Thermal Fracture and Fatigue of Anodized Aluminum Coatings for Space Applications

A concern for the use of anodized aluminum as Space Station thermal control surfaces is the potential degradation of critical optical properties due to oxide coating cracking. This cracking may be induced by differential thermal expansion during hot adhesive bonding of the radiator assembly or during 30 years of exposure to sunlight/darkness cycles in low earth orbit (LEO). This paper summarizes investigations into the effects of temperature and humidity on coating stresses and cracking. Experimentally measured coating residual stresses after anodizing and sealing are shown to be strongly dependent on humidity. Two sets of tests are used to study cracking directly: one determines the temperature (T_c at which cracks first appear during a single heating cycle, while the other involves rapid thermal cycling (RTC) between maximum and minimum temperatures. The effects of several test parameters on cracking are reviewed, with special attention to temperature, coating thickness, and humidity. Fatigue models are developed to describe the observed cracking and to serve as the basis for predictions of LEO cracking. The physical characteristics of cracks and related substrate plastic deformation in both T_c and RTC tests are discussed briefly.