**1. Introduction**

β-phase titanium alloys are characterized by high mechanical strength, good corrosion resistance, high biocompatibility and relatively low Young's modulus [1–6]. In addition, references [7,8] show that Nb and Zr are valuable alloying elements. Nb and Zr have an advantage over possibly toxic elements in body fluid, such as aluminum and vanadium, as they do not generate allergic reactions or genetic changes [9]. Consequently, β or near-β titanium alloys, such as Ti–13Nb–13Zr, have been developed for biological applications [1,10,11].

β-phase titanium alloys are referred to as second-generation titanium biomaterials. One of the rationales behind their development is to address the stress shielding associated with the high Young's modulus of the implant [2–6]. A lower elastic modulus is best suited for a biomaterial implant, as discussed by different authors [12–14]. Mohan and coworkers [15] indicated that the percentage of β phase greatly affects the mechanical properties of the Ti–Mo and Ti–Mo–Zr alloy systems. They showed that Mo improved the β phase stability when the percentage of Mo was increased from 12% to 15%. It was further noted that the elastic modulus of the alloys was lower compared to Ti–6Al– 4V and commercially pure Ti. The Ti–15Mo–6Zr alloy had the lowest elastic modulus,

**Citation:** Łukaszczyk, A.; Zimowski, S.; Pawlak, W.; Dubiel, B.; Moskalewicz, T. Microstructure, Micro-Mechanical and Tribocorrosion Behavior of Oxygen Hardened Ti–13Nb–13Zr Alloy. *Materials* **2021**, *14*, 2088. https://doi.org/10.3390/ ma14082088

Academic Editor: Marián Palcut

Received: 11 March 2021 Accepted: 16 April 2021 Published: 20 April 2021

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thereby confirming that the higher percentage of β phase is the main reason for the lower elastic modulus observed. However, the main drawback of all titanium alloys is their poor tribological performance, as demonstrated by the high coefficient of friction (COF) combined with a tendency for scuffing and adhesive wear [2,3]. Moreover, the friction of titanium alloys in a corrosive environment increases their wear rate [16,17], which is disadvantageous in their application as orthopedic bioimplants. Surface treatment is usually applied to enhance the properties of titanium alloys as bioimplants. Several techniques, including oxidizing [18], nitriding [19], ion implantation [20] and physical vapour deposition (PVD) coating [21], selective laser melting [22], Ar arc melting [12], spark plasma sintering [23], anodization [14], mechanical blending process [15] have been investigated. It is known that oxygen hardening can increase the hardness and tribocorrosion resistance [24–30].

The characterization of the oxygen diffusion zone and the impact of this zone on macroscopic properties are still subjects of intensive investigation. Dong and Li [28] reported that the boost diffusion oxidation (BDO) process was an effective way to harden a titanium surface. The BDO process involved two steps. In the first step, titanium samples were thermally oxidized in the air. During the second step, the air was removed from the reaction chamber, and pre-oxidized samples were further diffusion treated in a vacuum. The method was successfully employed by Zabler and coworkers [30] for the formation of oxygen diffusion hardening (ODH) zones in the commercially pure Ti (grade 2) and Ti–6Al–4V alloy. A well-controlled hardness and hardened zone depth were achieved. A disadvantage of the process is the formation of an oxide scale on the alloy surface. Thicker titanium oxide layers tend to deteriorate the alloy properties. Jamesh et al. [31] reported that the thermal oxidation process of commercially pure Ti above 850 ◦C led to oxide scale spallation. This behavior is dangerous in frictional contact, where the detached hard titanium oxide particles accelerate the abrasion.

Januszewicz and Siniarski [32] showed that the assistance of plasma glow discharge during oxidation is of great benefit as it prevents the formation of brittle rutile phase on the treated alloy surface by mechanically damaging the oxide scale by high-energy ions. In our previous work [33], we showed that diffusion hardening of the near-surface zone by interstitial oxygen atoms with using glow discharge plasma in Ar + O2 atmosphere is a very effective method to improve the mechanical and tribological properties of two-phase (α + β) Ti–6Al–4V titanium alloy. It was found that the treatment increased the surface hardness about three times, from 3.4 GPa to 10.6 GPa. The wear resistance in dry sliding contact with the Al2O3 ball was also significantly improved.

The oxygen diffusion hardening of the near-β Ti–13Nb–13Zr was performed by thermal oxidation in an oxygen-containing atmosphere [34]. The results suggest that zirconium plays a key role in the effective oxygen diffusion hardening at 500 ◦C for alloys of the Ti–Nb-Zr system. It was found that a sufficient Zr concentration (>6 wt.%) permits oxygen diffusion into the alloys. The near-surface hardness appears to increase further as Nb content increases. The high hardness of oxygen hardened Ti–13Nb–13Zr alloy is a result of an outer oxide layer composed of mixed metal (i.e., Ti, Nb, Zr) oxides on top of interstitial oxygen hardened alloy. This dense oxide layer is not only highly passive from a chemical/corrosion point of view but also resistant to abrasive wear, which is attributed to the mechanical stability of the oxide/substrate interface.

To our best knowledge, there are no data on the influence of oxygen plasma glowdischarge on the hardening of the Ti–13Nb–13Zr alloy and the influence of the surface treatment on microstructure and tribological properties in corrosive environments. Therefore, detailed studies on the effect of oxygen plasma glow discharge on the alloy usage properties are necessary. The friction wear corrosion results in metal degradation due to the simultaneous action of mechanical wear and (electro)chemical oxidation. The two mechanisms do not proceed separately and depend on each other in a complex way. In many cases, corrosion is accelerated by wear, and similarly, wear may be affected by corrosion phenomena. The nature of the passive film and the electrochemical conditions play a

significant role in friction, wear and corrosion mechanism [35]. When the implants are to be placed in bone tissue, bone ingrowth into porous implant surfaces starts and osteoblast cell adhesion on the substrate increases, which may improve osseointegration. Improved osseointegration provides mechanical stability by interlocking the surrounding bone tissue with the implant. Tribocorrosion is a material degradation process and involves chemical and/or electrochemical interactions between material and its environment during a tribological process. Friction wear corrosion is a degradation process resulting from the combined action of small movements between contacting parts and the environment's corrosivity [36].

The main purpose of this work is to investigate the effect of oxygen plasma glow discharge temperature on the microstructure development and micromechanical properties of the near-β Ti–13Nb–13Zr alloy. An influence of the applied surface treatment on tribocorrosion behavior of the alloy in Ringer's solution is also studied.
