OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 differences between single-component coatings and its combination in the multilayer coating. The evaluation of the crystal lattice parameter in the coatings and substrate, as well as its change during thermal cycling, was performed. The combination of the obtained data from in situ synchrotron studies made it possible to estimate the lattice distortions and thermal expansion parameters, which in turn enabled the evaluation of stress existence during thermal cycling. In-situ high-temperature X-ray diffraction may potentially serve as a fast method for studying thermal mismatch caused by the thermal expansion of the substrate and coating, as well as a method for further optimization of the obtained coatings with minimal differences in the physical properties of the coating materials. Methods and materials In-situ synchrotron X-ray diffraction measurements at high X-ray energies were performed during cooling and heating on the VEPP-3 beamline at Novosibirsk, Russia. The X-ray beam had a size of 1×2 mm, a wavelength of 0.1 nm, and a photon energy of 12.4 keV. The specimens were initially heated from 30 °C to 550 °C and then cooled to 30 °C at a rate of 60 °C/min; in total, 3 heating-cooling cycles were performed. X-ray diffraction patterns were recorded at intervals of 30 °C. The interplanar distance (d) and the full-width at half-maximum (FWHM) of the diffraction peaks were obtained by fitting the diffraction reflections with a Gaussian distribution function. The lattice thermal expansion is determined by the change in strain of the d-space with temperature variation. The first derivative of the lattice thermal expansion curve, divided by the initial interplanar distance of the corresponding reflection plane, is defined as the lattice thermal expansion coefficient. The peak width includes instrumental broadening, specimen size variation, grain size broadening, and micro-stresses [15, 16]. In this experiment, instrumental broadening and grain size broadening are assumed to remain constant with temperature variation, so any changes in FWHM are attributed to the evolution of stresses. Results and discussion Fig. 1 shows the dynamic changes in the lattice parameter during thermal cycling. Here and throughout, the left part of the figure refers to homogeneous coatings and substrates with single-component coatings (suffix “mono”), while the right part of the figure represents the components of the multilayer coating and substrate with the applied multilayer coating (suffix “CrZirN 8 rpm”). The start of the thermal cycling process is indicated by a diamond, and the end of the process is denoted by a star. In all cases depicted in fig. 1, an increase in the lattice parameter (calculated from the (111) reflection) is observed during heating, followed by a decrease during cooling. However, it is found that the lattice parameter changes globally when comparing its value at the beginning and after the testing. This is demonstrated by the fitted straight line that extends throughout the testing period. The slope of this line indicates the extent of the lattice parameter change. When comparing the lattice parameter of the same phase for the ZrN mono specimen (Fig. 1a) and the CrZrN 8 rpm specimen (fig. 1b), the differences in the change of the lattice parameter are insignificant and follow a similar trend, even though the actual value of the lattice parameter in the ZrN component of the multilayer coating initially differed significantly and was smaller by 0.03 Å. When comparing the CrN phase in the CrN mono specimens and the CrZrN 8 rpm specimens, the rate of change (slope of the straight line) of the lattice parameter differs by an order of magnitude. This suggests that in the CrN mono specimen (fig. 1c), nitrogen depletion occurs to a greater extent during thermal cycling compared to the multilayer coating (fig. 1d) despite the initial difference in the lattice parameter of 0.125 Å. The lattice parameter, calculated from the (101) reflection, as well as its changes for the polycrystalline substrate, does not differ significantly, but will be considered later. The constructed hysteresis loops of the lattice parameter versus temperature during thermal cycling (fig. 2) provide insights not only into the differences in the magnitude of the lattice parameter but also reveal that the largest change (reduction) in the lattice parameter for all coatings occurs after the first
RkJQdWJsaXNoZXIy MTk0ODM1