A study of initial oxidation of Fe₄₀Ni₄₀P₁₄B₆ met-glass using Auger electron spectroscopy

Abstract

The initial oxidation of Fe₄₀Ni₄₀P₁₄B₆ met-glass has been investigated by using a combination of Auger electron spectroscopy (AES) and Ar⁺ ion sputtering. The behaviour of oxygen uptake, sticking coefficient and the substrate composition as a function of oxygen exposure and its comparison with depth profiles of native and in-situ oxides suggests that within the experimental conditions of this work, the interaction of oxygen with met-glass involves a combination of reconstructive chemisorption and diffusion. The oxygen uptake could be distinctly classified into two different stages, fast below 15 L and slow above 15 L up to 500 L. The results are most easily interpreted by assuming the presence of defects in the surface region. The diffusion through defects being faster than chemisorption allows the latter to control the kinetics of total uptake in the first stage. During the second stage, diffusion becomes much slower than the chemisorption and takes over the kinetics. The interaction of oxygen with the surface induced marked segregation of iron for as-received (sputter cleaned) met-glass. For annealed samples boron was also found to segregate along with iron. Chemical shifts were observed in high energy peaks of all the substrate elements as is expected for reconstructive chemisorption. The mechanism leading to initial Fe₄₀Ni₄₀P₁₄B₆ oxide formation appears to be diffusion of Fe (and also B for annealed samples) through the rapidly oxidizing P-segregated surface and its subsequent oxidation at the surface. This process makes the superficial oxide Fe-rich (and also B-rich in the case of annealed samples) with the P-rich phase underneath. The effect of annealing the samples in vacuum prior to oxygen exposure at room temperature has also been studied. Annealing for 5 h at 580 K caused a decrease in oxygen uptake whereas the same treatment at 780 K resulted in an increase. These results are qualitatively consistent with the hierarchy of structural changes in Fe₄₀Ni₄₀P₁₄B₆ alloy with heat treatment at different temperatures.