Wear and friction of self-lubricating coatings applied to spur gears in fluid-free aerospace actuation gearboxes

Abstract

Aerospace actuation gearboxes operate in low-temperature environments where increased lubricant viscosity leads to significant no-load power losses. Replacing fluid lubricants with coatings applied to the gear teeth is one potential approach to improving gearbox efficiency. Here we develop an approach to determining average wear rates of coated gears using a power-recirculating test stand, profile measurements and a model of the tooth contact. Worn gears are inspected using scanning electron imagery, and energy dispersive X-ray and Raman spectroscopy to understand the wear mechanisms and failure modes. Average coefficients of friction are determined at 20°C and −40°C using a power-absorbing test stand and isolation of tooth friction losses by calculation. These methods are then demonstrated on a promising C/Cr composite coating.