Marián Mikula

The structure evolution and mechanical behavior of the reactive DC magnetron sputtered complex concentrated (or compositionally complex) nitride RM-TiZrHf-N _y coatings with variable nitrogen stoichiometry with RM = V, Nb, and Ta were compared with the reference (TiHfZr)-N _y coatings over a wide range of nitrogen concentrations. Despite the concentrations of the refractory metals deviated from equimolar concentration, homogeneous solid solutions with cubic structures were present in all coatings. Their mechanical properties depended on nitrogen concentration and RM type. The maximum values of hardness (~43 GPa) and indentation modulus (~530 GPa) were achieved in the reference TiZrHf-N _y coatings with (near-) stoichiometric composition. The levels of properties in the coatings with V, Nb, or Ta additions were ~10 % lower and the differences among them were within the scatter of measurements. Thus, the synergy effect from the incorporation of the additional refractory metals into the TiZrHf sub-lattice on mechanical properties was not confirmed.

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.

In this study, we introduced the method for the growth of titanium molybdenum oxide (TMO) nanotubes directly from metallic precursor solid state solution and provided their structural and chemical characterization. Precursor films with content of molybdenum from 32 to 82 at% were prepared using co-deposition magnetron sputtering. The optimization of deposition parameters allowed for the growth of a continuous nanotube array with a length up to 700 nm ± 10% by anodic oxidation. Scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) revealed nanotube formation with Ti_1−xMo_xO₂ composition, where x can reach the value of 0.5. Scanning transmission electron microscopy combined with EDS (STEM-EDS) confirmed the incorporation of Mo into the TiO₂ lattice and uniform elemental distribution across the nanotube at the submicron level. The nanobeam electron diffraction (NBD) and X-ray diffraction analyses (XRD) did not show any notable crystal phase formation for the titanium molybdenum oxide phase.

The use of silver-based antimicrobial materials has been growing recently. Considering the threat of developing silver-resistant bacteria, it is essential to address the endurance of such materials and the amount of silver released into the environment. Here we report on a durable, antibacterial Ag/TiB_x nanocomposite coating prepared by conventional magnetron sputtering. The coating consists of fine Ag clusters embedded in extremely hard, wear-resistant overstoichiometric TiB_x, which serves as a protective matrix. The highest E. coli growth inhibition of 97% is observed for the coating containing 24 at.% of Ag. A strong antibacterial effect is also maintained after 45 days of immersion in the Luria–Bertani + 5% HNO₃ solution. Despite a marked hardness decrease from 40 GPa for TiB_x to 6.4 GPa for Ag/TiB_x with 28 at.% of Ag, the coating maintains a good specific wear rate of 6 × 10⁻⁵ mm³/Nm. Moreover, the addition of Ag, which acts as a solid lubricant, decreases the coefficient of friction from 0.77 to 0.35, even at room temperature. Thanks to the combination of antibacterial properties and enhanced wear resistance, such material can find application as a protective coating for cutlery, door handles, water taps, and other daily-used objects in public areas.

Doping of transition metal diborides (TMB₂) films with soft metals (Ag, Au, Pt) can extend their application potential to tribological and biomedical fields. Here, a combination of direct current unbalanced magnetron sputtering (DC-UBMS) with high-power pulsed magnetron sputtering (HiPIMS) was used to synthesize silver-doped CrB_2+x thin films on unheated substrates. All Ag–CrB_2+x thin films were over-stoichiometric with a B/Cr ratio ranging from 2.05 to 2.30 and silver content varying from 3 at.% to 29 at.%. X-ray diffraction demonstrates the amorphous character of the structure in the case of films with silver content ranging from 0 at.% to 8 at.%. A nanocrystalline structure containing a cubic Ag phase is formed in the films with higher silver content. The highest hardness of 26.6 GPa accompanied by the highest value of elastic modulus of 362 GPa was measured in undoped CrB_2.3 films. As the silver content in the Ag–CrB_2+x thin films increases, the hardness and elastic modulus values gradually decrease to 7.8 GPa and 187 GPa, respectively. The friction properties of CrB_2.3 films, expressed by the coefficient of friction against a steel ball of 0.72, are insufficient and limit their use in demanding industrial applications. However, silver doping significantly reduces the friction coefficient when the lowest value of 0.39 is measured in moderately hard Ag–CrB_2+x films with an Ag content of 17 at.%. The scratch test shows satisfactory adhesion of films to substrates even without additional heating during deposition.

Hexagonal ZrB₂ belongs to the group of ultra-high temperature ceramics representing an important class of materials with the potential to meet the high demands of today’s industry. However, this potential is limited by inherent brittleness and poor tribological properties. Here, the combination of density functional theory and experiment is used to investigate the effect of silver alloying on the mechanical and tribological properties of hexagonal ZrB₂ thin films. Calculations indicate strong insolubility of Ag atoms in the ZrB₂ metal sublattice and a significant effect on the mechanical properties, pointing out an improvement in ductility and tribological properties but at the cost of reduced hardness. The experiments confirmed the theoretical predictions of the strong insolubility of silver, where the magnetron-sputtered Zr_1−xAg_xB_2+Δ films form a segregated nanostructure consisting of separated hexagonal ZrB₂ and cubic Ag phases. With increased Ag content, values of Young’s modulus decrease from E^ZrB2.31 = 375 GPa to E^{Zr0.26Ag0.74B0.89} = 154 GPa, followed by a decrease in hardness from H^ZrB2.31 = 30 GPa to a value of H^{Zr0.26Ag0.74B0.89} = 4 GPa. The suppression of crack formation is also shown with the material flow around cube corner indents, indicating enhanced ductility. The improvement of tribological properties was also confirmed when the coefficient of friction (COF) was reduced from COF^ZrB2.31 ~0.9 to a value of COF^{Zr0.26Ag0.74B0.89} ~0.25 for all counterpart materials—steel (100Cr6), Si₃N_4, and WC/Co.

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.