Degradation mechanisms and non-linear thermal cycling effects in a high-temperature light-absorber coating

Abstract

Absorber coatings in concentrating solar power (CSP) convert focused solar radiation into heat. Competitive and reliable CSP technologies require absorber coatings to be stable at high temperatures for many years of operation, and thus accelerated ageing tests are essential to evaluate and further develop these coatings. However, the ageing mechanisms are largely unknown, with previous studies mainly focusing on isothermal (IT) annealing or rapid-cycling (RC) thermal tests, not the more realistic cycle-and-hold (CH) thermal cycling caused by cloud shading. This investigation reports the effects of IT, CH and RC on degradation of Pyromark 2500 on Inconel 625. CH thermal cycling was a combination of IT (equivalent heating time at Tmax=850°C ) and RC (equivalent cycles). The results showed non-linear effects because: IT and RC samples were found to be stable (within 100 h or 1260 cycles) in terms of absorptance, despite having some differences in morphology; whereas CH samples experienced a drop in solar-weighted absorptance after 10 h (126 cycles) at Tmax followed by an increase after 100 h (1260 cycles). SEM and XRD analyses suggested that thermal cycling promoted sintering of the silica binder, producing an amorphous structure between fully formed crystalline grains, which were identified as nickel- and/or manganese-based chromite. The sintering occurred more severely for CH than RC due to holding the temperature at Tmax. Cracking was also more pronounced in CH than RC and IT, possibly due to stronger coating shrinkage. The results suggest that accelerated ageing tests should consider CH thermal cycling to better analyse coating degradation and predict its lifetime.