Gear Coatings for Loss of Lubrication Application

The purpose of this project is to demonstrate an innovative nanocomposite coating that provides a substantial improvement in gear lifetime for rotorcraft transmissions following loss-of-lubrication. The high temperature, self-lubricating coating is composed of a nanocomposite material that provides lubricity and resistance against scuffing and brittle fracture over a wide temperature range (> 650C). The coating should allow helicopter transmissions to run for at least 45 minutes after loss of lubrication, significantly improving rotorcraft survivability. The solid lubricant coating is composed of a graded nanocomposite material. The coating is a chameleon coating in that the lubricious properties only kick-in when the coating is at high temperature (> 150C), such as during loss-of-lubrication. During normal operation the coating helps protect the gear surface, but the lubrication is provided by the transmission oil. The coating provides a significant performance improvement compared to conventional DLC and carbide coatings due to its superior resistance to degradation and lubricious properties at elevated temperatures and its ability to automatically provide lubrication as needed. This advanced coating technology was demonstrated in Phase-I to provide excellent wear performance under non-lubricating conditions with low friction and less than 1% wear compared to uncoated and superfinished AISI 9310 gear steels at 650C in air. The coating also provides excellent loading capability without coating failure, compatibility with helicopter transmission oil, and durability with negligible wear and a low coefficient-of-friction of 0.07 when lubricated. The Phase-II development will primarily focus on coating optimization and uniformity on gears for optimized high temperature wear characteristics. The coating capabilities will be validated first in spur gear contact fatigue tests and then ultimately in full-scale rotorcraft gearbox tests under oil-out conditions.