Microstructure instability and spectral selectivity degeneration of the WTi-Al 2 O 3 multilayer solar selective absorbing coating

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The growth and oxidation behavior of nanoparticles in the Al 2 O 3 /W-Al 2 O 3 /WTi-Al 2 O 3 /Al 2 O 3 /W multilayer spectrally selective absorbing coating were investigated to understand the microstructure instability and spectral selectivity degradation at high temperatures. The experimental results demonstrate that after annealing at 650 °C for 400 h under vacuum condition, the coating's absorptance decreased from 0.93 to 0.90 and the emittance dropped from 0.09 to 0.07. However, when annealed under atmospheric conditions for 400 h, the absorptance initially decreased to 0.838 before recovering to 0.912, while the emissivity increased to 0.63. Consequently, the performance coefficient reached 0.29, significantly exceeding the 0.05 failure threshold and confirming coating failure. The microstructural analysis reveals significant crystallization, with columnar grains oriented perpendicular to the stainless-steel substrate. The extensive presence of grain boundaries facilitated atomic diffusion throughout the coating, consequently diminishing its thermal stability. Simultaneously, the β-W nanoparticles in the absorbing sublayers transition to the more stable α-W phase, driven by oxygen and elevated temperatures. This phase transition enhances near-infrared reflectivity and decreases emissivity. Meanwhile, Ti solute segregation and partial oxidation form a TiO 2 protective layer on α-W nanoparticles, inhibiting W diffusion and agglomeration. The noteworthy aspect is the progressive increase in TiO 2 layer thickness, coupled with nanoparticle agglomeration, leading to a decrease in absorptivity. The coating surface contains a thick Mn 2 O 3 layer, increasing roughness from 2.46 nm to 125 nm and enhancing emissivity. We provide morphological and chemical evidence for Mn 2 O 3 formation on the coating surface, indicating accelerated diffusion along columnar grain boundaries as the dominant factor.