Tribology Laboratory

people
research
facilities
publications


Courses

syllabuses
office hours
homework
Bulletin Board


Contact

Office
e-mail
mailing address
driving directions


Glossary

Links


The demands and challenges of highly efficient low heat rejection engines and other high temperature applications is motivating both scientist and engineers to find lubrication alternatives to fluid lubricants (which will not survive these application environments). Gas Phase Lubrication is a technology which is emerging as a reasonable candidate for high temperature applications which require continuous lubrication over extended operational times.

Gas Phase Lubrication

W. Gregory Sawyer 
Mechanical and Aerospace Engineering

University of Florida
Gainesville, Fl., 32611
 

Gas Phase lubrication is a technique that uses chemical reactions to build a solid lubricating film even as the existing film is being worn away. When this chemical reaction is activated by the gas environment we say it is a Gas Phase Lubrication process.

Introduction

The thermal requirements in many future high temperature applications will require the use of a lubricant that can endure high sliding speeds, high contact stresses, and extreme operational temperatures (temperatures above 800°C are often discussed). Existing solid lubricants are capable of enduring these ranges of temperatures, loads, and speeds. However, the wear (removal of material) for these solid lubricating films limits the life of these films. Gas phase lubrication is a process that can increase the lifetime of the lubricating film by replenishing the solid lubricant on the worn surfaces.

Formation of Lubricous Films

There are a number of different ways in which a gas can activate the formation of a solid film on a surface. Many solid films are formed by a reaction between the gas and the surface material, a process which consumes part of the surface material in the formation of the lubricous film. The reaction that occurs between tricresyl phosphate (TCP) and iron containing surfaces (such as steel) forms a lubricous polymeric film which is capable of providing low friction coefficients at elevated temperatures (600°C). The reaction between boric oxide and water vapor in the atmosphere will result in boric acid. Boric acid is a lubricous lamellar solid at room temperature. Polymers containing boric oxide fillers have shown excellent wear resistance properties and low friction coefficients as a result of this surface reaction with humid air. This process has an upper limit on temperature or ~185°C (at this temperature the boric acid will collapse back into boric oxide). Hydrogen Sulfide gas is capable of converting molybdenum and tungsten containing surfaces into molybdenum disulfide and tungsten disulfide respectively. Both molybdenum disulfide and tungsten disulfide are excellent lamellar solid lubricants, friction coefficients less than 0.1 are common.

Solid films can also be formed by a reaction process which occurs on the surface rather than with the surface. This involves a gas which decomposes and deposits a lubricating film on the surface. This is the process which occurs in the carbonaceous gas phase lubrication scheme. Carbonaceous gases such as acetylene, ethylene, or ethane decompose and form graphitic solid carbon which attaches to the surface. This decomposition process and the rate at which graphitic carbon is deposited has a strong dependence on the temperature of the environment. In general, the higher the temperature of the surroundings the faster the decomposition process will occur and the faster the graphitic film can develop. This process can provide both low friction (µ < 0.1) and good surface protection.

Tribopolymerization is a gas phase lubrication process in which various monomers are vaporized and transported to the contact with an inert gas such as nitrogen. Once near the contact, these monomers polymerize. This polymerization creates a complex mix of wear debris and polymers at the sliding contacts. This mix of polymers and wear debris is responsible for a reduction in friction coefficient and wear.

Gases can also be used to deliver fluid lubricants to a surface. An example of this is vapor phase condensation lubrication. Fluid lubricants are vaporized and carried to a cooler working surface using various carrier gases. Once at the surface these vapors condense into a fluid film which functions in a conventional elasto-hydrodynamic lubrication fashion.

Removal of Lubricating Films

Surfaces in sliding contact experience wear. Solid films deposited or grown on a surface for the purposes of lubrication experience wear just like any other surface. The Archard wear equation is a relatively simple, but elegant description of the factors which influence the rates of wear. The equation states that rate of material lost is directly proportional to the sliding speed between the surfaces, the load carried across the surfaces, and the softness of the film (inversely proportional to the hardness).

Formation of Lubricating Films

Solid lubrication is a viable alternative to fluid lubrication, and it is the lubrication scheme most likely to be successful at temperature extremes (high or low). Solid lubricants typically work because they are thin, soft, low shear strength films that are trapped in the sliding interface between two bodies in relative motion. Solid lubricants typically have short working lifetimes due to their high wear rates. This is a serious design limitation -- future engine designs are pushing for millions of operational miles. It is unlikely that any single film could

endure this. A traditional solution would be to service the application and reapply the lubricating film at specific service intervals. This is often infeasible and too costly of a proposition. An alternate solution is to pump or push new solid lubricants into the contact (possibly in a powder form). This transportation of new solid lubricant to the contact is rarely simple and never trivial.

This re-supply problem is elegantly addressed using gas phase lubrication. The gases deliver the necessary ingredients to form these solid lubricants during operation. In direct contrast to the difficulties of delivering solid lubricants to a contact, gases can be delivered to a contact with relative ease.

