The influence of boron content on the tribological performance of Ti–N–B coatings prepared by thermal CVD

In this work, Ti–N–B coatings have been deposited by thermal chemical vapor deposition (CVD). The effect of boron varying between 0 and 35.1 at.% on coating structure, mechanical and tribological properties was investigated. The coatings reveal a dual-phase structure of TiN and TiB2 above 1 at.% B. The addition of boron causes grain refinement and changes the structure from columnar for TiN to fine-crystalline. Coating hardness increases continuously from 20 GPa for TiN to 45 GPa at 35.1 at.% B. Ball-on-disc tests against alumina were conducted at 25, 500 and 600 °C. At 25 °C the friction coefficients increase from 0.75 for TiN to about 1.0 for boron contents up to 13.1 at.%, while values down to 0.26 were measured at 35.1 at.% B. The friction coefficients at 500 and 600 °C increase from 0.55 for TiN to 0.8–1.0 for the highest boron content. At 25 °C the lowest wear rates were obtained for the highest boron contents, while the opposite behavior was found for 500 and 600 °C. Extensive coating oxidation resulted in the highest wear rate at 500 °C for 35.1 at.% B, but the wear resistance improved at 600 °C. Raman spectroscopy of the oxide layer revealed the preferred formation of anatase at the expense of rutile for high boron concentrations.