Assessing the wear and friction properties of self-lubricating coatings in dry-running aerospace actuation gearboxes

Abstract

Actuation gearboxes used in high-lift systems on commercial airliners suffer from high zero load losses due to increased fluid viscosity in the low-temperature flight environment. These losses result in an increase in system power consumption and weight, whilst leakage of lubricating fluids reduces reliability. The replacement of fluid lubricants with modern self-lubricating thin films deposited onto the gear teeth provides a potential approach to the reduction in zero-load losses and improvements in reliability. However very little is known of the wear and friction performance of self-lubricating coatings on dry-running gears, and methods to assess their suitability are not yet established. In this research a power-recirculating test rig is developed to test gears treated with several promising PVD coatings. A wear model is constructed in which the contact between gear teeth is treated as a pair of loaded cylinders of constantly changing diameter and surface velocity in mixed sliding and rolling. Measurements of the pre-test and post-test profiles are used to generate wear maps, which are then compared to model outputs in order derive the wear rate. Detailed inspections and analyses of the worn tooth surfaces are then performed to determine the causes of coating failure. Further tests are performed on a power-absorbing test rig at low temperature to determine the overall power loss, which is then analysed to determine the losses attributable to tooth friction and a coefficient of friction is calculated. The most successful coating in the wear tests is found to be a sputtered coating consisting of Cr-doped a-C. In efficiency testing the coating’s friction coefficient is found to be higher than that obtained with uncoated gears and common fluid lubricants. However, the reduction in low temperature zero-load losses is found to result in an overall reduction in power loss. Whilst the anticipated efficiency improvement is realised, the coating lacks sufficient durability for the target service life due to high wear on the mid-addenda of the teeth and delamination at the start of active profile. The contact model is then used to propose modifications to the gear design that would improve coating life.

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Identifiers

ORCID for Thomas Morris Hilton Finch: orcid.org/0009-0002-2393-1927

ORCID for Tomas Polcar: orcid.org/0000-0002-0863-6287

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