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.