Site hosted by Angelfire.com: Build your free website today!
undefined
undefined

Landing on Planet Reality 101: The Co-op World

(Part I Tribology Section at Argonne National Laboratory)


This page was first created on: April 24, 2001 17:35(CST)
Last update: 5.01.2001 23:59 CST

Special Thanks to Dr. John Woodford and Dr. Bob Erck for corrections and further comments.

This page is intended to be an intro of tribology and PECVD . Please give me some feedback. I try to keep all the info as technically and politically correct as possible. However, there are still too many things for me to learn. Please correct me if you find something wrong and i'll update it. Thanks! :)
E-mail me or mark your comments in my guestbook!


Introduction

During my four-month-stay at Argonne National Laboratory, I was assigned to work at Tribology Section. The general public usually mistakes tribology as "tri-biology". In fact, tribology is not related to biological science at all. The following is another interesting misunderstanding provided by Dr. Woodford. A few years ago we had a tribology workshop at Argonne, but it couldn't be called "tribology"-it was described as a "friction and wear" workshop. Why? Because our Department of Energy sponsors had run into government officials who thought "tribology" was "the study of tribes," and couldn't understand why DoE was doing something that should be in the Department of the Interior/Bureau for Indian Affairs.

So what is tribology?

Tribology is a branch of interdisiplinary studies of physics and engineering. Dr. Woodford defines tribology as "a combination of mechanical engineering engineering (design considerations of friction & wear), chemistry/chemical engineering (lubricant design/formulation), physics (esp. surface science), and materials science/engineering (design of materials for wear resistance)."
Our Tribology Section studies the mechanisms of how traditional lubricants affect the mating surfaces of the two objects, and we have developed our unique Near Frictionless Carbon (NFC) Coating , which is probably the only institute developing such film at this moment. NFC is a type of Diamond-like Carbon (DLC). However, DLC is nothing new. In fact, it was founded in 1980s. Many progress have emerged since. Different methods and different composition of gas mixtures (hydrocarbons, hydrogen gas, and/or other gases) cause different properties of NFC. In the Tribology Section, we have come up with several species. The widely used ones are called NFC2, NFC6, and NFC7.

As a recap, the definition of the Tribology is:



Recall from high school physics, friction is the resistance when two things are rubbing against each other. For instance, the heat generated from rubbing hands. Long term friction usually leads to wear and tear of the contact surfaces of both objects, such as worn tires and brakes of automobiles, or worn heads in VCRs.
Friction Flash movie

What be done about friction and wear?
In order to make things last longer, lubrication is applied in between two contacting layers in order to minimize wear and tear.

Lubrications can be catergorized as follows:

  • liquid libricant, such as oil
  • solid lubricant, such as graphite or MoS2
  • gas lubricant, such as air


  • Near-Frictionless Carbon (NFC)

    What is Near Frictionless Carbon (NFC)

    In brief, NFC is an amorphous diamond-like carbon (DLC) thin film with special composition.

    Why use NFC? What's good about it?


    Some of the properties of NFC are:

  • chemically inert, i.e. non reactive
  • dielectric
  • low friction coefficient
  • wear resistant
  • Because of these excellent properties, there are many applications and still many potentials to be explored. In currently known applications, NFC can be applied in

    Potential uses to be explored:

    How does NFC coating work?

    At this stage of R&D phase, nobody fully understands how NFC coating works. Scientists suspect different mechanisms between NFC6, and NFC7, possibly due to the chemical bonding ( sp2, sp3 orbital). They cannot explain what gives NFC the property of ultra-low friction coefficient. Ironically, graphite ( similar structure to NFC) has poor friction coefficient under vacuum or dry, inert gas atmosphere. Yet, graphite has good friction coefficient when it is in open air or under reactive gas atmosphere. (*Side note: This is due to the outermost layer bond reacts with oxygen or moisture in air to form this inter layer, which make sliding possible and thus, less friction. This is known as "third body" in surface engineering.)

