Nanotechnology Now

Our NanoNews Digest Sponsors

Heifer International

Wikipedia Affiliate Button

Home > Press > Penn Research Shows Mechanism Behind Wear at the Atomic Scale

An illustration of a silicon AFM tip sliding over a diamond surface, with a TEM image of the tip inset. (Art: Felice Macera)
An illustration of a silicon AFM tip sliding over a diamond surface, with a TEM image of the tip inset.

(Art: Felice Macera)

Abstract:
Wear is a fact of life. As surfaces rub against one another, they break down and lose their original shape. With less material to start with and functionality that often depends critically on shape and surface structure, wear affects nanoscale objects more strongly than it does their macroscale counterparts.

Penn Research Shows Mechanism Behind Wear at the Atomic Scale

Philadelphia, PA | Posted on January 30th, 2013

Worse, the mechanisms behind wear processes are better understood for things like car engines than nanotech devices. But now, researchers at the University of Pennsylvania's School of Engineering and Applied Science have experimentally demonstrated one of the mechanisms behind wear at the smallest scale: the transfer of material, atom by atom, from one surface to another.

The research was conducted by Tevis Jacobs, a doctoral student in the Department of Materials Science and Engineering, and Robert Carpick, department chair of Mechanical Engineering and Applied Mechanics.

Their research was published in the journal Nature Nanotechnology.

On the nanoscale, wear is mainly understood through two processes, fracture and plastic deformation. Fracture is where large pieces of a surface break off at once, like when the point of a pencil snaps off in the middle of a sentence. Plastic deformation is what happens when the surface changes shape or compresses without breaking, like when the edge of knife gets dull or bent.

These mechanisms typically affect thousands or millions of atoms at a time, whereas nanoscale wear often proceeds through a much more gradual process. Determining the mechanisms behind this more gradual process is key to improving such devices.

"At the nanoscale, wear is a very significant problem," Jacobs says. "Nanotechnology is developing smaller and smaller parts for very tiny machines. Their contact interfaces wear out very quickly, sometimes surviving for hundreds of cycles when they need to survive for trillions or more."

One wear mechanism that had been hypothesized for the nanoscale is a process known as atomic attrition. There, atoms from one surface are transferred to the other surface via a series of individual bond-forming and bond-breaking chemical reactions. Other researchers have attempted to test this process by putting two surfaces in contact and sliding one against the other.

Those previous investigations involved Atomic Force Microscopes. Using an AFM involves dragging a very sharp tip mounted on a flexible cantilever over a surface while a laser aimed at the cantilever precisely measures how much the tip moves. By using the tip as one of the surfaces in a wear experiment, researchers can precisely control the sliding distance, sliding speed and load in the contact. But the AFM doesn't visualize the experiment at all; the volume of atoms lost from the tip can only be inferred or examined after the fact, and the competing wear mechanisms, fracture and plastic deformation can't be ruled out.

The Penn team's breakthrough was to conduct AFM-style wear experiments inside of a transmission electron microscope, or TEM, which passes a beam of electrons through a sample (in this case, the nanoscale tip) to generate an image of the sample, magnified more than 100,000 times.

By modifying a commercial mechanical testing instrument that works inside a TEM, the researchers were able to slide a flat diamond surface against the silicon tip of an AFM probe. By putting the probe-cantilever assembly inside the TEM and running the wear experiment there, they were able to simultaneously measure the distance the tip slid, the force with which it contacted the diamond and the volume of atoms removed in each sliding interval.


"We can watch the whole process live to see what happens while the surfaces are in contact," Jacobs said. "Then, after each pass, we use the TEM like a camera and take an even higher magnification picture of the tip. We can trace its outline and see how much volume has been lost, down to as small as 25 square nanometers, or about 1250 atoms.

"We are measuring changes in volume that are one thousand times smaller than can be seen using other techniques for wear detection."

While this new microscopy method can't image individual atoms moving from the silicon tip to the diamond punch, it enabled the researchers to see the atomic structure of the wearing tip well enough to rule out fracture and plastic deformation as the mechanism behind the tip's wear. Proving that the silicon atoms from the tip were bonding to the diamond and then staying behind involved combining the visual and force data into a mathematical test.

"If atomic attrition is what's happening," Carpick said, "then the rate at which those bonds are formed and the dependence on contact stress — the force per unit area — is well-established science. That means we can apply chemical kinetics, or reaction rate theory, to the wear process."

Now that they could measure the volume of atoms removed, the distance the tip slid and the force of the contact for each experimental test, the researchers could calculate the rate at which the silicon-diamond bonds form under different conditions and compare that to predictions based on reaction rate theory, a theory that is routinely used in chemistry.

"The more force the atoms are under, the more likely they are to form a bond with an atom on the opposing surface, so the wear rate should accelerate exponentially with additional stress," Jacobs said. "Seeing that in the experimental data was a smoking gun. The trend in the data implies that we can predict the rate of wear of the tip, knowing only the stress levels in the contact, as long as this wear mechanism is dominant."

