TiB₂ is considered a promising candidate among the family of transition metal diborides (TMB₂) to be applied in several high-performance applications under extreme conditions. In particular, the unique strength of TiB₂ is based on its high melting temperature (~3225 °C), high thermal shock resistance, and chemical inertness. However, the wide applicability of TiB₂ as a protective coating is still limited due to its poor high-temperature oxidation resistance by forming volatile, non-protective Ti- and B-based oxide scales. Si-alloying of TMB₂ provides a successful route to enhance the high-temperature oxidation resistance up to 1200 °C [1, 2]. Yet, the high Si content usually leads to the deterioration of the mechanical properties of these films [3].

Here, we investigated new alloying routes for sputtered TiB₂ coatings based on TMSi₂ secondary phases (TM = Ti, Ta, Mo) for achieving the challenging compromise between good mechanical properties and high oxidation resistance. We employed DC magnetron sputtering technique to synthesize TiB₂ alloyed coatings with different TMSi₂ phases from compound targets. The alloyed coatings were characterized in terms of chemical composition, phase constitution, and mechanical properties using high-resolution characterization techniques.Moreover, the oxidation kinetics within the alloyed Ti-(TM)-Si-B were studied at different temperature regimes up to 1400 °C, accompanied with detailed morphological characterization of oxide scales formed.

Keywords: Diborides; TiB₂; PVD; Protective Coatings; Oxidation; Mechanical Properties;

[1] T. Glechner, H.G. Oemer, T. Wojcik, M. Weiss, A. Limbeck, J. Ramm, P. Polcik, H. Riedl,Influence of Si on the oxidation behavior of TM-Si-B2±z coatings (TM = Ti, Cr, Hf, Ta, W), Surf. Coat. Technol., (2022) 128178.

[2] T. Glechner, A. Bahr, R. Hahn, T. Wojcik, M. Heller, A. Kirnbauer, J. Ramm, S. Kolozsvari, P. Felfer, H. Riedl,High temperature oxidation resistance of physical vapor deposited Hf-Si-B2±z thin films, Corrosion Science, 205 (2022) 110413.

[3] B. Grančič, M. Mikula, T. Roch, P. Zeman, L. Satrapinskyy, M. Gregor, T. Plecenik, E. Dobročka, Z. Hájovská, M. Mičušík, A. Šatka, M. Zahoran, A. Plecenik, P. Kúš,Effect of Si addition on mechanical properties and high temperature oxidation resistance of Ti–B–Si hard coatings, Surface and Coatings Technology, 240 (2014) 48-54.

Demanding high temperature mechanical applications create an opportunity to exploit the potential of transition metal diboride based thin films (TMB₂, TM = Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W) due to their very high hardness and temperature stability. However, the industrial use of these materials is limited by their brittle behavior under high-temperature mechanical loads which results in the initiation and propagation of cracks leading to permanent failure.

One of the approaches to suppress the inherent brittleness of TM diboride films is the concept of superlattices (SL) – i.e., formation of alternating chemically and/or structurally modulated nanolayers. Such SL structures ensure efficient dissipation of accumulated energy in the vicinity of an already formed crack due to its deflection, blunting and stopping at the interface between the flexible and stiff layers.

In our work, using density functional theory (DFT) calculations, we predict the structural stability and theoretical ductility of 184 TMB₂ SL systems, where we search for suitable candidates exhibiting improved fracture toughness.

In the next step, we experimentally focus on the magnetron co-sputtered TiB₂ – TaB₂ films with different bi-layer period where we performed micromechanical bending tests on the film cantilevers which confirmed the positive superlattice effect on improving the fracture resistance. In addition, for a more detailed explanation of the obtained results, the films are investigated using X-ray diffraction, and transmission electron microscopy. Mechanical properties (hardness and indentation modulus) are measured by nanoindentation techniques.

This work was supported by the Slovak Research and Development Agency (Grant No. APVV-21-0042) Scientific Grant Agency (Grant No. VEGA 1/0296/22), European Space Agency (ESA Contract No. ESA AO/1-10586/21/NL/SC), and Operational Program Integrated Infrastructure (Project No. ITMS 313011AUH4).

