OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 This leads to a significant increase in the size of the specimen [3, 9, 10], which is due to the porosity of oxides formation, which favors unhindered access of oxygen. Oxidation occurs not only during a long-term isothermal process [11, 12], but also during a short-term exposure to laser radiation [13]. It is noted in [14] that the oxidation rate increases with increasing temperature; however, the study [15] revealed a local inversion in the dynamics of oxidation in the temperature range of 528–630 °C. The oxidation rate also increases with the increasing heating rate [1, 5], which is confirmed by a shift in the curves of the dependence of mass on temperature. However, this may be due to the specifics of the experiment. It is noted in [6] that in the temperature range up to 650 °C a linear oxidation law is observed (dependence of the increase in mass on temperature), and above 800 °C it is quasi-parabolic. In other words, the oxidation rate increases linearly with increasing temperature above 800 °C. Increased oxygen [1, 12] and air flow rates accelerate oxidation [1], but at high flow rates this effect weakens due to cooling of the specimen. Recent studies have shown that WC-Co cemented tungsten carbides undergo several stages of oxidation when heated [16]: extremely weak oxidation of cobalt and carbides (up to 600 °C), the onset of oxidation (above 600 °C) of cobalt (CoO, Co3O4), and simultaneous oxidation of cobalt and carbides (above 700 °C) with the formation of tungsten oxides (WO2, WO3) and double oxides of tungsten and cobalt (CoWO4) [1, 7, 16]. A similar pattern is observed not only for sintered materials, but also for a mixture of powders [17]. A number of studies have noted that cemented carbides with a higher cobalt content oxidize more slowly than those with a lower content [1, 18, 19]. In [7], the authors discovered that the hardness of WC-10Co with particularly fine grains drops significantly during oxidation. However, the Vickers hardness of a surface cleaned of oxides after cooling may either not change at all or even increase [20]. During oxidation, the bending strength of cemented carbides decreases [12, 21, 22], which is largely due to the degradation of the surface layer. This was indirectly confirmed in [23], where the influence of oxidation on the development of surface cracks was noted. Coatings have a protective function [3, 24] and prevent oxidation of the cemented carbide base. However, when heated (due to thermal stresses caused by differences in the thermal coefficient of linear expansion), cracks appear in the coatings, allowing oxygen access to the base. In addition, coatings themselves are also oxidized [25]. Not only specially prepared specimens with a ground surface were used, but also commercially produced cutting inserts [26], including those with coatings [24] were used to study the oxidation process of cemented carbides. The results presented coincide with the results obtained on special specimens. Oxidation can be used to purposefully reduce the strength of a cemented carbide in order to improve its machinability [27]. In the work [8] it is even proposed to use oxidation to recycle worn multifaceted cutting tool inserts. However, when using cemented carbides as cutting tool materials, its high-temperature oxidation can play an important [28–30] or decisive role in the wear of cutting tools [26, 31, 32]. Having studied the literature, we can see that the above experiments mainly used isothermal heating with a large step of temperature change (500, 600 °C, etc.). The accuracy of the determined oxidation onset temperatures was not sufficient, and the influence of cobalt content on the value of these temperatures and on the dynamics of oxidation with wide variations in the composition of WC-Co cemented tungsten carbides was not established. The quantitative study of oxidative processes can be performed in various ways: by weight changes of the specimen – thermogravimetric analysis (TGA); by temperature difference – differential thermal analysis (DTA); by changes in heat flux – differential scanning calorimetry (DSC); by changes in the parameters of thermal radiation – infrared spectroscopy [15]; by changes in the properties of reflected radiation – optical or scanning electron microscopes (SEM). However, these methods do not always provide trustworthy information about the oxidation rate, and its accuracy is limited by the nature of the experiment. For instance, in thermogravimetric analysis, due to the absorption of gases and the appearance of volatile oxides in addition to solid oxides, the change in the mass of the specimen can be ambiguous.
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