To meet the increased thermal stresses of future advanced aircraft engines, new lubricants will have to be developed to replace the currently used ester-based liquid lubricants. If a suitable conventional replacement cannot be found, a different lubrication method will have to be used. The conventional method circulates bulk lubricant (stored in a sump) through a lubricating system containing cooling and filtering elements. Solid lubricants have been studied as a replacement for bulk liquid lubricants, and have been found to provide reasonable lubrication for lightly loaded systems. Solid lubricants, however, have proved inadequate for highly loaded, high-speed applications. Vapor/mist phase lubrication (VMPL), on the other hand, may be a viable alternative.
VMPL has been used successfully to lubricate high-temperature bearings or gears. It can be used as an emergency backup system or as the primary source of lubrication. With VMPL, minimal weight is added to the system and minimal debris is formed. It works over a wide temperature range. The concept is illustrated in the following figure.
Vapor/mist phase lubrication method.
The basis for the VMPL method stems from the idea that a "suitable" organic vapor or fine mist can react inside high-temperature sliding surfaces to form a lubricious film. The primary compounds used for VMPL studies have been organophosphates, which work well with only certain ferrous material. Their success is due to an initial, rapid formation of a predominant iron phosphate film, which is followed by the formation and growth (by cationic diffusion) of a lubricious pyrophosphate-type coating over the iron phosphate. As long as iron is present at a wearing surface, the VMPL method using organophosphates works well. Evidence, however, has been reported that continued organophosphate interaction with ferrous bearing material can lead to depletion of surface iron and to eventual lubrication failure. If VMPL is to work for prolonged periods of time, the depletion of surface iron must be circumvented.
Work at the NASA Lewis Research Center has resulted in a new organophosphate formulation that prevents surface iron depletion. This new formulation incorporates a soluble iron additive that reacts with the organophosphate to generate an iron phosphate film. This formulation was used to successfully vapor/mist phase lubricate a spur gearbox in a preliminary study. In a related study, this formulation was used to lubricate a silicon nitride pin-on-plate couple that used a reciprocating friction and wear apparatus at 500 °C.
Lewis contact: Dr. Wilfredo Morales, (216) 433-6052,
Wilfredo.Morales@grc.nasa.gov
Authors: Dr. Wilfredo Morales and Dr. Robert F. Handschuh
Headquarters program office: OAT
Programs/Projects: P&PM
Special recognition: A special achievement award was given for this work, and a patent was issued.
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