Microstructure and optical–infrared properties of vacuum-annealed Ag–TiN–Al2O3 coatings

This study investigates the structural and optical–infrared (IR) characteristics of Ag–TiN–Al2O3 multilayer coatings deposited via magnetron sputtering on silicon substrates and subjected to vacuum annealing at 300°C, 400°C, and 500°C. X-ray diffraction analysis confirmed the formation of crystalline Ag and TiN phases, with Al2O3 remaining amorphous. Surface morphology revealed significant grain coarsening at 400°C, correlating with enhanced crystallinity. UV–vis spectra indicated improved visible reflectance at this temperature, while Fourier transform infrared spectroscopy measurements demonstrated a pronounced reduction in IR emissivity, reaching minimum values of 0.020 (3–5 μm) and 0.070 (8–14 μm). These values outperform reported single-layer and bilayer systems such as TiN, Cu–ZrB2, Ag–ZrB2, and Al2O3/ZrB2, confirming the superior IR stealth capability of the multilayer. However, further annealing at 500°C led to oxidation-induced structural degradation, reflected in a marked increase in emissivity. The Al2O3 top layer effectively restricted oxygen diffusion up to 400°C, preserving the multilayer’s integrity. These results confirm the thermal durability and multifunctional potential of the Ag–TiN–Al2O3 system for advanced IR stealth and thermal control applications under moderate thermal loads.