Cermet coatings are composite materials consisting of metal nanoparticles embedded in a ceramic dielectric matrix. By controlling the content, size, and distribution of the nanoparticles, excellent spectral selective absorption can be achieved. However, when exposed to high-temperature environments above 500 ℃, the nanoparticles within the coating exhibit unstable behaviors such as agglomeration, oxidation, and growth, which significantly reduces the coating’s service life. To address the instability of metal nanoparticles at high temperatures, solutions include adding high-melting-point alloy elements or applying an anti-oxidation layer to prevent thermal diffusion-induced grain oxidation. However, these existing studies often treat the mixture of nanoparticles and ceramic dielectrics as an equivalent medium, neglecting the influence of nanoparticle distribution characteristics. This oversight fundamentally explains previous research failures in enhancing the thermal stability of cermet solar selective absorbing coatings. In this study, we propose a method using high-entropy nitrogen oxides as light-absorbing components. By employing a uniform nanoparticle distribution instead of a random distribution, we aim to improve the high-temperature performance and service life of solar spectrum selective absorbing coatings.

In this paper, we propose the construction of layered microstructures within an AlCrNbSiTiON-based high-entropy nitride-oxide solar spectrum selective absorbing coating, which leads to the development of a novel absorptive sub-multilayer “light trap” composite structure. By leveraging the unique grain structure within the absorptive layer and its spatial distribution, we achieve a synergistic effect from multiple spectral absorption mechanisms, thereby enhancing light absorption by the coating. Additionally, the hysteretic diffusion effect, induced by high entropy and complex elemental composition, along with the amorphous structure, delays grain growth and segregation caused by elemental diffusion. We investigate the photothermal conversion coating prepared by sputtering, examining the effect of the layered structure on the overall optical properties of the coating. Through microscopic characterization, we analyze the formation mechanisms of the special structures within the coating and the principles that enhance its thermal stability.

The stabilization mechanism of layered cermet coatings can be attributed to the multispectral selective absorption properties of the layered microstructure. For cermet coatings with uniformly distributed nanoparticles, sunlight absorption primarily occurs via the local surface plasmon effect of the nanoparticles. In layered microstructures, the stratified nanoparticles not only trap photons within the cermet coating, significantly increasing the number of photon reentry contact points and enhancing the intensity of interaction with sunlight (Fig. 7) but also integrate the absorptive capabilities of both cermet and dielectric-metal-dielectric structures. Synergistic multi-spectral absorption mechanisms, including nanoparticle scattering absorption, interlayer interference absorption, and surface plasmon polariton absorption, effectively mitigate the adverse effects caused by weakened local surface plasmon effects. It is worth noting that in the layered microstructure, nanoparticles may experience unstable growth, which leads to a weakening of the local surface plasmon effect and subsequent attenuation of the coating’s selective absorption performance. However, the reduced spacing between nanoparticles enhances the multilayer interference absorption mechanism, thus effectively turning the instability into a beneficial factor. Consequently, the spectral selective absorption performance of the coating can be maintained or even improved without deterioration. The sketch (Fig. 7) illustrates the decomposition diagram of the coating prepared in this study. When sunlight irradiates the coating surface, it first passes through the anti-reflection layer. The transmitted solar radiation then interacts with the upper columnar crystal structure, undergoing multiple reflections. During these reflections, intrinsic absorption occurs, and a portion of the solar radiation is absorbed. The columnar crystal structure acts as a light trap, which makes it difficult for light to “escape” and reflect downward, thereby further enhancing absorption. This structure effectively traps solar radiation, and the vertical complex crystal surfaces significantly reduce upward reflection, improving the spectral absorption rate. After initial absorption and reflection, sunlight enters the middle sublayer structure. In this sub-multilayer region, each layer has a distinct composition. One layer consists of an amorphous matrix with evenly embedded high-entropy nitride particles, while another layer is primarily composed of an amorphous structure. Within this sublayer, various absorption mechanisms are at play, including traditional cermet mechanisms such as small particle resonance absorption, scattering, and electron transitions, as well as additional mechanisms like local plasmon absorption. Combined with the light-trapping mechanism proposed in this study, the synergistic action of these multiple absorption mechanisms enhances the coating’s excellent solar absorption capability.

In this study, an AlCrO/AlCrNbSiTiON/Cr solar spectrum selective absorbing coating is prepared using multi-arc ion plating equipment, and its thermal stability at 500 ℃ under standard atmospheric conditions is investigated. The excellent optical properties and thermal stability of the coating are analyzed based on its phase composition and microstructure. Finally, the unique structure of the coating is examined in light of the characteristics of the multi-arc ion plating process. The absorptivity and emissivity of the deposited coating can reach 0.931 and 0.169, respectively, which demonstrates excellent optical properties. Phase composition analysis reveals the presence of face-centered cubic grains in the coating, which indicates that a high-entropy system is formed during sputtering. However, the coating also contains a significant amount of amorphous structural components. Microstructure analysis reveals that, after heat treatment, the coating’s absorption layer forms a high-entropy nitrogen oxide?amorphous composite structure. From bottom to top, this structure consists of amorphous regions, a high-entropy grain?amorphous composite layer, and a high-entropy columnar grain?amorphous composite layer. At high temperatures, the local surface resonance absorption of the coating is weakened by particle growth within the layers. However, as larger particles gradually fill one of the layered regions, they enhance the interference absorption effect, thereby maintaining stable light absorption performance at elevated temperatures. Additionally, the stratified grain distribution effectively suppresses optical property degradation caused by grain segregation, which ensures the long-term stability of the coating during extended service.