Effect of Carbon-Doping on Microstructure and Nanomechanical/Tribological Behavior of Ti–B–C Coatings onto H13 Steel

Due to its high hardness, chemical and thermal resistance, TiB₂ has become a great candidate to be used as a protective coating. However, high residual stresses after the deposition and brittleness have become the main obstacles for implementation at industrial levels. In the present work, the incorporation of graphite was studied as an alternative to improve the performance of the TiB₂ coatings and study the influence in the microstructure, Nano mechanical and tribological properties. Ti–B–C coatings were deposited with different carbon contents of 10, 20, 28 and 38 at%. XRD results showed that the carbon atoms enter within the crystal lattice of the TiB₂ forming a solid solution, and consequently, deforming crystal and modifying its lattice parameter of 3.2237–3.3414 Å. HRTEM images and selected area electron diffraction patterns analysis display the low crystallite or degree of amorphosity due to the carbon concentration (C_1.9 mol). Compressive residual stresses decrease in the coatings containing the higher amounts of carbon. The formation of a TiB₂-C solid solution contributed to the increment of nanohardness (H = 25 GPa) and improvement of the resistance to plastic deformation (H³/E²) of coatings. Regarding the tribological behaviour of the coatings, higher friction coefficient than those obtained on the uncoated substrate were observed. However, a reduction of the wear rate was also evident. The presence of a high amount of debris and severe wear of the counterpart material indicates a highly aggressive tribological contact. Roll-like debris with a shape of needles was found within the tribological tracks perpendicular to the sliding direction.