Ultra-precision fabrication of a nickel-phosphorus layer on aluminum substrate by SPDT and MRF.

Metal mirrors are rarely used in visible or ultraviolet systems due to the ultra-precision fabrication difficulties. In this work, a plane aluminum alloy substrate (ϕ100 mm) surface deposited with a nickel-phosphorus (NiP) layer by the electroless deposition technique is prepared. The NiP layer is processed by single point diamond turning (SPDT) technology to the accuracy of 60 nm in RMS, and the surface roughness reaches 4.157 nm in Ra. A kind of water-based magnetorheological polishing fluid for the ultra-precision of the NiP layer is developed, and magnetorheological finishing (MRF) is applied to the final finishing of the mirror. The developed fluid that contains small size (1.5 μm) carbonyl iron powder (CIP) and 50 nm nano-cerium possesses material removal of 1.8 μm/min, and surface roughness of 1 nm is determined as the optimal fluid formula. The surface residual error is improved from 60 to 10 nm, and the surface roughness decreases from 4.157 to 0.851 nm after MRF in 1.5 h with one polishing cycle with the developed MR polishing fluid. Finally, the surface quality after MRF is tested by SEM and XRD, and the results manifest that the periodical tool mark is swiped out and the surface is not contaminated by MR polishing fluid. The experiment results and theoretical analysis of this work prove that MRF can satisfy the ultra-precision fabrication of NiP film on the metal mirror, and the surface quality can be applied in a visible or even ultraviolet optical system by using suitable MR polishing fluids.