Tribochemistry of Ultrahard Carbon Films

 

Introduction: 

Tribology studies the science and technology of surfaces in relative motion. It includes the principles of:

a)      friction

b)     lubrication

c)      wear

Tribology is derived from the Greek word “tribo,” which means “rub.”

The focus of this research is to learn about the physics at an interface using a tribological system as the tool. The aim is to understand what occurs at a sliding interface, that is:

      •Bonds (breaking, forming)

      •Temperature

The research also seeks to find what wear mechanisms are responsible across different regimes in parameter space; to understand the origins of the extreme tribological properties of carbon-based films:

      –Ultralow friction & wear

      –Low adhesion

Novel carbon films have several potential applications:

      –Tool coatings

      –Seals

      –Bearings

      –Aerospace components

Micro- and nano-devices need to be made from new materials to overcome tribological challenges:

      –MEMS/NEMS

      –Thermomechanical data storage

      –Wear of AFM tips for extended imaging periods,       nanofabrication, nanolithography.

 

  

Aims of Project:

Learn about the physics at an interface using a tribological system as the tool;  understand what occurs at a sliding interface (bonds breaking/ forming and temperature changes).

-Find what wear mechanisms are responsible across different regimes in parameter space

Goal: 

To understand the origins of the extreme tribological properties of carbon-based films (Ultralow friction & wear; Low adhesion). These films have a number of potential applications.

 

Material Overview:

Carbon possesses properties that make it an attractive material:

High hardness

Low adhesion

–Ultralow friction & wear

Ultrananocrystalline diamond (UNCD) has a low coefficient of friction and low wear rate, but only at sufficiently high relative humidity.

 

Background: (Grierson et al, “Origin of Ultralow Friction and Wear in Ultrananocrystalline Diamond,” American Physical Society, Peer Review Letters,  PRL 100, 235502 (2008)).

  

The impressively low friction and wear of diamond in humid environments is debated to originate from either the stability of the passivated diamond surface or sliding-induced graphitization/rehybridization of carbon.

 

Ultralow friction and wear for ultrananocrystalline diamond (UNCD) surfaces was found even in dry environments, and negligible rehybridization observed except for a modest, submonolayer amount under the most severe conditions (high load, low humidity).

 

This supports the passivation hypothesis, and establishes a new regime of exceptionally low friction and wear for diamond.

 

The remarkably low friction and wear of diamond, particularly in humid environments, is postulated to be due to either rehybridization, or passivation of dangling bonds formed during sliding.

 

 

Experiment:

          Self-mated reciprocating wear of UNCD in varied relative humidity (RH) environments

        Four tracks made

        Examine behavior in different wear regimes

          Post-wear characterization of tracks

          Compare mechanics and chemistry of worn versus unworn surfaces

Major experimental steps:

  1. X-ray Absorption Near-Edge Spectroscopy (XANES).
  2.  PhotoElectron Emission SpectroMicroscopy  (PEEM)
  3. Raman Spectroscopy.

 

 

 

 

 

 

 

 

                

 

                 

         

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

   

 

 

  

 

 

 

Conclusions:

 

Self-mated diamond interfaces under dry conditions are not bad interfaces [have extremely low friction (0.02), and low wear after sufficient run-in (load, RH dependent)]

The formation of a graphite layer is NOT the mechanism of low friction under any conditions tested

Preliminary analysis indicates that oxidation occurs, particularly at higher contact pressures; chemical passivation may be the key

 

Future Work: 

•Test with constant contact pressure

Test different exposure times (sliding speeds)

Experiment with different gaseous species (e.g. F, OH, O2) 

 

 

Research Group:

 

This project was an initiative of Andy Konicek, PhD dissertator, University of Pennsylvania; Dr. Robert Carpick, Associate Professor, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania and Principal Investigator, Carpick Research Group; D. S. Grierson (University of Wisconsin-Madison),       P. U. P. A. Gilbert (University of Wisconsin-Madison), W. G. Sawyer (University of Florida), and A.V. Sumant (Argonne National Laboratory, Argonne, Illinois). Nanotribology is one of the various research areas that the Carpick Research Group is involved in.

 

Research Center: School of Engineering and Applied Science, University of Pennsylvania main campus.         Working with the Carpick Research Group was tremendous and fascinating. The group undertakes various initiatives to investigate friction far and wide, including its dependence on the structural, chemical, mechanical, vibrational, and electronic properties of materials.

They seek to develop and use advanced scanning probe microscopy tools to investigate the mentioned properties for novel materials such as diamond thin films, textured polymers, self-assembled organic monolayers, and nanostructured materials.

The Group also makes extensive use of surface spectroscopy and material characterization tools so they  can fully understand the structure, composition, and bonding of the materials under study, and their change in response to tribological sliding. The summer 2008 Carpick Research Group members that I worked with included:

  • Dr. Robert W. Carpick, (Associate Professor in the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania)
  • Dr. Matthew Hamilton, Postdoctoral Associate (U. Pennsylvania, Mechanical Engineering and Applied Mechanics).
  • Dr. Papot Jaroenapibal (Postdoctoral Associate, U. Pennsylvania, Materials Science and Engineering).
  • Mr. Andrew R. Konicek (Ph.D. dissertator, U. Pennsylvania, Physics).
  • Mr. Graham Wabiszewski (Ph.D. candidate, U. Pennsylvania, Mechanical Engineering and Applied Mechanics).
  • Mr. Sammy Saber (B.Sc. candidate, U. Pennsylvania, Materials Science and Engineering).
  • Mr. Vivak Adiga
  • Mr. Kendrick Hernandez.  

 

 

 

 

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