Hardening Ab initio molecular dynamics A B S T R A C T Rocksalt-structure (B1) (V,Mo)N alloys are... more Hardening Ab initio molecular dynamics A B S T R A C T Rocksalt-structure (B1) (V,Mo)N alloys are inherently hard and tough ceramics. However, the mechanical properties and thermal stability of (V,Mo)N solid solutions at temperatures ⪆ 700 • C of relevance for practical applications have not been previously investigated. In this work, we synthesize single-phase B1 polycrystalline V 0.57 Mo 0.43 N 0.95 coatings to investigate the effects induced by temperature on the nanostructural evolution and hardness (H) of the material. Nanoindentation measurements show that the as-deposited film (H = 23 ± 3 GPa) becomes ≈30% harder (up to 31 ± 2 GPa) upon annealing at 730 • C. Experimental characterization and analyses, based on dispersive X-ray spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), reveal that the age-hardening effect originates from decomposition of the solid solution into coherent strained cubic VN-rich/MoN-rich domains. The experimental results are complemented by the composition/temperature (V,Mo)N phase diagram-constructed upon ab initio molecular dynamics free-energies-which indicates that the separation observed in the solid solutions is of spinodal nature. Films annealed at temperatures exceeding 850 • C undergo structural coarsening, with formation of hexagonal Mo x N y and cubic VN phases, which cause a decrease in hardness to ≈22 GPa. Our present findings indicate that (V,Mo)N coatings may offer outstanding mechanical performances during operation at elevated temperatures.
Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/... more Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/ propagation leading to brittle failure during application, is a primary industrial requirement. In this work, experimental methods supported by ab initio density functional theory (DFT) calculations and electronic structure analyses are used to investigate the mechanical behavior of magnetron sputtered Ti-Al-Ta-N hard coatings. The as-deposited Ti 1-x-y Al x Ta y N (y ¼ 0-0.60) films exhibit a single phase cubic sodium chloride (B1) structure identified as TiAl(Ta)N solid solutions. While the hardness H of Ti 0.46 Al 0.54 N (32.5 6 2 GPa) is not significantly affected by alloying with TaN (H of the quaternary nitrides varies between 26 6 2 and 35 6 4 GPa), the elastic stiffness monotonically decreases from 442 to 354 GPa with increasing Ta contents, which indicates improved toughness in TiAlTaN. Consistent with the experimental findings, the DFT results show that Ta substitutions in TiAlN reduce the shear resistance due to the enhanced occupation of metal-metal bonding states while preserving strong metal-N bonds. The metal-N bonding character, however, is progressively modified from prevalently ionic (TiAlN) toward more covalent (TiAlTaN).
Hardening Ab initio molecular dynamics A B S T R A C T Rocksalt-structure (B1) (V,Mo)N alloys are... more Hardening Ab initio molecular dynamics A B S T R A C T Rocksalt-structure (B1) (V,Mo)N alloys are inherently hard and tough ceramics. However, the mechanical properties and thermal stability of (V,Mo)N solid solutions at temperatures ⪆ 700 • C of relevance for practical applications have not been previously investigated. In this work, we synthesize single-phase B1 polycrystalline V 0.57 Mo 0.43 N 0.95 coatings to investigate the effects induced by temperature on the nanostructural evolution and hardness (H) of the material. Nanoindentation measurements show that the as-deposited film (H = 23 ± 3 GPa) becomes ≈30% harder (up to 31 ± 2 GPa) upon annealing at 730 • C. Experimental characterization and analyses, based on dispersive X-ray spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), reveal that the age-hardening effect originates from decomposition of the solid solution into coherent strained cubic VN-rich/MoN-rich domains. The experimental results are complemented by the composition/temperature (V,Mo)N phase diagram-constructed upon ab initio molecular dynamics free-energies-which indicates that the separation observed in the solid solutions is of spinodal nature. Films annealed at temperatures exceeding 850 • C undergo structural coarsening, with formation of hexagonal Mo x N y and cubic VN phases, which cause a decrease in hardness to ≈22 GPa. Our present findings indicate that (V,Mo)N coatings may offer outstanding mechanical performances during operation at elevated temperatures.
Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/... more Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/ propagation leading to brittle failure during application, is a primary industrial requirement. In this work, experimental methods supported by ab initio density functional theory (DFT) calculations and electronic structure analyses are used to investigate the mechanical behavior of magnetron sputtered Ti-Al-Ta-N hard coatings. The as-deposited Ti 1-x-y Al x Ta y N (y ¼ 0-0.60) films exhibit a single phase cubic sodium chloride (B1) structure identified as TiAl(Ta)N solid solutions. While the hardness H of Ti 0.46 Al 0.54 N (32.5 6 2 GPa) is not significantly affected by alloying with TaN (H of the quaternary nitrides varies between 26 6 2 and 35 6 4 GPa), the elastic stiffness monotonically decreases from 442 to 354 GPa with increasing Ta contents, which indicates improved toughness in TiAlTaN. Consistent with the experimental findings, the DFT results show that Ta substitutions in TiAlN reduce the shear resistance due to the enhanced occupation of metal-metal bonding states while preserving strong metal-N bonds. The metal-N bonding character, however, is progressively modified from prevalently ionic (TiAlN) toward more covalent (TiAlTaN).
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Papers by M. Truchly