From a lubrication point of view, extreme operational conditions may not commonly occur in every industry, but in some sectors such as defense and aerospace they are encountered quite often. These challenging conditions may include:
- Very high or very low temperatures
- Variable temperatures
- High or low surface speeds on shafts
- The presence of a vacuum
- Inaccessibility for maintenance or re-lubrication
- The presence of vibrations, extreme loads and stresses
- Contaminants generated by processes
Petroleum-based lubricating substances work effectively only when:
- Operating temperatures are in the broad range of -4°F to 212°F (-20°C to 100°C)
- Tribology parameters enable the lubricant film to be formed within interacting surfaces, according to the mnemonic MAL/S
- M: Material
- A: Accuracy of clearances and surface speeds
- L: Lubricant film parameters
- S: The load and operational severity
Conditions beyond the ability of mineral lubricants (i.e., oils and greases) require solid dry film lubricants.
Dry Film Lubricants
A dry film lubricant is defined as a solid substance that will reduce friction and prevent wear and tear (as well as corrosion, preferably) when interposed between two mating surfaces subjected to relative movement.
Two popular solid lubricants that cause corrosion when used directly are molybdenum disulfide (MoS2) and graphite.
In moist environments, MoS2 works as a cathode to some metals, generating a voltage as high as 0.5 volts. When added to grease, MoS2 can cause accelerated galvanic corrosion and rusting on ferrous metals. However, molybdenum (the metal) can be anti-corrosive.
Polytetrafluoroethylene (PTFE) has excellent corrosion resistance.
Solid Film Lubricants
Solid film lubricants produce a film that reduces sliding and rolling friction by substantially eliminating surface-to-surface contact between interacting (mating) surfaces. Depending upon the lubricating substances used, they may be appropriate for different temperature ranges and loading conditions. Their corrosion resistance, wear resistance and coefficient of friction also vary. Some of these are nuclear radiation resistant and work in a vacuum as well.
In order to be anti-corrosive, the solid lubricant substances must be bonded with resins, because they create and maintain a barrier on the substrate to protect against the agents of oxidation reactions and other forms of corrosion.
Resin-bonded dry film lubricants with corrosion inhibiting capability have been used in military applications and have the following three basic components:
The resin binders must also have the following characteristics:
- Stability at low and high operating temperature ranges, and must remain hard at operating temperatures
- The ability to form a film on the surface when applied
- Compatibility with solid lubricants and the intended surfaces to be protected
- Resistance to wear and the creation of harmful wear debris (to ensure a long service life of the solid lubricant because re-lubrication may not be feasible)
- The ability to provide corrosion protection even in thin film conditions
Selecting a Dry Film Lubricant with a Pigment
The formulation of dry film lubricants with resin binders is similar to the formulation of corrosion prevention paints and coatings. In this case, the lubricant also works as a pigment, determining color aesthetics when required. The lubricant pigments help film formation, which separates the mating surfaces that are in relative motion, thus ensuring a lower coefficient of friction as well as wear and corrosion resistance.
Dry film (solid) lubricants are selected on the basis of their required performance and the environment they must endure. PTFE, MoS2, graphite, fluorinated ethylene propylene (FEP), tungsten disulfide (WS2), antimony oxide (Sb2O3), indium (In) and boron nitride (BN) are some common dry film lubricants.
Atmospheric moisture is essential for graphite to function as a lubricant, while the desirable properties of MoS2 are adversely affected by the presence of moisture. Moisture has no impact on the corrosion or wear resistance of PTFE.
MoS2 is the obvious choice when a high load carrying capability (up to 250,000 pounds per sq. inch) is a requirement. Graphite has a load capability of 50,000 pounds per sq. inch, whereas PTFE has a load carrying capability of only 6,000 pounds per sq. inch.
While graphite excels in thermal capacity (up to 1200°F), PTFE and MoS2 can withstand temperatures of 500°F and 750°F respectively. At higher temperatures the solid lubricants tend to oxidize and decompose.
Graphite is used in the joints of rail tracks, locks, firearms, open gear fasteners operating at very high temperatures, and bearings. However, it has the disadvantage of being electrically conductive, which can be a source of corrosion as well.
Fluoropolymers like PTFE have a low coefficient of friction, but have limited load carrying capacity and thermal capacity.
Boron nitride (hexagonal) and MoS2 have been applied in the internal parts of space vehicles.
Tungsten disulfide is often used for ball bearings in space vehicles, but is more expensive. It has better frictional properties at higher temperatures and higher loads, as compared to MoS2.
The Importance of the Dry Film Lubricant's Particle Size
The particle size of dry film lubricants must be carefully controlled because the particle size should be matched to the substrate's surface roughness. For example, hobbed open gears should have larger particles of dry film lubricants, whereas finely finished bearing surfaces and shafts should have finer dry film lubricants.
|Lubricant||Recommended Particle Size|
|Graphite||2.5 to 10 microns|
|MoS2||2 to 6 microns|
Table 1. Recommended particle sizes for various lubricants.
Different types of resins are used as bonding agents for creating a bond between the surface to be protected and the dry film lubricant.
Inorganic binders such as silicates are not moisture resistant and cannot provide corrosion resistance. However, a Boric Oxide (B2O3) binder along with lead sulfide (PbS) as a lubricating pigment has been found to ensure wear and corrosion protection in the high temperature range of 1000°F (538°C). However, it does not function as a lubricant at temperatures below 1000°F (538°C).
Common types of resin binders used along with solid film lubricants are generally classified as either:
- Thermosetting binders, or
- Thermoplastic binders
Thermosetting binders are those resins that require heat energy for curing. Thermoplastic binders can be cured through a solvent evaporation process (i.e., by ambient air curing). Due to their curing temperature requirements, thermosetting resins are sometimes not preferred for military applications.
