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

Abstract

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 degrees C) for 8 h in a vacuum atmosphere. Unlike those formed at 900 degrees C, boriding temperatures of 1000 and 1100 degrees C provided thicker and homogenous boride layers. However, the boriding temperature of 1100 degrees C induced cracking within the boride layer of the Ti-40Nb - 40Nb alloy. For these reasons, the optimum boriding temperature was determined as 1000 degrees C. Increase in the Nb content not only increased the fraction of beta-Ti phase in the microstructure of the sintered alloy at the expense of alpha-Ti, but also induced NbB2 2 in the structure of the boride layer along with TiB2. 2 . While Nb-poor alpha-Ti grains favoured the growth of TiB2, 2 , TiB2 and sdot;NbB2 2 and sdot;NbB 2 mixture preferentially developed over the Nb-rich beta-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 Ti-40Nb - 40Nb 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 2 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 alpha-Ti to minimize the wear of the tribocouple via reducing the abrasion at the counter body.