Air plasma sprayed multi-material composite coatings for enhanced light absorption and thermal emission

This study pioneers a transformative approach to solar thermal technology by leveraging air plasma-sprayed (APS) multi-material composite coatings. It is to achieve unprecedented light absorption and thermal emission, redefining the design paradigm for bi-layer coatings in solar thermal applications. For the first time, both single-layer (Mo-Mo₂C/ceramic, NiO/YSZ) and bi-layer (NiO/YSZ with an additional 8YSZ top layer) coatings on Hastelloy®X substrate were systematically compared using an extensive suite of characterisation techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), optical spectroscopy, infrared thermography, surface roughness, sheet resistance, electrical conductivity, dielectric constant measurements and water absorbency tests. The bi-layer NiO/YSZ + 8YSZ coating with the highest surface roughness demonstrated remarkable light absorption and thermal emission properties. With a minimal light reflectance of 0.1 and a high thermal emittance of 0.961, this configuration achieved superior solar energy capture and efficient heat re-emission, outperforming single-layer coatings with a moderate reflectance of 0.2 to 0.6. Additionally, the Mo-Mo₂C/ZrO₂ coating revealed unique spectral behavior with enhanced reflectance in the infrared region, indicating its potential for niche applications. Moreover, the NiO/YSZ + 8YSZ and NiO/YSZ coatings configuration also exhibited minimal water absorbency due to its fine microstructure, characterised by small pore sizes and low surface-connected porosity. These findings establish the bi-layer NiO/YSZ + 8YSZ coating as a groundbreaking advancement in thermal-sprayed materials, offering exceptional solar selective and thermal emission properties. This work underscores the transformative potential of APS techniques in developing next-generation coatings tailored for optimised solar thermal applications.