| 1. |
- Mahade, Satyapal, 1987-, et al.
(författare)
-
Incorporation of graphene nano platelets in suspension plasma sprayed alumina coatings for improved tribological properties
- 2021
-
Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 570
-
Tidskriftsartikel (refereegranskat)abstract
- Graphene possesses high fracture toughness and excellent lubrication properties, which can be exploited to enhance tribological performance of coating systems utilized to combat wear. In this work, suspension plasma spray (SPS) process was employed to deposit a composite, graphene nano-platelets (GNP) incorporated alumina coating. For comparison, monolithic alumina was also deposited utilizing identical spray conditions. The as-deposited coatings were characterized in detail for their microstructure, porosity content, hardness, fracture toughness and phase composition. Raman analysis of the as-deposited composite coating confirmed retention of GNP. The composite coating also showed good microstructural integrity, comparable porosity, higher fracture toughness and similar alumina phase composition as the monolithic alumina coating. The as-deposited coatings were subjected to dry sliding wear tests. The GNP incorporated composite coating showed lower CoF and lower specific wear rate than the pure alumina coating. Additionally, the counter surface also showed a lower wear rate in case of the composite coating. Post-wear analysis performed by SEM/EDS showed differences in the coating wear track and in the ball wear track of monolithic and composite coatings. Furthermore, Raman analysis in the wear track of composite coating confirmed the presence of GNP. The micro-indentation and wear test results indicate that the presence of GNP in the composite coating aided in improving fracture toughness, lowering CoF and specific wear rate compared to the monolithic coating. Results from this work demonstrated retention of GNP in an SPS processed coating, which can be further exploited to design superior wear-resistant coatings.
|
|
| 2. |
- Xu, Jinghao, et al.
(författare)
-
Short-term creep behavior of an additive manufactured non-weldable Nickel-base superalloy evaluated by slow strain rate testing
- 2019
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 179, s. 142-157
-
Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing (AM) of high γ′ strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 °C under strain rates of 1 × 10−5/s, 1 × 10−6/s, and 1 × 10−7/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ′ precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ′ strengthened polycrystalline Nickel-base superalloys.
|
|
| 3. |
- Grolig, Jan Gustav, 1986
(författare)
-
Coated Ferritic Stainless Steels as Interconnects in Solid Oxide Fuel Cells - Material Development and Electrical Properties
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Solid oxide fuel cells (SOFCs) are attracting increasing interest as devices with potentialuses in decentralized and clean electricity and heat production. Several challengeswith respect to materials have to be overcome to achieve efficiencies and life-spansthat are sufficient for long-term applications.An important element of an SOFC stack is the interconnect component, which connectstwo adjacent fuel cell elements. Interconnects, which are commonly composedof ferritic stainless steels, have to be corrosion-resistant, mechanically stable and costoptimized.This work aimed to investigate economic solutions for interconnect materials and tounderstand the underlying mechanisms of degradation and electrical conduction ofthese materials. Mainly two substrates, a commercially available steel (AISI 441) anda ferritic stainless steel that was optimized for an SOFC application (Sandvik SanergyHT) were combined with different barrier coatings and exposed to a cathode-sideatmosphere. A method was developed that allows for the electrical characterizationof promising material systems and model alloys, thereby facilitating a fundamentalunderstanding of the dominant electrical conduction processes linked to the oxidescales that grow on interconnects. The AISI 441 steel coated with reactive elementsand cobalt showed good corrosion and chromium evaporation profiles, while AISI 441coated with cerium and cobalt also had promising electrical properties. The SanergyHT steel was examined with coatings of copper and iron and copper and manganese,respectively. The corrosion and chromium evaporation profiles of Sanergy HT wereimproved by coating with copper and iron. The copper and iron-coated Sanergy HTshowed lower area specific resistance values than cobalt-coated Sanergy HT. Chromia,which is the main constituent of oxide scales, was synthesized using differentmethods. The electrical properties of chromia were found to be sensitive to not onlyimpurities, but also heat treatment. Finally the electrical properties of cobalt- andcobalt cerium-coated Sanergy HT steels were investigated. It was revealed that theaddition of cerium improved the conductivity of the interconnect by both slowingdown chromia growth and preventing the outward diffusion of iron into the spinel.
|
|
| 4. |
- Banerjee, Debashree, et al.
