Executive Summary
Solid lubricants provide lubrication in environments where liquid or oil-based lubricants are not practical, such as in high temperature or extreme pressure (EP) environments. These lubricants come with their own set of challenges, including cost, compatibility and reliability. Choosing the correct solid lubricant for a specific application also is critical. As most readers pointed out, the tribological response under different conditions is the greatest problem encountered when using solid lubricants.
Q.1 Please describe the most challenging problem(s) when considering a specific solid lubricant for a given application.

Cost effectiveness of solid lubricants/solid lubricant additives as compared to corresponding fluid/soluble ones.

Compatibility, load bearing capacity, frictional behavior, longevity and maintenance.

Molybdenum disulfide is the solid lubricant used in mobile equipment, and it is essential for machines with shock loads, like the arm in excavators, or the boom frame on loaders of all types. When using automatic lubricators, the grease cannot have more than 5% of this solid lubricant. The tendency to form a coat that blocks the flow of grease cannot be underestimated.

Are moly-based lubricants suitable for water-based applications, and up to what extent? Which solid lubricant is the most suitable for high-speed applications? Does any solid lubricant play a role in torque control application? What amount of solid lubricant is suitable for particular application?

The method of determining and maintaining the level of moisture in the solid lubricant.

The reliability and cost.

When a machine experiences a stress under a task at high temperature and pressure.

Film forming, compatibility with biobased oxidative unstable oils. Choice of viscosity modifier.

Compatibility with materials of construction.

Under extreme operating conditions of the dry plain bearings with reciprocation rotary motion when unit load approach up to 40-60 MPa and sliding velocity is very low, the solid lubricants do not work well, because they have very large plastic deformation and low resistance to stick-slip phenomena.

One of the most challenging problems is to get a good homogeneous grease when mixed with moly disulfide. Heat and mixing are often used to solve the problem.

Maintaining full surface coverage and replenishing any solid lubricant that has worn away. Availability of quality but cost-effective and reliable sources.

What is the greatest problem for currently used solid lubricants?
Thermal stability 29%
Tribological response under different conditions 71%
Cost 54%
Delamination from the substrate 32%
Controlling their reproducible growth 14%
Based on an informal poll sent to 15,000 TLT readers. Total exceeds 100% because respondents were allowed to choose more than one answer.

The only solid lubricants that I have ever used were applied by hand, such as putting wax on a piece of metal that was being machined.

I think shape, particle size, particle size distribution, compatibility with sliding materials, adjustment of dispersion and agglomeration and harmfulness.

Bad drying of solid lubricant, incompatibility resulting in softening or hardening, contamination leading to excessive wear, not using an optimized amount of lubricant, a bad film bonded lubricant and bad adhesion.

Low temperature properties.

Dispersing the solid lubricant inside the grease is very important. High pressure homogenizer should be used in order to achieve good dispersion of the solid lubricant.

Considering solid lubricants used in greases, typically intended to support lubrication in slow moving and/or oscillating applications, there are few usable screening methods to find out performance differences between different types and treat rates. It would be beneficial for the industry to have a test method that can evaluate performance and efficiency in the relevant conditions. Currently most methods are accelerated, to give quicker results, by use of high speed and/or high temperature and/or high loads. These acceleration methods often risk giving results not valid for the application challenges. In short, there are no good test methods to predict or investigate what solid lubricant to be used and how much of it.

Handling in automatic lubrication systems like single-, multipoint and centralized systems as part of a lubricant containing solid.

Coating wear resistance in gear contacts.

The presence of impurities in the solid lubricant.

Solid lubricant for critical condition.

How to master the durability of the solid lubricated interface? The durability of this interface is different from the wear of the solid lubricant coating: the wear of the coating is not always related to loss of frictional performances! With some materials, you may get high friction while the coating is not yet worn out, while with others you still have low friction despite the complete wear of the coating.

Impacts of the solid lubricant on the tribosystem and the lubricant. Solid lubricants can play a critical role in certain applications, but one must consider the whole picture before making choices.

Lubricating solids to be used when boundary and mixed film lubricating conditions prevail (mainly boundary lubrication). Lubricating solids are most effective under sliding friction conditions, although rolling friction can benefit as well in case of boundary lubrication. Lubricating solids do prevent metal-to-metal contact under boundary lubricating conditions and extreme and/or shock loads. Lubricating solids do not prevent impact damage under heavy shock load conditions—(extreme) high viscosity does.

Consistency.

Q.2 Please describe the most important design principles when developing solid lubricants for a given application.

Dispersion in oil medium and effectiveness over life cycle of the lubricant.

Low frictional wear, high temperature stability, uniform distribution and adhesion, application method and ease of use.

When used as an additive, the concentration on the base lubricant needs strict control.

High temperature and/or extreme pressure boundary lubrication operating conditions.

Providing limited moisture content inside solid lubricant, limited pollution level, low and stable coefficient of friction relative to load, sliding speed, temperature pollution level, high resistance to wear and chemical inertness in contact with environmental atmosphere.

Handling.

Antiwear and corrosion inhibition.

Cost of development and ability of the lubricant to solve problems.

To increase polymeric-based self-lubricating composites, their resistance to plastic deformation and eliminate stick-slip phenomena, we have applied reinforcing the composite mixture by glass or aramid fibers of various diameter, shape and length.

When developing dry lubricant films on aluminum surface involving metal-forming operation, an important approach is dissolving the solid lubes (e.g., boric acid) into alcohol solution, then spraying or brushing the solution on the metal surface. After the solvent is evaporated, dry film is developed.

Nominal contact pressures, speed range (including start-stop transients) and temperature of operation—especially in oxidizing atmospheres. How easily can the film be replaced if needed? Low shear strength, but at the same time, good adhesion to the substrate. Also, ensuring the appropriate surface texture of both mating surfaces.

Their ability to function as co-thickeners in various lubricating greases. For extremely high temperature applications, hexagonal boron nitride (hBN) is the thickener of choice for perfluoropolyether greases. Since polytetrafluoroethylene (PTFE) melts at approximately 326 C, it’s not viable for >300 C requirements. Unlike MoS2 and graphite, hBN is white, which is a desirable characteristic for food plant applications.

Temperature, adherence, process temperature and surface roughness.

Approved performances.

Particle size matters.

Of importance is particle size, structure of the solid lubricant, lubricity and chemical stability.

How harsh is the application to be forced to work with solids, which benefits can be generated when working with lubricants with solid particles compared to standard lubricants.

Environmental impact.

A coating needs to be well attached to the surface.

Speed, four-ball wear and four-ball weld.

Composite coating.

First, establish the operational conditions of the solid lubricant, including the type of motion and the surfaces that can be coated. Then, identify the type of interactions and degradations occurring in the system to lubricate (e.g., adhesion, abrasion, etc.). If possible, reproduce the type of degradations observed on the real system in a tribometer (which ideally would reproduce the operating conditions, especially similar type of motion). Based on these inputs, select a few candidates as solid lubricants and compare them on the tribometer.

Cost, cost, cost.

Understand the surface roughness and which solids would be most effective, purity of the solids, particle size and particle size distribution (average size + maximum size + size distribution).

Viscosity suitable thickener soap and selective EP additives.

Editor’s Note: Sounding Board is based on an informal poll sent to 15,000 TLT readers. Views expressed are those of the respondents and do not reflect the opinions of the Society of Tribologists and Lubrication Engineers. STLE does not vouch for the technical accuracy of opinions expressed in Sounding Board, nor does inclusion of a comment represent an endorsement of the technology by STLE.