An ‘interesting challenge:’ Make the thinnest-film lubricant ever
The Lab News asked Mike Dugger (1824) for some additional Sandia context to the ultra-thin lubricant achievement. Here’s his story:
Our colleagues in component design presented us with an interesting challenge last fiscal year. New stronglinks (safety and security components) for the W76 and W80 life extension program would use designs with tolerances down to 2.5 microns on some dimensions.
The solid lubricant films typically used in these mechanisms to minimize wear and ensure consistent friction performance over the life of the component would require that parts be left undersized to allow for the thickness of the lubricant. A lubricant layer so thin that it can be ignored in the design, and yet produce low and consistent friction behavior, was needed. Commercial sputtered solid lubricant films would meet this requirement, but a process external to the weapon complex presents some challenges in terms of lead times, inspection, and qualification for the stockpile.
Metal dichalcogenide lubricants used in stronglinks operate by “transfer film” formation. Shear occurs between weak c-axis bonds in the crystal, forming an atomically thin layer of the lubricant on both sliding bodies even if initially placed on only one. We reasoned that a particle of MoS2 blasted at a metal surface in a stream of nitrogen (to minimize oxidation) would fracture and form the transfer film. Based a fundamental understanding of how these materials work, the process should also be quite robust since additional lubricant will not tend to stick to the sulfur-terminated surface that forms. In other words, the lubricant thickness should be self-limiting.
The staff at Honeywell have done a great job implementing this idea, and our measurements show that we get friction coefficients around 0.03 in nitrogen, where these films will operate. The previous resin-bonded lubricant gave a friction coefficient of about 0.10 under the same conditions.
We have examined this film on complex mechanism parts as well as bearings, and it appears to perform well. This process should be amenable to any surface that can be reached with a line-of-sight gas stream. We are also working under an LDRD on other approaches for forming thin films conformally on hidden interfaces.