The STLE Compass, Released June 14, 2011 “Tribochemistry and Nanoscale Surface Interactions in Nanotribology” with David Huitink, Ph.D. student at Texas A&M KARA: Hello, I’m Kara Lemar. 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. This is another episode of The STLE Compass and today the focus is on nanotribology. Nanotribology, as defined in the TLT article (from January 2009), is the study of interfacial phenomena at the nanoscale. At the nanoscale, there are radically different conditions and tribological concerns when compared to a normal or macro scale. We will look at this interesting and growing field, and discuss the work that our interviewee is doing: using nanoscale contact to study friction and tribochemical reactions. This work involves studying the chemical and interfacial reactions taking place at the nanoscale as a result of friction and contact forces. David Huitink is a Ph.D. student at Texas A&M. He is a National Science Foundation Graduate Research Fellow and is the recipient of several awards for academic merit. David has authored 8 journal articles and a number of conference presentations from his research during his studies at Texas A&M. Additionally, he has recently founded the Texas A&M University Chapter of STLE and served as its president in 2010. He was also awarded the Young Tribologist Award at the 2010 STLE Annual Meeting. Today, David will give us some detail on his involvement with STLE, provide background on the field, shed some light on his work and the implications of this research. KARA: David, welcome to STLE Compass. DAVID: Thank you. KARA: We appreciate you taking the time; we know you’re very busy, so we appreciate that. First, let’s talk a little bit about your involvement with STLE. You’ve been involved with the Student Chapter, you’re a student member, so what got you interested and how is that going? DAVID: Well, it’s been going great. I started getting involved with STLE naturally out of working with Dr. Hong Liang. She’s on the Board of Directors Committee. So, with her attachment to STLE as well as having done some research in the field of tribology, it was very easy for me to start getting involved, going to meetings, being involved as a student member, plus there’s a lot of benefits for student members including scholarships and the Young Tribologists’ Award that have been offered to student members for several years now. Fortunately, I was able to take part in the scholarship part, and that was always great, being a student, having a little extra cash. And it’s really been a great benefit for me to get to know some people in the community of engineers at STLE and it actually has led to some internships and job opportunities through networking through the Society. KARA: That’s great. So, what motivated you to then go on and create the student chapter at Texas A&M? DAVID: I’d say there are two kind of motivating factors. One is just - it’s really a great venue for students to get involved because it’s a really connected Society and it has a lot of benefits and resources available, but yet, it’s not so large that you get kind of get lost in it, something like ASME, or one of the very large organizations that is sometimes a little too daunting to try and get attached to. But also, I mentioned having Dr. Liang as an advisor. She kind of impressed upon us that it would be a great idea if we were able to start some student organization so we could students on the Texas A&M campus more aware about STLE and some of the benefits that it offers. So that started the conversation between us. Then, I started looking into what it takes to get a student organization started and talking with some of the STLE Board Members about how that might work for STLE as an organization. One thing led to another and now we have a student organization here on campus for STLE. KARA: That’s fantastic. How many members do you have? DAVID: We have kind of a rolling membership because there’s a lot of students that come and go. But basically we make it open to anyone who’s interested in coming to our meetings. We try to hold technical talks where we have guest speakers come in. We actually had Dr. Bruce, a former STLE President, give a talk to us last fall. We also piggyback onto some of the Houston Chapter monthly meetings by teleconference. So, our membership usually fluctuates at the meetings between 10 and 20 or so, and most of them are graduate students who are working in tribology, and we have a couple different professors in the Mechanical Engineering Department that are interested in different areas of tribology. And so, it’s just a great way for us to get together and get to know one another, exchange ideas and also learn from professionals in the field. KARA: It makes sense, you band together, you get that networking and get a sense of a community, so we definitely appreciate what you’ve done. It’s fantastic. And you served as the President this last year? DAVID: Yes, that’s correct. So, we really got this student organization started last winter and really got things rolling in the summer. I served as the organizer of getting everything worked out and I served as the President last year, and we just recently had the elections for the current year and so we have a new student taking over as the President as well as a couple other officers, like Treasurer and that sort of thing. But it was a good experience. It led me to meet a lot of very interesting people and a lot of contacts through STLE. KARA: Sure. And it sounds like you’ve made some good progress. That’s great. So now that we have some background about you, let’s talk about your research. Can you give us an overview of nanotribology and how would you describe it? DAVID: Essentially, nanotribology is the response of