*2.4. Surface and Cross-Sections Evaluation of Samples*

### 2.4.1. Preparation of Samples for Surface and Cross-Section Tests

The samples T(1 ÷ 6), C(1 ÷ 6) and native material—MR—were integrated in phenolic resin (Verte 602, Lamplan, Gaillard, France) in a laboratory press (Opal 410, ATM GmbH, Mammelzen, Germany) and subjected to grinding on a metallographic grinding machine (Saphir 330, ATM GmbH, Mammelzen, Germany) using sandpapers with a gradation of 180, 400, 600, 800, 1200, 2000 and 4000 and polishing using diamond paste (DP-Paste M 1 µm, Struers, Inc., Cleveland, OH, USA). The samples were then etched in Kroll0 s reagent (a mixture of HNO<sup>3</sup> and HF acids in distilled water in a 6/2/92 volume ratio).

#### 2.4.2. Analysis of Surface Phase Composition

The samples MR, C, T(2,5), C(2,5) were subjected to phase structure tests using an X-ray diffractometer (X0Pert Pro, Philips, Amsterdam, The Netherlands) with a goniometer PW 3050/60 (θ–θ) with a Cu copper lamp (Kα1 = 1, 5405980 Å, Kα2 = 1.5444260 Å, Kα2/Kα1 ratio = 0.5). The test (with an 2θ angle range of 20–60◦ at 40 V, 30 mA and scanning speed 0.02/1.5 ◦ /s) permits the determination of the crystal structure of the material.

### 2.4.3. Raman Spectroscopy

Raman measurements were carried out using Renishaw InVia Raman microscope (Renishaw plc., Wotton-under-Edge, UK) equipped with an EMCCD detector (Andor Technology Ltd, Oxford Instruments, Belfast, UK). The Raman signal was acquired using laser radiation with a wavelength of 532 nm (27 mW). The laser beam was directed to the sample through a 50× objective lens. The wavelength of the instrument was calibrated using an internal silicon wafer, and the spectrum was centered at 520.5 cm−<sup>1</sup> . Raman spectra measurements were made for maps consisting of 100 points per sample. Based on this, an average spectrum was obtained and the standard deviation of the signal was determined—which is an indicator of the homogeneity of the layers.
