The STLE Compass, Released October 11, 2011
“Space Tribology”
With Chris DellaCorte, NASA
KARA: Hello and welcome to the STLE Compass, brought to you by the Society of Tribologists and Lubrication Engineers. The STLE Compass is your convenient and reliable resource for the latest developments in the tribology community. I’m Kara Lemar, Education Manager at STLE and in today’s episode, we’ll take a look at space tribology, starting off with a review of the general aspects of the field, and moving into current research and work that’s being done to eliminate friction, wear, and other tribological issues. Today we’ll talk to Dr. Chris DellaCorte. He is a senior research tribologist at NASA’s Glenn Research Center in Cleveland, Ohio. There he has been doing tribology research related to space and aeronautics systems since 1985. Over his career he’s had the opportunity to work on a variety of interesting tribological challenges including the Space Shuttle Main Engine turbopump bearings, high temperature solid lubricants and foil air bearings for Oil-Free turbochargers, aircraft turbines, and space power plants. In 2008, he worked on the team that determined the root cause for the failure of the large rotating joint (a.k.a the SARJ) used to point the solar panels on the International Space Station. More recently, he’s been addressing other tribological problems on the space station and Mars Science Laboratory (a.k.a. MSL or the Curiosity Rover). In his “spare time” he is developing the next generation ball-bearing materials using hard but super-elastic Nickel-Titanium alloys. Chris is an active STLE fellow, is the founding editor for TLT and currently serves on STLE’s Board of Directors.
KARA: So Chris, welcome to the STLE Compass.
CHRIS: Thanks.
KARA: First, let’s start with some definitions. What’s the difference between a liquid and solid lubricant?
CHRIS: Well that’s really a good question, Kara. Actually a better question to ask is what are the similarities between a solid and a liquid lubricant? I mean, the obvious thing is a liquid lubricant is a lubricant that’s liquid and a solid lubricant is a lubricant that’s solid. But I think it’s easier to define first what is a role of a lubricant. In the most basic terms, the role of a lubricant is to reduce friction and wear, right? But when we talk about lubricants, we in the tribology world really like to talk about shear forces. That is, when you rub two bodies together, how hard is it to slide or roll one surface over another. One of our former colleagues, the late Maurice Godet, had a really good way of explaining what lubricants are and when you understand that you can understand the difference between solids and liquids. But Maurice explained that whenever you have two bodies that contact one another, there is always something else in between the two bodies, a so-called “third body.” Whenever two surfaces interact, there is that interfacial region which really controls friction. He liked to talk about the third body being the shear layer. If it’s soft, then it provides a lubricating function. So, in respect if you had two surfaces rubbing on one another and you put a liquid in between, that liquid, if it had a low resistance to shear or slide, then it acts as a liquid lubricant. Conversely, if you have two bodies contacting one another and there’s a third body made out of a solid material in between that happens to be easy to shear, smear or slide, then it’s a solid lubricant. And in effect, they both reduce shear forces. To summarize briefly, a solid and liquid lubricant are kind of the same thing – they’re both third bodies that you put in between two other bodies that come into contact and you want that third body or that lubricant to be easy to shear.
KARA: So, what are some typical examples of a solid lubricant?
CHRIS: Well, there are the obvious solid lubricants, things that we’re all very familiar with, the soft polymers like Teflon, and then there are the inorganic materials like graphite and molybdenum disulphide. Those are the ones we’re all kind of familiar with. And again, they provide that role of being an easy to shear layer that separates two otherwise contacting bodies. But there are also some more subtle lubricants like soft metals such as lead, gold and silver, that we don’t think of as lubricants all that often, but we use them as such. Then there are the more exotic ones like fluoride coatings, boric acids and others. Some of the coatings behave as solid lubricants for different reasons. The fluoride coatings are soft materials that have high temperature capability and the boric acids are layered structure materials like graphite that when you mix them with water, they form like a deck of playing cards that sit between two surfaces in contact and are easy to slide over one another. And when you make a solid lubricant, it’s also kind of important to keep in mind that most of the commercial solid lubricants are really combinations of binders, which you might think of as a glue, mixed with solid lubricants that form a coating that provides a wear resistant sort of a matrix with the soft solid lubricants as a reservoir.
KARA: What are the advantages and limitations for each?
