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The bench-mark protective coating material utilized in the cutting and forming applications is TiAlN exhibiting high hardness as well as stiffness. Unfortunately, these favorable properties are associated with unwanted brittle deformation behavior resulting upon mechanical loading in the formation of cracks which limits the performance and lifetime of the coated tools. From a materials design point of view a rather unusual combination of properties - high hardness and stiffness together with moderate ductility - is therefore required for the next generation of protective coating materials.
In this work we have examined by ab initio density functional theory (DFT) calculations whether alloying the orthorhombic Mo2BC (o-Mo2BC) which was was proposed by Emmerlich [1] to exhibit both high hardness and moderate ductility with other elements to prepare a o-(Mo,X)2BC solid solutions, leads to improvements in stiffness and/or ductility, X = W, Ta, Nb, V, Zr, Hf, and Ti. o-(Mo,X)2BC models were prepared with the special quasi-random structures (SQS) method and elastic properties were calculated using the stress-strain approach utilizing the Quantum Espresso DFT code. Selected o-(Mo,X)2BC compounds were also prepared experimentally by means of the combinatorial magnetron sputtering to verify the theoretical findings.
Our results show that improvements of the mechanical properties are possible, with the best X candidates being W and Ta, however the elastic properties enhancement comes at the cost of reduced material stability.
Therefore, by performing ab initio calculations on a novel class of orthorhombic boron-carbide materials, we are able to design coating materials with high hardness and enhanced fracture toughness for cutting and forming applications with extended lifetime and performance.
[1] J. Emmerlich et al., J. Phys. D. Appl. Phys. 42, 185406 (2009).
