| 1. |
- Best, James P., et al.
(författare)
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Ni nanocluster composites for enhanced impact resistance of multilayered arc-PVD ceramic coatings
- 2018
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Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 354, s. 360-368
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Tidskriftsartikel (refereegranskat)abstract
- Optimally, hard protective coatings should effectively absorb impact energy to reduce the likelihood of failure events. In this work, an arc-PVD approach was utilised for the deposition of thick ceramic multilayer AlCrTiN/CrN-based coatings containing a distribution of metallic nickel inclusions throughout sequential CrN-based interlayers. The aim of such coatings is to provide resistance to impact loading in intensive application environments, such as drop forging of steel and turbine blades exposed to abrasive particles. The structure and micro-structural development of the ceramic was first investigated using transmission electron microscopy, where discrete Ni inclusions were observed as both larger discs (d ~ 600–800 nm, h ~ 200 nm) and smaller (d ~ 50 nm) nanoclusters. Confirmation of the distribution and nanocluster chemistry was achieved using atom probe tomography. For mono-block coatings, XRD data showed drastically reduced internal stresses as a result of the Ni inclusions, enabling the creation of thicker protective coatings which minimise substrate stress concentration upon loading. Nickel inclusion additionally provides a softening of the containing hard ceramic layer, allowing for tuning of mechanical properties. Supporting this idea, in situ micropillar compression measurements of the multi-layered coating systems showed that Ni clusters hindered crack propagation through the coating during failure, while the fracture strength could be increased by incorporating both Ti and Ni in the softer CrN-based layer. High-load impact testing highlighted the influence of Ni ‘shock absorbers’ in reducing circumferential cracking, which was further confirmed by an industrial die forging test that demonstrated a 15–22% life-time increase compared to a much thicker hard-chrome plated reference. The results of this study demonstrate an exciting way to produce Ni nanocluster integrated hard ceramic multi-layers using arc-PVD in a single processing step. Such tuneable thin-film composite systems show great promise in minimising damage from impact loading, even under severe working conditions such as in hot forging.
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| 2. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(författare)
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Microscale fracture of chromia scales
- 2019
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Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Native protective oxide scales offer resistance against corrosion for high temperature materials, which often work in extreme conditions of varying mechanical and thermal loads. The integrity of such layers is of critical importance, since their damage can lead to significant reduction in material life. Mechanical data such as fracture strain and elastic modulus are required to include oxides in material life estimation models for high temperature materials, but there is lack of such data. Their thickness is in the mm range, which makes mechanical testing for property determination difficult. Here we present a micro-mechanical testing method, based on bending of micro-cantilevers produced by focused ion beam milling, capable of circumventing the limitations of conventional approaches. We apply this method to chromia thermally grown on pure chromium, and measure fracture strains at room and high temperatures (600 °C). The measured fracture strains were found to be higher at room temperature, due to a larger fraction of transgranular fracture. Surprisingly, a large fraction of transgranular fracture was seen even in the presence of stress concentrations at grain boundaries. Removal of the stress concentrations accentuated the propensity for transgranular cracking at room temperature. Realistic values of room temperature elastic modulus were obtained as well. The observed mixed trans- and intergranular cracking points towards the need for dedicated investigations of both oxide grain boundary strength and cleavage resistance of single crystals in order to fully understand the failure mechanisms in thermally grown oxide scales.
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| 3. |
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| 4. |
- Kashani, Amir Hossein Navidi, et al.
