03 July 2001
New method for placing glassy-metal electroplates
By Bob Felton
A method that allows hard, glassy metals to be deposited onto substrates using electroplating, while retaining the material properties advantages of electroless plating, may soon see the light of day, said engineers at NASA's Marshall Space Flight Center.
|Three X-ray mirror shells (right) and a nested grouping of mirrors within one cylindrical tube (left).|
Manufacturers traditionally apply the nickel-cobalt-phosphorus plating used to make mirrors by dipping the part in a bath that evenly coats the exposed surface. Although the procedure yields high-quality coatings, it requires high temperature, has a slow deposition rate, and permits little process control during fabrication. NASA said the new procedure "enables stress-free plating, deposits glassy metal alloys at higher rates, and provides deposition at a much lower processing temperature than with electroless deposition. Plating rates are constant and predictable, and coatings can be extremely hard." Additionally, "This process also mitigates the need for constant chemical metal replenishment. You can replace the phosphorus at 1:1 consumption, unlike the 5:1 rate of electroless processes. Buildup of harmful by-products is minimal, and solutions can be left unattended for very thick deposit growth."
The electroplate process offers several benefits, NASA said:
- The coatings form at a temperature of 40° to 65°C, instead of 85° to 90°C.
- The coatings exhibit strengths on the order of 1,930 megapascals and hardness of 5052 on the Rockwell C scale.
- Fabricators control residual stresses, from tensile to compressive, in real time.
- Electroplating rates are faster-6.35 to 25.4 micrometers per hour-than electroless deposition.
- The process requires less maintenance and less ventilation than electroless plating, is less expensive, and exhibits less burn and fuming than conventional techniques.
When mounted on a platform similar to the Chandra Great Observatory, the telescope gathers X rays from a large area and focuses them into a very small area. Because X rays simply pass through most solids, however, the challenge was to develop a lightweight mirror with a surface so smooth the X ray would glance off if the angle of incidence were flat enough. The designers eventually produced mirrors that are truncated right cones with slopes so shallow they resemble tubes. A filter over the collecting end of the frustrum allows only X rays that will skip off the mirrored surface to enter.
To create the mirror, engineers placed a mandrel in a bath containing a nickel alloy; electroplating continued until the mandrel acquired a coating 0.25 millimeter thick. Later, the mandrel and the coating are separated, and the mandrel is reused to make additional mirrors. After polishing, a month-long process that smooths the surface to a roughness of less than 0.5 nanometer, the coating becomes a mirror.
Though developed to manufacture an X-ray telescope, the technology should have plenty of terrestrial applications, according to NASA, including compact discs, computers, a potential replacement for chromium, corrosion-resistant coatings, molds, and aircraft and military components.
NASA is now seeking companies to license the technology.
Additional information is available from any of the following individuals: Peter Liao, Research Triangle Institute, (919) 541-6124; Sammy Nabors, Marshall Space Flight Center, (256) 544-5226; or Dr. William Gathings, University of Alabama, (256) 824-6620. IT