ICMCTF 2025 Session MA3-3-WeM: Hard and Nanostructured Coatings III

Molybdenum oxide MoO_x films (x = 2, 3) are attractive for a variety of applications, but exclusive phase selection remains challenging due to the presence of multiple polymorphs. To achieve pure van der Waals epitaxy (vdWE), which relies on van der Waals forces for strain-free growth, we explored phase control of molybdenum oxide (MoOₓ), aiming to grow both non-layered monoclinic MoO₂ and layered orthorhombic α-MoO₃.

By adjusting the oxygen pressure p_O2 during reactive magnetron sputtering, we demonstrate control over phase selection, texture, and epitaxy in MoO_x films on f-mica and c-sapphire. Our results show non-layered monoclinic MoO₂ formation on both f-mica and c-sapphire at 500 °C for 0.1 ≤ p_O2 ≤ 0.25, outside which the films are amorphous. The result is similar at 400 °C except for layered orthorhombic α-MoO₃ formation at high p_O2 and a greater sensitivity of phase selection to p_O2. High p_O2 fosters large thin-sheet crystals with texture and/or epitaxy, while low p_O2 results in fine-grained flowery microstructure. Pole figure analyses reveal that the orthorhombic α-MoO₃ films on f-mica exhibit van der Waals epitaxy [1] wherein the out-of-plane 0k0 d-spacings are invariant with film thickness, indicative of negligible interfacial strain. In contrast, α-MoO₃ films on c-sapphire showed 0k0 fiber texture. In the case of MoO₂, epitaxy occurs on both f-mica and c-sapphire characterized by conventional epitaxy with strong interfacial bonding, promoting strain in the film but enables high crystalline quality and controlled orientation on both substrates [2]. The aforementioned results provide a framework for the controlled synthesis of MoO_x films with tunable orientation, epitaxy, strain, and microstructure for applications.

[1] E. Ekström, et.al., Materials & Design 229 (2023) 111 864.

[2] F.A. Lahiji, et.al., Thin Solid Films (2024) 140566.

Zr-Cu-based thin-film metallic glasses (TFMGs) have emerged as a promising class of materials due to their exceptional properties such as high glass-forming ability, superior elastic strain limit, enhanced hardness and plasticity. Moreover, these TFMGs offer the potential to be combined with nanocrystalline materials (transition metals or metal nitrides) to create heterogeneous dual-phase nanocomposite structures and thus achieving a better balance of toughness and hardness and/or unlocking new functionalities.

Therefore, this study explores the effect of gradual addition of W (negative mixing enthalpy with Zr but positive with Cu) and Nb (positive mixing enthalpy with both Zr and Cu) on microstructure and properties of Zr-Cu-based TFMGs. Two film series, W-Zr-Cu and Nb-Zr-Cu, were prepared, keeping Zr:Cu as 1:1 and gradually varying the W and Nb content in the respective series. Each deposition was done in Ar using three magnetrons equipped with Zr and W/Nb targets operated in the dc regime and a Cu target in the HiPIMS regime.

A systematic investigation revealed that W and Nb additions have a significant impact on microstructure and other properties of the films. The films remain amorphous with smooth surfaces (roughness < 2 nm) up to 65 at.% of W or Nb, displaying vein-like features typical of metallic glasses upon fracture. W-Zr-Cu films with 67 at.% W are characterized by a combination of featureless structures (amorphous-like) close to the substrate and thin columns in the upper part of the film. Films with even higher W contents grow in a V-shaped columnar microstructure corresponding to the bcc α-W crystalline structure. Nb-Zr-Cu films with 70 at.% Nb clearly exhibit a dual-phase structure with thin columns surrounded by vein-like features. Further increase in the Nb content above 70 at.% leads to the formation of a crystalline structure with parallel columns and very small voids. These voids tend to vanish with increasing Nb content. A gradual increase in hardness and reduced Young’s modulus is observed with increasing W content for the amorphous W-Zr-Cu films and the crystalline films show an enhancement in hardness of up to 15% compared to films with pure W due to solid solution hardening. In the case of Nb-Zr-Cu films with up to 70 at.% Nb, the hardness remains nearly constant. However, further addition of Nb results in a decreased hardness and this reduction might be attributed to a less dense structure of the films.

Cu and Cu-based films, known for their superior electrical and thermal conductivity, find primary applications in electronic devices and the electrical industry. However, the implementation of various strengthening mechanisms often compromises the conductivity. Therefore, it is crucial to carefully control the structural state and composition of these films to achieve an optimal balance between mechanical strength and conductivity.

