Anti-corrosion dry film lubricants

Dry film lubricants can lubricate and provide the corrosion resistance needed for machines operating in extreme conditions.

Extreme operating conditions may not commonly occur in all industries, but in some specific ones such as aerospace, military and naval, they are encountered quite often. These challenging conditions may include:

• Variable temperatures, from very high to very low
• High or low surface speeds on crankshafts
• The presence of vacuum
• Inaccessibility for maintenance or re-lubrication
• The presence of vibrations, extreme loads and stresses
• Abundant contaminants

Traditional petroleum mineral lubricant products (e.g. oils and greases) function effectively only when operating temperatures are within the range of -20°C to 100°C (-4°F to 212°F) and when they are met parameters such as the possibility of lubricating all interacting surfaces and free spaces, managing to withstand surface speeds and energies, load and operating gravity.

Conditions beyond the capability of mineral lubricants require dry film lubricants.

Solid lubricants

A solid lubricant is defined as a substance that reduces friction and prevents wear (as well as corrosion, preferably) when placed between two mating surfaces subject to relative motion.

Solid lubricants are selected based on the performance required and the environment they must withstand. 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.

MoS2 (molybdenum disulfide) is the obvious choice when high load capacity (up to 250,000 pounds per square inch) is required. In humid environments, MoS2 is negatively affected. Functioning as a cathode for some metals, it generates a voltage of up to 0.5 volts. When added to grease, MoS2 can cause accelerated galvanic corrosion and rust on ferrous metals.

Graphite requires atmospheric humidity to work correctly, but it is also corrosive because it causes selective leaching of iron from gray cast iron as the ferrous particles are removed and replaced by graphite, weakening the structure. Graphite has a load capacity of 50,000 pounds per square inch, is used in railroad track joints, 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 further source of corrosion.

Fluoropolymers such as PTFE have a low coefficient of friction and no negative influence with humidity, but have limited load carrying capacity and heat capacity (6,000 pounds per square inch).

While graphite excels in terms of heat capacity (up to 650°C / 1200°F), PTFE and MoS2 can withstand temperatures of 260°C (500°F) and 400°C (750°F), respectively. At higher temperatures, solid lubricants tend to oxidize and decompose.

Boron nitride (hexagonal) and MoS2 can be applied in the internal parts of spacecraft.

Tungsten disulfide is often used for spacecraft ball bearings, but it is more expensive. It has better friction properties at higher temperatures and higher loads, compared to MoS2.

In order to be used correctly, solid lubricating substances must be bound with resins, so that they create and maintain a barrier on the substrate that protects it from the agents of oxidation reactions and other forms of corrosion.

To obtain lubricating products with anti-corrosive properties and suitable for heavy-duty applications, compounds must be formulated starting from:

• A solid lubricant
• Resin binders
• A solvent
• Additives

The resin binders must also have the following characteristics:

• Stability at low and high operating temperature ranges
• The ability to form a film on the surface when applied
• Compatibility with solid lubricants and surfaces intended to protect
• Resistance to wear and the creation of harmful wear debris (to ensure long solid lubricant life as re-lubrication may not be feasible)
• The ability to provide corrosion protection even under thin film conditions

Particle size

The particle size of solid lubricants must be carefully controlled as it must match the roughness of the substrate surface. For example, lubricants with larger particles should be used for open toothed gears, while lubricants with finer particles should be used for bearing surfaces and finely finished shafts.

Recommended dimensions:

• Graphite: 2.5 to 10 microns
• MoS2: 2 to 6 microns
• PTFE: < 1 micron 

Resin binders

Different types of resins are used as binders to create a bond between the surface to be protected and the solid 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 provide wear and corrosion protection in the high temperature range of 538°C (1000°F). On the other hand, it does not function as a lubricant at temperatures below 538°C (1000°F).

Common types of resin binders used in conjunction with dry film lubricants are generally classified as:

• Thermosetting binders
• Thermoplastic binders

Thermosetting binders

Thermosetting binders are those resins that require thermal energy for polymerization. They can be polymerized through a solvent evaporation process (i.e. by polymerization in ambient air). Due to curing temperature requirements, thermoset resins are sometimes not preferable for military applications.

The most popular thermosetting resin binders for these applications are phenolic resins, urethanes, epoxy resins and silicone resins.

Wear debris generated by epoxy resins reduces long-term lubrication performance and therefore life may be short; Epoxy resins have good adhesion to metal substrates.

Phenolic resins should not be used in alkaline environments, 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.

Silicones also generate harmful debris, although the bond strength with metals is good. A combination of phenolic compounds with epoxy resin has been found to produce reasonable bond strength with metals for general applications.

Ceramic resins require high-temperature curing, which can interfere with the metallurgical properties of the substrate. Thermal curing requirements can be balanced by incorporating a curing component along with the resin, but this requires careful formulation and mixing just prior to application.

A carefully formulated and cured dry film lubricant can provide reasonable corrosion resistance, wear protection, and longer lubricant functional life with low friction. MoS2 lubricant with thermosetting resin provides effective wear resistance and corrosion prevention even in a nuclear radiation environment.

Considering that each resin has its own disadvantages and advantages, you can choose to use mixtures of resins in order to achieve a precise final result.

Thermoplastic binders

Air-drying thermoplastic binders, such as acrylic resins, require a solvent component. Curing is achieved by allowing the solvent to evaporate so that the dry lubricating film dispersed in the resin forms a hard coating on the desired 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.

Acetates, vinyls, and alkyds are the other type of resins that air dry, each with different properties.

Products bonded with thermoset resin generally have greater wear resistance than products bonded with air-cured thermoplastic resin.

Solvents are chosen based on the solubility of the resin and the rate of evaporation under normal environmental conditions. The toxic nature of these chemicals, occupational health considerations and regulatory requirements are also taken into account.

Water-based resin dispersions are gradually replacing the solvent component wherever feasibility has been established.

The ratio of solid lubricant to resin binder determines wear life, friction coefficient and corrosion resistance. Increasing the lubricant reduces friction, while decreasing its concentration improves corrosion resistance and the durability of the coating.

Additives are instead added to the formulations to improve their fluidity, protection from corrosion, wettability, dispersion and anti-sedimentation capacity, and the final aesthetic effect.

Application

Even the best formulations can fail if the surface pretreatment and application process is not conducted systematically. For steel surfaces, the best results are achieved when pretreatment includes vapor degreasing, 220 mesh aluminum oxide (Al2O3) blasting, and a phosphating treatment.

Coating application methods are: conventional spray, dip, electrostatic spray, brush and roller coating. The choice of method depends on the total surface area, the number of parts to be coated, the complexity and the size of the parts. Film thickness should be between 5 and 15 microns and should be precisely controlled for good corrosion prevention.

Typical results of the final products

Molybdenum disulfide (MoS2) dispersed in phenolic resin

• Can be cured at approximately 300°F (149°C)
• Maximum operating temperature 520°F (271°C)
• High load capacity
• Good corrosion resistance
• Moderate durability

PTFE in phenolic resin

• Can be cured at approximately 404°F (207°C)
• Maximum operating temperature 520°F (271°C)
• Low load capacity
• Excellent corrosion resistance
• Good durability

Molybdenum disulfide (MoS2) / Graphite in silicone resin

• Can be cured at 500°F (260°C)
• Maximum operating temperature 667°F (353°C)
• High load capacity
• Fair resistance to corrosion
• Moderate durability