Discovery is key to metal wear in sliding parts (w/ Video)

July 24, 2014
This sequence of images reveals surprising fluid-like behavior in a solid piece of metal sliding over another, forming defects leading to wear in metal parts. (Top) Two image frames of the material flow showing how these defects are spawned in the wake of the contact. (Bottom) Scanning electron microscope pictures of the corresponding wear surfaces showing a tear and a crack. Wear particles are formed when the tears and cracks detach from the surfaces. Credit: Purdue University School of Industrial Engineering.

Researchers have discovered a previously unknown mechanism for wear in metals: a swirling, fluid-like microscopic behavior in a solid piece of metal sliding over another.

The findings could be used to improve the durability of metal parts in numerous applications.

"Wear is a major cause of failure in engineering applications," said Srinivasan Chandrasekar, a Purdue University professor of industrial engineering and materials engineering. "However, our findings have implications beyond wear itself, extending to manufacturing and materials processing."

The findings are the result of a collaboration of researchers from Purdue, the Indian Institute of Science in Bangalore, India, and M4 Sciences, a company in West Lafayette, Indiana.

"Using high-resolution imaging of sliding contacts in metals, we have demonstrated a new way by which wear particles and surface defects can form," said Purdue postdoctoral research associate Anirban Mahato, who is working with Chandrasekar; Narayan Sundaram, an assistant professor at the Indian Institute of Science; and Yang Guo, a research scientist at M4 Sciences.

Findings are detailed in a research paper to appear Wednesday (July 23) in Proceedings of the Royal Society A, a publication of the Royal Society in the United Kingdom.

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The researchers, using a microscope, high-speed camera and other tools, had previously revealed the formation of bumps, folds and vortex-like features on sliding metal surfaces. The new findings build on the previous paper, published in 2012 in Physical Review Letters, to show how the behavior leads to cracks and wear particles.

The findings were counter-intuitive because the experiment was conducted at room temperature, and the sliding conditions did not generate enough heat to soften the metal. Yet, the swirling flow is more like behavior seen in fluids than in solids, Chandrasekar said.

The team observed what happens when a wedge-shaped piece of steel slides over a flat piece of aluminum or copper. The metals are commonly used to model the mechanical behavior of metals.

"We speculated in the earlier paper that the swirly fluid-like surface flow discovered on sliding is likely to impact wear in sliding metal systems," he said. "Now we are confirming this speculation by direct observations."

The observations show how tiny bumps form in front of the wedge, followed by the swirling movement. When the wedge angle is shallow, the flow is laminar, or smooth. However, it changes to a swirly flow when the angle is adjusted to a less-shallow angle, mimicking what happens in actual sliding metal parts. As the wedge slides across the metal specimen, folds form between the bumps, and then the folds transform into tears and cracks in the wake of the wedge, eventually falling off as wear particles.

"A single sliding pass is sufficient to damage the surface, and subsequent passes result in the generation of platelet-like wear particles," Chandrasekar said.

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The behavior was captured in movies that show the flow in color-coded layers just below the surfaces of the copper and aluminum specimens.

The defects range in size from 5 to 25 microns and are similar to those found in sliding components such as parts in automotive engines, compressors and numerous types of equipment and machinery. 1"In the past we only saw these features after they had formed, and we attributed them to various possible mechanisms," he said. "Here, we show a mechanism for how they are formed. The defect features observed also occur in surfaces created by manufacturing processes like grinding, polishing, burnishing, peening, drawing, extrusion, rolling, and so on, which are all commonly used in making structural and mechanical components in the ground transportation, aerospace, sheet- and wire-metals processing, and energy systems sectors."

Ongoing research will explore potential routes to reduce wear arising from this type of mechanism. Metals are made of groups of crystals called grains. Future work will study how a material's grain size and ductility influence this type of wear, how these types of surface defects in manufacturing processes can be eliminated through the modified design of tools and dies, improved models for sliding wear and wear-control strategies.

"We want to look at this mechanism in materials that have smaller crystals - in the 5-30 micron range," Chandrasekar said. "We want to show that the mechanism is more general and extends down to even finer-grained metals."

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5 / 5 (4) Jul 24, 2014
Damn. Those are some pretty slick movies/overlays. Good work.

Jul 24, 2014
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1 / 5 (3) Jul 24, 2014
Craftsmen have been using that effect for hundreds of years at least... Scientist are just now figuring it out? It seems that none of those scientist involved have any metal machining skills in their backgrounds. Any trained machinist could have told them about it.
4.4 / 5 (7) Jul 24, 2014
Scientist are just now figuring it out?