Thermosetting Resin Binders
A mixture of resins can also be considered. However each resin has its own drawbacks and advantages. When selecting a mixture of resins the specific end use must be kept in mind. The wear debris that is generated by epoxies reduces the lubrication's performance over the long-term, and hence service life can be short. However epoxies have good adhesion to metallic substrates. Phenolic resins should not be used in alkaline environments.
Phenolic resins do not generate harmful wear debris and are suitable for high vacuum applications. However their adhesion to metals is not as good as that of epoxy resins. Even silicones generate harmful debris, although the bonding strength with metals is good. A combination of phenolics with epoxy resin has been found to produce reasonable bonding strength with metals for general applications. Ceramic resins need high temperature curing, which can interfere with the metallurgical properties of the substrate. Curing temperature and the baking duration is an important consideration when selecting a resin.
Thermal curing requirements can be balanced by incorporating a curing component along with the resin. But this requires accurate formulation and mixing just prior to application.
An accurately formulated and carefully cured solid dry film lubricant with a thermosetting resin bonded solid lube system can ensure reasonable corrosion resistance, wear protection and a longer lubricant functional lifespan of low friction. The thermosetting resin-bonded solid film lubricant with MoS2 provides effective wear resistance and corrosion prevention even in a nuclear radiation environment.
Thermoplastic Resin Binders with Solvents
The air-drying type of thermoplastic resin binders, such as acrylic resins, will need a solvent component. Curing is achieved by allowing the solvent to evaporate so that the dry film lubricant dispersed in the resin will form a hard coating on the intended surface. These have a lower temperature limit of -200°F (-129°C) and an upper limit of 300°F (149°C). They can be formulated as convenient pressurized sprays. (Thermoplastic binders are discussed in The Composition of a Paint Coating.)
Acetates, vinyls and alkyds are the other air-drying type of resins, each having different properties. (See An Overview of Alkyd Resins for more about alkyd resins.) Thermal cured (baked in oven) thermoset resin-bonded products generally have a higher wear life (i.e., durability) as compared to air-cured thermoplastic resin-bonded products.
Ratio of Lubricant Pigment to Resin Binder
Changing the ratio of the solid lubricant to the resin binder can have an effect. The amount of solid lubricant to be dispersed in the resin during formulation is an important consideration that determines the wear life, friction coefficient and corrosion resistance. Increasing the lubricant pigment reduces the friction, whereas decreasing the concentration improves the corrosion resistance and durability.
Solvents are chosen on the basis of resin solubility and the evaporation rate under normal ambient conditions. The toxic nature of these chemicals, occupational health considerations and regulatory requirements (e.g., OSHA and EPA) are also considered.
Water-based dispersions of resins with lube pigments are gradually replacing the solvent component wherever feasibility has been established.
Additive chemicals are selected for their ability to enhance flow ability, corrosion protection, wettability and dispersion capability (anti-settle down tendency). Considerations of aesthetics and how well they facilitate easy application also factor into the coating formulation as well.
How to Apply a Formulated Resin-based Product
Even the best formulations can fail in service if the surface preparation and application process are not conducted systematically. For steel surfaces, best results are obtained when pretreatment includes vapor degreasing, grit blasting with 220 mesh aluminum oxide (Al2O3) and a phosphate treatment.
Application of a Formulated Resin-based Product
Resin-bonded dry film lubricant application methods are similar to corrosion resistant coating application methods. In most cases, the film thickness needs to be within a range of .0002 to .0005 inches, and needs to be precisely controlled for good corrosion prevention.
Methods of coating application such as conventional spray, dipping, electrostatic spray, brush and roll coating, are all applicable for resin-bonded solid lubricants as well. Final selection of the method is dependent upon the total surface area, number of parts to be coated, complexity and dimension of the parts.
Typical Final Products Results
Molybdenum Disulfide (MoS2) Dispersed in Phenolic Resin
- Can be cured around 300°F (149°C)
- Max operating temperature 520°F (271°C)
- High load carrying capacity
- Good corrosion resistance
- Moderate durability
PTFE in Phenolic Resin
- Can be cured around 404°F (207°C)
- Max operating temperature 520°F (271°C)
- Low load carrying capacity
- Excellent corrosion resistance
- Good durability
Molybdenum Disulfide (MoS2)/Graphite in Silicone Resin
- Can be cured at 500°F (260°C)
- Max operating temperature 667°F (353°C)
- High load carrying capacity
- Fair corrosion resistance
- Moderate durability
When dry film lubricants such as molybdenum disulfide and graphite are directly used for lubrication, the surfaces are prone to corrosion, as these don’t provide corrosion protection.
Thermosetting resins such as phenolics, urethanes, epoxies and silicones are used as resin binders for dispersing dry film lubricants and are cured at high temperatures. Each has its own merits and best use cases. Dry film lubricants dispersed in phenolic resins don’t produce harmful wear debris, but their bonding strength with the metal surface is inadequate, whereas epoxies have better bonding strength and but produce harmful wear debris. When epoxies are combined with phenolics, the bonded dry film lubricant acquires satisfactory bonding strength as well as a reduction in wear debris.
When formulating the product, the ratio of lubricant to the binding resin needs to be carefully controlled. A higher proportion of dry film lubricant can reduce the coefficient of friction, but will reduce the durability and corrosion protection.
Air cured thermoplastic resins such as acrylics are convenient for field application, but the durability in extreme conditions does not match the properties of thermosetting resins.
To obtain the best performance, the steel surfaces must first be vapor degreased, grit blasted and phosphated before spraying the resin-bonded dry film lubricant.