tribologists to the scientific community’s push to understand the physics and chemistry of materials at the nanoscale. Particularly what their interactions are in terms of atomic and molecular interactions because most people are aware that with nanotechnology, that’s kind of coming of age over the past several years, and there’s a lot of research going on. There’s how we can use some of these fundamental ideas of interactions at the nanoscale to make better engineered devices and so forth. In terms of tribology, that means looking at the role of surface chemistry and interatomic forces in the friction behavior and wear of materials in contact. KARA: And why is it important to look at things at the nano level? DAVID: As I mentioned, things start to behave a little differently when you get down to the nanoscale. It’s not that they’re really truly different, but now different factors are more important than what they may have been at the macroscale. This idea is somewhat lost on some of the very traditional engineers that have been working on engines or gearboxes and that sort of thing, but the truth is, even in those very large systems, the mechanisms that drive friction or wear are really rooted at this nano- or molecular level of these materials interacting with each other. Even in these huge systems, if it’s a wind turbine gearbox or something else, you have these asperities on your parts or components that are interacting with one another. So, even though you have a giant gear, you have little localized nanoscale particles that are contacting with each other and causing sorts of different behavior and reactions that occur at the surface. KARA: Okay, so how do you determine what is really happening at the nanoscale? DAVID: So this is really the biggest challenge of nanotechnology and nanoscience in general. Because it’s so small, it’s difficult to see what’s really going on. You have to be fairly clever about the way you go about doing the analysis and observations when you’re dealing with things at the nanoscale. One of the things you have to start with is using some very fancy equipment. Since nanoscale falls below the visible light spectrum, or the defraction limit as physicists might say, you have to use electron microscopes if you want to do any visual observation of surfaces and you have to start thinking about different types of spectroscopic analyses and things that are really high tech and maybe futuristic in terms of what some people might think, but really just special tools to help you understand what’s really going on. One way that we’ve been looking at our labs a lot, is using an atomic force microscope, or an AFM, and this is basically a tool that allows you to measure surface features and also affect surface conditions depending on how you configure the tool, at a very small scale. This is helpful because this tool can also be used as a nano-sized tribometer, and so you can look at what’s going on at a very molecular level during a tribological reaction. Basically we have these tools available to us and they’re really expensive sometimes, and so it makes it difficult to always be able to do the most advanced analyses, and so it takes a little bit of elbow grease, per se, to be creative and think about how can I approach this problem a little bit differently, how can we look at surfaces in a new way and try to understand what’s really going on. KARA: As far as impact, how does this impact the general public and where might this research, or the technology that you’ve been using, be applied? DAVID: Some people might say why are we concerned about the nanoscale? Because as most people are aware of it, engines and gearboxes are very large, and they wonder how it will impact things if we know how it’s behaving at the nanoscale. But this is very, as I mentioned, a fundamental understanding of how surfaces interact and behave. This is something that, if you understand the fundamental physics or chemistry that is involved, there’s no telling what you could possibly do. You could create all sorts of enhanced surfaces, very low friction surfaces, or maybe something that you could even tune the surface to attain some sort of advanced functionality. But this sort of surface reaction, something that’s rooted in everyday processes, whether it’s how you butter your frying pan before you cook food so that stuff doesn’t stick to your pan, or it could be about changing your engine oil – how can you make your engine last longer and have better performance over the long haul? But these kind of ideas are rooted in electronic interactions, or how the molecules and their electrons interact with one another. So, by gaining a better understanding the nature of these interactions, as scientists, we can develop better engineered interfaces that can help extend life or performance and that sort of thing. KARA: So this is a basic structure that you want to understand, when you multiply it to the macroscale, it makes a huge difference in what you’re doing. DAVID: Right. It’s a cascading sort of thing. If you understand the fundamental physics of what’s going on, then you can manipulate those interactions in such a way that, even at the macroscale, you can have much better performance. KARA: Okay. So what research are you currently working on? DAVID: So, in our lab, we’ve been taking an approach to looking at the nanotribological behavior. As I mentioned, this can be a difficult task because you have to have the tools available, and you have to have a little bit of creativity about how you get those measurements, but particularly, what I’ve been looking at is studying the chemical reactions that can result from a mechanical impulse, which is sometimes referred to as mechanochemistry, or more specifically in terms of