ICMCTF 2024 Session MA5-2-ThA: Boron-containing Coatings II

Titanium diboride is currently the most widespread boride coating used in industry. Its most common application is machining non-ferrous metals, due to its outstanding properties such as high hardness 40–50 GPa, high elastic modulus ≥ 500 GPa, high chemical inertness, high melting point above 3000°C, and low propensity for sticking to soft metals. The main drawbacks of diborides are their generally low oxidation resistance (between 600-700°C for TiB₂) and brittleness. As productivity demands from customers rise, the cutting speed and feed rate of the tool increase as well, resulting in elevated temperatures at the contact point between the workpiece and the tool. Therefore, there is a strong incentive to increase the oxidation resistance and/or reduce the coefficient of friction of the coating. This study investigates the effect of alloying diboride materials with transition metals, altering the boron-to-metal coating stoichiometry (x = B/Me), mechanical properties, tribological properties, and oxidation resistance of the coating. A Platit Pi411 coating machine equipped with LACS® technology was used to synthesize the coatings. This technology enables magnetron sputtering to be performed from a central cylindrical cathode (SCiL®) while simultaneously running a cathodic arc evaporation process from cylindrical cathodes located in the chamber door (LARC®). For this study, the metal boride (MeB_x) coatings were deposited by concurrent magnetron sputtering of a MeB₂ target and cathodic arc evaporation of a Me target (Me = Ti, TiSi, Cr and Zr) to tune the coating stoichiometry and composition. Nanoindentation tests revealed that this alloying strategy can decrease the B-to-Me ratio from 2.0 to 1.5, resulting in a hardness drop from about 45 GPa to 35-40 GPa. Isothermal annealing tests conducted in air at 600°C for 1 hour showed that decreasing the B-to-Me ratio of the coating effectively doubles the oxidation resistance of the coating. In addition, it was found that the use of ternary boron alloys leads to an even more pronounced increase in oxidation resistance, up to threefold.

Transition metal diboride films are characterized by high mechanical hardness, wear resistance and chemical stability at elevated temperatures. Combination of these advanced properties makes them applicable as protective coatings for alloy-machining tools. Typical nanocomposite structure of these films, consisting of hexagonal P6/mmm columnar grains surrounded by boron tissue phase, leads to high hardness above 37 GPa [1]. However, formation of the boron tissue phase is not convenient in terms of brittle fracture and oxidation resistance, which are the main drawbacks of diboride films. Therefore, one of the ways to improve the properties is to reduce the boron to metal ratio and aim for understoichiometric films [2]. Here, it is important to understand, how the boron deficiency will be accommodated by the films’ structure, because it can significantly affect the mechanical properties [3]. In this work, we study the effect of boron understoichiometry on structure in case of ternary V_1-xW_xB_2-Δ films prepared by magnetron sputtering and ex-situ annealed up to 1200°C. We present results of detailed structural analysis by high-resolution transition electron microscopy, which revealed interesting structural features, including several types of planar defects and coexistence of coherently linked orthorhombic and hexagonal phase accompanied by chemical decomposition.

[1] P.H. Mayrhofer, C. Mitterer, J.G. Wen, J.E. Greene, I. Petrov, Self-organized nanocolumnar structure in superhard TiB2 thin films, Appl. Phys. Lett. 86 (2005) 1–3, https://doi.org/10.1063/1.1887824.

[2] J. Thӧrnberg, B. Bakhit, J. Palisaitis, N. Hellgren, L. Hultman, G. Greczynski, P.O. Å. Persson, I. Petrov, J. Rosen, Improved oxidation properties from a reduced B content in sputter-deposited TiBx thin films, Surf. Coat. Technol. 420 (2021), https://doi.org/10.1016/j.surfcoat.2021.127353.

[3] K. Viskupová, B. Grančič, T. Roch, Š. Nagy, L. Satrapinskyy, V. Šroba, M. Truchlý, J. Šilha, P. Kúš, M. Mikula, Thermally induced planar defect formation in sputtered V1-xMoxB2-Δ films, Scripta Materialia, Volume 229, 2023, 115365, ISSN 1359-6462, https://doi.org/10.1016/j.scriptamat.2023.115365.

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).

Magnetron sputtered transition metal diboride TMB_2-x coatings with significant substoichiometry are often amorphous in the as deposited state [1, 2]. Subsequent annealing at high temperatures can lead to formation of various crystalline phases [3]. In our work, we study the effect of annealing up to 1300°C on structure of substoichiometric TaB_1.2 films. Analysis by High Resolution Transmission Electron Microscopy revealed crystallization into orthorhombic Cmcm structure with high density of annealing twin lamellae. Using ab-initio calculations we show that the formation of stacking faults on {110} planes are energetically favorable and accumulation of such defects can lead to formation of orthorhombic Pnma phase with similar formation energy as Cmcm phase. Moreover, the thermally induced structural changes were accompanied by coatings’ hardness increase from 27 to 34 GPa.

[1] B. Grančič, M. Pleva, M. Mikula, M. Čaplovičová,L. Satrapinskyy, T. Roch, M. Truchlý, M. Sahul, M. Gregor, P. Švec,M. Zahoran, P. Kúš, Stoichiometry, structure and mechanical properties of co-sputtered Ti_1-xTa_xB_2±Δ coatings, Surf. Coat. Technol. 367 (2019) 341–348, https://doi.org/10.1016/j.surfcoat.2019.04.017.