CHRIS: Well, in a general sense, the liquid lubricants have an advantage because they can easily be replenished and offer a longer service life. Of course, the limitations for a liquid lubricant is that it has to be sealed inside whatever you’re trying to lubricate whether it’s a bearing, a transmission, a mechanism or an engine. The solid lubricants are simpler. They work better than liquid lubricants at extremely low and extremely high temperatures, but they are not as easy to replenish and they kind of have a limited life. You put a coating on a component and put it into surface and when the coating is worn out your friction and wear go up. And then of course, you’ve got things like greases, which are really a hybrid. Grease is sort of like a sponge made out of soap fibers at the macroscopic level that are infused with oils and sometimes infused with solid lubricants as well. Greases kind of combine the best of both worlds. You’ve got your liquid and solid lubricants there held in a matrix so they don’t have to be sealed quite as well as a lubricant. It’s “different horses for different courses,” as they say.
KARA: You mentioned that solid lubricants have a limited life – what happens when it does run out? Do you just re-coat it?
CHRIS: Well, you can re-coat it, if you have the luxury of getting into your machine and doing a re-lube job. Unfortunately, most of the time that you use a solid lubricant, it’s a component that you don’t have a chance to ever touch it again, at least not without going to great expense. Take a fairly straightforward case such as the window mechanism of a car – you might put a solid lubricant coating on some of the components in there and build the car around the mechanism and never touch it again. So, generally speaking, solid lubricants are not designed to be replenished.
KARA: What are the best applications for each – liquid and solid lubricants?
CHRIS: Well I don’t know if you can ever say “best applications,” but I’ve been giving this a lot of thought over the last 5-10 years, especially because I get asked a lot, “What kind of lubricant is best for a particular application?” And my philosophy is as follows: if you’ve got some kind of machine or mechanism that has to provide some kind of motion – if you can design your mechanism so that you can get motion through the flexing of a spring, that’s actually preferable to the second way of getting motion, which is rolling contact. The reason I talk about that flexing thing is because I work for NASA and we oftentimes design pointing mechanisms for space telescopes and some of those mechanisms only have to move a few degrees one way or another and we often do that just by bending of a beam. I always think if you can avoid a rubbing or rolling contact, that’s better than having to have something that needs to be lubricated. I always tell people that flexing is better than rolling and rolling contact is better than sliding. If you’ve got to have a mechanism that has sliding contact, greases and liquid lubricants are better in terms of performance than solid lubricants. If you have to use solid lubricants, then the conventional solid lubes, like I mentioned before, graphite and Teflon and what have you, are typically better performers than the exotic materials I talked about. The thing to bear in mind is that specific applications often preclude the preferred approach or the preferred material. In other words, you may have an application that if you have a liquid present and it vaporizes at ultra-high vacuum, like we have in space, or the lubricant can’t take the temperatures, then you can’t use oils and you have to use solid lubricants. But just so that everyone keeps in mind that all moving surfaces in contact, even rolling contact, need lubrication. There’s no such thing as a pure rolling contact at the microscopic level, even materials that roll over one another, there are some atoms in that rolling contact that are sliding. And just like Maurice Godet pointed out to us, when you’ve got atoms that are rubbing against one another, you’re going to wind up having a third body there. If it’s not intentionally a lubricant that’s there, it’s going to be a wear debris particle that forms when two surfaces rub. As far as lubricants are concerned, solid lubricants are good, liquid lubricants are good, but different lubricant approaches for different applications.
KARA: Last time we heard from you in TLT, you were revealing a new material for bearings – NiTinol. What’s happened since then, any new developments in that arena?
CHRIS: Oh sure. We’ve been making lots of progress. You know, I’ve been working in the field for a while and it’s really rare for something new to come along. A few years back, working with some colleagues in industry, they brought me what I thought was a new technology. It’s a bearing material based upon Nickel and Titanium, which is affectionately termed NiTinol. Just to give you some background on NiTinol – it’s actually an old NAVY technology that was put forward – and that’s where the name came from – it’s Nickel Titanium Naval Ordinance Lab because a guy named Buehler at the Naval Ordinance Lab came up with it in the 1950s and it’s an interesting alloy because at the same time, it’s hard and it’s non-magnetic and it has no iron in it so it doesn’t rust. The idea had been abandoned decades ago to use it because it was just so hard to work with. But applying modern processes that have developed over the years for ceramic materials, we actually wind up being able to make this into things like bearings. It’s an interesting alloy because it’s a corrosion proof material that’s also a member of the superelastic family that includes the well known shape memory alloy NiTinol 55. We just add a little extra Nickel to the NiTinol 55 and it eliminates that shape memory effect, but the material retains its super-elastic properties, meaning it’s kind of like a hard rubber. We’ve done some research to indicate that we can make bearings that are not only corrosion immune, but highly resistant to shock damage – kind of the holy grail for space flight hardware. This stuff is interesting also because unlike some of the emerging ceramic hybrid materials, the NiTinol 60 is electrically conductive and the really interesting and exciting thing for me, from the perspective of a researcher, is that we’ve only been working with this 60 Nickel Titanium, which is 60 weight percent Nickel and 40 weight percent Titanium – it has no other additives in it. And we’ve just started formulating new versions of it that have a little pinch of this and a little dash of that and we’re able to refine its properties and enhance them even further. I’m really looking forward to further developments with these alloys as time goes on.