Capacity of the Lubrication Process

The capacity of a gas phase lubrication process to provide adequate lubrication to a contact requires the presence of a well attached and lubricous film in the contact. Adequate lubrication and protection of the components depends on the existence of this film. Therefore, the competition between the formation of lubricating films and the removal of these films must be won by the deposition process (film formation rates must exceed film removal rates). A major difficulty in gas phase lubrication is assuring that this is the case.

Although the delivery of the gases to the surfaces is relatively straight forward the chemistry that is responsible for the formation of the lubricant is not. The application of gas phase lubrication systems will require answers to many difficult questions such as:

·How much lubricant gas should be supplied to the contact?

·How much of the lubricant gas is being converted into a solid film?

·How much of the lubricating solid film is remaining in the contact?

Analytical solutions for determining the state and condition of the film in the contact are currently unavailable. Current efforts are relying on careful laboratory experimentation and good scientific judgment. A model based on competitive rates of film deposition and removal has shown some success at predicting the boundaries that divide the adequate and inadequate lubrication regimes. However, these models do not provide information on the condition of the surface film as a function of time.

Competitive Rates Modeling of Gas Phase Lubrication

As previously described, the deposition rates of films formed from a gas reaction process involve many factors. The competition between the deposition and removal processes and the coupling between the state of the existing film (film thickness or fractional coverage) and the accumulation rates of the film add significant complexity. It is unlikely that any models which treat the deposition and removal processes separately will be successful.

A simple model based on experimentally observed transitions from adequate to inadequate lubrication has been developed. This equation says -- The rate at which lubricant is accumulating is equal to the rate at which it is being deposited minus the rate at which it is being removed. At a transition from adequate to inadequate lubrication the accumulation rate of lubricant is negative (the film is being depleted). At a transition the film is of insufficient coverage and incapable of providing a significant friction reduction. In the mathematical model further simplifications occur at the transition by setting the accumulation term to zero and equating the deposition and removal rates. Experimentally causing a series of transitions from adequate to inadequate lubrication and then fitting the transition data to the mathematical models results in a mathematical expression that can be used to predict regions of adequate lubrication.

Summary

Gas phase lubrication is a process that uses chemical reactions to deposit or generate a well attached and protective surface film. These films can be formed in a number of different ways -- chemical reactions with the surface, decomposition reactions on the surface, and polymerization in the contact. These films can be modeled and treated as solid lubricant coatings subject to wear and material removal. The possibility of continuous operation is a result of continuing film formation -- a result of the gas and surface chemistry.

References

Vapor Phase Lubrication


E.E. Graham and E.E. Klaus, "Lubrication from the Vapor Phase at High Temperature.", ASLE Transactions, 29 (1986), pp 229-234

B.F. Hanyaloglu, E.E. Graham, T. Oreskovic and C.G. Hajj, "Vapor Phase Lubrication of High Temperature Alloys." Lubrication Engineering, 51 (1995)503

A. Rao, "Vapor-Phase Lubrication: Application-Oriented Development", Lubrication Engineering, 52 (1996) 856-862

Carbonaceous Gas Phase Lubrication

J.L. Lauer, T.B. Blanchet, B.L. Vlcek and B.L. Sargent, "Lubrication of Si3N4 and Steel Rolling and Sliding Contacts by Deposits of Pyrolyzed Carbonaceous Gases." Surface and Coatings Technology, 62 (1993) 399

N.J. Barnick, T. A. Blanchet, W. G. Sawyer, James E. Gardner "High Temperature Lubrication of Various Ceramics and Metal Alloys via Directed Hydrocarbon Feed Gases", Wear 214, (1998) 131-138.

W. G. Sawyer, T. A. Blanchet and S. J. Calabrese, "Lubrication of Silicon Nitride in a Simulated Turbine Exhaust Gas Environment", Tribology Transactions, April (1997).

Tribopolymerization

J. C. Smith, M. J. Furey and C. Kajdas, "An Exploratory Study of Vapor-Phase Lubrication of Ceramics by Monomers", Wear, 181-183 (1995) 581-593.

Hydrogen Sulfide

I. L. Singer, T. Le Mogne, C. Donnet, and J. M. Martin, "Friction Behavior and Wear Analysis of SiC Sliding Against Mo in SO2, O2, and H2S at Gas Pressures Between 4 and 40 Pa", Tribology Transactions, vol. 39 (1996) 950-956.

Vapor Condensation Lubrication

R. W. Bruce, T.J. Kasun, and L. D. Wedeven "Lubrication of High Rolling Speed Ceramic Contacts with Two Percent Slip at 815°C (1500°F)" Tribology Transactions, vol 40 (1997) 589-596.

Boric Oxide Composites

B. R. Burroughs, J. H. Kim, T. A. Blanchet, "Boric Acid Self Lubrication of Boric Oxide Filled Polymer Composites" submitted to Tribology Transactions

Competitive Rates Modeling

T.A. Blanchet, J.L. Lauer, Y.F. Liew, S.J. Rhee and W.G. Sawyer, "Solid Lubrication by Decomposition of Carbon Monoxide and Other Gases." Surface and Coatings Technology. 68/69 (1994) 446.