Printed Athermal Mirror (PAM) based on AlSi40 and NiP coating for space application

Most optical instruments for space applications in the areas of earth observation, science and optical communication require high performance optical mirrors. Driving requirements are typically low Surface Form Error (SFE), high structural stiffness over a wide temperature range, a low micro roughness and low mass. A promising material combination for optical mirrors is the usage of the aluminium alloy AlSi40 as mirror substrate and electroless Nickel phosphorous (NiP) as coating for the optical surface. The main advantage of this combination is the CTE compatibility of AlSi40 and NiP over a large temperature range. In addition, NiP coated surfaces can be easily diamond turned and polished. This material combination thus enables relatively fast production cycles even for a mirror surface roughness in the few-nm range. Within the ESA GSTP SME4ALM program, the feasibility to produce optical mirrors from AlSi40 by additive manufacturing (AM) was demonstrated. Main objectives were the determination of AlSi40 AM parameters and material properties as well as the demonstration of advantages of AM (e.g. topology optimization, monolithic design, lattice structures). By consequent improvement of the Laser Powder Bed Fusion (LPBF) manufacturing process, similar material properties to bulk AlSi40 were obtained. Up to 40% improvement were achieved for relevant material properties such as stiffness and SFE. Based on the good results of the first project, a follow up program was launched by ESA (also in the context of ESA-GSTP) with the objective to design and print an optical mirror with TRL ≥ 5. This paper will summarize the development program and results of the follow-up project, in particular: · Design and testing of lattice structures · Definition of a cleaning process · Transfer of AlSi40 processing strategies to industrial-scale machines · Design and testing of a mirror demonstrator, which is representative for space applications.