Accelerated lifetime test considering with optimal surface roughness for gas foil bearings

This study experimentally evaluates the lifetime of PTFE coating on Inconel-X750 specimens, a material commonly used in gas foil bearings (GFBs), based on surface roughness using a reciprocating test apparatus from a tribological perspective. The primary objective is to verify the enhancement of coating lifetime for GFBs, specifically focusing on the top foil coatings employing solid lubricants. The surface roughness of Inconel specimens was manipulated in five cases within the range of 0.1–1.0 µm using an electrochemical etching method. A coating of consistent thickness was applied using a Polytetrafluoroethylene (PTFE) solvent-based coating material. Lifetime evaluation was conducted by ASTM G133-05, with normal direction load as a variable to simulate accelerated stress conditions. To simulate the line contact environment of the rotor of the turbo machinery, test specimens were fabricated using a Roll-on-Flat method. A test roller was implemented to maintain the perpendicular angle between the roller and the coated specimens, allowing for automated alignment. The experimental verification demonstrated that the lifetime of the coating reached its maximum in the specific surface roughness range of the test specimen, with a R_a of 0.3 to 0.4 μm (Case: 3). In the SEM (Scanning Electron Microscope) image analysis of the cross-section of Inconel specimens coated with PTFE, it was verified that the region where PTFE particles adhere varies according to the surface roughness of the sample. This results in a tangible increase in the bonding force (mechanical interlocking) between the coating and the Inconel sample. The test results could be utilized to predict the lifetime of Inconel X-750 specimens with PTFE coating through accelerated life test (ALT) evaluation using the Weibull-linear function model. The ALT test results in this study were determined by correlating the actual rotor and bearing interactions under Hertzian-contact pressure conditions. Friction coefficient characteristics and sliding distances observed in a reciprocating test rig were converted into estimations representative of the actual bearing-rotor environment. In conclusion, it was predicted that the cyclic lifetime of GFB at the optimal surface roughness (Case 3) of the Inconel specimens would be approximately 1.4 × 10⁵, confirming an extension of the bearing lifetime by over 18 times compared to Case 1 without surface treatment.