(författare)
-
Elevated thermoelectric figure of merit of n-type amorphous silicon by efficient electrical doping process
- 2018
-
Ingår i: Nano Energy. - : Elsevier BV. - 2211-2855 .- 2211-3282. ; 44, s. 89-94
-
Tidskriftsartikel (refereegranskat)abstract
- The currently dominant thermoelectric (TE) materials used in low to medium temperature range contain Tellurium that is rare and mild-toxic. Silicon is earth abundant and environment friendly, but it is characterized by a poor TE efficiency with a low figure of merit, ZT. In this work, we report that ZT of amorphous silicon (a-Si) thin films can be enhanced by 7 orders of magnitude, reaching ∼0.64 ± 0.13 at room temperature, by means of arsenic ion implantation followed by low-temperature dopant activation. The dopant introduction employed represents a highly controllable doping technique used in standard silicon technology. It is found that the significant enhancement of ZT achieved is primarily due to a significant improvement of electrical conductivity by doping without crystallization so as to maintain the thermal conductivity and Seebeck coefficient at the level determined by the amorphous state of the silicon films. Our results open up a new route towards enabling a-Si as a prominent TE material for cost-efficient and environment-friendly TE applications at room temperature.
|
|
| 5. |
- Wójcik, Natalia A., et al.
(författare)
-
The influence of Be addition on the structure and thermal properties of alkali-silicate glasses
- 2019
-
Ingår i: Journal of Non-Crystalline Solids. - : Elsevier BV. - 0022-3093 .- 1873-4812. ; 521
-
Tidskriftsartikel (refereegranskat)abstract
- Be-Na-(Li)-Si oxide glasses containing up to 15 mol% of BeO were prepared. Their structure was characterized by X-ray powder diffraction and Raman as well as infrared spectroscopic techniques, while their chemical compositions were examined by Inductively Coupled Plasma Optical Emission Spectrometry. All materials were found to be amorphous and contain Al contaminations from minor dissolution of the alumina crucibles. The results of Raman and IR spectroscopies showed that BeO addition to Na-(Li)-Si glass systems resulted in the formation of [BeO4/2](2-) tetrahedra which are inserted into the silicate glass network, demonstrating the intermediate glass-forming role of BeO. In parallel, the effective destruction of Si-O-Si bridges was observed by vibrational spectroscopy. The glass transition temperature was studied by Differential Thermal Analysis and found to range from about 431 degrees C to 551 degrees C. A significant increase in T-g by 70 degrees C was found as SiO2 was substituted by up to 15 mol% BeO.
|
|
| 6. |
- Hanning, Fabian, et al.
(författare)
-
The Effect of Grain Size on theSusceptibility Towards Strain Age Crackingof Wrought Haynes® 282®
- 2020
-
Ingår i: SPS 2020. - Amsterdam : IOS Press. - 9781643681467 - 9781643681474 ; 13, s. 407-416
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The effect of grain size on the suceptibility towards strain age cracking (SAC) has been investigated for Haynes® 282® in the tempeature range of 750 to 950°C after isothermal exposure up to 1800s. Grain growth was induced by heattreating the material at 1150°C for 2h, resulting in a fourfold increase in grain size. Hardness was significanlty reduced after heat treatment as compared to millannealed material. Large grain size resulted in intergranular fracture over a widertemperature range than small grain size material. Ductility was lowest at 850°C, while lower values were observed to be correlated to increased grain size. The rapid formation of grain boundary carbide networks in Haynes® 282® is found to be notable to compensate for higher local stresses on grain boundaries due to incresedgrain size.
|
|
| 7. |
- Qin, Xiao, 1993, et al.
(författare)
-
Microstructure and texture evolutions in FeCrAl cladding tube during pilger processing
- 2023
-
Ingår i: Journal of Materials Research and Technology. - 2238-7854. ; 25, s. 5506-5519
-
Tidskriftsartikel (refereegranskat)abstract
- The microstructure of FeCrAl cladding tubes depends on the fabricating process history. In this study, the microstructural characteristics of wrought FeCrAl alloys during industrial pilger processing into thin-walled tubes were investigated. The hot extruded tube showed ∼100 μm equiaxed grains with weak α∗-fiber in {h11}<1/h12> texture, while pilger rolling process change the microstructure to fragmented and elongated grains along the rolling direction. The pilgered textures could be predicted with the VPSC model. The inter-pass annealing at 800–850 °C for 1 h results in recovery and recrystallization of the ferric matrix and restoration of ductility. The final finished tube shows fine recrystallized grains (∼11 μm) with dominant γ-fiber in three dimensions. Pilger rolling enhanced α-fiber while annealing reduced α-fiber and enhanced γ-fiber. Microstructural evolution in the Laves precipitates followed the sequence of faceted needle-like → spherical → faceted ellipsoidal. Thermomechanical processing resulted in cladding tubes with an area fraction of ∼5% and a number density of 5 × 10−11 m−2 in Laves precipitates, which is half that of the first-pilgered tube. Laves precipitates pin the grain boundaries to control the microstructure and prevent grain coarsening.
