| 1. |
- Oikonomou, Christos, 1980, et al.
(författare)
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An XPS investigation on the thermal stability of the insulating surface layer of soft magnetic composite powder
- 2016
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Ingår i: Surface and Interface Analysis. - : Wiley. - 1096-9918 .- 0142-2421. ; 48:7, s. 445-450
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Tidskriftsartikel (refereegranskat)abstract
- Ferromagnetic powder particles coated with electrical insulating inorganic layers constitute composite materials used as powdered iron cores in electromagnetic applications. The aforementioned surface layers of these soft magnetic composites (SMC) are responsible for their high electrical resistivity. The perseverance of the coating through annealing processing step is crucial to the performance of finalized SMC products. In the present study, the thermal stability and microstructure of an iron phosphate based surface layer from a commercially available SMC powder were investigated by coupling various analytical techniques. The material was annealed at different temperature regimes, both in oxidizing (air) and inert (N-2) conditions in order to isolate the effect of temperature from the atmosphere composition on the chemical state of the coating. X-ray photoelectron spectroscopy (XPS) was utilized to assess the composition of the surface layer at all conditions. Moreover, phosphate-based chemical standards were processed and analyzed in a similar manner so as to facilitate the interpretation of the observed XPS spectra from the SMC powder. High resolution scanning electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffraction were implemented in order to fully characterize the material under question. The results indicated the transition of the insulating layer from an amorphous state to fully crystalline under annealing in inert atmospheric conditions. Moreover, it was observed that phosphates are still present in the coating in mixture of valance states. Conversely, a thick iron oxide scale was formed under treatment in air, and no phosphorus signal was detected, indicating a total decomposition of the layer. Copyright (c) 2016 John Wiley & Sons, Ltd.
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| 2. |
- Oikonomou, Christos, 1980, et al.
(författare)
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Development of methodology for surface analysis of soft magnetic composite powders
- 2012
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Ingår i: Surface and Interface Analysis. - : Wiley. - 1096-9918 .- 0142-2421. ; 44:8, s. 1166-1170
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Tidskriftsartikel (refereegranskat)abstract
- Soft magnetic composites (SMCs) are designed for the sustainable industrial production of uniform isotropic components with three-dimensional magnetic properties. The nanocoating covering the surface of water-atomized iron powder is the paramount feature of SMC technology that gives the material the high resistivity needed for minimizing the negative effects of eddy currents. An analysis of its initial state/composition on a micro level and any changes during further compaction and heat treatment is thus of great importance. Therefore, a method for the evaluation of composition and thickness of such insulating coatings was developed in this study. High resolution imaging combined with surface analytical and depth profiling techniques (high resolution scanning electron microscopy (HR SEM) with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy) were used. Analyses were performed on commercially available SMC powder grades having a phosphate-based coating. Depth profiling was carried out by ion etching and X-ray photoelectron spectroscopy analysis. The analyses revealed the presence of oxygen (O1s), iron (Fe2p), and phosphorous (P2p), indicating these elements as the main components of the insulating coating. Two approaches for the evaluation of the coating thickness were tested based on the relationship between the normalized intensity of the iron metal peak and etch depth where the phosphorus and oxygen intensities reach half the difference of their maximum and minimum values. Results indicate that the thickness of the coating extends to the ~30 nm depth into the surface. For HR SEM imaging, utilization of an InLens detector for secondary electron imaging and a low acceleration voltage (~5 kV) is recommended for HR SEM + energy dispersive X-ray spectroscopy analysis of coating morphology and composition.
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| 3. |
- Oikonomou, Christos, 1980, et al.
