By combining experimental and theoretical methods, we have conducted a detailed study of the tern... more By combining experimental and theoretical methods, we have conducted a detailed study of the ternary diboride system (W 1-x Al x) 1-y B 2(1-z). Tungsten rich solid solutions of (W 1-x Al x) 1-y B 2(1-z) were synthesized by physical vapor deposition and subsequently investigated for structure, mechanical properties and thermal stability. All crystalline films show hardness values above 35 GPa, while the highest thermal stability was found for low Al contents. In this context, the impact of point defects on the stabilization of the AlB 2 structure type is investigated, by means of ab initio methods. Most notably, we are able to show that vacancies on the boron sublattice are detrimental for the formation of Al-rich (W 1-x Al x) 1-y B 2(1-z) , thus providing an explanation why only tungsten rich phases are crystalline.
Coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughne... more Coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughness, similar to the reported effect in indentation hardness. We found-by employing in-situ micromechanical cantilever bending tests on free-standing TiN/CrN superlattice films-that the fracture toughness increases with decreasing bilayer period (Λ), reaching a maximum at Λ ~ 6 nm. For ultrathin layers (Λ ~ 2 nm), the fracture toughness drops to the lowest value due to intermixing and loss of superlattice structure. Both, fracture toughness and hardness peak for similar bilayer periods of TiN/CrN superlattices. Hard coatings are used to protect engineering components, e.g. cutting tools, from severe external loads and harsh environments [1]. Thereby, the coatings should ideally be strong and tough. Multilayer coatings composed of two coherently stacked, alternating materials with a periodicity length in the nanometer range, referred to as superlattice films, have been reported to possess exceptional high hardness values exceeding that of their single layered constituents by some hundred percent. In the 1980s, Helmersson et al. [2] reported on a hardness enhancement of up to ~250% compared to single-layered materials for the single-crystalline coherent TiN/VN superlattice (SL) structure grown by physical vapor deposition on single crystalline MgO (100) substrates. Thereby, the peak hardness was found for a periodicity length of ~ 5 nm. Later, an hardness enhancement was observed for a row of other SL film systems grown on MgO (100), but also on (native oxide) of Si (100) and polycrystalline steel substrates [3]. Besides high hardness values, a sufficiently high fracture toughness is needed to ensure the integrity of bulk and coated engineering components. Unfortunately, these material properties are commonly mutually influential (especially for materials showing plastic behavior), as a high strength often implies a low fracture toughness and vice versa [4]. In the last decades various strategies have successfully shown how to break down this relationship, spanning from grain refinement toughening based on the classical Hall-Petch relation used in a variety of steels [5,6]-to recently found nanoscaled twinning mechanisms being operative in high-entropy alloys-enabling exceptional high fracture-resistance even at cryogenic temperatures [7]-and several other mechanisms presented in Ref. [4]. Strategies for enhancing the (fracture) toughness of ceramic coatings (see review by Zhang et al. [8]) include: incorporating a ductile phase; toughening through a nanocrystalline microstructure, composition, or structure grading; multilayer structur-ing; phase transformation toughening; or apparent toughening by implementation of compressive stresses, most of them being already effectively applied in industrial products. However, the exceptional effect of a superlattice structure on the fracture toughness has yet not been reported. Here, we study the influence of the superlattice structure on the fracture toughness. Therefore, we have conducted micromechanical experiments on freestanding superlattice coatings with different bilayer periods (Λ). The isomorphous face-centered cubic (B1) TiN/CrN superlattice grown on Si (100) substrates served as a model system. The constituents TiN and CrN represent one of the most widely used nitrogen based hard coating materials and their shear moduli (~180 GPa [9] and ~ 135 GPa [10], respectively) are significantly different, which promotes the superlattice effect [11]. TiN/CrN multilayer films with equal thick layers-bilayer periods ranging from ~2 to 200 nm, and total film thicknesses of ~2 μm-were synthesized by dc unbalanced reactive magnetron sputtering. All films were grown on Si (100) substrates (7 × 20 × 0.38 mm 3) in an AJA International Orion 5 magnetron sputtering system equipped with one two-inch Cr and one three-inch Ti target (both from Plansee Composite Materials GmbH, 99.6 at.% purity). Prior to the deposition, the substrates Scripta Materialia 124 (2016) 67-70
By combining experimental and theoretical methods, we have conducted a detailed study of the tern... more By combining experimental and theoretical methods, we have conducted a detailed study of the ternary diboride system (W 1-x Al x) 1-y B 2(1-z). Tungsten rich solid solutions of (W 1-x Al x) 1-y B 2(1-z) were synthesized by physical vapor deposition and subsequently investigated for structure, mechanical properties and thermal stability. All crystalline films show hardness values above 35 GPa, while the highest thermal stability was found for low Al contents. In this context, the impact of point defects on the stabilization of the AlB 2 structure type is investigated, by means of ab initio methods. Most notably, we are able to show that vacancies on the boron sublattice are detrimental for the formation of Al-rich (W 1-x Al x) 1-y B 2(1-z) , thus providing an explanation why only tungsten rich phases are crystalline.
Coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughne... more Coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughness, similar to the reported effect in indentation hardness. We found-by employing in-situ micromechanical cantilever bending tests on free-standing TiN/CrN superlattice films-that the fracture toughness increases with decreasing bilayer period (Λ), reaching a maximum at Λ ~ 6 nm. For ultrathin layers (Λ ~ 2 nm), the fracture toughness drops to the lowest value due to intermixing and loss of superlattice structure. Both, fracture toughness and hardness peak for similar bilayer periods of TiN/CrN superlattices. Hard coatings are used to protect engineering components, e.g. cutting tools, from severe external loads and harsh environments [1]. Thereby, the coatings should ideally be strong and tough. Multilayer coatings composed of two coherently stacked, alternating materials with a periodicity length in the nanometer range, referred to as superlattice films, have been reported to possess exceptional high hardness values exceeding that of their single layered constituents by some hundred percent. In the 1980s, Helmersson et al. [2] reported on a hardness enhancement of up to ~250% compared to single-layered materials for the single-crystalline coherent TiN/VN superlattice (SL) structure grown by physical vapor deposition on single crystalline MgO (100) substrates. Thereby, the peak hardness was found for a periodicity length of ~ 5 nm. Later, an hardness enhancement was observed for a row of other SL film systems grown on MgO (100), but also on (native oxide) of Si (100) and polycrystalline steel substrates [3]. Besides high hardness values, a sufficiently high fracture toughness is needed to ensure the integrity of bulk and coated engineering components. Unfortunately, these material properties are commonly mutually influential (especially for materials showing plastic behavior), as a high strength often implies a low fracture toughness and vice versa [4]. In the last decades various strategies have successfully shown how to break down this relationship, spanning from grain refinement toughening based on the classical Hall-Petch relation used in a variety of steels [5,6]-to recently found nanoscaled twinning mechanisms being operative in high-entropy alloys-enabling exceptional high fracture-resistance even at cryogenic temperatures [7]-and several other mechanisms presented in Ref. [4]. Strategies for enhancing the (fracture) toughness of ceramic coatings (see review by Zhang et al. [8]) include: incorporating a ductile phase; toughening through a nanocrystalline microstructure, composition, or structure grading; multilayer structur-ing; phase transformation toughening; or apparent toughening by implementation of compressive stresses, most of them being already effectively applied in industrial products. However, the exceptional effect of a superlattice structure on the fracture toughness has yet not been reported. Here, we study the influence of the superlattice structure on the fracture toughness. Therefore, we have conducted micromechanical experiments on freestanding superlattice coatings with different bilayer periods (Λ). The isomorphous face-centered cubic (B1) TiN/CrN superlattice grown on Si (100) substrates served as a model system. The constituents TiN and CrN represent one of the most widely used nitrogen based hard coating materials and their shear moduli (~180 GPa [9] and ~ 135 GPa [10], respectively) are significantly different, which promotes the superlattice effect [11]. TiN/CrN multilayer films with equal thick layers-bilayer periods ranging from ~2 to 200 nm, and total film thicknesses of ~2 μm-were synthesized by dc unbalanced reactive magnetron sputtering. All films were grown on Si (100) substrates (7 × 20 × 0.38 mm 3) in an AJA International Orion 5 magnetron sputtering system equipped with one two-inch Cr and one three-inch Ti target (both from Plansee Composite Materials GmbH, 99.6 at.% purity). Prior to the deposition, the substrates Scripta Materialia 124 (2016) 67-70
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Papers by Rainer Hahn