Coating Development

Coatings have an important role to play in two specific areas of the LHR engine; insulation and tribological wear resistance. The surface coating on the combustion chamber may also have an important effect on emissions.

The use of thermal barrier coatings to protect hot metal engine components is not new. It has been an area of interest in marine applications for some time. The coatings that have been used are relatively thin and are used primarily for corrosion resistance. It is possible to do a substantial amount of insulation with plasma sprayed coatings provided they are applied thick enough and have good thermal insulation properties. Figure 2 shows the potential gains in heat loss reduction and fuel consumption. Presently the program is funding the development of an insulation with a conductance goal of .5 Btu/hr-sq.in-F. This can be achieved by a coating of plasma sprayed Zirconia approximately 0.1 inches thick. This thick coating needs a substantial amount of development to be successful.

The primary use of wear resistant coatings in the LHR engine is to maintain low levels of friction and wear and to supplement other lubricant methods in the high temperature areas of the cylinder. At present wear coatings as well as methods of high temperature solid, liquid and vapor lubrication are being investigated.

NASA research in high temperature self-lubricating films is being evaluated for use as a cylinder liner protective coating. Originally the coatings were developed for use on gas lubricated bearings. The solid films protect the bearing surfaces during starting and stopping transients. This is also directly applicable to a gas lubricated turbocharger bearing.

Earlier work done on dry friction and wear characterization with pin on disk tests showed that ion-implantation of certain elements into the ceramic liner and ring materials had the potential to reduce friction and wear. While not a coating in the true sense of the word, it is of interest and is worth mentioning. Figure 3 shows the results of the pin on disc tests of friction versus time. It can be seen that ion-implantation of the ceramic materials reduced friction after a brief run-in time. As a result of these tests, it was decided to pursue the results further. The program is presently funding testing of the concept on actual piston rings and liners.

Work in solid and vapor lubricants is being carried out in one of the programs major engine component development contracts. A high temperature liquid lubricant development contract is presently in the later stages of procurement.

Overall program funding for the project for FY 87 is $4.5M. It is expected to remain relatively constant at that level for the future. Figure 4 shows the overall program schedule predicated on that funding level. Plans are to continue with the work that has been identified in this paper. Coating development for both insulation and tribological needs is considered a vital element of the potential solutions to making the low heat rejection engine a reality.

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