ICMCTF 2026 Session MA2-2-WeM: Hard and Nanostructured Coatings II

Magnetron sputter deposition has been demonstrated to be a suitable technique for synthesizing metallic glasses as thin films (TFMGs). Thanks to the non-equilibrium conditions of low-temperature plasma and extremely high cooling rates at the atomic scale on the substrate, TFMGs can be prepared with a much wider composition variety and solubility than bulk metallic glasses (BMGs). Moreover, TFMGs exhibit properties and characteristics that surpass those of BMGs as well as conventional metallic and ceramic coatings, particularly in achieving an optimized balance between ductility and strength.

The amorphous structure of TFMGs, characterized by short- and medium-range atomic ordering, combined with their exceptional properties, offers opportunities to create dual-phase architectures incorporating both TFMGs and crystalline materials. These architectures have the potential to overcome the limitations inherent to each constituent phase while enhancing existing properties or even enabling novel functionalities through synergistic phase interactions.

Dual-phase crystalline-amorphous coatings based on TFMGs can be relatively easily prepared in multilayer architectures comprising alternating crystalline and TFMG sublayers. We demonstrated this concept with multilayer Zr-Cu-N coatings consisting of hard ceramic ZrN and ductile glassy ZrCu sublayers. The coatings exhibited enhanced damage tolerance due to effective crack deflection at sublayer interfaces, yielding superior fracture stress and toughness values. Incorporating ZrN-Cu nanocomposite surface sublayers further imparted antibacterial functionality, expanding their potential applications.

The formation of dual-phase crystalline-amorphous coatings based on TFMGs in a nanocomposite architecture presents significant challenges. However, we successfully synthesized such coatings in the Zr-Cu-N and Zr-Cu-B systems using a one-step process of reactive and non-reactive magnetron co-sputtering, respectively. The coatings prepared under optimized conditions were nanocomposites comprising nanocrystalline ZrN or ZrB₂ and glassy ZrCu phases, representing a novel class of nanocomposite coatings combining ceramic and TFMG phases.

Introduction: Ceramics are widely used in various structural and functional applications; however, their intrinsic brittleness at room temperature remains a critical challenge, often leading to early-stage catastrophic failures. This brittleness arises primarily due to the high critical-resolved shear stress required to initiate dislocation movement and the limited number of operational slip systems. Addressing this limitation is crucial for the development of ceramics with improved mechanical properties.This study aims to develop a novel strategy for enhancing the deformability of ceramics by leveraging negative stacking fault energy (SFE). The approach seeks to reduce the energetic barriers to dislocation motion and expand the number of available slip systems, ultimately improving room-temperature plasticity while maintaining high strength and toughness. In this work,a TiN/TaN superlattice was fabricated and subjected to in-situ micro-mechanical testing to evaluate its mechanical response. Post-mortem transmission electron microscopy (TEM) was employed to analyze deformation mechanisms at the atomic scale, providing insights into the role of negative SFE in promoting dislocation activity, atomic plane faulting, and twinning.The TiN/TaN superlattice exhibited remarkable room-temperature compressive plasticity (~43%), attributed to extensive atomic plane faulting and twinning facilitated by negative SFE. This behavior enabled an exceptional combination of plasticity, strength, and toughness, demonstrating the feasibility of overcoming the brittleness barrier in ceramics.

References: Huang, Y., et al. (2025). "Harvesting superior intrinsic plasticity in nitride ceramics with negative stacking fault energy." Acta Materialia: 120774.

Acknowledgement:The authors would like to thank Dr. Christian Mitterer, Velislava Terziyska (Montanuniversität Leoben) for the film deposition. We sincerely thank Rainer Hahn and Helmut Riedl (TU Wien) for their invaluable assistance with the micropillar compression tests. We also appreciate Verena Maier-Kiener (Montanuniversität Leoben) for her support with the nanoindentation tests and Michael Meindlhumer (Montanuniversität Leoben) for his help with the microcantilever bending tests. Additionally, we acknowledge David Holec, Thomas Leiner, and Lukas Hatzenbichler (Montanuniversität Leoben)for their contributions to the DFT calculations. Also, thanks to Zequn Zhang (ESI) and Yonghui Zheng for their help with microscopy. The financial support (Y.H., Z.C., and Z.L.Z.) by the Austrian Science Fund (PAT 1946623) is highly acknowledged.