tribology, you can call it tribochemistry, because in this particular instance, we’re talking about not just simply mechanical impulse, but a rubbing and sliding contact sort of impulse. You know, this might be a foreign idea to some, but it’s actually been around for a while. A couple examples of this tribochemistry is the use of ZZDP and MoDTC, a molybdenum compound, additives that are used in engine oil. These are used for wear reduction and friction improvement in engines and by themselves, these compounds actually don’t do that much. But now when you start to rub the surface together with these compounds between the surfaces, they actually chemically change the substances into different things. The ZDDP for instance interacts with metal compounds on the surface to create a coating that’s made of zinc and iron that’s in the metal as well as some sulfides and oxides. And basically it creates a protective coating over the surface so that now you have a very wear-resistant material on the surface. Additionally, the molybdenum compound, when it’s exposed to this kind of tribochemical environment, it actually transforms into moly-disulfide, which I’m sure many STLE people have heard about this, but it’s a very low friction material and it helps to improve sliding performance of materials. As for what I’m doing, it’s not so much involved with the ZDDP and the MoDTC, but it’s actually involved in looking at how these sort of reactions occur, how do they progress, how can we understand what’s going on a little better so that we can use it for an engineered purpose, and how these materials form at interfaces. KARA: Very nice. And we have talked about the moly-disulfide and ZDDP, we’ve had a couple interviews about that but what about powder coating, or solid lubrication – is that on the same line? DAVID: Solid lubrication is just using these various compounds or solid elements to help lubricate a surface but in terms of ZDDP and the MoDTC, it’s not the additives that are in the oil that actually help the surface, it’s the products that result from putting the additives in the oil and then having the rubbing environment. Because without the rubbing environment, it’s just an organo-metallic or organic-alkali compound, which don’t do a whole lot. They do affect the oil itself a little bit but not significantly and don’t cause the benefits that are resulting from these friction coatings or wear layers in the moly-disulfide, which is a solid lubricant, as you mentioned. KARA: In your work, what kind of challenges have you encountered and what have you had success with? DAVID: The challenges, as I mentioned, are largely related to how you develop these methods of understanding what’s going on. Because now we’re talking about something that’s happening at a very molecular type of level and how do you use the tools that are available to try to understand what’s going on? Especially when we’re looking at these tribochemical reactions – this is something that occurs in a fairly violent environment. When you’re talking about rubbing a surface, it’s very difficult to observe the surface while you’re rubbing it, so you have to be creative about what are some of the ways that we can accomplish this. So, in tribochemistry, what can happen is you can have a chemical reaction that takes place, or you can even have structural transformations to different crystal structures if you have metals or various crystalline materials. So you have to basically plan very well to take these measurements and use some good judgment and hopefully have a little bit of good luck so that you can get some decent results. So, in research, sometimes there’s a lot more luck than maybe you’d think when you see the newspapers. KARA: Well, you try to play it off – it’s more your hard work than necessarily good luck. DAVID: Ha - sure. But a couple successes we’ve particularly been working on are some in-situ, that means measuring as it is occurring types of tests. So we can try to see what’s going on while we’re imposing these impulses in the tribological environment. One particular that was highlighted in TLT, a student abstract article a few months back, based on a poster I put together at last year’s (STLE) Annual Meeting, it was using an in-situ nanoindenter, so basically this is an indentation tool that is incorporated inside of a transmission electron microscope. What we did is we coated our indenter probe with gold and then we indented it into a silicon substrate. It sounds somewhat mundane when you just say it like that, but what actually is involved is you have to carefully align the surfaces and before you can even do the measurement, you have to use a focused ion beam tool to machine the surface of the silicon at a nanoscale so you have an electronically transparent material. That means that the electrons from the transmission electron microscope can actually pass through the silicon because it’s thin enough, so you have to use this special tool to make it small enough before you can actually start to go and observe what’s happening. Then the challenge becomes is how do you align features that are less than a couple hundred nanometers, something you can’t even see with a normal microscope, and then how do you align them so that you can perform the experiment. So this is a big challenge in and of itself. But fortunately, we were able to overcome some of those challenges and actually get some good measurements where we could observe what happened as you pushed the gold-coated probe into the silicon. And what we found was that there was a mechanochemical formation of a compound called gold silicide, which kind of makes sense that you have something that’s made of gold and silicon if you’re pressing it together. But generally you would think that when