Effect of tribo-layer developed during turning of Ti–6Al–4V ELI alloy on its low-temperature gas nitriding

• Published in: Applied surface science Vol. 602; p. 154327
• Main Authors: Toboła, Daniel, Morgiel, Jerzy, Maj, Łukasz, Pomorska, Małgorzata, Wytrwal-Sarna, Magdalena
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-11-2022
• Subjects: Gas nitriding; SEM; TEM; Ti-6Al-4V; Tribo-layer; Turning; XPS; Gas nitriding; Ti-6Al-4V; Turning; SEM; XPS; Tribo-layer; TEM
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2022.154327

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A tribo-layer, produced on parts made of titanium alloys fabricated using various machining techniques, is a natural outcome of these processes. Its removal, improving surface quality before nitriding, may seem not always justified as the latter treatment again causes surface roughening increasing with temperature. Therefore, the aim of the present paper was to investigate the surface integrity aspects of Ti–6Al–4V ELI alloy subjected to turning, i.e. presence of grooving and tribo-layer on the following gas nitriding process but performed at the low temperature of only 540 °C. Roughness and hardness measurements were supplemented by complementary microstructural investigations using SEM, TEM and XPS. These established that the coating of tribo-material on the as-machined alloy consists of oxidized amorphous layers with numerous thicker clumps. Nitriding at 540 °C, for 8 and 16 h, causes a gradual (from top to bottom) transformation of the tribo-layers into mostly crystalline TiO2 rutile phase. Completion of this stage makes the thinner part of tribo-layers permeable to nitrogen, starting nucleation and growth of TiN in the substrate. This process is accompanied by a release of nitrogen from the nitrided region and formation of a thin layer of α-Al2O3 in-between TiO2 rutile and TiN. This shows that efficient nitriding of machined titanium alloy parts, which produces an uniform TiN layer, should be achieved only after a full removal of the tribo-material from their surfaces.