**2. Materials and Methods**

A near-β phase titanium alloy with a chemical composition of 13.6 Nb, 13.6 Zr, 0.06 Fe, 0.04 C, 0.01–0.02 N, 0.001 H, 0.11 O (in wt.%), provided by Xi'an Saite Metal Materials Development Co., Ltd., Xi'an, China was used for oxygen hardening. The β transformation temperature of this alloy is 735 ◦C [37]. The samples were cut from the bar (27 mm diameter) into discs with a thickness of 1–2 mm. The samples were ground with sandpaper up to 3000-grit until a flat surface was obtained. Next, they were pre-polished with Al2O3 suspension (grain size 1 μm) and finally polished with silica suspension (0.04 μm) until a mirror finish. The prepared alloy had a fine acicular martensitic morphology composed of α (hexagonal close-packed; hcp) and some α" (orthorhombic, Cmcm space group) laths in β (body-centered cubic; bcc) grains (Figure 1). A detailed description of the microstructure of the as-received alloy has been presented in our previous works [24,25]. The grain size, estimated by image analysis, was in the range of 20–80 μm.

**Figure 1.** Microstructure of the as-received alloy.

The samples were placed in a quartz tube furnace. After air evacuation, argon of 99.999% purity was purged into the tube. The pressure was adjusted to 10 Pa by flowing argon (flow rate of 30 standard cm3/min). When the samples reached the desired temperature of oxidation, i.e., 700 ◦C or 850 ◦C or 1000 ◦C, a glow discharge was started. The glow discharge was generated at 1250 V and an electric current of 45 mA. During discharge, high purity oxygen (99.999%) was mixed with argon and pumped to the furnace in 20 cycles of 1 min duration (flow rate of 6 standard cm3/min). During oxygen pumping, the pressure was increased to 12 Pa. After oxidation, cooling to room temperature with working Ar glow discharge was provided.

The microstructure of the oxygen hardened alloy was characterized by light microscopy (LM, ZEISS Axio Imager M1m microscope, Oberkochen, Germany), scanning electron microscopy (SEM, FEI Nova NanoSEM 450 microscope, (Eindhoven, The Nether-

lands) and transmission electron microscopy (TEM, JEOL JEM-2010 ARP microscope, (Tokyo, Japan) techniques. The phase constitution was studied by selected area electron diffraction (SAED). The Java Electron Microscopy Software (JEMS, version 4.4131U2016, Pierre Stadelmann, Switzerland) was used to interpret the SAED patterns. The lamella from the cross-section of the coated alloy was prepared for the TEM investigation. The samples were thinned using FEI Quanta 3D 200i scanning electron microscope equipped with a Ga+ ion gun, Pt precursor gas injection systems (GIS) and OmniProbe micromanipulator for in situ lift-out [38]. Ion beam accelerating voltage of 30 kV and ion currents in the range of 15 nA–30 pA were applied. The lamella was transferred via a micromanipulator to a TEM half ring, where a focused ion beam (FIB) milling to electron transparency was performed (ion currents of 500–30 pA). FIB deposition process from Pt precursor was used to attach the manipulator probe to the lamella and attach it to the grid. The phase identification was supplemented by energy-dispersive X-ray spectroscopy (SEM-EDS, TEM-EDS) microanalysis and STEM-EDS line analysis.

The open-circuit potential (OCP), linear sweep voltamperometry and electrochemical impedance spectroscopy (EIS) were carried out using a potentiostat Autolab PGSTAT302N (Metrohm Autolab B.V., Utrecht, The Nederlands). The reference electrode was a saturated calomel electrode (SCE), and a platinum plate was used as the counter electrode. Ringer's solution was used as the electrolyte for the corrosion study. The chemical composition of the Ringer's solution was: 8.6 g/L NaCl, 0.3 g/L KCl, 0.25 g/L CaCl2. Measurements were performed at pH 7.4 in deaerated solutions at 37 ◦C. The polarization test was performed at a scan rate of 1 mV/s from −1.3 V to +2.2 V vs. SCE. For the EIS measurements, the amplitude was 10 mV; the frequency was 10<sup>5</sup> Hz to 10−<sup>3</sup> Hz. The EIS measurements were performed at the OCP potential. The EIS data were fitted using ZView software.

The hardness and elastic modulus of the as-received and oxygen hardened Ti–13Nb– 13Zr alloys were determined by micro-combitester (CSM Instruments). The indentation tests were performed using a Vickers diamond indenter. The indenter was loaded with a force of 200 mN and kept for 5 s before un-loading. An analysis of the mechanical properties of the alloy was carried out using the loading/unloading curve by Oliver and Pharr method [39]. It allowed for the determination of the elastic modulus (EIT) and hardness (HIT). For each sample, ten consecutive experiments at randomly selected places were performed, and the average of 10 measurements was calculated.

The abrasive wear resistance and the friction coefficient of the as-received and treated alloy were determined by friction tests based on the ISO 20808 standard [40]. The tests were carried out in dry sliding contact with an Al2O3 ball (6 mm diameter) using a ball-on-disc tribotester (ITeE Radom, Radom, Poland). In a typical ball-on-disc arrangement, a counterelement in the form of a ball was pressed against a rotating disc (sample) made of titanium alloy. The tribological tests were repeated three times, with the same parameters: ball load Fn = 5 N, sliding speed v = 0.07 m/s, sliding distance s = 1000 m, room temperature 23 ◦C and relative humidity 55%. The wear rate Wv = V/Fn × s was determined as the ratio of the volume of material removed during friction (V) to the load (Fn) and the sliding distance (s). The volume was calculated based on the size of the cross-sectional area of the wear groove. The groove profile was measured with a stylus profilometer in six places around the wear track.

Friction-wear tests with simultaneous measurement of corrosion potential during friction in Ringer's solution were also performed. To record the changes in the corrosion potential, a 2-electrode system with the working electrode (titanium alloy) and reference electrode (calomel electrode in 3 M KCl) was designed. The schematic of the tribocorrosion device is shown in Figure 2. A specially designed system ensuring an efficient and stable electric contact with the alloy sample was used. In this system, the ball with the holder was moving, whereas the titanium alloy sample was stationary. To fix the ball and sample positions, special polymer holders were used. The same measurement parameters were applied as in the case of dry friction tests. The friction process began after a stable corrosion

potential for the titanium alloy was reached. When the friction was activated, a change in the potential was recorded as a result of the wear of the alloys' surface layer.

**Figure 2.** Schematic of the tribological device with parallel corrosion potential measurement during wear tests of the titanium alloy in sliding contact with a ball.

Before each test, the alumina ball and titanium alloy were ultrasonically cleaned in ethanol and left to dry. Additionally, the ball used in the tribocorrosion test was also washed in Ringer's solution. The holders were subjected to an identical cleaning procedure. The whole measurement took place in a plastic vessel filled with Ringer's solution.
