Molybdenum Disulfide and it’s applications for lubrication
The science of Molybdenum Disulfide is well established in the industrial community. Scientific and industrial literature describes Molybdenum Disulfide (MoS2) as a naturally occurring relatively inert, chemically stable, practically non-corrosive, non-toxic mineral commercially mined in Colorado, Idaho and South America. It is extracted from the ground, separated from other materials, cleaned, crushed purified and separated into particles of different size through screens of progressively finer mesh.* Molybdenum Disulfide has a particular affinity to adhere to metals with a peculiar mechanical-thermal chemical bonding that is not clearly understood to the scientific community (neither is aspirin or anesthesia understood). Even though we may not completely understand MoS2 on the subatomic level, industry and science does understand and can describe its physical properties and how it behaves as a dry lubricant on the microscopic level.
*Super Fine grade Moly is 99.7% (guaranteed 98%) pure. Particle size is from less than 0.05 to 8 microns (median 1.5).Technical Grade particle size:1 to 100 microns (median 30.0). (l micron = l millionth of l meter). Super Fine is expensive. One industrial supplier quotes about $3500 per 200lb drum. One scientific supply house quotes $40 for 4oz.[img-frame align=’left’]http://www.bobistheoilguy.com/wp-content/uploads/2011/03/figure1.jpg[/img-frame] The Scanning Electron Microscope (SEM) at 150Xmagnification shows that even a highly polished metallic surface is truly rough and irregular on the microscopic level. Under the SEM the surface of a bearing appears like the Himalayan Mountains as seen from aircraft at 40,000 ft. Deep steep sided valleys separate the rough crags and peaks. In scientific language, this roughness with jagged high peaks and deep irregular valleys are termed asperities.
Now mentally visualize what happens to a bearing, under conditions of extreme pressure and heat passing over this rough surface. The metal is going to be abraded away, leaving behind sheared off metal particles that are packed down the valleys of the asperities. When metal deposits get down into these asperities. Unfortunately, it is the nature of metal, copper, and lead to keep building up on itself creating more groves.
Floating metal fragments are not good. A minimal amount of metal wear is not serious, but it does affect the valleys of the asperities and over time will significantly change the dimensions of the asperities, like a river will a canyon. Like the river in the canyon, bit by bit, rocks brake loose, so are metal fragments, causing more wear than before.[img-frame align=’left’]http://www.bobistheoilguy.com/wp-content/uploads/2011/03/figure2-300×117.jpg[/img-frame] Elimination of this condition by chemical cleaning is needed to help reduce the increased wear.
The electron microphotograph has clearly shown Molybdenum Disulfide particles intruding down into the valleys of the asperities, covering the offending sharp crags or summits. This will appear similar to glaciers filling valleys and covering mountain peaks. The SCM measures the distance between the peaks as greater than 30 microns. Super Fine moly particles will completely fill and pack down into the valleys of the asperities, to form a uniform buffering of 0.0005 – 0.001 coating on the bearing surfaces.
Under the SEM microscope, we can see individual particles of MoS2. They have the appearance of rounded clamshells better known as plates. The plates appear stacked one on top of another. These plates adhere to each other. Molybdenum Disulfide has been described as conglomerate particles composed of associated platelets (small plates). These platelets have a mild attraction for each other. The platelets cleave off from each other rather easily with lateral (side ways) mechanical pressure. With mechanical pressure, moly platelets will slide and glide over each other in a shingling effect.
An analogy would be to picture moly particles as a deck of playing cards. If one puts downward pressure on them and then moves the hand to the right or left, the cards (moly platelets) will be spread out in a shingling effect of overlapping cards (platelets).
In addition, the particles of Molybdenum Disulfide are both relatively soft and non-abrasive. The shingling effect and the softness of the elements is specifically why MoS2 is ideal for lubrication uses.
In a working analogy, imagine a sheet metal loading chute on which heavy boxes are moved from one end of the chute to the other. On that chute are decks of playing cards stacked side by side across the width and down the length of the chute. Throw a heavy box on top of the cards and chute, and push the heavy box down and off the chute. This is what would happen. The heavy box easily slides over the tops of the slick playing cards, the cards shingle under the weight of the box, cards slide over one another, but the box moves easily along down the chute. The more energy in the push; the faster the box moves. When we get to the end of the chute, the box goes zooming down and off the chute along with a lot of the cards. Boxes will keep on sliding easily down the chute as long as there are slick playing cards underneath shingling along with the box. When the supply of cards is at last exhausted, we come back to the steel surface of the chute. The box does not slide so easily any more. And so it is with moly, the same thing happens to moly conditioned surfaces, ie, bearings, cams, rings, etc.
There is no such thing as a “moly build up”. Moly either goes into the asperities and crystalline structure of the metal surface, or it stays suspended in the base lubricant. After a metal surface is “moly conditioned”, you will notice during oil analysis that the moly additive level won’t be as low as it was upon the first application. This is due to the plating action that occured during the intial contact of moly and that surface, once plated it just passes by in the suspended state.
Note: Graphite and many other metallic materials often used in some other lubricating applications are less suitable. Graphite is a carbon compound and use of a graphite lubricant can contribute to a faster erosion. When two different metals such as copper and iron alloy, interface in the presence of atmospheric moisture and excellent conductor, (like carbon graphite), galvanic electrical currents are to some degree produced, resulting in metallic oxidation or corrosion and pitting. The automotive industry does not add graphite to oils where heavy PSI loads are encountered in conjunction with higher temperatures.
The relatively hard carbon graphite particles may have an abrasive effect in environments of high heat and pressure. Molybdenum Disulfide is ideal for these high temperature-high pressure applications. MoS2 will survive pressure into the neighborhood of a half million PSI and two thousand degrees F. in an oxygen depleted environment with practically no corrosion and erosion. MoS2 is relatively soft and is not corrosive and non-abrasive.
In microphotograph Figure 2, we see a clean, highly polished surface. It is being covered with particles of powdered MoS2 which adheres with burnishing on impacting to the metallic surface. Notice that the powdered moly invades and fills the valleys to the extent that it forms a buffering coating over the tops of the mountainous asperities. The general thickness of dynamic films of powdered MoS2 is in the area of l to 4 microns,(l to4 one millionths of 1meter) as measured by interferometry techniques. The use in bearings impacting moly particles over a metallic area achieves a fairly uniform coating over a short period of time.
In comparison, if that same polished metallic field sprayed with a soluble moly suspended in a lubricant (as in an aerosol application or motor oils), that metallic area would be completely and uniformly coated. Most bonded moly applications generally form a film thickness from 0.0005 to 0.001 (5/10,000 to 1/1,000).
Mentally picture bonded moly particles being sprayed on the rough mountainous surface. In the aerosol composition, the bonded moly particles are individually suspended and separated in a solution of solvents, waiting to be released and applied to the substrate field. When released, the bonded moly particles descend on to the materials surface similar to black snow falling on a recently plowed agricultural field. They adhere/bond to each other and to most clean materials with which they contact. The particles/flakes accumulate – filling the furrows (valleys of the asperities) and eventually cover the peaks. As particles continue to accumulate, they are bonded to the alloy forming the desired 0.0005 to 0.001 buffering zone. The buffering coating completely shields the under lying alloy from direct metal to metal contact, abrasion, corrosive agents, and extreme temperatures from combustion.
The bonded particles in the coating re-establish their properties of being able to delaminate and shingle along the lines of applied force and pressure. This shingling effect is again shown in the electron-photograph Figure 4. The picture center clearly shows the shingling effect of moly particles. This shingling effect of relatively soft moly platelets is what gives metal sufaces the gliding surface over which to travel without metallic wear and without moly buildup. The metal surface has no metal to metal contact – only moly to moly contact. That is; so long as the moly is present. Moly must be suspended in the lubricant so it can replenish any moly carried away under load. However, if there is insufficient moly in suspension, the moly supply is exhausted in growing bald spots. Again we would have metal to metal contact along with its associated wear. This is analogous to sledding down a hill in the winter where the cars have packed the snow down to a slippery glaze. You are really zooming along -slick and smooth – until you hit a bare spot with the cement or blacktop showing. Result, perish the thought!
The final result of the shingling effect of powered moly is a thin layer of moly powder deposited on a metallic surface. This coating is comprised of overlapping moly platelets measured by the scanning electron microscope to be 2 microns in thickness. Soluble moly layers are 0.0005 to 0.001. It has been suggested that this thickness is relatively constant. Any moly over this amount will be carried off under the pressure of an object passing over this surface and kept in suspension, ie, motor oil. The effect of the playing card like bridge, spanning the valleys or voids is important in that we have greatly increased the bearing surface. With peak to peak or point to point contact, tremendous pressures and temperatures are localized and concentrated on those points of metal to metal contact. Now, with the moly bridging or coating the entire surface, the bearing surfaces are relatively and uniformly in contact. The bearing pressure is now evenly distributed over a greater number of contact points. Also, the other metal alloys are not in contact with the each other.