Microstructure, mechanical and tribological properties of TiBC coatings by DC magnetron sputtering onto AISI M2 steel using independent TiB2 and graphite targets

Despite TiB2 being a ceramic material widely known for its high hardness, chemical resistance and high thermal stability; its use in coatings for industrial conditions has been limited due to low adhesion, which is associated with high residual stresses. Recently, several investigations have shown that the incorporation of carbon within TiB2 coatings reduces residual stresses and increases adhesion, making Ti-B-C coatings excellent candidates for applications at high temperatures. In this research, DC UBMS Ti-B-C coatings were deposited onto AISI M2 steel substrates. The power density of TiB2 was fixed at 2.4 W/cm2 and four different graphite power densities were made in order to obtain four different power ratios (WC/WTiB2) and carbon content in the Ti-B-C coatings. XRD and AFM results suggest the formation of nanocomposite coatings, with TiB2 nanocrystals embedded in an amorphous carbon matrix. SEM images revealed a glass-like structure for coatings with a low power ratio, which became a dense and featureless-like structure with a higher power ratio. A progressive decrease in hardness and Young's modulus was observed with the increase in carbon content. Hardness up to 27 GPa was reached with lower carbon content and decreasing under 5 GPa with the higher carbon content. The Young's modulus exhibited the same feature. Tribological properties of the coatings were investigated using a pin-on-disk tribometer. All coatings showed μ higher than the AISI M2 substrate, and only the coatings with a power ratio of 1.30 showed μ over 0.30. Regarding the wear rate, it remained almost constant for all the Ti-B-C coatings.