India-ink polishing following single-point diamond turning yields superior aluminum optics.
A process for making precise, high-quality curved or flat mirror surfaces on bare aluminum substrates has been devised. The process consists of (1) diamond turning to establish the desired surface figure, followed by (2) a polishing subprocess that is mostly conventional except for the composition of the polishing compound. This process can maintain a surface figure accurate to within a peak-to-valley error of as little as 1/8 wavelength (at a wavelength of 6,328 Å) and can produce a finish characterized by a root-mean-square roughness of <8 Å. Hence, the process creates possibilities for the fabrication of precise scientific-instrument mirrors (see figure) that, because they could be made entirely of aluminum, would be lightweight, relatively inexpensive, and thermally stable over wide temperature ranges.
Because of the relative softness of aluminum, heretofore, there has been no way to polish bare aluminum to an optical quality adequate for precise scientific instrumentation. Under optimum conditions, diamond turning can be used to obtain a surface figure within an error of no less than about 0.5 wavelength and surface roughness of no less than about 50 Å on an aluminum substrate. To obtain higher optical surface quality, it has been necessary to deposit a thin coat of electroless nickel on a diamond-turned aluminum substrate, then conventionally polish the nickel coat. The disadvantages of this approach are that plating nickel onto aluminum is difficult and expensive, bimetallic thermal stresses can distort the optical surface of the mirror, and there is risk of polishing through the nickel coat in one or more spots. In the latter case, it is necessary to strip the entire coat, redeposit a new nickel coat, and begin polishing anew.
In both conventional practice and the present process, polishing involves the use of a lap that is coated with a polishing compound and liquid carrier and is moved semirandomly and repeatedly over the substrate surface. Conventionally, the liquid carrier is often water. The major distinguishing feature of the present polishing subprocess is that India ink (either alone or diluted with water) is used as both a polishing compound and the liquid carrier. (India ink has been used in the past in this way to polish metals, but not, until now, as an ingredient in an integrated diamond-turning/polishing process for finishing aluminum to precise surface figure and high optical quality.)
The present polishing subprocess consists of two stages. In the first stage, the diamond-turned surface is polished with a mixture of 0.25-µm diamond powder, distilled water, and India ink. In the second stage, polishing is started with pure India ink, and then the ink is slowly diluted with water. Polishing is continued until the desired specification is achieved.
The success of this process has been attributed partly to the carbon particles in India ink. These particles are small and hard enough to provide the correct action between the substrate and lap so as not to cause severe scratching or cold flow of the substrate. In addition, the liquid portion of the ink includes an oily base that acts as an excellent lubricant during polishing. After years of experimentation with known conventional polishing materials and combinations thereof, India ink is thus far the only material known to be effective for polishing bare aluminum.
This work was done by James J. Lyons and John J. Zaniewski of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Manufacturing & Prototyping category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Goddard Space Flight Center; (301) 286-7351. Refer to GSC-14147.