OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 4 2024 The free surface energy γs of the material before and after laser treatment was determined by the Owns – Wendt – Rabel – Kjelble (OWRK) method [30], using known reference data on surface tension, dispersed γd and polar γp components of the test liquids (water and glycerin) and the obtained data on the contact angle. The surface topography of TiNi alloy and steel before and after laser treatment was studied using the contact method with a profi lometer for tribological tests (Tt-Tribotechnic, France) equipped with a high resolution diamond needle (7.55 nm along the Z axis) without a sliding element. The roughness Ra, averaged over the entire length of the baseline equal to 3 mm, was measured in accordance with GOST 2789-73. At least 5 measurements were performed for each specimen. The structure and elemental composition of the materials surface before and after treatment were estimated using data obtained using a scanning electron microscope (SEM) (VEGA 3 TESCAN, Czech Republic) equipped with an energy dispersive analyzer (EDS). The phase composition of TiNi and steel specimens in the initial state and after laser treatment was determined from diff raction patterns obtained on a DRON-type X-ray diff ractometer (Burevestnik, St. Petersburg, Russia) with fi ltered CuKα radiation in the range of scanning angles 2Θ 30°–110°. Qualitative and quantitative analysis of X-ray diff raction (XRD) patterns was carried out using the PDWin and CDA software packages (OJSC Burevestnik, St. Petersburg, Russia). Results and discussion Measuring and determining the contact angle with deionized water is the simplest method for studying the wettability of material surfaces. Fig. 3 shows the graphs of the dependence of the contact angle of TiNi and steel surfaces and the duration of UV laser treatment. The insets (Fig. 3) show typical images of water droplets on the specimen surfaces before and after laser treatment. In the initial state, the contact wetting angles for TiNi and steel specimens are similar, measuring 75.0 ± 5.1° for TiNi and 75.4 ± 5.4° for steel. Ultraviolet laser treatment alters the hydrophilicity of both TiNi alloy and steel surfaces. The contact angle decreases as the UV laser treatment duration increases. Already after 10 s of treatment, a signifi cant reduction in the contact angle is observed for both materials compared to its initial state. For TiNi specimens, the contact angle decreases more than twofold, while for steel specimens, it decreases by approximately 30 %. A sharp decrease in the contact angle is seen up to 120 s of treatment. With a further increase in the duration of treatment, the contact angle for TiNi specimens remains virtually unchanged at 11–13°, while for steel specimens, it continues to decrease gradually, reaching a minimum value of 22.6 ± 4.2° after 600 s of laser treatment. As observed in Fig. 3, increasing the duration of UV laser treatment reveals diff erences in the kinetics of contact angle changes for TiNi and steel specimens. The contact angle of TiNi decreases faster and more signifi cantly compared to steel as the treatment duration increases. Moreover, the minimum values of the contact angle also diff er. For the same duration of ultraviolet laser treatment, the contact angle of TiNi specimens is 1.5-2 times lower than that of steel ones. Therefore, ultraviolet laser treatment of TiNi and steel specimens’ surfaces eff ectively alters its hydrophilic behavior, making the TiNi alloy more hydrophilic than steel with the same laser treatment parameters. Since the wettability of materials is regulated by a thin surface layer (the fi rst atomic layers of the surface), any change in the physicochemical properties of this layer can signifi cantly aff ect it [31]. Currently, there is no consensus among researchers regarding the mechanisms of changing hydrophilicity through various surface modifi cation methods. Numerous hypotheses exist about the causes of changes in the degree of hydrophilicity of materials, and these hypotheses are often contradictory. Surface wettability is greatly infl uenced by the phase and chemical composition of the surface, the surface microgeometry factor, its texture, roughness, structure, as well as the surface polarity, which is one of the important characteristics aff ecting the affi nity for water [21, 22, 27, 32]. One hypothesis suggests that a decrease in the contact angle, indicating an increase in hydrophilicity, may result from cleaning the surface of materials from organic contaminants [4, 33–35]. On the one hand, it is known that contamination of the metal surface with organic compounds with a predominance of hydrocarbon groups in the molecule leads to surface hydrophobization, therefore, removal of these organic contaminants from the surface of materials can lead to a moderate increase in surface hydrophilicity [35]. At the same time, a number of studies report the absence of an eff ect
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