Superlattice architecture presents a promising strategy for the simultaneous enhancement of hardness and fracture toughness in hard ceramic films. We demonstrate the success of this approach in transition metal diboride films and report the structural and mechanical properties of films composed of nanocrystalline ZrB_2+xand disordered TaB_2-y layers. Superlattice films with a wide range of bilayer periods (Λ = 1.8–31.5 nm) were prepared by magnetron sputtering. Deposition was performed at 300 °C with a floating bias to minimize interdiffusion. The formation of sharp interfaces for all Λ values is confirmed by X-ray reflectivity. The films consist of strongly understoichiometric TaB_1.4 layers, which lack long-range ordering, and overstochiometric ZrB_2.6 layers with a preferential (001) crystalline orientation. With decreasing Λ, we observe a change in preferential orientation and the formation of a true superlattice structure, evidenced by satellite peaks. This indicates crystallization of the TaB_1.4 layers, as confirmed by STEM data which shows both layers exhibiting a (001)-oriented hexagonal structure. This is a result of two effects: locally induced stabilization by the underlying ZrB_2+xlayer and boron diffusion at the interface, enhanced by the boron concentration gradient and the bombardment of Ar neutrals reflected from the targets. This transition is accompanied by a remarkable increase in hardness from 34.1 ± 1.9 to 47.2 ± 2.3 GPa as Λ decreases to 3.4 nm. The observed hardening exceeds estimations based on Koehler’s strengthening mechanism for two layers with a shear modulus difference of only 39 GPa. Improved mechanical properties are observed also from DFT calculations for defect-free ZrB₂/TaB₂ cells (Λ = 1.4–8.2 nm), which reveal a stabilizing effect with decreasing Λ and a significant increase in stiffness, peaking at Λ = 2.7 nm. At the same time, the fracture toughness K_IC, obtained from notched cantilever bending tests, increases from 3.3 ± 0.2 MPa·m^1/2 (average of both monolithic films) to 4.6 ± 0.3 MPa·m^1/2 for the superlattice film with Λ = 1.8 nm. This improvement is attributed to coherent stresses at the interfaces due to lattice mismatch. The suppression of brittle response under mechanical load is also confirmed by cube-corner indents, which show shorter radial cracks with decreasing Λ. This work demonstrates that the superlattice approach is highly effective in transition-metal diborides and highlights the crucial role of stoichiometry. It was supported by the Slovak Research and Development Agency (Grant No. APVV-21-0042 and APVV-24-0038), Scientific Grant Agency (Grant No. VEGA 1/0473/24) and COLOSSE project (No. 101158464).

The addition of Al to cubic transition-metal nitrides (TMNs) has been a cornerstone in the development of hard, protective coatings for high-performance applications. Increasing the Al content in cubic TMNs typically enhances hardness, oxidation resistance, and thermal stability. However, under thermodynamic equilibrium, AlN and cubic TMNs are immiscible, as AlN favours the wurtzite (w-) structure. The formation of hexagonal w-AlN, in turn, is generally detrimental to mechanical performance and abrasive-wear resistance.

Theoretical and experimental studies have shown that high compressive residual stresses, high deposition rates, and low substrate temperatures can extend the metastable cubic solubility limit (along with Al subplantation or coherency-strain-stabilised multilayer architectures, which, however, are outside the scope of this study). This behaviour is well captured by metastable phase-diagram calculations for magnetron-sputtered (Ti,Al)N and (V,Al)N films [1,2].

High reported Al solubility levels in cubic TMNs are often inferred solely from the absence of w-AlN reflections in X-ray diffractograms. We investigated a series of sputter-deposited (Al,Cr)N films with Al metal fractions up to 0.72, which exhibited only fcc reflections in X-ray diffractograms. However, selected-area electron diffraction revealed the presence of w-AlN, indicating that the true solubility limit is lower. We discuss the influence of residual stress, substrate temperature, and deposition kinetics on the stabilisation of metastable cubic solid solutions, and propose indirect, experimentally accessible indicators for the onset of wurtzite-phase formation. These findings refine the understanding of Al solubility in (Al,Cr)N and, more broadly, in cubic TMNs.

References:

[1] S. Liu et al., Acta Mater. 165 (2019) 615–625.

[2] S. Liu et al., Acta Mater. 196 (2020) 313–324.