Boriding of Ti-xNb alloys: Influence of Nb on the features of boride layer

In this study, Ti-xNb (x = 0–40 wt%) alloys produced by the powder metallurgy were borided with the aim of clarifying the effect of Nb on the structural and mechanical properties of the boride layer. After smearing the paste prepared from nano boron powder on the surfaces of the alloys, boriding was conducted at three different temperatures (900, 1000 and 1100 °C) for 8 h in a vacuum atmosphere. Unlike those formed at 900 °C, boriding temperatures of 1000 and 1100 °C provided thicker and homogenous boride layers. However, the boriding temperature of 1100 °C induced cracking within the boride layer of the Ti40Nb alloy. For these reasons, the optimum boriding temperature was determined as 1000 °C. Increase in the Nb content not only increased the fraction of β-Ti phase in the microstructure of the sintered alloy at the expense of α-Ti, but also induced NbB₂ in the structure of the boride layer along with TiB₂. While Nb-poor α-Ti grains favoured the growth of TiB₂, TiB₂·NbB₂ mixture preferentially developed over the Nb-rich β-Ti grains. As the result of this, the hardness of the boride layer tended to decrease with increasing Nb content of the substrate. For example, the average hardness of the boride layers formed on Nb-free Ti and Ti40Nb alloy were measured as ∼2674 HV_0.025 and ∼ 2460 HV_0.025, respectively. But regardless from the hardness, the boride layers provided a good protection for the underlying substrates against dry sliding contact and triggered abrasive wear on the contact surface of the counterface (WC-Co ball). The presence of NbB₂ in the boride layer led to a reduction in abrasive wear of the counterface. This finding revealed that in any wear-related application, where borided Ti alloys were intended to be used, it is better to choose high Nb-containing Ti alloys instead of α-Ti to minimize the wear of the tribo-couple via reducing the abrasion at the counter body.