KARA: So, has this material been commercialized? Where might it be used?
CHRIS: Well, the NiTinol 60 is in the process of being commercialized. We’ve got some patent applications in to Washington, D.C. and we’re working with industry right now to make our first complete bearings and we’re conducting lots of the supporting tests that will guide us in how to use it – things like fracture strength, rolling contact fatigue resistance, and that sort of thing. Right now, everything is looking really good. We expect the initial NiTinol 60 bearings to be used to fix an ongoing challenge on the International Space Station’s waste water treatment system. As you can well imagine, in order to turn the waste water on the station into water that we can drink again, there are all sorts of chemical processes involved and one of them means taking the waste water and making it highly corrosive and then heating it up and boiling it in a vacuum chamber. The chamber has to rotate at the same time. The bearings that are in that system are really suffering. The first bearings were made out of stainless steel and they corroded like crazy. The second set of bearings are made out of a corrosion-proof soft cobalt alloy and they’re wearing out. So, we’re hoping that this NiTinol 60 will solve the corrosion and wear problem all in one fell swoop. So, we’re kind of looking for niche applications in which the unique capabilities of this material are going to solve problems and whatever we learn there, we’ll transfer to industry.
KARA: Sure. Once you find out its successful, you can take it and apply it to other things.
CHRIS: Exactly.
KARA: So, what are you working on now?
CHRIS: Well, in addition to pushing the superelastic bearings into use, we’re working on some tribology challenges inside Stirling Engines, which are little pistons engines that convert heat from any source including nuclear power into electricity. They’ve got tight clearances, gas bearings and coatings and things of that nature inside and we’re working on some life test issues that have come up. Those are for unmanned space missions. And really, I’ve spent a lot of time over the years just to increase the general recognition, even within NASA, that the best way to fix space tribology problems is to avoid them through good up-front design practices, and I think the same thing is true for all engineering disciplines. Just to give you an example, we’re hosting a class for NASA colleagues in about a week and half, to give them training on the latest ball bearing design and analysis tools. Hopefully, we’re going to be engineering new machinery for the space program to make sure that we don’t overload ball bearings from the get go.
KARA: Certainly. Design is really important and that’s what I’ve heard from a lot of people is that starting with a good product is a lot easier than trying to go back and fix it with lubricants or other means.
CHRIS: Exactly. What I try to tell people, companies that make bearings, seals and gears, they know how to make these precision components and when properly applied, they don’t fail. So, when you’ve got a new machine and your bearing keeps failing, the chances are the problem is not in the bearing, but in the machine design itself. So, we’re trying to train people on how to be able to understand what mechanical components can do for us and what lubricants can do for us and not try to push them beyond what we know the limits are.
KARA: What’s the impact of your research? What are some applications?
CHRIS: I think that’s a good question and I think it’s hard to really know whether what you’ve done is making a difference, especially in the aerospace field, where it takes so long for technology to make it from the lab into the marketplace, but looking back now, I can see that some of my early solid lubricant technologies are making headway in industrial applications. We did work early on in high temperature solid lubricants that are now used in furnaces as bushings for conveyor systems. We know that there are power plants all over the world that are using some of the coatings that we developed here for steam turbine control valves and those kind of applications reduce downtime, reduce costs and ultimately improve productivity. We worked for many years on oil-free turbines that are using foil gas bearings and high temperature solid lubricants for start-stop and those are now commercially available in size classes from 15 to 250 kilowatts of electrical power and getting bigger. Just recently, we’re hearing that some of our work on oil-free turbochargers is emerging for cars and trucks, fuel cell blowers, wastewater treatment blowers, and lots of high efficiency industrial machines. Displacing conventional machines all around the world, so it’s kind of encouraging to see the progression – we do research, write papers, go to conferences and present our work and its nice to see people that are actually following up and learning from what we published, making new products and moving technology forward.
KARA: Definitely. Something tangible from all the work that you put in – it’s got to be rewarding.
CHRIS: Yes, it really is.
KARA: So, talking more about another project of yours – SARJ, have you heard anything else about that relubrication project?