(författare)
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Effect of heat treatment in air on surface composition of iron-phosphate based soft magnetic composite components
- 2014
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Ingår i: Materials Science & Engineering B: Solid-State Materials for Advanced Technology. - : Elsevier BV. - 0921-5107. ; 189, s. 90-99
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Tidskriftsartikel (refereegranskat)abstract
- tSoft magnetic composite materials (SMC) manufactured by conventional powder metallurgical tech-niques for electromagnetic applications constitute individually encapsulated ferromagnetic powderparticles with an insulating surface layer, bonded together into 3D finished structures. The produc-tion procedure includes compaction of the SMC base powder mixed with a lubricant substance and apost-annealing treatment that aims to relieve the stresses induced in the component during pressing.In the present study, the effect of the heat treatment process to the nature of the insulating layer wasinvestigated under different temperature regimes using analytical techniques. Its surface chemistry wasdetermined based on the XPS depth profiling technique, and its morphology and structure were evaluatedusing HR-SEM and XRD. Differences between interior and exterior areas of the samples suggested thedevelopment of an oxide scale in the outer regions that prevents its further bulk oxidation at temperaturesabove 500◦C, while below that temperature incomplete de-lubrication takes place.
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| 4. |
- Oikonomou, Christos, 1980, et al.
(författare)
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Effect of Powder Properties on the Compressibility of Water-Atomized Iron and Low-Alloyed Steel Grades
- 2013
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Ingår i: Euro PM2013 Proceedings. ; 2, s. 205-212
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Konferensbidrag (refereegranskat)abstract
- Compaction of iron and steel water-atomized metal powders is a multiplex process. Lack of solid knowledge concerning the respective mechanisms governing the process and factors affecting them hinders its simulation. Solid solution strengthening is widely considered one of the most influential factors in compressibility, especially for prealloyed grades. Results indicate the significance of powder geometry, size distribution, morphology, etc., on the powder compressibility at different compaction stages. Compaction tests in the range of 50 to 800 MPa for four different size fractions for iron and Cr-Mo prealloyed powder grades are presented. The effect of the microstructure and internal inclusions was evaluated using HR-SEM combined with EDX and EBSD mapping. Experimental results enabled the quantification of the compaction behavior based on parameters associated to the geometrical and mechanical properties for each fraction. A two stage densification process was observed for both grades that includes initial particle rearrangement and subsequent bulk mechanical deformation.
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| 5. |
- Oikonomou, Christos, 1980, et al.
(författare)
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Evaluation of the thickness and roughness of homogeneous surface layers on spherical and irregular powder particles
- 2014
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Ingår i: Surface and Interface Analysis. - : John Wiley & Sons. - 0142-2421 .- 1096-9918. ; 46:10-11, s. 1028-1032
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Tidskriftsartikel (refereegranskat)abstract
- The present study describes the development of a theoretical model for estimating the thickness of homogeneous surface layers on spherical and irregular shaped powder particles using the XPS depth profiling. As opposed to flat surfaces, such an approach for substrates of specific geometry is not straightforward. One needs to consider those geometrical factors associated with the experimental setup and sample's roughness, which impose an angle dependence on the photoelectrons peak intensity, X-ray flux and ion etch rate over the surface in question in order to evaluate it. The novelty of the current model lies in the introduction of geometrical freedom in connection to the experimental arrangement, which can be tailored to match the needs of contemporary instruments. The model was evaluated experimentally by analyzing the surface oxide layers on metal powder grades of different morphology. Complementary analytical techniques such as high resolution (HR) SEM and focused ion beam (FIB) were used in combination to further characterize the surface layers prior to the XPS investigations. The results reveal that the estimation of the oxide/metal interface from the measured relative metal intensity signal as function of etch depth using the model is in extremely good agreement with the corresponding values from direct measurement of HR SEM on FIB cross-sectioned powder samples. The model deviates slightly only for the irregular shaped powder, which can be regarded as means for quantification of surface roughness of the material. The model is used as a basis for a computer software that estimates the thickness of surface layers for powdered materials.
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| 6. |
- Oikonomou, Christos, 1980, et al.
