*2.2. Coatings Characterization*

Energy dispersive spectrometry (EDS) was used for the analysis of the elemental composition (EDS, Quantax70, Bruker, Tokyo, Japan). The morphology was examined by scanning electron microscopy (SEM, Hitachi TM3030Plus, Tokyo, Japan). Phase composition was studied by the X-ray diffraction method (XRD, SmartLab diffractometer, Rigaku, Tokyo, Japan), using Cu Kα radiation, from 10◦ to 100◦ with a step size of 0.02◦/min. For the thickness and surface roughness determination, a surface profilometer (Dektak 150, Bruker, Billerica, MA, USA) was used. Surface roughness was measured for each investigated sample on five line-scans, each on a distance of 4000 μm.

The mechanical properties of the coatings were determined using a Hysitron Premier TI nanoindentation unit equipped with a Berkovich indenter tip of 100 nm radius and a total included angle of 142.3◦, respectively. Prior to any sample testing, the *Z*-axis calibration was performed in the air and the machine compliance was assured using a fused quartz standard calibration sample with known hardness (H = 9.25 GPa ± 10%) and elastic modulus (E = 69.6 GPa ± 10%). In order to perform the nanoindentation experiments, a 15 × 15 μm<sup>2</sup> area was previously scanned using the same Berkovich diamond tip at a normal force of 2 μN to investigate the surface roughness for the subsequent indents. Additionally, the indents were intentionally located at least 5 μm apart from each other, whilst an applied force of 10 mN was employed for every nanoindentation test. The force–displacement curves were recorded using a gradual force increase up to 10 mN in a 7-s time interval, followed by a 2-s dwell time at the maximum force of 10 mN and a gradual force decrease within the next 7 s, until the complete tip retraction from the coatings surface.

The electrochemical behavior of the investigated specimens was analyzed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), using a VersaSTAT 3 Potentiostat/ Galvanostat system. The measurements were performed in 90% DMEM + 10% FBS, at 37 ± 0.5 ◦C, using a typical three electrode setup with a Pt grid counter electrode (CE) and a saturated calomel (saturated KCl) (SCE) as the reference electrode (RE), while the working electrode consisted of uncoated CoCr and TiCN or TiSiCN coated CoCr substrates, respectively.

The open circuit potential (EOC) was monitored for 1 h, starting after the sample's immersion. Linear polarization, Tafel and potentiodynamic curves were performed by applying a potential of −20 to 20 mV vs. EOC, −50 to 250 mV vs. EOC and −1 V vs. EOC to 2 V vs. RE, respectively, with a scanning rate of 0.167 mV/s. For the linear polarization measurements, the testing conditions were selected based on preliminary results, in such a way that the applied potential gave a linear behavior. The selected value was used to accommodate all the investigated systems. The EIS measurements were performed over a range of frequencies (0.1 ÷ 10<sup>4</sup> Hz), by applying a sinusoidal signal of 10 mV RMS vs. EOC.