In this work, we prepared nanocrystalline Cu-Zr films with a minor Zr content ranging from 0 to 2.7 at.% using non-reactive direct current magnetron co-sputtering of separate Cu and Zr targets in pure Ar at a pressure of ~0.5 Pa without substrate bias and external heating. The effects of Zr alloying on the structure, surface, mechanical, and electrical properties were systematically investigated using X-ray diffraction, electron microscopy, atomic force microscopy, indentation, and the four-point probe method. We demonstrate that Zr alloying within the investigated composition range is an effective approach for modifying the structural state and properties of sputter-deposited films, with the most notable changes observed between 0.3 and 1.3 at.% Zr. Beyond this range, only minor changes in the microstructure and mechanical properties are observed, while the solubility, electrical resistivity, and surface roughness continue to rise.

High entropy alloy (HEA) nitride thin films have attracted considerable attention from the global industrial and academic communities due to their excellent mechanical properties. HEA multilayer nitride films also exhibit good interfacial stability, outstanding mechanical performance, and superior corrosion resistance. In this study, AlCrNbSiTiN/AlCrN nitride multilayer thin films were deposited using a high power impulse magnetron sputtering (HiPIMS) system with AlCrNbSiTi and AlCr targets in a mixed argon and nitrogen atmosphere. By adjusting the residence time of the substrates in the plasma regions of the AlCrNbSiTi and AlCr targets, multilayered thin films with varied bilayer periodic thicknesses ranging from 6 to 40 nm were fabricated. For the multilayer thin film with 15 nm bilayer period thickness, the thickness ratios of AlCrNbSiTiN and AlCrN single layer were adjusted to evaluate their influence on the hardness and corrosion resistance of films. XRD analysis indicated that all AlCrNbSiTiN/AlCrN multilayer films, as well as single-layer AlCrNbSiTiN and AlCrN films, exhibited a face-centered cubic crystal structure. Notably, the AlCrNbSiTiN/AlCrN multilayer film with a 15 nm bilayer period demonstrated a high hardness of 28 GPa and excellent corrosion resistance in 0.5 M H₂SO₄ aqueous solution, with a corrosion impedance value of 1.19 × 10⁶W.cm². The influence of AlCrNbSiTiN to AlCrN thickness ratios on the mechanical properties and corrosion resistance of AlCrNbSiTiN/AlCrN multilayer thin film with 15 nm bilayer period was explored in this work.

Nowadays, Cr₃C₂-based cermet hardface coatings manufactured by advanced HVOF processes are well recognized for their corrosion and erosion resistance, particularly at high temperatures. Their lightweight nature and high temperature capability make them an attractive alternative to WC-based alloy coatings and hard Cr plating coatings. The objective of this study is to develop optimal Cr₃C₂-NiCr coatings by comparing different feedstock materials, including feedstock with nanocrystalline and/or submicron sized Cr₃C₂ phases. The focus of the investigation is on understanding the impact of feedstock features such as particle size, morphology, and nanocrystalline carbide sizes, as well as sliding abrasive wear conditions on the coating properties and sliding wear performance. The results of the study indicate that the sliding wear resistance of the Cr₃C₂-NiCr coatings is highly influenced by the features of the Cr₃C₂ carbides. With the special interest of nano-crystaline and or submicron-sized carbides, the presence of nano, submicron and a few microns sized carbides in the coatings was revealed to improve their density, residual stress and hardness, leading to a significant reduction in wear rates under test conditions. Furthermore, the size of the abrasive SiC grit on the counter surface plays a significant role in determining the sliding wear behavior of these coatings. Based on the analysis of the test data, the mechanisms behind the performance of the Cr₃C₂-NiCr coatings have been investigated and used to interpret their sliding wear behaviors. This study has identified and recommended optimized materials for improved coating properties based on the key findings and results analyses. These findings and model analyses contribute to the understanding of the relationship between feedstock features, sliding abrasive wear conditions, and the wear rates of HVOF-sprayed Cr₃C₂-NiCr coatings. Hence, the optimized manufacture method by advanced HVOF method will meet the on-going need for a robust alternative solution to hard chromium plating (HCP) method.

Multilayer coatings offer significant advantages in protecting materials’ surfaces by shielding the underlying materials hierarchically from damage and wear. The layering morphology and structure of multilayer coatings directly affect their wear resistance capacity. Using a systematic set of experiments and molecular dynamic simulations, we studied the effect of layering thickness on the macroscale wear response of DLC/WC multi-nanolayer coatings. Our study revealed the existence of a critical bilayer thickness where maximum scratch hardness and wear resistance can be achieved. Our large-scale molecular dynamics simulations showed that reducing the WC layer thickness to a certain limit increases the scratch hardness due to the confinement of dislocation motion. However, when the thickness of WC layers falls below 2 nm, the deformation mechanism transitions from the interface-induced dislocation confinement to the interface-mediated amorphization of WC layers, reducing the scratch hardness of the coating. This finding offers a procedure for optimizing the macroscale wear performance of multi-nanolayer coatings.