatings exhibit a highly dense morphology and offer pronounced age-hardening ability up to 40 GPa after annealing at 700 °C for 1h. The obtained results present a distinct influence of the reactive gas partial pressure on the structural development as well as the Al/Ti ratio of the coatings synthesised, suggesting a maximum solid solubility between 0.55 ≤ xmax ≤ 0.6. Furthermore, altering the substrate bias voltage underlines the detrimental effect of highly energetic ions - especially regarding heavier, doubly charge Ti2+ - on the phase stability of c-Ti-Al-N, by promoting wurtzite phase growth upon applying VS= -100 V. Nevertheless, further deposition variations on the film growth rate reveal, that enhanced deposition rates are rather more important for the cubic phase formation than the bombardment of energetic Ti2+-ions. Finally, seminal information is gathered on the structural evolution of high AlN containing thin films when using a substrate bias synchronised to the HiPIMS discharge with changing duration and time delay.

Project Head Helmut Riedl:
CDL-SEC