(Ti,Al)O2 Whiskers Grown during Glow Discharge Nitriding of Ti-6Al-7Nb Alloy.

Krzysztof Szymkiewicz,Małgorzata Pomorska,Jerzy Morgiel,Łukasz Maj

doi:10.3390/ma14102658

Krzysztof Szymkiewicz, Małgorzata Pomorska + Show 2 more

Open Access

https://doi.org/10.3390/ma14102658

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Abstract

Plasma nitriding of titanium alloys is capable of effective surface hardening at temperatures significantly lower than gas nitriding, but at a cost of much stronger surface roughening. Especially interesting are treatments performed at the lower end of the temperature window used in such cases, as they are least damaging to highly polished parts. Therefore identifying the most characteristic defects is of high importance. The present work was aimed at identifying the nature of pin-point bumps formed at the glow discharged plasma nitrided Ti-6Al-7Nb alloy using plan-view scanning and cross-section transmission electron microscopy methods. It helped to establish that these main surface defects developed at the treated surface are (Ti,Al)O2 nano-whiskers of diameter from 20 nm to 40 nm, and length up to several hundreds of nanometers. The performed investigation confirmed that the surface imperfection introduced by plasma nitriding at the specified range should be of minor consequences to the mechanical properties of the treated material.

Highlights

A glow discharge plasma nitriding (PN) of parts made of Ti-6Al-4V and Ti-6Al-7Nb alloys is gaining an upper hand over gas nitriding (GN), as it allows the increase of surface hardness without compromising a high strength of the core material [1]
This is because the former process could be performed at much lower temperatures than the latter one, i.e., the PN secures formation of a continuous and slightly porous TiN surface layer even after processing duration of eight hours carried out at 600–850 ◦ C [2], while achieving the comparable results with the GN of the same duration must be performed at 800–1050 ◦ C [3]
The cross-section scanning and transmission electron microscopy (SEM/transmission electron microscope (TEM)) investigations confirmed the presence of a surface undulation representing the nodular features, and proved that they increase with the PN temperature both for the Ti-6Al-4V [8] and Ti-6Al-7Nb [2,9] alloys

Summary

Introduction

A glow discharge plasma nitriding (PN) of parts made of Ti-6Al-4V and Ti-6Al-7Nb alloys is gaining an upper hand over gas nitriding (GN), as it allows the increase of surface hardness without compromising a high strength of the core material [1] This is because the former process could be performed at much lower temperatures than the latter one, i.e., the PN secures formation of a continuous and slightly porous TiN surface layer even after processing duration of eight hours carried out at 600–850 ◦ C [2], while achieving the comparable results with the GN of the same duration must be performed at 800–1050 ◦ C [3]. The plan-view observations of the surface of the PN Ti-6Al-4V alloy with the scanning electron microscopy (SEM) method showed that it bears both nodular [6,7] as well as much finer point-like features [7]. None of the images published so far gave any indication of the nature of the point-like objects observed with the SEM method

Results

Discussion

Conclusion