Thin-walled structures made of strong glassy nickel and glassy nickel-cobalt alloys, with tailorable low residual stresses, and with high resistance to permanent plastic deformation, can be formed by use of an electrodeposition process. This process was developed to enable the fabrication of lightweight, high-quality x-ray mirrors that do not undergo unacceptably large distortions when differential thermal contraction upon cooling is used to release the mirror deposits from their electrodeposition mandrels. This process supplants an older pure-nickel electrodeposition process where it was necessary to form relatively thick deposits in order to make them strong enough to resist distortion in the presence of stresses imparted during release.
The process is based on the concept of selecting the composition of a plating bath and controlling the electric-current density to tailor the alloy composition and the level of stress in the electrodeposit. The plating bath contains a mixture of nickel salts, or nickel and cobalt salts with sodium hypophosphite. The bath also contains a surfactant and at least one complexing salt capable of combining with nickel, cobalt, and sodium. The pH of the bath is moderate; consequently, there is little risk of corrosion.
At low temperatures, cobalt contributes to compressive stress, while nickel contributes to tensile stress in the deposit. Thus, by suitable balancing of the ingredients in conjunction with the temperature and current density, it is possible to obtain zero net stress in the deposit throughout any desired thickness. Moreover, for a given suitable formulation of ingredients, the stress can be varied from compressive to tensile by varying the current density at a fixed temperature or by varying the temperature at a fixed current density.
The temperatures used in this process lie in the range from 35 °C, for glassy nickel-cobalt alloys, to 70 °C for glassy nickel; as a result, this process is safer than is an electroless process, widely used in airline repair centers, that involves temperatures from 85 to 90 °C. In this process, unlike in the electroless process, nickel, and cobalt when used, can be replenished by dissolution from anodes rather than by addition of chemicals. Little effort is required to maintain the plating bath in this process because any occasional chemical adjustments that must be made are much less critical than those needed in the electroless process.
A deposit formed in this process exhibits little or no plastic deformation at the parts-per-million level (microyielding) at stresses below 100 kpsi (0.7 GPa) and has an ultimate strength of greater than 200 kpsi (1.4 GPa). The deposit is also very hard (>50 Rockwell C), permitting diamond tool machining.
This work was done by Brian D. Ramsey of Marshall Space Flight Center and Darell E. Engelhaupt of the University of Alabama in Huntsville. For further information, please contact
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Darell E. Engelhaupt
Center for Applied Optics
University of Alabama
Huntsville, AL 35899
Refer to MFS-31377