you’re pressing or rubbing something together, you would just have one material wear off on the other. Instead, what we find is there is a chemical reaction that takes place and the compound that forms is actually what is known as meta-stable, which means that it doesn’t naturally occur in the environment and thermodynamically it doesn’t favor to be formed, but here, by pressing things together, we formed it. Additionally, we can reproduce it in a lab, but what it takes to create this compound typically is a high temperature diffusion process, and when I say high temperature, I mean really high temperature like over 1,000 degrees Celsius. So, simply by pushing these two materials together at a sufficient pressure, we actually have the compound formed without any sort of heat input or high temperature. It’s very interesting and it has some interesting implications too. Not only does it offer a new way of getting to this material and how the formation occurs, but it has intriguing implications for the application of metal interconnects and semi-conductor devices because we began looking at these two materials particularly for use in developing electronic structures on silicon devices for use in computers or something like that. What happens is that normally gold and silicon do not want to mix very well and what has to happen is there has to be some kind of interfacial material that’s placed between them, otherwise they’ll separate overtime. Typically that’s done with chromium. But here, when you have this formation of the gold silicide, you actually promote adhesion between the two materials so that they will stick together, and not only do you promote adhesion, but you actually promote better electron transfer between the silicon and the gold itself through the formation of these interfacial silicides. KARA: Then what would you consider to be the most important issues for this field? DAVID: As we were talking, I mentioned a little bit about the integrated circuits or silicon-based devices, and when you think about these devices or things like MEMS, microelectricalmechanical systems, the growing trend for these devices is to go smaller and smaller because as you go smaller, you increase the ability to produce multiples more cheaply and generally your power density goes up but your energy use per device goes down. So, we’re thinking about going more efficient and cheaper, basically. What happens as you go smaller and smaller is that these chemical and surface interactions become increasingly important. So now you’re not so much concerned about how these materials mix together on a macroscale, but how do they actually influence one another in terms of their electrons interacting with one another or how they mix and that sort of thing. So, when you consider tribochemical transformations that I was talking about with the formation of the gold silicide, this somewhat surprising phenomena may be one of the biggest challenges for scientists to grapple with because there is no real, unifying model to predict what transformations may occur or where they may occur. Moreover, these reactions fundamentally change the way the surfaces interact. So, that, even on the nanoscale drives how macroscale interactions will occur because if you have a new material or a new chemical surface in between two rubbing materials, now you have a totally different response. KARA: So, you would have to individually test each given material and see how that would proceed? You don’t have a model as of yet? DAVID: Right. So as it stands right now, there’s not a real good understanding of how these things progress. That’s what really the big challenge facing the state of nanoscale interactions right now is trying to really understand how these things occur and how mechanical forces or rubbing or sliding can affect things. Once we understand how that occurs better, it no longer becomes a trial and error thing like you mentioned, but now we can start to look at – when we’re interested in materials in a gearbox, or when we’re interested in materials in a MEMS device, how can we design them more effectively so we know they’re going to last for “x” number of years or indefinitely, that sort of thing. KARA: Get some more reliability out of the whole process. DAVID: Exactly. KARA: So, given all of that, what conclusion would you like listeners to take away from today’s discussion? DAVID: Basically the idea of nanotribology and the study of interfacial reactions is really working towards an understanding of the fundamental role of atoms and electrons in those tribological interactions. So, rather than just looking at abrasion, adhesion and those types of things that traditional material scientists and tribologists look at, we have to realize that there are other things going on including these tribochemical reactions and the inner diffusion of atoms that take place in a rubbing and contact environment that essentially govern what wear mode will occur that you see in the large scale like abrasion or adhesion, that sort of thing. So I think the most important issue that researchers will address in the coming years is trying to develop a greater understanding of how these interactions progress and how those interactions are related to the macroscale understanding of the friction and wear processes. KARA: Thank you so much for your time, for joining us today and for your insight. It certainly was enlightening. DAVID: Well thank you for having me. KARA: I’m Kara Lemar. For more news, information and research on nanotribology, please visit our website. You can get more detail on the Tribology Group at Texas A&M by visiting their website. Thank you for joining us today. This has been another episode of The STLE Compass, pointing you in the right direction.