Viktor Šroba

Time-resolved ion mass spectrometry is used to analyze the type and the energy of metal-ion fluxes during hybrid high-power impulse/direct-current magnetron co-sputtering (HiPIMS/DCMS) in Ar. The study focuses on the effect of HiPIMS plasma plumes on the cross-ionization of the material flux sputtered from the DCMS source. Al, Si, Ti, and Hf elemental targets are used to investigate the effect of the metal’s first ionization potential IPMe1 and mass on the extent of cross-ionization. It is demonstrated that the interaction with HiPIMS plasma results in the significant ionization of the material flux sputtered from the DCMS source. Experiments conducted with elements of similar mass but having different IPMe1 values, Si and Al (Si-HiPIMS/Al-DCMS and Al-HiPIMS/Si-DCMS) reveal that the ionization of the DCMS flux is favored if the sputtered element has lower ionization potential than the one operating in the HiPIMS mode. If elements having similar IPMe1 are used on both sources, the metal mass becomes a decisive parameter as evidenced by experiments involving Ti and Hf (Ti-HiPIMS/Hf-DCMS and Hf-HiPIMS/Ti-DCMS). In such a case, Ti+ fluxes during Hf-HiPIMS/Ti-DCMS may even exceed Hf+ fluxes from the HiPIMS cathode and are much stronger than Hf+ fluxes during Ti-HiPIMS/Hf-DCMS. The latter effect can be explained by the fact that heavier Hf+ ions require longer transit time from the ionization zone to the substrate, which effectively increases the probability of interaction between the Hf-HiPIMS plasma plume and the Ti-DCMS flux, thereby leading to higher Ti ionization. Thus, the common notion of low ionization levels associated with DCMS has to be revised if DCMS is used together with highly ionized plasmas such as HiPIMS operating at higher peak target currents. These results are particularly important for the film growth in the hybrid configuration with substrate bias pulses synchronized to specific ion types.

Alloying of transition metal diborides with other metals seems to be a suitable way to maintain their excellent mechanical properties under high-temperature loads. Here, we focus on the perspective tantalum diboride (TaB2) hard film, where we investigate the effect of aluminum alloying on the thermally induced structural evolution and mechanical properties of α-Ta1−xAlxB2 using a combination of density functional theory and experiments. Ab initio calculations predict a strong tendency toward the spinodal phase decomposition of the α-TaAlB2 solid solution into isostructural binaries in the entire concentration range at T = 0 K. However, an increase in temperature (T > 773 K) leads to stabilization of the α-Ta1−xAlxB2 solid solution at x < 0.5. The experimental results of magnetron co-sputtered Ta0.75Al0.25B2.14 and Ta0.69Al0.31B2.16 films with aluminum concentrations of 8 and 12.5 at. %, respectively, confirm the insolubility of aluminum in the TaB2 phase. The structure has a nanocomposite character formed by α-TaB2 nanofilaments surrounded by an Al-rich tissue phase. The films are structurally stable up to 1200 °C, but Al atoms already begin to diffuse from the boundary regions when the temperature exceeds 1000 °C. Al alloying causes a decrease in hardness, since the hardness of the reference as-deposited TaB1.21, Ta0.75Al0.25B2.14, and Ta0.69Al0.31B2.16 films is 34, 28, and 27 GPa, respectively. Exposing the films to high temperatures does not lead to a hardening effect; the hardness of Al-depleted films annealed at a temperature of 1200 °C decreased by approximately 10%. The decrease in Young's modulus from 420 GPa (TaB1.21) to 370 GPa (Ta0.69Al0.31B2.16) indicates a tendency toward the ductile behavior of Al alloyed films under mechanical load.

This work reports on various thermally induced planar defects formed in ternary V_1-xMo_xB_2-Δ coatings with different influence on mechanical properties. V_0.58Mo_0.42B_1.9 and V_0.37Mo_0.63B_1.2 films were prepared by magnetron sputtering and annealed in vacuum up to 1300°C. Depending on the stoichiometry, different types of planar defects were identified by scanning transmission electron microscopy (STEM). In the slightly understoichiometric V_0.58Mo_0.42B_1.9 system, anti-phase boundary defects (APB) were observed, which was associated with chemical separation of V and Mo. Accumulation of these defects led to formation of MoB-Cmcm orthorhombic phase in otherwise hexagonal VB₂-P6/mmm structure, with no apparent effect on mechanical properties. On the other hand, in case of the highly understoichiometric V_0.37Mo_0.63B_1.2 coating, no chemical separation was observed. Here, annealing resulted in crystallization from amorphous state into orthorhombic Cmcm phase with high density of twin lamellae, which was accompanied by a significant hardening.