|
|
| 8. |
- Shoja, Siamak, 1980
(författare)
-
Microstructure and plastic deformation of textured CVD alumina coatings
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is known that the wear performance of α-alumina coatings produced by chemical vapor deposition (CVD) is significantly influenced by the type and degree of texture. However, the main reasons behind this behavior are not fully understood. This thesis contains studies of two related topics for increasing the understanding of α-alumina coatings. The first topic concerns the microstructure and texture development of CVD α-Al2O3 coatings, and the second topic concerns calculations and analysis of the Schmid factors ( m ) for coatings with different textures. By combining different analysis methods (such as XRD, SEM, FIB/SEM, TKD, TEM, STEM, XEDS), and theoretical and experimental Schmid factor analysis by MATLAB and EBSD, the microstructure and plastic deformation of α-alumina coatings were investigated. The microstructures of three different CVD α-Al2O3 layers deposited onto a Ti(C,N,O) bonding layer were studied. Grain boundary diffusion of heavy elements from the substrate to the bonding layer/α-Al2O3 interface was observed. This may be the cause of a disturbance in the early growth of α-Al2O3. Additionally, it was found that the number of interfacial pores at the bonding layer/α-Al2O3 interface increased by introducing the H2S gas. The H2S gas also promoted an earlier development of the (0001) texture. The orientation of the grains was developed to the desired texture both as a gradual change over several grains and as an abrupt transformation from one grain to another. The probability of plastic deformation in different wear zones on the rake face of a cutting tool was investigated theoretically and experimentally by analyzing Schmid factors for textured α-Al2O3 coatings. Schmid factor diagrams were constructed using MATLAB/MTEX and used to extract frequency distributions for different slip systems and textures. The results were compared with lateral distribution maps of Schmid factors obtained from experimental coatings. It was observed that basal slip is most easily activated in the transition zone, followed by prismatic slip systems 1 and 2 in coatings with an (0001)-texture. The homogeneous plastic deformation behavior observed in this coating is also connected to mostly high Schmid factors in the m -value distribution. The differences between the m -value distributions for the three slip systems are not that pronounced in the (01-12) and (11-20) textures, and the distributions are relatively wide. The low wear rate and more homogeneous deformation of the coating with (0001) texture compared to the other coating textures may be the result of the high plasticity, offered by the easy activation of basal slip and prismatic 1 slip, and the low spread of Schmid factor values at the transition zone. In conclusion, the results presented in this thesis form a knowledge platform that can be used to understand the microstructure and wear mechanisms of textured CVD α-alumina coatings.
|
|
| 9. |
- Krakhmalev, Pavel, 1973-, et al.
(författare)
-
Microstructure, solidification texture, and thermal stability of 316 L stainless steel manufactured by laser powder bed fusion
- 2018
-
Ingår i: Metals. - : MDPI AG. - 2075-4701. ; 8:8, s. 1-18
-
Tidskriftsartikel (refereegranskat)abstract
- This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.
|
|
| 10. |
- Saarimäki, Jonas, et al.
(författare)
-
3D Residual Stresses in Selective Laser Melted Hastelloy X
- 2017
-
Ingår i: Residual Stresses 2016: ICRS-10, Materials Research Proceedings 2 (2016). - : Materials Research Forum LLC. ; , s. 73-78
-
Konferensbidrag (refereegranskat)abstract
- 3D residual stresses in as manufactured EOS NickelAlloy HX, produced by laser powder bed additive manufacturing, are analysed on the surface closest to the build-plate. Due to the severe thermal gradient produced during the melting and solidification process, profound amounts of thermal strains are generated. Which can result in unwanted geometrical distortion and effect the mechanical properties of the manufactured component. Measurements were performed using a four-circle goniometer Seifert X-ray machine, equipped with a linear sensitive detector and a Cr-tube. Evaluation of the residual stresses was conducted using sin2ψ method of the Ni {220} diffraction peak, together with material removal technique to obtain in-depth profiles. An analysis of the material is reported. The analysis reveals unwanted residual stresses, and a complicated non-uniform grain structure containing large grains with multiple low angle grain boundaries together with nano-sized grains. Grains are to a large extent, not equiaxed, but rather elongated.
|
|
