Scanning AES and XPS analysis of a thin Au emissivity barrier on Ni alloy

Scanning Auger electron spectroscopy and x-ray photoelectron spectroscopy w ere used to analyze discolored gold plating on nickel alloys. Gold plated n ickel alloy sheets have been used as a thermal shield inside nacelle housin gs for various jet engines. The thin gold film is applied to serve as a low emissivity coating to reflect thermal radiation. Inconel 625 sheet was gol d plated and exposed to 590 degreesC in air for 924 hours to achieve an app ropriate service use reference point. The visual appearance of the gold thi n-film surface had noticeably dulled after this prolonged exposure. In some cases, several dark spots a few microns in size also appeared on the dulle d gold surface. Our hypothesis was that nickel or some alloy constituent ha d diffused through the gold film and changed the color of the gold surface. X-ray photoelectron spectroscopy and Auger electron spectroscopy were used to differentiate the composition of the gold plated Inconel samples prior to thermal exposure and after the prolonged exposure. Scanning Auger microg raphs showed that the composition of the dulled gold surface had changed du e to the diffusion of nickel from the substrate alloy through the gold thin film. Nickel was absent at the surface of the unexposed samples while sign ificant nickel concentrations were detected on the discolored gold surface and with the highest nickel levels detected in the dark spots on the gold s urfaces. Auger depth profiles made on the exposed gold film verified that a discrete gold layer remained on top of the Inconel with a broad Ni-Cr-Au z one beneath this gold layer. (C) 2001 Kluwer Academic Publishers.