Thermal cycling stability of silicon particle reinforced aluminumThursday (09.11.2017) 10:55 - 11:15 Part of:
Ultra precise metal mirrors are particularly used for scientific instrumentations of large telescopes at cryogenic temperatures. The irreversible dimensional stability over temperature of the materials applied is crucial for the optical performance at the addressed temperature. Suitable thermal treatments as well as an athermal behavior of the materials applied are the key tools for dimensional stable metal optics. For minimizing the reversible shape changes (bimetallic bending), the coefficient of thermal expansion (CTE) can be manipulated by the chemical composition of the materials used. In case of the substrate, silicon particle reinforced aluminum with approx. 40 % silicon (Al-40Si) matches the CTE behavior of the amorphous polishable layer made of a nickel-phosphorus alloy (electroless Nickel).
The paper demonstrates the thermal cycling stability of Al-40Si. Mirrors made of such Metal-Matrix-Composite (MMC) are thermally cycled in different cryogenic temperature ranges and resulting irreversible changes of shape deviation in the nm-range are analyzed by Fizeau Interferometry. Additional thermal cycles lead to a progressive smaller increase of shape deviation and a higher thermal cycling stability, respectively. Results of Al-40Si mirrors are correlated with dilatometric studies in the same temperature range. The stresses in the aluminum matrix and the silicon particles are analyzed by Neutron Diffraction at 4 K, 77 K, and 293 K. Thermal mismatch stresses lead to crystallographic defects due to the different CTE of aluminum and silicon. Crystallographic defects are analyzed by Transmission Electron Microscopy (TEM). The paper demonstrates a suitable thermal treatment of Al-40Si for cryogenic ultra-precise metal optics applicable in astronomy as well as space.
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