Surface analysis of iron and steel nanopowder

Journal article, 2018

High sinter density is desired in powder metallurgy components as the requirement
for performance is increasing day‐by‐day. One of the promising ways to achieve
improved densification during sintering is through the addition of nanopowder to
the conventional micrometer sized metal powder. It is well known that the surface
chemistry of the powder has a decisive effect on sintering and consequently the properties
of the components produced. Extensive research has hence been conducted to
elucidate the surface chemistry and its influence on sintering for powder used in conventional
powder metallurgy. Nanopowder, owing to high surface to volume ratio, can
contribute to the activation of sintering at lower temperatures and enhance the sinter
density. In this context, the surface chemistry of the nanopowder is also expected to
exhibit substantial influence on sintering. The present investigation is aimed at establishing
a methodology to study the surface chemistry and oxide thickness of
nanopowder. For this purpose, iron nanopowder of 3 different size fractions: 35 to
45, 40 to 60, and 60 to 80 nm with core‐shell structure were studied. Different
approaches were adopted to evaluate the shell thickness of the iron nanoparticles.
The methodology was developed and tried on low alloy steel nanopowder to measure
oxide thickness. X‐ray photoelectron spectroscopy, thermogravimetry, and high‐resolution
scanning electron microscopy techniques were used to study the nanopowder.
Results from different core‐shell models for iron nanopowder were found to be consistent
except in the case where depth profiling was taken into account. The results
were in agreement with the values obtained from thermogavimetry‐surface area
correlation.