OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 the FWHM variation, where the peaks’ maxima and minima coincide at different temperatures, can be attributed to the occurrence of a low lattice thermal expansion coefficient in all directions of the specimen. Through the analysis of in-situ data presented above, we can understand the correlation between thermal expansion and internal stress. The FWHM of X-ray diffraction peaks is often used to measure internal stress caused by lattice defects [19]. In this study, the gradual increase in FWHM demonstrates the gradual increase in lattice distortions caused by interfaces between the substrate and coatings, as well as between individual layers of the multilayer coatings. As shown in figs. 3 and 5, the increase in FWHM coincided with a decrease in the thermal expansion coefficient, indicating that the accumulation of internal stress leads to volumetric expansion, which compensates for some of the lattice shrinkage of the original phase during cooling. It is worth noting that the FWHM values for the single-component coatings and its substrate decreased with increasing temperature (figs. 5, a, c, d). The reduction in peak broadening indicates the relaxation of residual stresses [13]. On the other hand, the FWHM increased with increasing temperature for the ZrN phase in the multilayer coating (fig. 5, b). Some studies attribute this effect to the presence of residual stresses coupled with an induced phase transformation [20]. Alternatively, under our thermal cycling conditions, the ZrN phase in the multilayer coating may experience compression from the second component of the multilayer coating, CrN, thereby inducing a stressed state. Additionally, nitrogen evaporation, which leads to changes in the chemical composition of the coating, can also affect the lattice parameter and create internal stresses [21]. The overall residual stress in the components includes internal stress, thermal stress, and external stress. During thermal processing, the change in each component of residual stress affects both the substrate and the coating layers due to differences in thermal expansion coefficients (CTE) and is attributed to the variation of thermal stress with temperature. Since the CTE of the ZrN and CrN phases in the single-component coatings was lower than that of the VK8 alloy substrate (figs. 3, a, c, d), an increase in tensile stress is observed in the ZrN and CrN layers upon heating [22]. However, the situation changes when analyzing the CTE of the present phases in the multilayer coating. As shown in figs. 3b, 3d, and 3e, the substrate and the CrN phase exhibit the highest CTE values (60–80×10-6 C-1), while the ZrN phase has the lowest CTE, with values of 30×10-6 C-1 during heating and 40×10-6 C-1 during cooling. Therefore, between the ZrN, CrN, and WC phases in the multilayer coating, conditions are created during heating that induce compressive stress in the CrN phase and tensile stress in the ZrN phase [23]. Conclusions This study investigated the behavior of thermal expansion and lattice parameter changes in relation to the development of internal stresses during thermal cycling. Based on the experimental data, the following conclusions can be drawn: 1. The lattice parameter of all coatings decreased during thermal cycling, indicating nitrogen evaporation and changes in the chemical composition of the coatings under thermal load. The multilayer coating showed the least variation in the lattice parameter, indicating the presence of diffusion barriers for nitrogen. 2. The analysis of the crystal lattice distortions of the coating components of the studied specimens did not reveal significant differences between single-component and multilayer coatings; 3. All studied coatings displayed thermal expansion comparable to that of the substrate. The slightly higher thermal expansion of the substrate in the single-component coatings resulted in tensile stresses at the interface. Between the ZrN and CrN phases of the multilayer coating and the WC phase of the substrate, conditions were created during heating that induced compressive stresses in the CrN phase and tensile stresses in the ZrN phase. As a result, it can be expected that the service life of multilayer coatings will be higher compared to single-component coatings.
RkJQdWJsaXNoZXIy MTk0ODM1