Synthesis and Characterization of Multilayered CrAlN/Al₂O₃ Tandem Coating Using HiPIMS for Solar Selective Applications at High Temperature

The effect of applying a negative bias during deposition of a previously designed multilayer solar selective absorber coating was studied on two types of substrates (316L stainless steel and Inconel 625). The solar selective coating is composed of different chromium aluminum nitride layers deposited using a combination of radiofrequency (RF), direct current (DC), and highpower impulse magnetron sputtering (HiPIMS) technologies. The chemical composition is varied to generate an infrared reflective/ absorber layer (with low Al addition and N vacancies) and two CrAlN intermediate layers with medium and high aluminum content (Al/Cr = 0.6 and 1.2). A top aluminum oxide layer (Al2O3) is deposited as an antireflective layer. In this work, a simultaneous DCpulsed bias (-100 V, 250 kHz) was applied to the substrates in order to increase the film density. The optical performance, thermal stability, and oxidation resistance was evaluated and compared with the performance obtained with similar unbiased coating and a commercial Pyromark paint reference at 600, 700, and 800 degrees C. The coating remained stable after 200 h of annealing at 600 degrees C, with solar absorptance (a) values of 93% and 92% for samples deposited on stainless steel and Inconel, respectively, and a thermal emittance e25 degrees C of 18%. The introduction of additional ion bombardment during film growth through bias assistance resulted in increased durability, thermal stability, and working temperature limits compared with unbiased coatings. The solar-to-mechanical energy conversion efficiency at 800 degrees C was found to be up to 2 times higher than Pyromark at C = 100 and comparable at C = 1000.