Super slurry from Belarus polishes U.S. optics

A high-tech mix used in shock absorbers and clutches in Russian military vehicles has been recast as an award-winning U.S. technology for polishing precision optics, vital components in today's products such as camcorders, CD players, surgical lasers, bar-code scanners, and telescopes.

A team of scientists in Minsk, Belarus, in 1988 came up with the idea of using the slurry to polish glass, an application that piqued the interest of scientists at the University of Rochester's Center for Optics Manufacturing (COM). Initially doubtful, optics expert Stephen Jacobs of the Laboratory for Laser Energetics (LLE) visited the principal investigator, William Kordonski, in Belarus to see his discovery, and was immediately impressed. The visit resulted in a research collaboration between COM and Byelocorp Scientific Inc., a firm that develops new technologies; the agreement brought Kordonski and members of his team to the United States. Together COM, Byelocorp and Kordonski's team launched a research program at COM in 1994 to perfect the technology. The collaboration has resulted in three patents and commercialization of the technology, known as magnetorheological finishing (MRF), in the optics industry.

Kordonski now works for QED Technologies, a Rochester company that formed following the success of the MRF research program at COM. Last year QED, in cooperation with COM and Byelocorp, introduced the first commercial product using MRF. The machine, the Q22 Magnetorheological Finishing System for polishing high-precision spherical and aspherical lenses, was recently chosen by Photonics Spectra magazine as one of 25 winners of the 1999 Photonics Circle of Excellence Award.

"This has been an exciting and satisfying program to work on. That's extremely fast for a technology to be invented and turned into a commercial product," says Jacobs, who is senior scientist at COM and LLE as well as associate professor of optics and materials science.

The MRF process relies on a unique 'smart fluid' which changes viscosity and responds to external conditions. The fluid is a slurry of water and other materials, mainly carbonyl iron powder and an abrasive material, cerium oxide. When exposed to a magnetic field, the mixture temporarily hardens and conforms to the surface of the glass to be polished, making it ideal to polish components of any shape. The water in the slurry softens the glass surface, making it easier for the abrasive particles to scratch off silica molecules and carry away the debris. A pump draws the slurry into a mixing bottle where a circulation system keeps the fluid homogenous.

"MRFupclo" Photo credit: Center for Optics Manufacturing, University of Rochester A lens mounted on a spindle is swept through an abrasive slurry in the magnetorheological finishing (MRF) process perfected at the University of Rochester's Center for Optics Manufacturing. The polishing action takes place within the magnetic field as the slurry stiffens and conforms to the glass surface. In this photo the slurry is spinning on a wheel, polishing a lens mounted on a spindle. Controlled by the magnet, the slurry becomes a temporary polishing tool. The slurry returns to its viscous state after passing through the magnetic field. Full size image available through contact

Such fluids are already used commercially as expensive shock absorbers to cushion the rides of long-haul truckers, but the fluids have remained a niche item in the commercial sector because of their high cost. "What we do at COM is really the only other commercial application for MRF. This is a very high-tech application, and the cost of the fluid is relatively inexpensive compared to the value added to the product," Jacobs says.

"Q22" Photo credit: QED Technologies Full size image available through contact

"Q22large" Photo credit: QED Technologies These two photographs show the Q22 MRF System shooting a thin ribbon of magnetorheological fluid onto a vertical wheel, where it passes through a magnetic field, stiffens and polishes an optical piece mounted on a spindle. The material is being ejected from the nozzle on the left side of the image; the lens being polished is visible directly beneath the gold section of the spindle, just above the wheel. A magnet located underneath the polishing wheel controls the slurry's viscosity. Full size image available through contact

Magnetorheological finishing is good news for an industry faced with a dwindling number of master opticians, as fewer and fewer young people take up the craft of glass polishing. With the new technology, a computer program analyzes the errors on the surface of the lens and the shape of the polishing spot to determine the necessary polishing motion and duration. "The MRF process is fast and precise. It does an excellent job polishing glass and surpasses the conventional method," Jacobs says.

The flexibility of the finishing process also makes it handy for producing oddly shaped optical components known as aspheres, highly efficient lenses that are normally very difficult and expensive to produce. The technology has been incorporated as a finishing step for the latest machine developed by COM, a conformal grinder; just last month COM received a prototype of this machine, which makes grinding free-form shapes possible for the first time.

"Magnet" Photo credit: Sandy Hill/University of Rochester The iron content in magnetorheological fluid makes it responsive to magnetic fields. The slurry is considered to be a "smart fluid" because it can change shape to perform some function, in this case polishing glass. This photograph illustrates how a puddle of magnetorheological fluid stiffens when a magnetic force is applied, as seen by the sample of the gray slurry at the center. The fluid, which also contains abrasive particles, becomes a temporary polishing tool as long as the. Full size image available through contact