On the response of Ti–6Al–4V and Ti–6Al–7Nb alloys to a Nitron-100 treatment

Abstract

Titanium alloys have been widely used in automotive, biomedical and aerospace industries due to their high strength-to-weight ratio, outstanding corrosion resistance and biocompatibility. Although α–β alloys such as Ti–6Al–4V (workhorse of aerospace industry) and Ti–6Al–7Nb (much used for surgical implants) exhibit a good combination of mechanical properties, their tribological properties are still limited as they tend to seize and gall when in contact with other surfaces. In this paper, the response of Ti–6Al–4V and Ti–6Al–7Nb α–β alloys to a Nitron-100 treatment is investigated. This treatment, which enhances the load-bearing capacity of titanium alloys, consists of two sequential processes: plasma nitriding using a glow discharge under triode configuration and deposition of a TiN coating. Scanning electron microscopy (SEM), optical surface profilometry (OSP), Knoop microhardness measurements and glow discharge optical emission spectroscopy (GDOES) were used to characterise both titanium alloy materials prior to and after the Nitron-100 treatment. Although the Nitron-100 treatment significantly improved the load-bearing capacity of both alloys, the Ti–6Al–4V alloy exhibited a deeper hardened case but lower surface hardness than the Ti–6Al–7Nb alloy for identical processing conditions. Their different response to the Nitron-100 treatment could be attributed to the different chemical compositions (replacement of vanadium by niobium) of these two-phase (α–β) titanium alloys containing the same amount (i.e. 3.6at.%) of niobium or vanadium. Due to the larger β-phase stabilising effect of niobium, a higher fraction of β phase resulted in the Ti–6Al–7Nb alloy at the nitriding temperature (750°C). Under the Nitron-100 treatment conditions used in this work, results indicate that Nitron 100-treated Ti–6Al–4V alloy (evidenced by its higher load-bearing capacity) may be preferred to be used in engineering applications involving high contact loads (e.g. aerospace applications such as aero-bearings). Nitron 100-treated Ti–6Al–7Nb alloy may be more suitable in applications requiring high surface hardness but relatively lower contact loads (e.g. biomedical applications).