Innovative Kansas City Plant process yields lubricant of unprecedented thinness — with help of Sandia
This story, reprinted from the Kansas City Plant’s internal newsletter, Connections, offers a vivid account of a collaboration between Sandia and the Kansas City Plant to solve a sticky friction challenge. — Editor
Mark Smith doesn’t like to say no. So when customers at Sandia National Laboratories came to him with a requirement that — as far as any of them knew — had never been met, he considered it a challenge.
The requirement was for an extremely thin layer of lubricant — thinner than has ever been achieved either within the weapons complex or in industry — to be applied to small bearings and parts for the W76 and W80.
“Oil can’t be used on these parts because oil flows and could eventually spread to parts of the system where it might interfere with performance,” said Smith, principal engineer in materials engineering at the Kansas City Plant. “Also, for systems that are likely to remain in stockpile for years at a time, oil can settle, leaving parts unprotected by lubrication.”
What the lab asked for was a layer of lubricant to be applied at a maximum thickness of .05 mils, or 50 microinches.
For comparison, typical paints and coatings go on at 1.0 to 3.0 mils thick. Bonded solid film lubricants, which are applied as a solid lubricant powder mixed with a liquid adhesive, are generally thinner, at 0.6 to 0.9 mils; but that’s still more than 10 times too thick. The Kansas City Plant’s production paint department, using their expert techniques, can apply those adhesively bound lubricants at 0.1 to 0.3 mils, but that’s also too thick to meet the new requirement.
“We were trying to solve a ‘micro’ problem with a ‘macro’ solution,” said Smith. Faced with these impediments, he determined that an entirely new approach was called for.
Initially, Smith, who has extensive expertise in spray coating with powder, considered incorporating the lubricant into a powder coating that could be sprayed onto the substrate. But because the goal was thinness, and lubricant by itself is as thin as you can get, Smith tried something unusual — spraying the solid lubricant directly onto the substrate without using an adhesive.
The lubricant he used, molybdenum disulfide (MoS2), is a dry powder in the form of microscopic flakes or plates. Senior engineering technologist Mike Hester dry-blasted the MoS2 at high pressure onto clean substrates in a dry nitrogen atmosphere. When the dust settled, and the loose material was washed away, Smith and Hester were greeted by a pleasing sight: the substrate had a smooth, even, and very thin layer of MoS2 embedded into its surface.
It was Mike Dugger, Ph.D., distinguished staff member at Sandia Albuquerque, who had initially suggested to Smith the possibility of blasting the lubricant directly onto the substrate. Dugger is a tribologist — an expert in friction and lubrication — with years of experience analyzing lubricants and their properties.
And he’s frankly impressed with the Kansas City Plant’s success. “This process generates a film of unprecedented thinness and excellent friction behavior,” said Dugger.
Lubricant is measured using a coefficient of friction: the lower the coefficient, the less friction exists. “This process provides an excellent fiction coefficient,” said Dugger. “Typically, for the kinds of products we build in the weapons complex, we get friction coefficients of 0.15 to 0.12. This process is significantly lower. The lowest we’ve measured with the new process is .03, which is so low it is getting hard to measure.”
The thinness of the MoS2 lubricant is what pleases Smith most about the process. “That’s what we were after,” he said. “It’s a good lubricant, and it’s as thin as you can get. Adhesive and binder are what make the lubricant thicker, and we’ve eliminated them.”
He’s also extremely pleased with the simplicity of the process. It’s easy and inexpensive to perform and, unlike many other methods of applying lubricant, it uses no hazardous solvents or other pollutants.
“This process can be easily applied to production throughout the nuclear weapons complex,” said Smith. “It will lend itself to new weapon designs that require or can make good use of extremely thin permanent lubricants.”
The Air Force is highly interested in the new lubricating process and has asked the Kansas City Plant to consider reprocessing 2,000 gyroscopes — containing 20,000 individual parts — for them. The MoS2 dry-blast coating will allow the gyros to remain in long-term storage without losing lubrication protection.
“What we really need is a material that can be applied with a thickness so small that we don’t have to allow for it in the dimension of the part,” said Dugger, “and this lubricant is providing that. I’ve looked at a lot of solid lubricants over the years, and this is one of the best I’ve seen.”