Tribology of Surface Films on High Temperature Alloys for Metal Seals

Abstract Metal seals designed for severe environments and requiring very low leak rates often use ductile surface films. A ductile film provides a low shear strength material to localize friction forces and plastic strain to the surface layer. Conversely, hard coatings require that plastic deformation in the substrate beneath the coating or wear occurs in the contact material. 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 helping to minimize contact marks that can become leak paths during service, second, it could deform plastically in a manner to form a leak barrier against adjacent surface features in application assembly. With respect to the second benefit, elastic Hertzian contact stress theory causes the expectation that the resulting wear scar is 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 nickel (Ni) superalloy commonly used in metal seals for severe environments. Specifically, the films in this work are a physical vapor deposition gold (PVD Au), a mechanically applied tungsten sulfide (WS2), and a thicker film of electroplated Ni (EP Ni) applied to wrought IN625 Ni-alloy. 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 forces. The measured responses were coefficient of friction (CoF) and 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 of the films exhibited increasing z-depth of the contact ball with increasing normal force but the CoF responses were not the same for all surfaces. For the single-stroke tests, the thin films, PVD Au and WS2 had similar CoF values that were relatively stable with increasing normal load. The EP Ni had a distinctly higher z-depth and CoF that increased with increasing normal load in single-stroke tests. The effects of film deformation, substrate interaction, and wear particle creation are discussed as they affect the single-stroke test responses. Although uncoated IN625 consistently had the lowest CoF, it also had the lowest z-depth response, indicating the lowest capability for surface deformation that could act as a seal against typical surface features. EP Ni exhibited the highest CoF and z-depth, and it produced abrasive wear particles under all sliding loads. Among the surface films likely to best provide a ductile seal, the PVD Au and WS2 have the lowest and most robust responses for CoF and z-depth, indicative of a primarily elastic response to the sliding contact. The results indicate that PVD Au and WS2 are preferred over EP Ni as metal seal coatings. Also the PVD Au and WS2 have small but greater plastic deformation than uncoated IN625, therefore these two thin films are preferred over uncoated IN625 for better sealing performance.