Scuffing Resistance and Starved Lubrication Behavior in Helicopter Gear Steels Coated with Nanocomposite Surface Coatings with and without a Hard Sublayer

Abstract

Helicopter drivetrain components depend on oil lubrication to maintain elastohydrodynamic lubrication between sliding surfaces and are susceptible to a loss of lubrication event. A loss of lubrication event starves the mechanical interfaces of oil, pushing them from elastohydrodynamic toward mixed and boundary lubrication regimes. In those regimes, scuffing initiation and thermal runaway occur, resulting in a loss of torque transmission. Lubricious nanocomposite coatings made of a molybdenum disulfide and graphitic carbon are studied in combination with a titanium carbon nitride (TiCN) hard sublayer to improve scuffing resistance. Two experimental methods using a rotating ball-on-disc tribometer were conducted to evaluate the scuffing performance of the coatings. The first method is a load capacity experiment that incrementally increases load until either a maximum stress of 2.38 GPa or scuffing occurs. The second method is a loss of lubrication experiment that maintains a constant speed and contact stress with a running-in period before the lubricant supply is turned off. The elapsed time between the initiated scuff and the oil supply removal is the key experimental result. Experimental results show the time to scuffing initiation after loss of lubrication supply is extended with the application of a nanocomposite coating on a hard sublayer above the application of a hard coating alone and the uncoated medium superfinish baseline. Additional experiments were performed to study the effect of surface roughness and nanocomposite coating composition, but conclusions were limited due to a weaker adhesion strength of the TiCN sublayer applied with a lower deposition temperature to maintain steel substrate hardness.