Could a trained machinsit have quantized the flow? No
Could a trained machinist use the knowledge to devise structures that produce less of a bulge (less wear)? No

This is not about "is there a bulge", or "does metal flow"? This is about whether there is turbulence in the flow and how is it quantified and WHY it happens (which no trained machinist knew about).
Unbiased Observer
1 / 5 (2) Jul 24, 2014
antialias... let me fix that for you.

1) Maybe. Some are actually quite brilliant and have additional background in material science. Not every machinist is just a jockey. Careful with overly broad generalizations.
2) Absolutely, ever hear of surface finish? Poor example.

You seem to be confusing a couple of issues that I really don't have the time to deal with at the moment, but your last paragraph is not really... applicable. The why is answered by various defects and grain boundaries (for example).

While I do not follow tribology and its direct associated derivatives with any frequency I might have to check up on this paper.

I do not say that it isn't a good setup, conclusion, or experiment (I haven't read the paper yet), but some of your points are just not really good arguments.
4 / 5 (4) Jul 25, 2014
antialias... let me fix that for you.

It ain't broken.

Some are actually quite brilliant and have additional background in material science. Not every machinist is just a jockey. Careful with overly broad generalizations.

I haven't seen a single paper by a machinist making quantifiable statements ot atomic levels - or has published demonstrable experiments. Have you?
They're not stupid - but 'having a good feel' or being able to make a fuzzy statement is not scientific work.

The why is answered by various defects and grain boundaries (for example).

Exactly. That's a fuzzy statement. That this was so is not something that surprised scientists (they knew about this too). But WHY it happens and HOW it happens and to what quantifiable degree.

Qualitative statements vs quantifiable statements. It's the difference between saying "it's red" and telling you exactly what type of red and why.
not rated yet Jul 25, 2014
This is really interesting work. The implications for evolvement of metallurgy, manufacturing and lubricants is astonishing. I am sure there will be many different materials scientists scrambling to harness this information and expand on it; especially those in disciplines of critical consequence. Aeronautics and race cars come to mind. Also, how might this be of benefit as we delve further into 3D metal printing and laser sintering? How does that process effect the crystallography and surface performance- lots of work to be done and questions answered.
3.7 / 5 (3) Jul 25, 2014
AP, In the REAL world - not some school or college environment, company engineers don't do 'papers' except for the people they work for. People don't worry about publish or perish crap that college people do. They get a paycheck for gaining real usable information that the company can make a profit on. As far as articles that contain the information about this kind of stuff goes, there are many articles about it but they are in commercial engineering magazines and books which is generally not stuff you will find at some school or college library.

Normally I agree with your opinions most of the time but in this case you are obviously out of your knowledge skill range. When this is done to metal it's called 'work hardening' and what happens to the metal crystals have been known for years. It's not new info. Every company of any size that makes metal cutting tools has a 'scientist' on staff working on this kind of stuff.
4 / 5 (4) Jul 27, 2014
Exactly. they dabble. They do stuff that kinda sorta works and makes money...and leave it at that. No systematic progress - just trial and error and the occasional happy accident.
If we were to leave things to that kind of approach we'd still be banging rocks together.

Normally I agree with your opinions most of the time but in this case you are obviously out of your knowledge skill range.

Nope, because I am an engineer by education, a scientist by training (and later education), and now am working in the industry where just that 'dabbling' happens. It's pitiful to watch what tech companies get away with because it makes profit
(and the products are so convoluted and legal-defended that no startup could hope to get a competitive product into the market without many millions of dollars worth of investment first - even though one could be built MUCH better).

Big company engineering is the ball and chains on humanity's progress.
Unbiased Observer
not rated yet Jul 28, 2014

So that makes you a computer or electrical engineer by education, with training as a material scientist?
5 / 5 (1) Jul 28, 2014
Unbiased, what part of quantized data do you not get? Knowing that the effect exists is one thing. Being able to measure how much of it is happening and from the exact cause is entirely different.

Yes, maybe the way antialias is putting it may sound rather brash and arrogant, but he is merely stating a simple truth. It's quantized data that a machinist wouldn't be able to get out of their experience of working with the materials. They would know how to minimize the effects and produce high quality work. But they wouldn't be able to tell you HOW MUCH pressure would cause how much damage and wear at the molecular and even at the atomic level.

That is where the researchers in this article come into play. And their job is to hopefully reduce the effects of normal wear and tear, so that machinists will have and be able to produce machines and parts that last longer without having to "reinvent the wheel" so to speak.
5 / 5 (1) Jul 28, 2014
So that makes you a computer or electrical engineer by education, with training as a material scientist?

No, Otto (still using your sockpuppets, I see...pitiful, dude...just pitiful).
That makes me an electrical engineer by degree, a software engineer by training, and a human biology scientist by (later) education.

But you are missing, as ususal, the distinction between quantitative and qualitative (look up those words...might help you with your beliefs in cold fusion and the like).

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