For Immediate Release
October 27, 1995

WHEN IS A LUBRICANT NOT A LUBRICANT? THIN FILMS IN SMALL-SCALE DEVICES CREATE POTENTIALLY HARMFUL INTERACTIONS

In most complex mechanical systems, lubricants help reduce friction and protect moving parts against wear. But research published October 27 in the journal Science suggests that under extreme conditions, lubricants in systems such as computer disk drives may behave in unexpected ways that in certain cases can harm the very systems they are intended to protect.

Using molecular dynamics simulations, researchers at the Georgia Institute of Technology predict that ultra-thin films of the organic lubricants used in nanometer-scale devices may act more like solids than liquids when subjected to high pressures. The simulations also warn of possible damage from cavitation effects, as well as fatigue failure caused by repeated surface deformation.

Molecular dynamics simulations show asperities on sliding disks approaching one another at high speed.

"We believe these results could have some impact on the design and way of thinking about devices like high density disk drives that have moving parts in very close proximity lubricated by thin films," said Dr. Uzi Landman, director of Georgia Tech's Center for Computational Materials Science.

The work helps expand the understanding of elastohydrodynamic lubrication phenomena from large-scale mechanical systems to "nanoscale" devices. Knowing how the behavior of lubricants can change and affect the lubricated surfaces under these conditions is becoming more important as the need for miniaturization leads to development of ever smaller components.

Using supercomputers to model the complex physical processes involved, the researchers studied the behavior of a thin-film hexadecane (C16H34) lubricant flowing between two gold disks sliding past each other at a relative velocity of 10 meters per second.

Under these conditions -- similar to the operation of a computer disk drive -- the flow of liquid lubricant through the narrow space between the surfaces creates pressure high enough to cause temporary elastic deformation of the disks. Repeated deformation could ultimately lead to fatigue failure and the development of pits or cracks on the disk surface, Landman noted.

More troubling are the possible effects associated with tiny surface nonuniformities that exist even on disks that appear to be smooth. These "asperities" can take the form of bumps or ridges that rapidly reduce the amount of space between the two surfaces as they move toward each other on the sliding disks. Because the long-chain lubricant molecules are able to flow out of the narrowing gap between those surfaces at a limited rate, the convergence of the asperities dramatically increases the pressure exerted on the metal surfaces.

Landman and colleagues Jianping Gao and W. D. Luedtke studied three instances in which asperities passed close enough together to affect the lubricating film. The first involved a separation of approximately 17.5 Angstroms; the second a "near-overlap" of just 4.6 Angstroms, and the third a situation in which the ridges overlapped and collided.

In the first instance, the increased confinement in the region between the approaching asperities caused the lubricant to organize into distinct layers that resembled the ordered structure of a solid. As the pressure continued to increase, the viscosity of the liquid film increased as the molecules flowed out of the gap one layer at a time. This quantized layered structure of the lubricant molecules caused an oscillation in the force required to maintain the relative sliding velocity between the disk surfaces.

In the second instance of a much smaller separation, the pressure imposed on the lubricant became large enough to elastically deform and flatten the asperities, helping to smooth the surface of the disks. Pressures of 200,000 to 300,000 atmospheres could be created as the lubricant is squeezed out of the gap.

In the third instance, all of the lubricant molecules were forced out of the gap and the asperities collided, briefly forming a metallic junction and transferring material before separating. The high pressures created by the collision caused the liquid lubricant to change into a near-solid phase.

"You come to the point at which the lubricant molecules are arrested," Landman explained. "There is not enough time for them to escape the confined region that is being produced. This can cause the lubricant to undergo a phase change to an ultra viscous fluid or a soft solid. Under extreme circumstances, the liquid can become a glassified solid that develops a significant resistance to shear. This could bring about seizure and a disk crash."

Another potentially harmful effect is the cavitation that can occur after the asperities collide. When they separate as the disks continue their rotation, lubricant molecules are slow to fill the widening gap, creating a cavitation phenomena.

"At these velocities, the lubricant may not be able to backflow and fill the void," Landman explained. "This brings about the formation of cavities in the thin film lubricants. These cavities may have harmful effects because their collapse releases sufficient energy to damage the surface through the propagation of supersonic shock waves and the release of heat."

In addition to the cases of non-uniform surface asperities, Landman said the normal starting and stopping of a disk drive could also alter the space between moving surfaces and create increased pressure. As reader heads are placed closer and closer to disk surfaces to gain spatial resolution, this problem could become more of concern.

In the process of deformation and collision, the action of the elastohydrodynamic lubrication may liberate wear particles. How these particles affect the device and how they might degrade the lubricant are subjects Landman hopes to study further.

The research was sponsored by the U.S. Department of Energy, the National Science Foundation, and the Air Force Office of Scientific Research. Computations were performed at the National Energy Research Supercomputer Center in Livermore, CA., at the Pittsburgh Supercomputing Center, and at the Georgia Tech Center for Computational Materials Science.

RESEARCH NEWS AND PUBLICATIONS OFFICE
Georgia Institute of Technology
75 Fifth Street, N.W., Suite 100
Atlanta, Georgia 30308

MEDIA RELATIONS CONTACTS:
John Toon (404-894-6986);
Internet: john.toon@edi.gatech.edu;
FAX: (404-894-4545)

TECHNICAL:
Dr. Uzi Landman (404-894-3368); FAX: (404-853-9958)

WRITER: John Toon