Advantages and Applications of Solid Lubricants

Solid lubricants can be utilized as free-flowing powders, as additives in some oils and greases, and as key ingredients in high-performance anti-friction coatings and anti-seize pastes. These special lubricant additives and powders fill in and level surface asperity valleys and peaks thanks to their adherence to the substrate and coherence between them.

The solids deliver efficient boundary lubrication, improving friction and minimizing wear under extreme operating environments. Contrary to grease or oil fluid films for hydrodynamic lubrication, boundary films created by solid lubricants are capable of maintaining a uniform thickness irrespective of speed, temperature and load.

Types and Characteristics of Solid Lubricants

Solid lubricants are available in various compositions with different properties. This application note describes the common types of solid lubricants utilized in Molykote brand anti-friction coatings and anti-seize pastes.

Copper is a soft metal with high plasticity and low shear strength, providing it good lubricating capabilities.

Benefits of soft-metal solid lubricants include:

• High-temperature stability
• High load-carrying capacity

However, they cannot be utilized with stainless steel at temperatures more than 1,000 °C due to the chance for galvanic corrosion.

Graphite with a layered lattice structure and weak bonding between layers delivers superior lubricity as long as the presence of moisture.

Benefits of graphite solids include:

• Good lubrication in high humidity
• Protection against fretting corrosion
• High-temperature stability
• Low coefficient of friction under high loads

Polytetrafluoroethylene (PTFE) comprises carbon and fluorine atoms and is recognized as one of the most slippery manmade materials due to its low surface tension.

Advantages of PTFE solids include:

• Good sliding-friction reduction
• Good chemical resistance
• Low load-carrying capacity
• Low coefficient of friction at low loads
• Colorless film lubricity

Molybdenum disulfide with a lamellar structure can be sheared easily in the motion direction. It is possible to match particle size and film thickness to match surface roughness.

Benefits of molybdenum disulfide solid lubricants include

• Excellent adhesion
• Wide service-temperature range
• Protection against fretting corrosion
• Decreased friction with increasing loads
• Stick-slip prevention
• High load-carrying capacity

Molybdenum disulfide solid lubricants cannot be utilized in wet conditions as moisture increases friction.

White solids are different types of inorganic compounds and capable of forming a reactive lubricating layer for better wear protection.

Other advantages of white solids include

• Resistance to high temperatures
• Good protection against fretting corrosion

Product Selection by Ingredients

Normally, anti-friction coatings will comprise around 30% solids blended with a resin binder and solvent carrier, while anti-seize pastes will consist of 40% to 60% solids in a base-oil carrier. Oils and greases may contain up to 10% of solids for lubrication during shock-load, start-up and shutdown conditions.

Solid lubricants such as graphite and molybdenum disulfide normally demonstrate higher load-carrying capacity of up to 1,000 N/mm². They can play a role in preventing cold-welding and galling and are capable of providing lifetime lubrication under dirty and dusty conditions.

Although the load-carrying capacity of PTFE is on the lower side (up to250 N/mm²), it facilitates achieving a low coefficient of friction in sliding-load conditions. Metal pastes can be utilized as anti-seize compounds on threaded connections. Metal-free white pastes are ideal option for extreme heat applications and for preventing stress-corrosion cracking and solder embrittlement.

Performance Improvement with Different Combinations

It is possible to combine different types of solid lubricants in a coating or paste formulation to deliver certain synergistic results such as improved fretting-corrosion protection, optimal friction control and wear prevention, and increased load-carrying capacity. The advantages of using different combinations of solid lubricants are better than that of individual solid lubricants.

This information has been sourced, reviewed and adapted from materials provided by Dow Corning.

For more information on this source, please visit Dow Corning.