Synthesis and Characterization of Ta–B–C Coatings Prepared by DCMS and HiPIMS Co-sputtering

This study reports on the deposition and properties of Ta–B–C coatings by the co-sputtering of tantalum, boron carbide, and graphite targets using High Power Impulse Magnetron Sputtering (HiPIMS). It was possible to affect the microstructure of the deposited coatings by altering the deposition temperature or by the application of RF induced self-bias on the substrates without changing their chemical composition. The only identified crystalline phase from the Ta–B–C system present was TaC. The boron content in the coatings shows that the TaC crystallite size can be changed by a factor of 10 by changing the power to the boron carbide target. Mechanical properties of the coatings measured directly after the synthesis yield hardness higher than 40 GPa. After the relaxation of internal stress in the coatings (after one year) and changes in the structure, the hardness of all coatings was close to 36 GPa. According to ab initio calculations, the B incorporation in the fcc lattice of TaC in combination with C vacancies lead to lower (higher) shear-to-bulk modulus ratio (Poisson’s ratio), providing a good basis for improved ductility. All in all, Ta–B–C system shows a good potential as a novel hard protective coating

Synthesis and Characterization of Ceramic High Entropy Carbide Thin Films from the Cr-Hf-Mo-Ta-W Refractory Metal System

We use reactive DC magnetron sputtering to showcase synthesis strategies for multicomponent carbides with the NaCl-type fcc structure and illustrate how deposition conditions allow controlling the formation of metallic and ceramic single phases in the Cr-Hf-Mo-Ta-W system. The synthesis is performed in argon flow and different acetylene flows from 0 to 12sccm, at ambient and elevated temperatures (700 °C), respectively, hindering/promoting the adatom diffusion. Structural and microstructural investigations reveal the formation of the bcc metallic phase (a =3.188–3.209Å) in films deposited without acetylene flow, also supported by ab initio density function theory (DFT) analysis of lattice parameters as a function of the C content. Experimentally, a bcc-to-fcc phase transition is observed through the formation of an amorphous coating. Contrarily, samples deposited in higher acetylene flow show an fcc multielement carbide phase (a =4.33–4.49 Å). The crystalline films reveal columnar morphology, while the amorphous ones are very dense. We report promising mechanical properties, with hardness up to 25± 1GPa. The indentation moduli reach up to 319± 6GPa and show trends consistent with DFT predictions. Our study paves the path towards the preparation of Cr-Hf-Mo-Ta-W multicomponent carbides by magnetron sputtering, showing promising microstructure as well as mechanical properties

Tuning structure and mechanical properties of Ta-C coatings by N-alloying and vacancy population

Tailoring mechanical properties of transition metal carbides by substituting carbon with nitrogen atoms is a highly interesting approach, as thereby the bonding state changes towards a more metallic like character and thus ductility can be increased. Based on ab initio calculations we could prove experimentally, that up to a nitrogen content of about 68% on the non-metallic sublattice, Ta-C-N crystals prevail a face centered cubic structure for sputter deposited thin films. The cubic structure is partly stabilized by non-metallic as well as Ta vacancies – the latter are decisive for nitrogen rich compositions. With increasing nitrogen content, the originally super-hard fcc-TaC0.71 thin films soften from 40 GPa to 26 GPa for TaC0.33N0.67, accompanied by a decrease of the indentation modulus. With increasing nitrogen on the non-metallic sublattice (hence, decreasing C) the damage tolerance of Ta-C based coatings increases, when characterized after the Pugh and Pettifor criteria. Consequently, varying the non-metallic sublattice population allows for an effective tuning and designing of intrinsic coating properties.Swedish Foundation for Strategic Research (SSF)VR-RFI1111