What should lubrication engineers understand about solid lubricants before recommending them for an application?

TLT Sounding Board May 2014


Solid lubricants are useful for conditions when conventional lubricants and additives are inadequate for the application. Typical materials include graphite, molybdenum, boron nitrate and PTFE. Solid lubricants frequently work well as grease additives. Application methods include spraying, dipping and brushing; dry powder tumbling; and bonding. Yet, as with all forms of lubrication, solid lubricants require an expert to know when, where and how. TLT readers responding to this month’s survey say that before recommending these materials, a lubrication engineer needs to understand the lubrication and delivery systems, effects of temperature (including the loss of cooling), purity, particle size and foam characteristics. Many respondents said it’s important to determine if solid lubricants are the best answer and whether an alternate form of lubricant might be a better solution. “Can they really do the work they are specified for? Is there any previous record or history of performance?” one reader asked.

The lubricating system and how to apply. The type of solids use in the lubricant.

Composition, content, properties, viscosity of oil, basis, baking.

Temperature application.

Particle size distribution, particle sizes, purity.

Must overcome a problem that’s not easily solved by another technology.

Availability, coverage, delivery method, temperature, speed, required intervals.

Foam characteristics.

Application techniques and their effect on durability of the lubricating film, lubricant compositions and compatibility with operating environments.

Delivery methods, performance criteria.

PV rating, friction coefficient, wear coefficient, temperature capability, contamination capability.

Temperature, revolutions per minute and operating conditions.

What are the effects of the loss of cooling? How will particles interfere with the lube and tribosystem? Are there filters, etc.?

When compared to fluid lubricants, solid lubricants have less ability to carry away heat and contaminants.

Dust collection and how to keep workers from inhaling them.

Lube life expectations and control of heat generation.

Extreme cleanliness needed for van der Waals bonding.

Temperature limitations of solid lubricants.

Temperature—high and low NLGI number. Expected maintenance interval versus failure mechanism of the lubricant. 

Geometry of parts, temperature development and normal load.

Feasibility and practicality, as well as observable results.

How to calculate base oil viscosity. Need to understand that you cannot replace an oil with a grease with the same base oil viscosity.

They must understand chemistry and long-term effects on machinery screen additives.

Do you prefer to run standard tests (such as ASTM) or in-house developed, application-specific tribology tests?
In-house developed, application-specific tribology tests 49%
ASTM Standard 54%
Other 9%
Based on responses sent to 13,000 TLT readers. Total exceeds 100% because some respondents chose more than one answer.

Please describe an application where a solid lubricant performed well and replaced a previously used liquid lubricant.
According to the Frenkel-Kontorova model, there exists a lubricant which could help achieve incommensurability as the ratio of the lattice constants at the contact meet a critical value and, therefore, realize superlubricity. This lubricant is solid material. Keeping that in mind, it’s reasonable to believe that more solid lubricants would replace liquid lubricants which currently are used.

Lubrication of bearings on brick carts that go through the furnace.

Kiln Tyre and roller lubrication.

Lubrication of difficult-to-reach moving surfaces and bearings.

High-temperature vacuum industrial application where grease can’t fail.

Boundary/mixed film conditions. Shock loads.

Antifretting material in a motor bearing bore.

Fillers in liquid lubricants to increase load-carrying capacity, e.g., in cold pilgering for pipe forming.

Have only seen it used in one application on large (24-inch by 14-foot) Kiln bull gears. The application did not work as well as the previous synthetic product, therefore it was replaced. Most likely cause for failure was a misunderstanding of lubricant intervals and proper delivery method.

Pollution in application.

To replace grease where dust and dirt can be a problem.

In my experience, solid lubricants work well as additives in grease, offering some load-carrying, antiwear advantages in certain applications.

High-heat applications. Steel ladles—the carrier burns off at 800 F, leaving solid a lubricant film that operates at temperatures up to 1900 F.

Tyre kiln slipper pads.

Slow-speed oscillatory application.

Fretting or start/stop motion.

Changed oiled bronze bushings on low-pressure pump to high-pressure grease system.

The use of solid lubricant in the wheel bearings of carts used to cure insulation in an oven.

Cold forging of stainless steels. Quite tedious, but the process still works.

Pins and bushings where liquids and grease migrated away from the wear zone.

Rod mill bearings which were previously lubricated by oil mist.

In your organization, the use of lubricants made from renewable resources is:
Becoming more common as the best solution for the application, regardless of cost 24%
Becoming more common due to management directive 27%
Not being used due to performance issues 30%
Not considered because we have no data on performance 24%
Not being used more often due to cost considerations 9%
Based on responses sent to 13,000 TLT readers. Total exceeds 100% because some respondents chose more than one answer.
Editor’s Note: Sounding Board is based on an e-mail survey of 13,000 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.