Session: 18-06: Metallurgy, Coating, & Repair
Paper Number: 154126
Tribology of Surface Films on High Temperature Alloys for Metal Seals
For metal seals designed for severe environments and requiring relatively low leak rates, the use of ductile surface films is considered. Although fluid lubricants and greases are well established material for the improvement of wear resistance in high friction operations and for localized sealing, they are not applicable in under conditions of high temperature, fluid migration, or contamination. A ductile film provides a low shear strength material to localize friction forces to the surface material. Conversely, hard coatings require that plastic deformation occurs in the substrate beneath the coating or wearing the contact material sacrificially. The possible benefits of a ductile film on the metal seal are two-fold; first it could reduce friction and wear during installation of a static seal to reduce contact marks that provide leak paths, second, it could deform plastically in a manner to form a leak barrier against adjacent surface features. With respect to the second benefit, elastic Hertzian contact stress theory causes us to expect the wear scar to be only slightly larger than the contact radius, which would serve the purpose of acting as a sealant. This work evaluates the effect of several films applied to IN625, a Ni superalloy commonly used in metal seals for severe environments. Specifically, we evaluated a low shear strength PVD gold, a lamellar solid WS2 which has low interaction energy between lamellar layers, and a thicker film of electroplated Ni. Uncoated IN625 was also included as a baseline material. The surfaces were evaluated by single stroke and reciprocating wear testing using a C276 Ni superalloy contact ball under various normal loads. The measured responses were coefficient of friction (COF) over time and mean z-depth of the contact ball as a measure of wear. In addition, the surface texture was examined using confocal imaging. Consistent with behavior expected for a ductile film on a rigid surface contacted by a spherical rigid ball, all the films exhibited z-direction wear, either at a sufficiently high normal load or after a sufficient number of reciprocating cycles. All the surfaces exhibited a break-in period where z-depth increased rapidly followed by a slower wear rate period. The WS2 film had lowest COF indicating best ability to mitigate friction forces and wear but PVD gold exhibited the greatest deformation likely needed for improved sealing. In comparison the Ni-based surfaces had intermediate COF and low z-depth, consistent with films with higher hardness and strength. The WS2 lamellar solid film had the lowest COF and high z-depth penetration, considered to be a good combination for sealant behavior, although the surface deformation appears to be primarily z-direction compaction into the substrate texture and not lateral deformation.
Presenting Author: Elaine Motyka Technetics Group
Presenting Author Biography: Elaine Motyka is a Principal Material Engineer at Technetics Group. She is part of the R&D team, working on structure-property relationships to new product development and applications development. Elaine has expertise in coatings, tribology, powder metallurgy, sintering & heat treating, corrosion, tribology, and materials testing. She received her B.S. in Mechanical Engineering from Worcester Polytechnic Institute and her M.S. in Materials Engineering from Rensselaer Polytechnic Institute. She has been with Technetics Group for 15 years supporting customers in industries such as aerospace, power generation, semi-conductor, oil & gas, and most recently hydrogen production.
Authors:
Elaine Motyka Technetics GroupErin Volpe Technetics Group
Stefan Roeseler Technetics Group
Ryan Plessinger Technetics Group
Tyler Noyes Technetics Group
Becca Jones Technetics Group
Shannon Depratter Technetics Group
Jonathan Kweder Technetics Group
Tribology of Surface Films on High Temperature Alloys for Metal Seals
Paper Type
Technical Paper Publication