The Effect of TVAC Cycles on Mechanical Properties of Coated AA7075 via Hard Anodization and Pulsed Laser Deposition: A Comparative Study for Space Applications

In the space environment, aluminum alloys used in structural applications are exposed to severe conditions such as debris impacts and extreme temperature fluctuations. These phenomena can significantly compromise surface integrity, leading to increased wear, reduced mechanical performance, and premature failure of components. To address these challenges, various surface treatments have been developed to form protective oxide films on aluminum surfaces. Hard anodization is the traditional and widely adopted method, producing thick, hard and columnar aluminum oxide through an electrochemical reaction. Recently, Pulsed Laser Deposition (PLD) has emerged as a potential alternative, offering the possibility of depositing dense, uniform thin films with strong adhesion to the substrate. The aim of this work is to assess the reliability of both coating techniques when samples are subjected to TVAC cycling. To achieve this objective, AA7075-T6 coated samples underwent both thermal cycling in vacuum and tribo-mechanical tests. To simulate the Low Earth Orbit (LEO) exposure, the environment in which the “6S CubeSat” of the PoliSpace project is subjected, a Thermal Vacuum Chamber (TVAC) was used, following “ECSS-E-ST-10-03C” standard. By maintaining a high vacuum level, the samples underwent a specifically designed thermal cycle, with the maximum and minimum temperatures chosen based on the thermal study of the cited project. These temperatures were the ones at which premature failure of some components of the CubeSat was observed [1]. Subsequently, the mechanical behaviour of the coatings was characterized both before and after TVAC exposure, focusing on properties critical to in-orbit durability. Vickers and Berkovich tests were conducted to monitor changes in surface hardness for hard anodized and PLD coatings, respectively. Vickers micro-hardness tests followed the procedures outlined in “ISO-4516-2002”, while Berkovich nano-hardness tests were selected for the PLD coatings due to their limited thickness. The friction coefficient and coating wear must be considered for the CubeSat rails, as these components are critical for the satellite’s release from the rocket’s storage chamber. To assess the performance of our coatings in relation to these factors, a pin-on-disk wear test was conducted under dry conditions. Coating adhesion, a crucial factor for the stability of space structures in the presence of sliding space debris, was evaluated through scratch tests following “ASTM-C-1624-05”. These tests provided insights into interface stability, coating adhesion under increasing applied load, and the failure mechanism. A quasi-static load ramp was applied to avoid impact-like loading conditions.