(författare)
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Surface Chemical Analysis of Soft Magnetic Composite Podwers
- 2012
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Ingår i: Materials for Tomorrow 2012.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Soft Magnetic Composite (SMC) components produced based on traditional Powder Metallurgical (PM) techniques are strong candidate materials for electromagnetic applications. Their advantages are based on profitable and energy efficient production methods, shape complexity realization and uniquely uniform and isotropic 3D magnetic properties. SMC powder grades consist of encapsulated iron powder particles with an ultra-fine, electrically insulating surface coating. Component manufacturing procedure involves compaction of the admixed SMC powder with a lubricant to a final shape, as well as a subsequent heat-treatment that aims on the relaxation of stresses induced during the compaction. The concept of SMC is based on taking advantage of the dielectric properties of the surface coating and creating a laminated stack in a powder form. In this manner, products with superior magnetic performances can be produced as opposed to the more traditional laminated steels and ferrites, due to the higher reduction of eddy currents especially at high frequency applications. It is obvious that the insulating coating is the paramount feature of the SMC technology. Its morphology, thickness, cohesion to the powder particle and durability during compaction and heat-treatment, are critical for the properties of a finished component. Hence, a methodology based on surface sensitive analytical techniques was developed in order to address these matters on a micro-level for such insulating coatings. Standard methods previously used for the powder surface characterization have been significantly modified due to the non-conductive nature of the surface coating, which led to charging during analysis. Charge compensation was confronted by adding slow electrons and software correction. Additionally two commonly used sample preparation techniques were implemented and compared. Surface chemical analysis and coating’s morphology characterisation were performed on commercially available water atomized SMC powder grades that were coated with a phosphate based inorganic glass. High resolution imaging combined with spectroscopic techniques and depth profiling (HR-SEM, EDX and XPS) provided complementary results for the coating characterisation. The chemical states of the main constituent elements of the coating, namely oxygen (O1s), iron (Fe2p) and phosphorous (P2p), were determined. Furthermore, two approaches to evaluate the coating thickness were followed based on the relationship between the normalized intensity of the iron metal peak and the etch depth where phosphorus and oxygen intensities reach half the difference of their maximum and minimum values. The results indicated the thickness of the coating to have an average value of approximately 30 nm. Moreover, HR SEM imaging as well as EDX analysis conditions for optimum analysis was established using low acceleration voltage (~5 kV) and inLens detector for secondary electron imaging. Results revealed a uniform coating morphology without the presence of any artifacts with good overall coverage and coherence to the iron core.
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| 7. |
- Pauzon, Camille Nicole Géraldine, 1994, et al.
(författare)
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Effect of layer thickness on spatter properties during laser powder bed fusion of Ti–6Al–4V
- 2023
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Ingår i: Powder Metallurgy. - : Informa UK Limited. - 0032-5899 .- 1743-2901. ; 66:4, s. 333-342
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Tidskriftsartikel (refereegranskat)abstract
- High layer thicknesses for laser powder bed fusion are promising for productivity increase. However, these are associated with increased process instability, spatter generation and powder degradation, crucial for alloys sensitive to oxygen. The effect of increasing layer thickness from 30 to 60 µm is studied focusing on Ti-6Al-4V spatter formation during LPBF and its characterisation, with scanning and transmission electron microscopy, combustion analysis and X-ray photoelectron spectroscopy. Results indicate that spatters are covered with a uniform Ti-Al-based oxide layer and Al-rich oxide particulates, the thickness of which is about twice that present on virgin powder. The oxygen content was about 60% higher in spatters compared to the virgin powder. The study highlights that increasing the layer thickness to 60 µm permits to reduce the total generation of spatters by ∼40%, while maintaining similar spatter characteristics and static tensile properties. Hence, this allows to increase build rate without compromising process robustness.
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| 8. |
- Raza, Ahmad, 1993, et al.
