ICMCTF 2025 Session MA3-2-TuA: Hard and Nanostructured Coatings II

This talk presents advanced methods in combinatorial synthesis and microstructural design to achieve extraordinary properties in multielement alloys and layered coatings. Using the CrCuTiW alloy system as a primary example, we demonstrate how large compositional variations and the limited miscibility between elements lead to diverse self-assembled multicomponent phases,combining solid solutions, nanocomposites, and metallic glasses. These structures exhibit unexpected combinations of hardness and elastic modulus, demonstrating the potential for unique property tailoring.

In a second example, a cross-sectional combinatorial synthesis of nanostructured CrMnFeCoNi alloy is employed to address the thermal stability of this metastable alloy. This approach enables an in-depth analysis of segregation kinetics in the primary phase at moderate temperatures (50-450°C) resulting in the formation of a variety of coexisting phases that enhance alloy strength while maintaining ductility and fracture toughness. This approach demonstrates its capability to provide insights into the thermal behavior of complex, metastable microstructures and allows for controlled property enhancement.

Additionally, the talk emphasizes a bio-inspired approach to compositional and microstructural design withina layered Zr-Cu-N system, where antibacterial properties are combined with enhanced fracture toughness and stress resistance. These multifunctional coatings represent a new class of sustainable materials, suitable for both hard and smart coating applications.

This study rigorously investigates the transformative potential of the Pulsed-DC Powder-Pack Boriding (PDCPB) process to catalyze boride layer formation on AISI H13 steel at remarkably reduced temperatures (600°C, 650°C, and 700°C) under substantial current densities (~952mA·cm⁻²) and significantly minimized exposure times of 1800s, 2700s, and 3600s. Enabled by the implementation of a custom high-capacity power supply, this innovation generates the essential electric field to support boriding at unprecedented low temperatures. Traditionally, achieving similar results in AISI H13 required treatments at temperatures exceeding 900°C with exposure times of at least 14400s, underscoring the extraordinary advancement represented by this approach.

Through meticulous microstructural and physicochemical analyses using SEM–EDS and XRD, the study reveals substantial findings: at a mere 600°C, PDCPB successfully produced dense, biphasic FeB+Fe₂B layers with thicknesses ranging from ~8µm to ~17µm, uniformly distributed across the sample surfaces. Remarkably, and contrary to established reports on borided AISI H13, the substrate retained its α-phase microstructure without transformation to the α'-phase, and the interface between the boride layer and substrate remained free of any diffusion zone. This breakthrough not only introduces significant commercial scalability for low-temperature boriding but also opens possibilities for further innovations, potentially achieving effective boriding near the 530°C threshold. The insights presented mark a seminal advancement in boriding technology, with vast implications for industrial applications and the future of materials engineering.

Brittleness and poor fracture toughness are limiting factors for the application of hard protective coatings. To resolve these issues, we explore multilayer superlattice (SL) coating designs based on HfN_1.33 and Hf_0.76Al_0.24N_1.15. We achieve high-quality single-crystal films and superlattices with superior mechanical characteristics by epitaxial growth on MgO(001) substrates using ion-assisted reactive magnetron sputtering at high temperatures.

The structure and properties of monolithic single-crystal HfN_1.33 and Hf_0.76Al_0.24N_1.15 are studied to evaluate the SL-coating performance. Overstoichiometric HfN_y exhibits metal-like ductility in micropillar compression tests, with easy dislocation nucleation and movement along multiple {111}<110> slip systems, which results in significant strain hardening and a doubled ultimate strength at 17% strain, compared to the yield point at 2%. The improved ductility is attributed to point defects—vacancies and nitrogen interstitials—forming a checkerboard superstructure of hyper-overstoichiometric and near-pristine domains. In contrast, HfAlN shows improved hardness and yield strength in pillar compression, however, it fails by strain-burst with fractures on the {110}<110> slip system. These properties stem from strain fields, pinning dislocations, which develop between coherent Hf- and Al-rich nanodomains, formed by surface-initiated spinodal decomposition. In addition, the domains similarly self-organize into a checkerboard superstructure.

Thus, by combining overstoichiometric HfN_1.33 and Hf_0.76Al_0.24N_1.15 in SLdesigns with equal layer thicknesses but varying bilayer period of 20 nm, 10 nm, and 6 nm, fascinating three-fold superstructured SLs are created by checkerboard superstructuring in 1) the HfN layers and 2) the HfAlN layers, as well as 3) the multilayer structure itself. While the interfaces provide dislocation pinning to maintain an equally high hardness as Hf_0.76Al_0.24N_1.15, about 20% higher than HfN_1.33, other multilayer effects and inherent ductility of HfN_1.33 enhance the toughness through coherency strains, crack-tip blunting or deflection. The SLs are analyzed using X-ray diffraction, reciprocal space maps, high-resolution z-contrast scanning transmission electron microscopy, selected area electron diffraction, nanoindentation, and micropillar compression tests. Post-mortem imaging of the pillars reveals the underlying plastic deformation mechanisms. Superlattice effects enhance mechanical performance, combining properties of both materials for coatings with high hardness and improved toughness, ideal for advanced protective applications.