Substrate-dependent thermally driven morphological evolution of Pt$_{0.9}$Ni$_{0.1}$ thin films on sapphire and langasite
We examine the thermal evolution of 100-nm-thick Pt$_{0.9}$Ni$_{0.1}$ alloy films on sapphire and langasite substrates with a 10-nm Zr adhesion layer. Sequential vacuum annealing from room temperature to 800 $^\circ$C produces a substrate-dependent morphological pathway that is quantified by atomic force microscopy, image segmentation, height-distribution analysis, and effective coarsening metrics. Both types of films remain densely granular up to 400 $^\circ$C, undergo a sharp coarsening transition between 400 and 600 $^\circ$C, and evolve into coalesced faceted crystallites by 800 $^\circ$C. The mean projected feature area increases from $7.84 \times 10^2$ to $1.09 \times 10^5$ nm$^2$ on langasite and from $3.32 \times 10^2$ to $1.67 \times 10^5$ nm$^2$ on sapphire. Sapphire therefore develops the larger faceted crystallites, while langasite exhibits the larger roughness maximum at 600 $^\circ$C. Area distributions reveal that the 400-600 $^\circ$C transition does not uniformly shift the initial granular population but instead replaces it with a distinct large-feature ensemble. Height and roughness measurements show a pronounced maximum at 600 $^\circ$C, followed by partial smoothing at 800 $^\circ$C, consistent with rapid coalescence followed by faceting-limited growth. An effective Arrhenius analysis based on $\Delta\langle r^2\rangle$, with $\langle r^2\rangle = \langle A\rangle/\pi$, identifies the 400-600 $^\circ$C interval as the dominant coarsening window. These results show that the thermal stability of PtNi/Zr films on sapphire and langasite is governed by the coupled influence of alloy mobility, interfacial energetics, and substrate-dependent morphological selection.