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

Scanning Auger electron spectroscopy and x-ray photoelectron spectroscopy were used to analyze discolored gold plating on nickel alloys. Gold plated nickel alloy sheets have been used as a thermal shield inside nacelle housings 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 gold plated and exposed to 590°C in air for 924 hours to achieve an appropriate service use reference point. The visual appearance of the gold thin-film surface had noticeably dulled after this prolonged exposure. In some cases, several dark spots a few microns in size also appeared on the dulled gold surface. Our hypothesis was that nickel or some alloy constituent had 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 micrographs showed that the composition of the dulled gold surface had changed due 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 significant nickel concentrations were detected on the discolored gold surface and with the highest nickel levels detected in the dark spots on the gold surfaces. 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 zone beneath this gold layer.