(författare)
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Degradation of AlSi10Mg powder during laser based powder bed fusion processing
- 2021
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Ingår i: Materials and Design. - : Elsevier BV. - 1873-4197 .- 0264-1275. ; 198
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Tidskriftsartikel (refereegranskat)abstract
- Knowledge concerning powder degradation during additive manufacturing (AM) processing is essential to improve the reusability of the powder in AM and hence maximize feedstock powder reuse and economy of the process. AlSi10Mg powder degradation in Concept Laser XLINE 2000R machine over the total period of 30 months was analyzed in order to understand the extent and mechanism affecting powder aging. Thereby, detailed analysis of the powder morphology, microstructure and surface chemistry was performed by SEM, TEM and XPS. The results show an increase in volume fraction of heavily oxidized spatter particles up to 3% in 30 months. XPS analysis of the powder surface chemistry indicates that powder particles are covered by uniform oxide layer, formed by Mg- and Al-based oxides, average thickness of which increased from ~4 nm in case of the virgin powder up to about 38 nm in case of the reused for about 30 month powder, established by XPS. Analysis of the oxide characteristics were consistent with the observed oxygen content in the sampled powder. Columnar oxide scale formation on spatter particles was revealed as well, reaching up to 125 nm in thickness measured using STEM. Results of the XPS and STEM-EDX analysis of oxide composition are shown to be in agreement with the thermodynamic calculations confirming that oxide scale on sputter particles is formed by MgAl2O4 spinel and Al2O3 (corundum) oxides.
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| 9. |
- Alzati, L., et al.
(författare)
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PM parts sintered state improvement achieved by carbon source selection
- 2013
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Ingår i: 2013 International Conference on Powder Metallurgy and Particulate Materials, PowderMet 2013. - 9780985339739 ; , s. 516-524
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Konferensbidrag (refereegranskat)abstract
- In this work primary synthetic graphite, natural graphite and carbon black as carbon sources for PM parts sintering are compared. In order to highlight the different behaviour in demanding applications, special focus is on Cr-alloyed PM steels. Interrupted sintering trials have been run on specimens that were analysed both by advanced techniques (HR SEM, EDX) and Light Optical Microscopy, highlighting how sintering process rate can be positively affected by carbon source selection. Confirmation from tests run on Iron-Copper-Carbon is also presented. Implications for production of different typologies of PM parts is are also discussed. Test results on Key properties like Dimensional Stability (within one lot and lot-to-lot), Mechanical Performance (in terms of macro-Hardness, Tensile Rupture Strength) and solid-state Lubrication are also displayed.
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| 10. |
- Bidulsky, Robert, et al.
(författare)
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Interparticle Neck Connections in Innovative Insulated Iron Powder Compounds
- 2015
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Ingår i: Acta Physica Polonica A. - 0587-4246 .- 1898-794X. ; 128:4, s. 647-650
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Tidskriftsartikel (refereegranskat)abstract
- Goal of the present paper is the analysis of the interparticle neck connections in a system made of insulated iron powder compounds with different additions of an Al-Mg-Si-Cu alloy (0.25, 0.5 and 0.75 wt%). The introduction of the aluminium alloy powder has been made in order to improve the mechanical properties, evaluated as thetransverse rupture strength, without decreasing the agnetic properties (evaluated in terms of iron loss and coercivity force). The fracture analysis of investigated systems puts into evidence the breaking of interparticle neck connections. Heat treatment (at the temperature of 500C) contributes to the early stages of interparticle neck developments. The chosen aluminium alloy presents a sort of pre-sintering behaviour at 500C, with the possibility of mass-transport processes around the insulated iron powder compounds. The air heat treatment applied aims at providing an increase in the mechanical behaviour of the material, with a final good rigidity after the cooling process. Fracture surfaces and transverse rupture strength values show that, at 500 MPa, the strength and the area related to the inter-particle necks can be correlated to the occasional broken insulated point-to-point surfacesthat hinder the development of inter-particles necks.
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