Comparing 69Ga+ and C60+ for ToF-SIMS sputter depth profiling in a Cr2O3 formed during oxidation of a Ce coated FeCr steel substrate

Conference poster, 2012

Ferritic (FeCr) stainless steel is often used as interconnects in solid oxide fuel cells (SOFCs) at temperatures between 650-850°C. The chromia (Cr2O3) scale formed upon high temperature exposure has good electron conductivity, but can be further improved by using suitable coatings. Since the electron conductivity in the Cr2O3 scale is crucial for interconnects, the ability to accurately determine the distribution of elements of low concentration are highly important. Hence this study concerns Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiling in a 200 nm thin Cr2O3 scale formed on a Ce-coated ferritic stainless steel after exposure at 850°C during 1h. The aim is to investigate the effects of using different sputtering ions. Depth profiles were made using two different ToFSIMS instruments and the obtained sputter depth profiles were compared to a TEM-EDX map of the scale cross section. In the IONTOF TOF-SIMS V instrument C60 + at 10 keV and 700 pA was used for sputtering and Bi3 + at 25keV and 0.15 pA was used for analysis. In the PHI TRIFT II instrument a pulsed Liquid Metal Ion Gun (LMIG) with 69Ga+ at 15 keV and 600 pA was used. In both instruments depth profiles were obtained by sputtering a surface area of 50x50 μm2 and analyzing an area of 25x25 μm2. Furthermore, the effect of having oxygen present in the chamber to a pressure of 1x10-6 torr during analysis in the PHI TRIFT II instrument was investigated. The results showed that the oxide scale was composed of two equally thick layers, an outer Cr/Mn rich layer (100 nm) and an inner Cr rich layer (100 nm). The Ce from the surface coating was observed between those two layers. Additionally, a thin layer (~10nm) rich in Fe/Mn was observed at the outermost oxide surface and Ti was observed in the inner Cr rich layer. The depth profiles obtained by the two SIMS instruments showed similarities in the way of revealing the outer Cr/Mn rich layer and the inner layer richer in Cr as well as the outermost thin Fe/Mn rich layer. However, the Ce enrichment was more clearly revealed using the IONTOF instrument and the enrichments of Mn and Ti at the oxide/metal interface were more obvious in the PHI TRIFT II depth profile. The presence of oxygen enhanced the yield of Fe and Cr when entering the steel matrix. The sputter craters were investigated using a Zeiss ULTRA 55 Scanning Electron Microscope (SEM). A topographic surface results after sputtering with 69Ga+. The presence of oxygen in the chamber significantly decreased that topography although without enhancing the depth resolution in the profile. In the IONTOF C60+ sputter crater a relatively smooth topography was observed. However, a large amount of carbon was deposited, which is believed to deteriorate the depth resolution when analyzing thick layers. Nevertheless, it can be concluded that C60+/Bi3+ renders better depth resolution in thin layers compared to 69Ga+, while carbon deposition from C60+ sputtering makes 69Ga+ a more suitable alternative for thick layers.