Characterization of electroless nickel plating on aluminum mirrors

Electroless nickel is often plated on the surface of aluminum mirrors to improve the ability to polish the mirror surface. Electroless nickel plating can cause a bimetallic effect, creating distortion of a mirror surface if it is heated or cooled. Published data listing the thermal coefficient of expansion and Young's modulus as a function of temperature for electroless nickel is not readily available. This study examined using bimetallic bars to measure the Young's modulus and thermal coefficient of expansion of electroless nickel as it is cooled from room temperature to 100 K. A test chamber was developed which can accurately measure the rotations of a bimetallic bar as it is cooled. Elementary beam theory equations for a bimetallic beam were developed. These equations indicate that by testing beams with a variety of beam thicknesses, one should be able to determine modulus and thermal coefficient of expansion data for electroless nickel. The results show that the method fails to find accurate values. Very small measurement errors cause large changes in the modulus values. By using typical values for Young's modulus and the measured beam rotations, values for thermal coefficient of expansion for electroless nickel were obtained.