For now, those predictions can only be made about the wear of silicon on diamond in a vacuum, though the selection of those two materials was not accidental. They are common in nanoscale devices and tools for nanomanufacturing.

The math behind the atomic attrition mechanism could eventually be applied in a fundamental way.

"The goal of this avenue of research is to get to the point where you tell me the materials in contact, and you tell me the period they are in contact and the stresses applied and I will be able to tell you the rate at which atoms will be removed," Jacobs said.

"With a fundamental understanding of wear, you can cleverly design surfaces and choose materials to make longer lasting devices," Carpick said.

This fundamental, predicative understanding of wear could vastly improve nanomechanical design, increasing functionality and decreasing costs.

The research was supported by the National Science Foundation's Nanomanufacturing Program and Penn's NanoBio Interface Center.

####

For more information, please click here

Contacts:
Evan Lerner

215-573-6604

Copyright © University of Pennsylvania

If you have a comment, please Contact us.

Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

Bookmark:
Delicious Digg Newsvine Google Yahoo Reddit Magnoliacom Furl Facebook

Related Links

Download article:

Related News Press

News and information

Zap! Graphene is bad news for bacteria: Rice, Ben-Gurion universities show laser-induced graphene kills bacteria, resists biofouling May 22nd, 2017

Leti Will Demo World’s-first WVGA 10-µm Pitch GaN Microdisplays for Augmented Reality Video at Display Week in Los Angles: Invited Paper also Will Present Leti’s Success with New Augmented Reality Technology That Reduces Pixel Pitch to Less than 5 Microns May 22nd, 2017

Sensors detect disease markers in breath May 19th, 2017

Graphene-nanotube hybrid boosts lithium metal batteries: Rice University prototypes store 3 times the energy of lithium-ion batteries May 19th, 2017

Govt.-Legislation/Regulation/Funding/Policy

Zap! Graphene is bad news for bacteria: Rice, Ben-Gurion universities show laser-induced graphene kills bacteria, resists biofouling May 22nd, 2017

Graphene-nanotube hybrid boosts lithium metal batteries: Rice University prototypes store 3 times the energy of lithium-ion batteries May 19th, 2017

Stanford scientists use nanotechnology to boost the performance of key industrial catalyst May 18th, 2017

Oddball enzyme provides easy path to synthetic biomaterials May 17th, 2017

Molecular Nanotechnology

First 3-D observation of nanomachines working inside cells: Researchers headed by IRB Barcelona combine genetic engineering, super-resolution microscopy and biocomputation to allow them to see in 3-D the protein machinery inside living cells January 27th, 2017

Captured on video: DNA nanotubes build a bridge between 2 molecular posts: Research may lead to new lines of direct communication with cells January 9th, 2017

Tip-assisted chemistry enables chemical reactions at femtoliter scale November 16th, 2016

Scientists come up with light-driven motors to power nanorobots of the future: Researchers from Russia and Ukraine propose a nanosized motor controlled by a laser with potential applications across the natural sciences and medicine November 11th, 2016

Discoveries

Zap! Graphene is bad news for bacteria: Rice, Ben-Gurion universities show laser-induced graphene kills bacteria, resists biofouling May 22nd, 2017

Sensors detect disease markers in breath May 19th, 2017

Graphene-nanotube hybrid boosts lithium metal batteries: Rice University prototypes store 3 times the energy of lithium-ion batteries May 19th, 2017

Plasmon-powered upconversion nanocrystals for enhanced bioimaging and polarized emission: Plasmonic gold nanorods brighten lanthanide-doped upconversion superdots for improved multiphoton bioimaging contrast and enable polarization-selective nonlinear emissions for novel nanoscal May 19th, 2017

Materials/Metamaterials

Stanford scientists use nanotechnology to boost the performance of key industrial catalyst May 18th, 2017

Self-healing tech charges up performance for silicon-containing battery anodes May 15th, 2017

Discovery of new transparent thin film material could improve electronics and solar cells: Conductivity is highest-ever for thin film oxide semiconductor material May 6th, 2017

CCNY physicists demonstrate photonic hypercrystals for control of light-matter interaction May 5th, 2017

Announcements

Zap! Graphene is bad news for bacteria: Rice, Ben-Gurion universities show laser-induced graphene kills bacteria, resists biofouling May 22nd, 2017

Leti Will Demo World’s-first WVGA 10-µm Pitch GaN Microdisplays for Augmented Reality Video at Display Week in Los Angles: Invited Paper also Will Present Leti’s Success with New Augmented Reality Technology That Reduces Pixel Pitch to Less than 5 Microns May 22nd, 2017

Sensors detect disease markers in breath May 19th, 2017

Graphene-nanotube hybrid boosts lithium metal batteries: Rice University prototypes store 3 times the energy of lithium-ion batteries May 19th, 2017

NanoNews-Digest
The latest news from around the world, FREE



  Premium Products
NanoNews-Custom
Only the news you want to read!
 Learn More
NanoTech-Transfer
University Technology Transfer & Patents
 Learn More
NanoStrategies
Full-service, expert consulting
 Learn More











ASP
Nanotechnology Now Featured Books




NNN

The Hunger Project