Surface roughening of Ti-6Al-7Nb alloy plasma nitrided at cathode potential

Abstract A plasma nitriding of titanium alloys is very efficient method of their surface hardness and wear resistance improvement without compromising a core strength of the treated elements. The surface roughening increasing with the temperature of this treatment is a major obstacle on the way toward production of the fine quality products. Confronting of the roughness measurements performed at different scales with complementary plan-view surface observations (OM, SEM/SE, FIB/SE, AFM) and cross-section (TEM) ones, focused on description of the surface defects, allowed the mechanism of their development to be explained. Namely, the TiN layer produced at lower temperatures (∼620 °C) grows predominantly with a flat front, roughly reproducing the substrate morphology, while at intermediate and higher temperatures (680 °C−830 °C) it is a subject of severe self-spalling of the upper part of this layer. It means that simultaneous hardening and preservation of a high surface quality of Ti-6Al-7Nb after CP-PN is possible only during the plasma nitriding performed at the temperatures close to 600 °C. Additionally, a strong self-spalling of the upper part of the TiN layer during this treatment at higher temperatures means that only its remaining part, growing toward the core, could eventually serve as a corrosion protection barrier.