Increasing customer demand requires new process applications with needs for reliable lubrication and resistance to galling under stringent conditions of temperature, pressure, vacuum, corrosive environments, process sensitivity to contamination, product life, and maintenance requirements. The various applications within power generation and industrial and chemical processing have advanced lubrication needs. Specifically, this blog is related to the science of molybdenum disulfide coatings in compression fittings. In demanding environments, such as power generation plants, where tubing connections are exposed to stringent requirements and extremes, consistent leak-tight performance is critical and molybdenum disulfide coated fittings provide a solid solution.
Molybdenum disulfide is a naturally occurring black colored solid compound that feels slippery to the touch. It readily transfers and adheres to other solid surfaces with which it comes into contact. Its mineral form – called molybdenite – was commonly confused with graphite until late in the 1700s. Both were used for lubrication and as writing material for centuries. Wider use of molybdenite as a lubricant was impeded by naturally occurring impurities that significantly reduced its lubricating properties. Methods of purifying molybdenum disulfide and extracting molybdenum were developed late in the 19th century, and the value of molybdenum as an alloying addition to steel was quickly recognized. The demand for a domestic source of molybdenum during World War I resulted in the development of the Climax mine in Colorado, which started production in 1918 and continued into the 1990s.
The availability of high purity molybdenum disulfide spurred extensive investigations into its lubrication properties in various environments during the late ’30s and ’40s. These investigations demonstrated its superior lubrication properties and stability under extreme contact pressures and in vacuum environments. These investigations resulted in extensive applications in spacecraft.
Download our white paper Molybdenum Disulfide - The Ideal Solid Lubricant and Anti-Galling Material for a closer look into the science behind high-performance compression tube fittings required for demanding environments.
Molybdenum disulfide’s exceptional lubricity is a consequence of its unique crystal structure, which is made up of very weakly bonded lamellae. The lamellae tend to align and adhere to contact surfaces, particularly under conditions of sliding and pressure, as shown here. This “burnishing in” of the molybdenum disulfide gives it its exceptional performance life.
Since molybdenum disulfide is a solid phase, it is not “squeezed out” like liquid lubricants under conditions of extreme pressure. The lamellae are very “hard” to forces perpendicular to them. This combination of properties provides a very effective “boundary layer” to prevent the lubricated surfaces from contacting each other.
The surfaces of objects are generally rough on a microscopic scale. When two objects are in contact with each other, they actually “touch” at very small regions of contact (i.e., asperities).
These contact regions have considerably less area than the bulk surface area, typically in the range of 0.5 to 0.001 percent of the bulk area for a machined metal surface, and consequently, the stresses at these contact points are considerably higher than the stresses calculated for the bulk surface area. When these objects slide relative to each other the frictional forces add to the stresses at the contact points, and the resultant stresses may be sufficient to cause deformation of the contact points.
When stainless steel objects slide against each other under high load, they will “gall” or “seize” due to the deformation at the contact points. The objects will actually “cold weld” themselves to each other, which is indicated by the transfer of material from one object to the other on the sliding surfaces. This causes a very rapid increase in friction, quickly to the point that further sliding is impossible without damage to the objects. To prevent this, it is necessary to introduce an “anti-galling” or “anti-seizing” agent between the surfaces. This is a substance that can maintain separation of the surface asperities under high compressive loads – that is, to provide a “boundary layer” between the surfaces. Anti-galling materials are generally very thick grease-like substances or solid materials in powder or plated layer form. Molybdenum disulfide is an ideal anti-galling compound because of its combination of high compressive strength and its adherence (ability to fill or level) to the sliding surfaces.
There are many methods of applying molybdenum disulfide to a surface, from “high tech” techniques such as vacuum sputtering, to simply dropping loose powder between sliding surfaces. The most versatile technique is the application of the powder mixed with a binder and a carrier to form a bonded coating. The binder may be a polymeric material or several other compounds, and the carrier may be water or a volatile organic. The characteristics of the molybdenum disulfide powder, the binder, the carrier, and particularly the application process must be carefully developed and controlled to optimize the performance of a specific product. A properly developed bonded coating of molybdenum disulfide can provide exceptional lubrication performance over a temperature range up to approximately 500°C, under very high pressure and corrosive exposure conditions for extensive lifetimes.
During fitting make-up, the ferrule(s) is driven forward into the body seat and tubing surface as the nut is turned per the makeup instructions. The ferrules seal at contact points with the fitting body seat and tubing surface. The fitting is carefully engineered such that the ferrule and tubing do not rotate with the nut, moving only in the axial direction. This is critical to forming a high integrity leak-free tubing connection during the first make-up and subsequent remakes. Therefore, the sliding takes place between the back of the ferrule and the flange of the nut under very high pressure. This region of contact between the nut and the ferrule must have excellent lubrication for the proper action to occur during make-up to ensure ease of assembly, low make up torques, and optimum fitting function. The nut is “pulled” against the ferrule during make-up by the threads, which are also sliding under very high pressure and require high-performance lubrication.
Stainless steel compression fittings have the additional problem of preventing galling in these areas of sliding under very high contact pressures. This requires a “boundary layer material” – a substance that maintains separation of the surfaces during sliding.
Our instrumentation engineers have developed and used a bonded molybdenum disulfide coating on the nuts of our premium CPI™ compression fitting products for 30+ years. These fittings are readily recognized in the field by the “black nut” – the molybdenum disulfide coated nut, and have been providing exceptional service in many demanding applications.
The molybdenum disulfide coating has been carefully formulated and processed to optimize the performance of the CPI™ compression fitting. This fitting, with its single ferrule and the molybdenum disulfide, coated nut, has easy initial make-up with very low torque, consistent remake, and exceptional, leak-tight performance under demanding power generation applications including pressure, temperature, corrosion exposure, and vibration.
We also offer a “Moly Inside” nut for use with our CPI™ and A-LOK® compression fittings. This is an optional version of our premium compression fitting products with the molybdenum disulfide coating only where it is needed, on the inside surfaces of the nuts. This offers the same low make-up torque and consistent remake-ability of the standard CPI™ compression fitting in both the CPI™ and A-LOK® versions due to the use of molybdenum disulfide on the critical mating surfaces of the interior threads and flange of the nut, but without the molybdenum disulfide on the external surfaces to rub off onto hands, gloves, or other equipment for industries requiring a “clean” appearance.
Download our white paper Molybdenum Disulfide - The Ideal Solid Lubricant and Anti-Galling Material for a closer look into essential attributes of high-performance compression tube fittings required for extreme environments.
Article contributed by Kevin Burke, marketing manager, Instrumentation Products Division, Parker Hannifin.
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