An Investigation into the Nature of the Oxide Layer Formed on Kovar (Fe\u201329Ni\u201317Co) Wires Following Oxidation in Air at 700 and 800\xa0\xb0C

Paul M Yates,Julie A Yeomans,Christopher F Mallinson,Mark J Whiting,Phillip M Mallinson

doi:10.1007/s11085-017-9772-y

Paul M Yates, Julie A Yeomans + Show 3 more

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https://doi.org/10.1007/s11085-017-9772-y

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Abstract

This work provides new insight and evidence that challenges and extends the accepted view of the oxidation behaviour of Kovar (ASTM-15). Specimens of 2 mm diameter Kovar wire were oxidised in air at 700 or 800 °C for 10 min. The resulting oxide layers were analysed by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, scanning transmission electron microscopy and Raman spectroscopy. Oxide layers of approximately 2 and 4 µm thickness were formed at 700 and 800 °C, respectively. These were found to contain iron, cobalt and traces of nickel. The combination of analysis techniques revealed that the oxide contains Fe2O3 in addition to (Fe, Co, Ni)3O4, a spinel oxide, in contrast to the combinations of Fe3O4, Fe2O3 and FeO that are typically reported. The oxide layer was found to be complex, consisting of multiple layers with different compositions, which is overlooked in the existing literature.

Highlights

The alloy Kovar, ASMT-15 (Fe–29 wt% Ni–17 wt% Co), is widely used in glassto-metal sealing because of its electrical properties and in particular its thermal expansion, which closely matches frequently used borosilicate glasses [1]
The resulting oxide layers were analysed by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, scanning transmission electron microscopy and Raman spectroscopy
This study aims to explore a specific oxidation treatment relevant to a common glass-to-metal seal and to characterise the oxide layer produced

Summary

Introduction

The alloy Kovar, ASMT-15 (Fe–29 wt% Ni–17 wt% Co), is widely used in glassto-metal sealing because of its electrical properties and in particular its thermal expansion, which closely matches frequently used borosilicate glasses [1]. It has been common practice, for more than 60 years, to pre-oxidise Kovar prior to sealing. The composition of the oxide layer formed on Kovar is often assumed to be independent of oxidation temperature [11, 12], and a wide range of temperatures have been studied in the literature. There is, some evidence that the oxidation temperature affects the type of oxide formed [6,7,8]

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