Surface analysis of iron and steel nanopowder

Abstract

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.

Effect of Nanopowder Addition on the Sintering of Water-Atomized Iron Powder

“…A surface analysis of both the nanopowder and the reduction of surface oxide has been discussed elsewhere. [9] The powder blends and the lubricant were compacted at compaction pressures of 400, 600 and 800 MPa. Figure 2 shows the compressibility curves for the compacts of all the powder blends.…”

“…As shown in the fractographs (Figure 8), nanopowder was sintered in the temperature range between 500°C and 700°C. As the sintering was conducted in a pure hydrogen atmosphere, which reduces iron oxide in the range of temperatures between 300°C and 500°C, [9] the oxide scale covering the nanoparticles was reduced. The oxide layer plays a crucial part in sintering, as sintering is a surface phenomenon.…”

Investigation of surface and thermogravimetric characteristics of carbon‐coated iron nanopowder

“…In our previous study, it was seen that when such a metallic shoulder is observed, the scale thickness is generally 3 to 4 nm. 4 Zinc is considered to be an impurity and could be from the manufacturing process.…”

“…The present study aims to characterize the surface products of the carbon-coated iron nanopowder using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and high-resolution scanning electron microscopy (HR-SEM) and to correlate with the findings from thermogravimetric analysis. The oxide thickness determination for nanopowder has been studied before, 4 whereas this study emphasize on the changes that the nanopowder undergoes during the heating phase of the sintering from the perspective of surface chemistry.…”

Analysis of Iron Oxide Reduction Kinetics in the Nanometric Scale Using Hydrogen

“…Iron nanopowder is hence used in the present research as an additive to improve the density of water-atomized iron powder compacts. The iron nanopowder was found to be covered with an oxide scale of 3 nm thick [7]. For the sintering process to be efficient, this surface oxide on the metal particle must at least in part be reduced.…”