    A more thorough explanation is provided by Dr. Woodford:

    Although graphite is a fine solid lubricant in open air, it is pretty atrocious in inert atmospheres or vacuum. It turns out that the out-of-plane pi and pi* orbitals do interact very strongly with each other. But they also tie onto water molecules, oxygen, and the like, so that adjacent graphite planes are actually separated by adsorbed material. Thus, shear past each other easily. (That is, the lubricity of graphite is a function of the intercalation of water, oxygen, and/or other reactive species.) When reactive species are absent, the graphite planes do not shear past each other so readily, and friction increases. Now it's possible to produce a chlorine-intercalated graphite via the reaction SiC + 2+n Cl2 --> SiCl4 + C[2n Cl], and the graphite so produced does exhibit good inert-atmosphere friction behavior. So what's the reason for low friction in NFC? I don't know for certain, but would be extremely surprised if it didn't depend on hydrogen passivation of surface carbon dangling bonds.

    If NFC is such a great material, why don't we make everything out of NFC? Why is NFC used as coating only?

    Ah ha! It is likely because of NFC's internal stresses. If the whole object were made out of NFC, it would crack easily by its own weight.
    Also, NFC is a very brittle material. It is not as tough as tool steel.

    According to "Friction, Wear, Lubrication-- A Textbook in Tribology" by Ludema, substrate (i.e. the core of the material block) is usually expected to provide mechanical strength, ductility, conductivity, and several other functions. While, the surface is to serve as resisting wear and corrosion, and having acceptable appearance.

    How is NFC coating produced?

    Before you start to deposit the coating, the substrate surface should be clean and smooth, and preferably mirror-like smooth. This requires polishing, and there are different methods to polish. You can do it by hand (which can take hours, up to days, depends on how fine polishing you need) or with a machine as such
    This machine has a table rotating at different speeds, and people can just hold the sample to get polished., which has a table rotating at different speeds, and people can just hold the sample to get polished.
    You have to polish with different grain size polish papers step by step before promoting to a finisher polish cloth. Polishing is challenging because it is time consuming and little abrasive contamination can send you back to previous step. after using different size of grit papers, from 80, 120, 220, 320, 600, 1200, 2400, 4000, 5 micron, 1 micron, to 0.3 microns, you finally have a shiny, mirror-like metal piece. then, you do final rinsing with acetone and wipe dry and it's ready to be coated. :)

    In general, DLC coating can be coated via two methods, physical vapour deposition (PVD), and chemical vapour deposition (CVD). Within each, there are several sub-catergories. According to Modern Tribology Handbook,Bhushan, CRC Press, the most common methods are as such, laser-assited CVD, Plasma-Enchanced CVD (PECVD), hot-filament CVD, microwave CVD ,DR-arc jet, and combustion flame.
    In our tribology section, we use plasma-enchanced CVD to produce NFC coating. This is a view of our old chamber system. The new machine is being setup and should be available in mid-May. :)

    Some Limitations on Current Coating Technology

    It's challenging to make smooth and uniform NFC coating . Because of its chemical inertness, NFC doesn't form bonds easily with other materials, which makes it adhere poorly to something. A solution for that is to add a bond coat. Bond coat is an intermediate layer between the substrate and coating, it acts as super glue to adhere both substrate and coating layer together.

    Shield Effect

    Hollow Cathode discharge

    One can think of hollow cathode discharge as lightning within the vacuum chamber. This happens when charges between two electrode plates (act as capacitors) build up. As time passes by, the voltage difference become significant enough, that electric field is established between the two surfaces. With the plasma particles striking and colliding with each other, hollow cathode discharge thus exists.

    Shadowy Effect

    NFC deposits at line of projection, just like the properties of light. Since deposition happens at stright line, Therefore, if samples are placed too close together, interference would happen and uneven coatings might result.

    That's why we need fixtures and aluminium foil to void the internal cavity of the samples. Besides eliminating the possibility of hollow cathode discharge, fixtures and alumnium foil also serve as to raise the samples as high up as possible, to be as close to source as possible.

    On top of these, it's challenging to get uniform coatings. This is due to ultra-sensitivity of NFC coating deposition. If the piece is not placed at the correct orientation or little off the pedestal, a gradient of NFC coating thickness can result!

    Talking about coating thickness, it is very critical to have the right thickness of NFC thin film. Too thick of a NFC coating can flake off when the internal stresses gets too high and cannot hold the coating in place.

    The violet illumination is due to plasma being ignited by RF (radiofrequency) power. The violet illumination is due to plasma being ignited by RF (radiofrequency) power."

    Now, that you have learned about the properties of NFC thin film, and challenges of coating them. It's time for us to ensure quality control of these NFC thin films.

    Common Testings on Quality of NFC thin film coatings

    Fretting Test


    fretting Machine - Mr. Porky!

    The definition of Fretting is: