How do you choose between Graphite or Moly, Molybdenum disulfide (MoS2), for a particular lubrication application?
Graphite or Moly (molybdenum disulfide) dry lubricants are common in operations where machinery operating conditions are extreme such as high shaft speeds, very low rotation or sliding machinery, extremes of temperature and pressure, inaccessibility of lubricated surfaces, and more in industrial applications. In the discussion below, we provide some background on the differences and reasons to choose one over the other.
There are other dry lubricants, Teflon or Polytetrafluoroethylene (PTFE), being the primary alternative to Moly and Graphite. PTFE lubricants have many great applications where being inert and non-conductive are desired characteristics. Some disadvantages include that when overheated it can release mildly toxic fumes. In addition, its abrasion resistance is not great, it can be higher priced, and it can’t withstand temperatures above 326 C where it melts. This discussion assumes you want a lubricant similar to Graphite or MoS2 and need more information to decide which one to apply.
Reducing Frictional Between Surfaces
Both Graphite and Moly are dry film lubricants that are solid materials that reduce frictional resistance between surfaces. Each material has different properties. Which can give it an advantage in a particular application. They both generally have a crystalline lattice structure which is described as plates sliding over each other. This gives them both excellent load-bearing capability in the sliding or rolling mode. They both come with a variety of application methods from painting on to aerosol delivery. They can be used in a stand-alone form or be added to other oil-based lubricants or even metals such as lead and copper to achieve the exact lubrication properties.
Moly Inhibits Corrosion on Stainless and has Slightly Better Wear Resistance
MoS2 occurs naturally being mined from granite and then refined to achieve the high purity you find in lubricants. MoS2 is a great lubricant for high vacuum and extremely high vacuum applications where Graphite may have a requirement for entrained moisture vapor
MoS2 has a temperature limit of 400°C (752°F) where it starts to oxidize in an atmosphere with oxygen.
MoS2 usually has a lower coefficient of friction than Graphite, but this is not always the case and is not a huge differentiator. One of the best properties of Moly is its ability to inhibit corrosion when used as a coating on stainless steel. Also, Moly is considered to have slightly better wear resistance, particularly under extreme pressures.
Graphite is a great Lubricant for temperatures up to 1450F and Anti-Seize up to 2400F
Graphite goes back a very long way, probably used in pencils in the mid-1500s. It’s a soft, crystalline form of carbon with excellent sliding lubrication properties. It is gray to black, opaque, has a metallic luster, and is flexible but not elastic. It has some properties of metals and some non-metals. Like most metals, it’s electrically conductive and is a good thermal conductor also. Like non-metals, it is inert.
Graphite provides excellent lubrication up to 1,450 F with water vapor available. It can be used as an anti-seize compound up to 2400°F with an oxidation product of CO2. Because graphite requires water vapor for its lubricity, it is not used in a high vacuum environment including for deep-space use.
Summary
Both Moly and Graphite are excellent dry lubricants with very similar properties in many areas. Graphite tends to be a better choice for extremely high-temperature applications. MoS2 offers improved corrosion resistance for stainless steel in those areas where that is a concern. Moly may have a slight advantage in some applications where wear resistance under high pressure is a consideration.