Influence of Coating Technologies on Frictional Properties of Foil Bearings During Start/Stop Conditions

The main objective of this paper is to study and compare the performance characteristics of foil bearings for a typical turbo-blower during start/stop conditions. The test bearings feature three-segment bump foils and a full smooth top foil with a nominal diameter of 70 mm and an aspect ratio of 1. All foils are made out of Inconel 718 and have a thickness of 0.2 mm (0.008”). Three coatings, applied to the top foil, have been evaluated during the test campaign: Teflon, Molybdenum-Titanium Nitride (MoTiN) and Molybdenum-Aluminum-Titanium Nitride (MoAlTiN). MoTiN and MoAlTiN were applied using physical vapor deposition (PVD) technolgy. Each bearing was instrumented with sixteen thermocouples located within the bearing sleeve 1 mm away from its inner diameter.  Thermocouples allow the measurement of the bearing temperature in the axial and circumferential directions.  Bearing displacement was measured using 8 proximity probes located at each side of the bearing (four per side). Overall vibrations of the bearing under test were measured via two accelerometers located on top of the bearing housing in the vertical and horizontal directions. In addition, a torque arm mechanism was used to measure the bearing shear force; hence deduce the friction coefficient, friction torque and total power loss. The paper discusses friction and wear results obtained after one hundred start-stop tests for each bearing. A tribological and microstructural analysis is also presented and discussed.