The samples were tested at a certaintemperature in range of 200–800 °C, but the temperature did not change duringtests. This test, thermal aging test, is able to test high temperature stabilityof the sample to some extent, as well as study the mechanism of theperformance of the coating at high temperature. According to these researchers,element diffusion, oxidation, phase transformation, andsurface morphology of the coating are the main factors in the change of theperformance of the coating at high temperature. The element diffusion ofCu, which worked as infrared reflector for solar selective absorbing coatingsdue to its low emittance, has been reported.The diffusion of Cu from thesubstrate changed the composition of the coating, which resulted in thechange of the optical property of the coating. Barshilia et al.reported their strategy that using TiAlN as a diffusion barrierfor copper and obtaining sound result that the coating was able to maintainperfect optical properties even annealed at 600 °C for 2 h. Oxidation isalways the result of coating annealed in the air at high temperature.Oxides, formed during the annealing, changed the composition of thecoating, and caused the change of the optical property of the coating, too.