CHRIS: Well, it’s interesting that you bring that up. Listeners may have caught a short article I wrote a couple years ago about “The world’s most extreme lube job.” As some of you know the International Space Station, on orbit has some pretty unique mechanisms, and one of the mechanisms we call the SARJ, which stands for Solar Alpha Rotary Joint. It’s a big bearing that allows the space station solar cells to rotate and stay pointed to the sun while the station orbits the globe. We had a failure on one of those bearings shortly after launch and I was lucky enough to be on the team to figure out why it failed, and it turned out that it failed because the solid lubricant that was used was not really up to the task. So, the astronauts went out on the wing, literally wiped off the wear debris, and replaced some of the bearings and added some good vacuum grease and it’s been up there running happily since we regreased it in 2009, and just a few months ago we sent our astronauts back out there to do an inspection and everything looked great. We had them add a little extra grease just for good measure, but we think we’re going to be okay for a long time now. We haven’t put it to bed yet, completely. We’ve got some ground tests that we’ve got going to accelerate the depletion of the grease so that we’ll have an idea of how long the grease job will last before we need to send astronauts out on the wing again.
KARA: So, planning ahead.
CHRIS: Exactly.
KARA: How will things change going forward with the Shuttle Program coming to an end?
CHRIS: Well, you know the shuttle really did its job and did its job well. The primary purpose of the space shuttle was to be able to launch large payloads up to the space station to construct the space station and the space shuttle was also designed to have astronauts spend more time in space doing experiments and also to service satellites like the Hubble space telescope, but now that the space station is fully operational, the need for the shuttle had been diminished, so the ideas was to bring it to an end and have other ways of getting up to the space shuttle. While we have a hole in our launch capability today, we fully expect a commercial launch capability to develop rapidly and we’ll be able to rely on our international partners to bring cargo and people up to the space station, too. It’s interesting, when you think about it in perspective – one of the things we lost by not having the space shuttle is the ability to bring large pieces of hardware back to earth and one of the things we brought back from the space station in the last shuttle mission was a broken cooling pump used for the cooling system on the space station. Many of us here on the ground can’t wait to open that thing up to find out why it broke. While we lament that we don’t have that “down-mass” capability anymore, that the shuttle provided, we also know that we’re pretty good at finding work arounds and we know that in the next five to ten years we’re going to have all kinds of capabilities for launching new stuff and bringing new stuff home. We always seem to find new ways to do what we need to do. We always do. We’re not too concerned.
KARA: And I think that’s the impression a lot of people have. You’re part of the space program, you’re in it for a reason and you find all kinds of solutions to a problem or challenge and you overcome it. That’s what I think of in regards to the space program.
CHRIS: Exactly.
KARA: Any new developments in the field of space tribology that you want to share with the audience?
CHRIS: Well, I think there are a couple things. You brought it up in the last question, very well, I think. NASA – we’re all about trying things that haven’t been done before. Every time we go after a new mission, we oftentimes have to develop new technologies and a lot of times the existing solutions don’t always work. When we try new things, we face setbacks during the development period and sometimes those setbacks are kind of publicized widely as “NASA is making mistakes again,” but we have setbacks because we stretch. We try things that haven’t been done before. Ultimately, we come up with solutions and those solutions wind up being new tools in the toolbox for the future and many of these tools spin off and out of NASA missions and wind up in other fields. Right now, we’re doing the Mars Science Laboratory and the James Webb Telescope and they have really challenging mechanisms that have to operate cold and hot and operate unattended with very low friction and wear. You can look for all those papers and patents coming out of our solutions in the coming years and understand that we’re pushing technology forward.
KARA: What would you say to those in the field – what should they take away from today’s discussion?
CHRIS: Avoid reinventing the wheel unless you absolutely have to. What I mean by that is most of the terrestrial tribology problems have been around for a long, long time and there is a great wealth of past experience that has been published and reported on in the STLE literature and in the literature of our companion societies. The trick is to take advantage of what people have done before you rather than spending your time and energy reinventing the wheel. That way, you can spend your time and energy pushing the technology forward and if you do discover a novel solution to a really vexing tribology problem, please share it with others. Come to a conference and give a presentation. Publish a paper and become more engaged in the tribology community. I know, looking back over my career, being more engaged in the tribology community has been good for me, good for NASA, and I think it can be good for you as well.
KARA: Beyond attending meetings and publishing, meeting subject matter experts like yourself who have been there and done that, addressed problems in the field – you’re a wealth of knowledge to tap into as well.
KARA: Thank you Chris for joining us today and for your insight.
CHRIS: You are very welcome. It’s my pleasure.
KARA: I’m Kara Lemar. For more news, information and research on space tribology, you can visit our website. NASA’s website is also a good resource. Thank you for joining us today. This has been another episode of The STLE Compass, pointing you in the right direction.

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