Aging mechanisms for high-temperature solar absorber coatings under extensive thermal cycling

Solar absorber coatings play a crucial role in the efficiency and longevity of concentrating solar power systems. Thermal stresses caused by fluctuating solar conditions from the passage of clouds contribute to the degradation and failure of these coatings. This paper investigates how different factors in a thermal cycling test regime impact aging mechanisms. Two different thermal cycling tests were examined - rapid cycling (RC) and cycle and hold (CH) - and compared with isothermal test results. Varied test parameters include temperature ramp rate, depth of cycling, and holding time at peak temperature. Aging mechanisms were examined in the context of two distinctly different coatings, the well-known Pyromark 2500 coating and a recently developed coral-structured coating with hierarchical features. For Pyromark 2500, the results indicate that test conditions inducing higher thermal stresses accelerate the sintering and crystallisation rate. In particular, the combined cycle and hold test is shown to accelerate key aging mechanisms, including oxide layer growth, elemental diffusion, phase formation and crystallisation, and has significant influence on optical degradation and failure rates. The coral-structured coating demonstrated superior optical, morphological, and chemical stability compared to Pyromark 2500 samples under all test conditions. It is recommended that, where Pyromark 2500 is selected as a benchmark to compare to new coatings, a cycle and hold test is included in the testing program.