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

Ti–B–C–N nanocomposite coatings with different C contents were deposited on Si (1 0 0) 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 8 nm was found at a C2H2 flux rate of 1 sccm. However, the hardness, elastic modulus and fracture toughness values were not consistent with the grain size. They got to their maximum of 35.7 GPa, 363.1 GPa and 2.46 MPa m1/2, respectively, at a C2H2 flow rate of 2 sccm (corresponding to about 6 nm 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.