No abstract

Surface chemistry of the titanium powder has particularly growing interest due to the increasing application of titanium components prepared by powder metallurgy, in particular metal injection moulding and additive manufacturing. Due to the high chemical activity, number of titanium oxides, calcium and complex Ca-Ti-oxides can be expected on the component/ medical implant surface, depending on powder and component manufacturing and posttreatment, but are very difficult to analyse due to the lack of the experimental data and analysis methodology. Therefore, a methodology for the analysis of the surface chemistry of the Ti-powder by XPS utilising internal standard reference was developed. The obtained methodology was used for the surface analysis of titanium powder and identification of its surface oxide composition. The results show that the powder surface is covered by TiO 2 layer in the form of rutile with a thickness of 4.4 nm. Carbon and nitrogen impurities were also found present on the powder surface.

Press and sinter is a core technology in powder metallurgy in which a metal powder is mixed with a lubricant and other additives and subsequently compacted at large mechanical pressures to create the desired shape. The component is then delubricated and finally sintered to strengthen the material. The end result of the sintering depends on both the physical properties of the powder such as particle size, morphology, and size distribution, as well as chemical properties like surface chemical composition and presence of surface oxides. In this study, the surface characteristics of 3 fractions of water‐atomized iron powder, sieved to −20, −45, and −75 μm, were investigated. This was done by using X‐ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy equipped with energy‐dispersive X‐ray spectroscopy. The powder was sintered while performing thermogravimetric analysis at 1350°C for 30 minutes in pure hydrogen. Results showed that powder of all 3 size fractions were mainly covered by a thin iron oxide layer but with some presence of submicron‐sized oxide particulates. X‐ray photoelectron spectroscopy and thermogravimetric analysis showed reasonable agreement on the oxide layer thickness, while a size dependence was found with an enrichment of oxide particulates on the finer powder fractions. Fracture surfaces of the sintered material were analyzed with X‐ray photoelectron spectroscopy and scanning electron microscopy + energy‐dispersive X‐ray spectroscopy, which indicated that full reduction of all oxides was achieved. The sintering conditions were further related to the surface properties of the powder and its size fraction.

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.

Multicomponent layered systems with tailored magnetic properties were fabricated via current annealing from homogeneous Fe67Pd33 thin films, deposited via radio frequency sputtering on Si/SiO2 substrates from composite target. To promote spontaneous nano-structuring and phase separation, selected samples were subjected to current annealing in vacuum, with a controlled oxygen pressure, using various current densities for a fixed time and, as a consequence, different phases and microstructures were obtained. In particular, the formation of magnetite in different amount was observed beside other iron oxides and metallic phases. Microstructures and magnetic properties evolution as a function of annealing current were studied and interpreted with different techniques. Moreover, the temperature profile across the film thickness was modelled and its role in the selective oxidation of iron was analysed. Results show that is possible to topologically control the phases formation across the film thickness and simultaneously tailor the magnetic properties of the system.