[2] K. Viskupová, B. Grančič, T. Roch, L. Satrapinskyy, M. Truchlý, M. Mikula,V. Šroba, P. Ďurina, P. Kúš, Effect of reflected Ar neutrals on tantalum diboride coatings prepared by direct current magnetron sputtering, Surf. Coat. Technol. 421 (2021), https://doi.org/10.1016/j.surfcoat.2021.127463.

[3] K. Viskupová, B. Grančič, T. Roch, Š. Nagy, L. Satrapinskyy, V. Šroba, M. Truchlý, J. Šilha, P. Kúš, M. Mikula, Thermally induced planar defect formation in sputtered V_1-xMo_xB_2-Δ films, Scripta Materialia, Volume 229, 2023, 115365, ISSN 1359-6462, https://doi.org/10.1016/j.scriptamat.2023.115365.

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).

Alloying with silicon or Si-based phases is an efficient approach to improve the oxidation resistance of transition metal diborides (TMBs). It is well established for bulk ceramics [1,2] but was also recently verified for thin film TMBs such as CrB_2, HfB₂, or TiB₂ [3,4]. Adding these strong oxide formers to diborides results in highly dense and protective SiO₂-based scales, whereas the amount of Si required differs between ternaries (pure Si addition) and quaternaries (alloying via disilicides), respectively. In more detail, alloying of TaSi₂ and MoSi₂ into TiB₂ thin films not only reduces the amount of Si needed to provide excellent oxidation resistance but also minorly influences the mechanical properties. For quaternary TiB₂ based coatings, hardness values of 36 GPa (TaSi₂) and 27 GPa (MoSi₂) compared to around 38 GPa of the binary system have been achieved. Interestingly, for ternary Ti-Si-B_2±z films the mechanical properties vary in a wide range ceasing 20 GPa while exhibiting similar oxidation stabilities. All these coatings crystalized in the α-AlB₂ structure type with a preferred 0001 orientation being decisive for highest hardness.

Meanwhile the oxidation behavior and mechanical properties have been thoroughly described for ternary and quaternary TMBs so far [3,4] the fracture characteristics of these coating materials are rather unexplored. Based on a recent study [4] the fracture toughness (K_IC) of binary TiB_2±z is known to be highly dependent on the amount of tissue phase present at the grain boundaries.

Therefore, this study aims to unravel the fracture resistance of Si containing ternary and quaternary TMBs, focusing on alloying routes with disilicides. A set of different coating compositions has been deposited by non-reactively DC magnetron sputtering using a broad set of composite targets TiB₂, TiB₂/TiSi₂ (90/10 & 80/20 mol%), TiB₂/TaSi₂ (90/10 & 80/20 mol%) and TiB₂/MoSi₂ (85/15 & 80/20 mol%). The elastoplastic behaviour involving also fracture characteristics such as K_IC are evaluated by micro-mechanical testing methods, such as cantilever bending or pillar compression testing, as well as nanoindentation. For an in-depth understanding, these results are correlated with detailed structure-morphological investigations using XRD, SEM, TEM, or ERDA/RBS.

[1] GB. Raju, et al,. J Am Ceram Soc. 2008;91(10):3320–3327.

[2] GB. Raju, et al., Scr Mater. 2009;61(1):104–107.

[3] T. Glechner, et al., Surf. Coat. Technol. 434 (2022) 128178.

[4] A. Bahr, et al.,Materials Research Letters. 11 (2023) 733–741.

Ultra-high temperature ceramics based on transition-metal diborides (TMB₂) attract attention due to their excellent chemical and mechanical properties. However, their practical application is limited by their inherent brittleness. The Y-B system offers a number of stoichiometric phases with a wide range of mechanical properties. The YB₂ has the lowest Young's modulus among the TMB₂, while the YB₆ is predicted to be a ductile ceramic. On the other hand, the YB₄ has the highest hardness among the Y-B phases, a melting temperature of 2800 °C, and a high flexural strength of 317 MPa. However, only a limited amount of knowledge is available about its properties in the form of a thin film. In this work, we focus on YB₄ thin film prepared by High Target Utilization Sputtering (HiTUS) technology using a stoichiometric YB₄ target. We report the evolution of the chemical composition, nanostructure, and mechanical properties after vacuum annealing up to 1300 °C. The EDS/WDS analyses show a slight over-stoichiometry of the sputtered film with a B/Y ratio of 4.7, a low oxygen content below 2 at.%, and a stable chemical composition with no boron loss. The as-deposited film is X-ray amorphous with a hardness of 23.4 GPa and a low Young's modulus of 281 GPa. After annealing at 800 °C, a partial crystallization occurs, and small regions with the tetragonal YB₄ phase in the amorphous matrix can be recognized. Further annealing leads to the grain coarsening of the YB₄ phase and formation of smaller grains of the cubic YB₆ phase. According to the XRD and HR-STEM, the coherence domain size increases from 26 nm to 44 nm for the YB₄ phase and from 7 to 53 nm for the YB₆ phase. The changes in the nanostructure are reflected in the mechanical properties of the film. The hardness increases to 26.4 GPa at 1000 °C, while a relatively low Young's modulus of 322 GPa is maintained. The ductile/brittle response to the mechanical loading is examined by cube-corner indentation. In the case of as-deposited film, a material pile-up at the edges with no crack formation is observed, indicating a ductile behavior of the X-ray amorphous film. After annealing, a crack formation is observed at the corners of the indents, which is a sign of a more brittle response. To conclude, the exceptional properties of YB₄ ceramic are confirmed in the form of thin sputtered films and are maintained even after high temperature loading. Therefore, the YB₄ thin film is a suitable candidate for example in superlattice architecture, where it would play the role of a medium-hard but less brittle layer in order to improve the overall toughness and thermal shock resistance of the coating.