Detailed analysis of the DSC curves recorded for Ni-P and Ni-P-Re coatings demonstrated that in addition to the crystallization peak,extra exothermic peaks can be observed in temperature above and below the main peak. Fig.1 presents an example of such DSC curve obtained for Ni-P-Re coating of the following composition:3.7 wt% of Re,5 wt% of P,91.3 wt% of Ni. It should be noted that extra peaks have low intensity and sometimes the lack of their registration was associated with insufficient thickness of the coating and thus the scale of this effect, which is directly proportional to the amount of the material, and it is below the sensitivity of the calorimeter. To verify the presence or absence of these effects, the increase in the efficiency of the process or the time of the deposition should be implemented, which will be undertaken in the future.
Fig.1.DSC curve registered for Ni5.0P3.7Re alloy coating deposited on Cu registered with the heating rate of 25℃/min.
In-situ heating studies(Figs.2 and 3) conducted in a transmission electron microscope for a thin film obtained from this coating enabled identification of thermal effects recorded on the DSC curve(Fig.1). An increase of temperature from ambient to 400 ℃caused the following phenomena: grain growth(compare Figs.2a and b), crystallization of the Ni5P4 phase as well as Ni3P(see Fig.3a and b). The first peak at 375 ℃ can be, therefore, associated with the crystallization of the metastable Ni5P4 phase. However, the Ni3P phase was also identified at a temperature lower than it would appear from the calorimetric curve, while the main crystallization peak of the Ni3P phase was recorded at 434℃. This is due to the broadening of the main crystallization peak(Fig.1), which suggests that there are some fluctuations in phosphorus content and areas more enriched in phosphorus will crystallize as Ni3P phase at slightly lower temperatures. In addition, the extension of thermal effects(Fig.1) can be associated with the overlap of the grain growth effect of the separated phases, which can be seen in the recorded bright field images shown in Fig.2. Electron diffractions in Fig.3 clearly show that Ni, Ni5P4 and Ni3P are the only identified phases of the coating after its heating to 525℃. Further heating was impossible to perform due to the equipment limitations.
Fig.2.TEM bright field images of Ni5.0P3.7Re coating before and after heating.Applied heating temperatures are marked in the figures.
Fig.3.Electron diffractions obtained for Ni5.0P3.7Re coating before and after heating.Applied heating temperatures are marked in the figures.
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