Synthesis of Cr-Al2O3 Cermet Selective Surfaces Coated by SnO2 Thin Film by Air Plasma Spraying

High efficient Cr-Al2O3 cermet selective surfaces coated by sol-gel SnO2 thin film for high-temperature application were synthesized by air plasma spraying (APS), followed further heat and polish treatment. The phase composition and micro morphology of coating samples were characterized by X-ray diffractions (XRD), scanning electron microscope (SEM) and roughmeter respectively. The solar absorptance (alpha) and thermal emittance (epsilon) were determined by optical spectrum instrument. The results show that Cr-Al2O3 cermet coating has a high absorptivity nearly 0.90. But unfortunately, the emissivity of these coatings is nearly 0.50 because of the big thickness and coarse surface prepared by APS technique. However, once coated SnO2 thin film, the composite coatings exhibit excellent selective absorbing property of alpha=0.89 and epsilon=0.12. Thus, SnO2 thin film plays a significant role in decreasing the emissivity of coatings as antireflection layer. And furthermore, the optical performance shows that more metal content and smooth surface are favorable for the selective absorbing property. Moreover, the coatings have excellent bond strength with stainless steel substrate and thermal shock resistance at high temperature.