Microstructure and mechanical properties of reactive magnetron sputtered Ti–B–C–N nanocomposite coatings

► The study revealed that, in the studied composition range, the deposited Ti–B–C–N coatings exhibited nanocomposite based on TiN nanocrystallites. ► The following two results are new and beneficial for practical application. ► The first, incorporation of small amount of C promoted crystallization of Ti–B–C–N nanocomposite coatings, which resulted in increase of nano-grain size and mechanical properties of coatings. ► However, further increase of C content dramatically decreased not only grain size but also the mechanical properties of coatings. ► The second, we measured and calculated the fracture toughness of Ti–B–C–N nanocomposite coatings, which revealed that the fracture toughness was consistent with hardness/elastic modulus, however not consistent with the grain size. Ti–B–C–N nanocomposite coatings with different C contents were deposited on Si (100) and high speed steel (W18Cr4V) substrates by closed-field unbalanced reactive magnetron sputtering in the mixture of argon, nitrogen and acetylene gases. These films were subsequently characterized ex situ in terms of their microstructures by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), their nanohardness/elastic modulus and facture toughness by nano-indention and Vickers indentation methods, and their surface morphology using atomic force microscopy (AFM). The results indicated that, in the studied composition range, the deposited Ti–B–C–N coatings exhibit nanocomposite based on TiN nanocrystallites. When the C2H2 flow rate is small, incorporation of small amount of C promoted crystallization of Ti–B–C–N nanocomposite coatings, which resulted in increase of nano-grain size and mechanical properties of coatings. A maximum grain size of about 8nm was found at a C2H2 flux rate of 1sccm. However, the hardness, elastic modulus and fracture toughness values were not consistent with the grain size. They got to their maximum of 35.7GPa, 363.1GPa and 2.46MPam1/2, respectively, at a C2H2 flow rate of 2sccm (corresponding to about 6nm in nano-grain size). Further increase of C content dramatically decreased not only grain size but also the mechanical properties of coatings. The presently deposited Ti–B–C–N coatings had a smooth surface. The roughness value was consistent with that of grain size.