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
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 , 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:
Lubrications can be catergorized as follows:
In brief, NFC is an amorphous diamond-like carbon (DLC) thin film with special composition.
Some of the properties of NFC are:
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:
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.
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.
Before you start to deposit the coating, the substrate surface should be clean and smooth, and preferably mirror-like smooth. This requires , 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,
In our tribology section, we use plasma-enchanced CVD to produce NFC coating. The new machine is being setup and should be available in mid-May. :)
It's challenging to make
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 , 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
Talking about coating thickness, it is very critical to have the right thickness of NFC thin film.
Fretting Test is performed in order to test out the long-term durability of NFC coat films. Usually, a coated fuel injector or a coated steel ball would rub against a coated or bare, polished, hardened, steel flat under the load of about 1-5 newtons. The experiment is run in inert gas atmospheres and different testing fluids in order to study how NFC coat film reacts with different testing fluids, and how the fretting behaves. The test is usually carried out for about 1 million cycles. After that, we would study the wear and tear of both substrates and hypothesis of what mechanisms or if any chemical reactions happen in between.
This is an image of how the fuel injector was interpreted under microXam.
This is an image of after test wear scar, you can compare with pre-test and calculate the wear off volume. Also, to study the mechanisms by studying the wear track and debreed location.
The The followings are some ongoing Research & Projects at Tribology Section
This page is written by Joanne Lee. For any suggestions and comments, please send me e-mails.
last update: 5.01.2001