The calculations were carried out using the resources by the Swedish National Infrastructure for Computing (SNIC)—partially funded by the Swedish Research Council through Grant Agreement (VR-2015-04630)—on the Clusters located at the National Supercomputer Centre (NSC) in Linköping"

Despite being intensively investigated and already established in industry as protective coatings for aluminium machining [1] or diffusion barriers [2], Titanium-Diboride (TiB₂) is still not fully understood. In particular, for the fracture behaviour and the corresponding material characteristic (intrinsic fracture toughness, K_IC) the literature presents only little descriptions. So far it is known that the mechanical properties (H and E) of TiB₂ are highly dependent on the crystal orientation [3] and the amount of tissue phase present [4], which in turn can be influenced by the choice of coating parameters. To figure out, if this also affects the fracture behaviour, we synthesized a broad stoichiometric variation from TiB_2.07 up to TiB_4.42 by DC magnetron sputtering and thoroughly determined their structural, morphological and mechanical properties. By using micro-mechanical test set-ups (in-situ microcantilever bending tests) the intrinsic fracture toughness was investigated in relation to the chemical and hence morphological variation. In addition, information about the stress states within the thin films were obtained by nanobeam (synchrotron) experiments. The presence of a B-rich tissue phase was confirmed by HR-TEM analysis, showing that the grain size is decreasing with increasing B content, which is accompanied by an increase in tissue phase fraction. The dominance of the tissue phase is progressing with increasing B, also forcing smaller column sizes (from ~10 to < 5 nm) [5]. A change in Boron content from TiB_2.22 to TiB_4.42 leads to a decrease in fracture toughness from 3.55 ± 0.16 MPa√m to 2.51 ± 0.14 MPa√m of these superhard films. In summary, this study underlines the influence of stoichiometry and the potential of tissue phase engineering on the mechanical properties of TiB_2+z based thin films.

[1] C. Mitterer, PVD and CVD Hard Coatings, in: Comprehensive Hard Materials, Elsevier, 2014: pp. 449–467.
[2] H. Blom et al., Reactively sputtered titanium boride thin films, J. Vac. Sci. Technol. A. 7 (1989) 162–165.
[3] H. Holleck, Material selection for hard coatings, J. Vac. Sci. Technol. A. 4 (1986) 2661–2669.
[4] P.H. Mayrhofer et al., Self-organized nanocolumnar structure in superhard TiB2 thin films, Appl. Phys. Lett. 86 (2005) 131909.
[5] C. Fuger et al., Revisiting the origins of super-hardness in TiB2+z thin films – Impact of growth conditions and anisotropy, Surf. Coat. Technol. (2022) 128806.

Current machining applications require new developments in the field of hard coatings to achieve excellent precision and effective processes. TiAlN thin films are well established due to their good thermo-mechanical properties. Recently, we reported TiAlLaBN films by sputtering. In the report the addition of La into the TiAlN lattice was shown to increase film hardness and H/E which correlates with film toughness. The microstructure of the film composes fine grain crystals with a size of 5 nm. However, the relationship between the microstructure of the TiAlLaBN film and its mechanical properties is still unclear. It is necessary to understand the relationship to maximize the cutting performance. The microstructure of the film can be varied by changing the deposition rate. Increasing the deposition rate may change the crystal grain size, leading to coarse grain microstructure. In this study, we investigate the changes in microstructure and mechanical properties by changing the deposition rate in order to improve the cutting performance.

Two different deposition approaches of sputtering and CAE (Cathodic Arc Evaporation) were used for changing the deposition rate. Ti-Al alloy added with 2 mol% LaB₆ was used as metal target. So as to prepare the metal target including unstable La, it was stabilized by combining with boron to form the LaB₆ compound. The deposition rate in CAE was 144 nm/min which was faster than in sputtering, 104nm/min. From the cross-sectional SEM observation and TEM images, the TiAlLaBN film of 144 nm/min was found to be a fine columnar structure with a width of 10 nm and length of 30 nm. The film hardness H and H/E of the fine-grained TiAlLaBN were H=40.1 GPa and H/E=0.095, then those of columnar structure were H=35.3 GPa and H/E=0.096, where E is elastic modulus. Both increased compared to H=31.3 GPa and H/E=0.065 for conventional TiAlN by CAE. The hardness of the TiAlLaBN film with fine-grained structure was found to be larger than that of columnar structure, while film toughness H/E was almost the same regardless of their different structures.

ZrBSiTaN films were fabricated on silicon and AISI 420 stainless steel substrates through direct current magnetron co-sputtering. ZrB₂, Ta, and Si targets were used. The power applied on the Si target and the nitrogen flow ratio of the reactive gas were the variables in the sputtering processes. The effects of Si and N contents on the mechanical properties and thermal stability of ZrBSiTaN films were investigated. The results indicated that all the as-deposited ZrBSiTaN films formed amorphous structures. The supplement of reactive gas with a nitrogen flow ratio of 0.4 resulted in that the ZrBSiTaN films exhibited a high N content of 60 at.%. The increase of Si content from 0 to 42 at.% in ZrBSiTa films decreased the hardness and Young’s modulus values from 19.1 to 14.3 GPa and 264 to 242 GPa, respectively, whereas the increase of Si content from 0 to 21 at.% in ZrBSiTaN films increased the hardness and Young’s modulus values from 11.5 to 14.0 GPa and 207 to 218 GPa, respectively. The amorphous BN and SiN_x phases played the vital role in the variations of structural and mechanical properties of ZrBSiTaN films. The thermal stability test was conducted at 800 °C for 10 min within purged Ar gas in a rapid thermal annealing furnace. The ZrBSiTa films oxidized with the residual oxygen in the vacuumed furnace, which was accompanied with the formation of ZrO₂, Ta₂O₅, and TaSi₂ phases, whereas the ZrBSiTaN films maintained amorphous structures. Further exploration on the oxidation behavior of ZrBSiTaN films will be studied.

Mo_1−xCr_xAlB thin films were deposited by combinatorial magnetron sputtering at temperatures between 450 °C and 700 °C. Subsequent analysis by XRD, EDX, ERDA, and TEM revealed the compositional phase formation range to be between 0.38 ≤ x ≤ 0.76 for orthorhombic MAB phase Mo_1−xCr_xAlB, consistent with previous ab initio predictions, and can hence be rationalized by composition-induced changes in chemical bonding. For samples with lower x, the formation of Cr₂AlB₂, Cr₃AlB₄, and Cr₄AlB₆ side phases has been observed, again in accordance with theoretical and experimental literature data.

In recent years, the machining of difficult-to-cut materials, such as heat-resistant alloy and titanium alloy, has been increasing for aircraft, energy and medical markets. When cutting exotic alloys, the workpiece material is likely to adhere onto the cutting edge of a tool, resulting in a sudden fracture of the cutting edge of the tool. The tool life is significantly shorter than that of tools for cutting general steel. This study was intended to investigate the mechanical property and cutting performance of AlTiBN coating in order to solve the serious problems. AlTiBN coating was deposited on cemented carbide by arc ion plating method using nitrogen as reaction gas and Al-Ti-B alloys with B contents of 0, 2, 5 and 10 at. % were used as target materials. The mechanical properties of the coatings were measured by nano-indenter and the microstructure was evaluated by X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The cutting performance was evaluated by milling test of Ni-based heat-resistant alloy, and then tool life and damage to the coating at the early stage of machining were evaluated.In conventional PVD coatings for cutting tools, Cr and Si has often been added to AlTiN in order to increase hardness and heat resistance, achieving high wear resistance. However, these methods increase hardness as well as Young’s modulus, which reduces toughness of coating. In heat-resistant alloy and Titanium alloy milling, coatings with a high Young’s modulus tends to develop internal cracks, leading to early destruction of the coatings. In this research, we developed an innovative coating with high hardness and low Young's modulus for difficult-to-cut materials milling. Optimizing B content of AlTiBN coating, we realized low Young's modulus of 504 GPa while maintaining high hardness of 38 GPa. This is probably because the pure AlTiN changed to a mixed crystal structure of cubic and hexagonal by the addition of B. When the cutting performance of this AlTiBN coating was evaluated in milling for Ni based super alloy, material equivalent to Inconel 718, it was found that internal cracks were suppressed at the initial stage of processing. As a result, cemented carbide tool with the AlTiBN coating achieved approximately 1.6 times longer tool life than that with B-free AlTiN coating. As described above, the AlTiBN film with high hardness and low Young's modulus has not only excellent wear resistance but also excellent toughness, so it is expected to contribute to the improvement of productivity in aircraft, energy and medical markets.