Challenges in quantifying Pt concentrations in Pd alloys by using secondary ion mass spectrometry: Strong grain orientation effects

Abstract

Palladium–platinum alloys were analysed by dynamic secondary ion mass spectrometry (SIMS) to investigate grain orientation effects that gave differences of up to 400% in the Pt/Pd count rate ratios, even within the same grain upon small rotations of a Pd sample with 1 wt% Pt. The sample had been homogenized by annealing, and the homogeneity was confirmed by X-ray analysis in scanning electron microscopy (SEM). Grain orientations were determined by electron backscatter diffraction (EBSD). Crater depths were measured by white light interferometry (WLI). SEM images from the bottom of SIMS craters made in the same grain after small rotations around the sample surface normally showed different patterns of microfaceting for some rotation angles, probably exposing low-index crystallographic planes. A complete understanding of the observed grain orientation effect is still lacking. However, factors such as ion mass, sputter rate, ion channelling, ion focusing, preferential sputtering, surface height, crater microfaceting and/or angle-dependent sputtering seem to play a role. For these Pd–Pt alloys, the strong grain orientation effect adds another level of complexity when attempting to quantify concentrations and obtain depth profiles by SIMS. Without proper sampling and/or averaging, one could reach very wrong conclusions when comparing results from different samples.