(författare)
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Morphology, mechanical properties, and oxidation behavior of stoichiometric Ti0.33-xAlxB0.67 coatings (x=0.04, 0.15, 0.21, and 0.28)
- 2024
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 270
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Tidskriftsartikel (refereegranskat)abstract
- Stoichiometric Ti0.33-xAlxB0.67 coatings with x = 0.04, 0.15, 0.21, and 0.28 were synthesized by magnetron sputtering and characterized regarding phase formation, mechanical properties, and oxidation behavior. By increasing the Al concentration from 4 to 28 at.%, the measured elastic modulus (496±19 GPa) and unit cell volume (25.646 Å3 ) decreased by 33 and 0.8 %, respectively. The Al concentration induced changes in measured elastic modulus and unit cell volume are in very good agreement with ab initio predictions, as the maximum deviations between experiment and theory, observed here, are 12 and 1.1 %, respectively. The corresponding hardness values decreased by 45 % from 22±1 to 12±1 GPa. The oxidation experiments were performed in ambient air at 700, 800, and 900 °C for 1, 4, and 8 h. Analysis by scanning transmission electron microscopy (STEM) revealed a bimodal, strongly Al concentration-dependent oxidation behavior where films containing ≤15 at.% of Al form a porous, non-passivating crystalline oxide scale containing Ti -rich as well as Al -rich oxide regions, while the formation of a passivating, dense, X-ray amorphous oxide scale was observed for films containing ≥ 21 at.% of Al. Coincident with the passive scale formation for Al concentrations ≥ 21 at.%, the elastic modulus decreases by ≥ 32.6 % compared to TiB2 and can be rationalized based on Al concentration induced bond weakening as revealed by the concomitant cohesive energy reduction of ≥ 22 %.
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| 5. |
- Kashani, Amir Hossein Navidi, et al.
(författare)
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Temporally-resolved decomposition of Ti0.12Al0.21B0.67 thin films at 1000°C
- 2024
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 487
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Tidskriftsartikel (refereegranskat)abstract
- The thermal stability of stoichiometric Ti0.12Al0.21B0.67 thin films synthesized by magnetron sputtering was investigated by vacuum annealing at 1000°C for 1 and 3 h. The as-deposited and post-annealed films were compared regarding changes in chemical composition, phase formation, and morphology. X-ray diffraction (XRD) data indicate the formation of a single-phase solid solution in the as-deposited Ti0.12Al0.21B0.67 thin film. After annealing for 1 h, scanning transmission electron microscopy (STEM), energy-dispersive X-ray mapping (EDX), and atom probe tomography (APT) investigations reveal segregation into Al- and Ti-rich (Ti,Al)B2 domains, consistent with spinodal decomposition. Furthermore, the formation of AlB12 with a concomitant reduction in Al concentration from 20.9 to 16.8 at. %, likely by evaporation, indicate the decomposition of Al-rich (Ti,Al)B2 domains during annealing for 1 h. Analysis of the film after annealing for 3 h shows evidence for continued spinodal decomposition as well as for further decomposition of Al-rich (Ti,Al)B2 domains, leading besides the formation of AlB12 to a reduction in Al concentration to 12.5 at. % by Al evaporation. The observed phase formation trend during in situ transmission electron microscopy (TEM) studies at 1100 degrees C is consistent with the above discussed decomposition processes. The here identified thermal stability limit, revealed with spatially resolved structure and composition probes, confines the application temperature range of Ti0.12Al0.21B0.67 in vacuum to temperatures <1000°C and underlines that thermal stability investigations solely based on XRD data result in an overestimated thermal stability.
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| 6. |
- Li, Hu, 1986-, et al.
(författare)
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Superior adhesion of graphene nanoscrolls
- 2018
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 1
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Tidskriftsartikel (refereegranskat)abstract
- An emerging material in the carbon family, a graphene nanoscroll (GNS) is composed of tubularly scrolled monolayer graphene and has shown superlubricity and large current sustainability, surpassing the properties of monolayer graphene itself. Here we report on the superior adhesion of GNS prepared with a high yield synthesis method that allows for mass production of high quality GNSs. Raman spectra indicate that the GNS still maintains the signature of monolayer graphene, implying the lacking of π-stacking between adjacent layers. Importantly, adhesion measurements using atomic force microscopy reveal these GNSs with height range of 120-130 nm show a 2.5-fold stronger adhesion force than pristine graphene. This result potentially indicates that the GNS has higher adhesion than monolayer graphene and even higher than the liquid-solid and hydrogen-bonding enhanced interfaces which are essential types of adhesions involved in the field of physical adhesions and thus, GNS could be a new candidate for super-strong and lightweight devices.
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| 7. |
- Michler, Johann, et al.
(författare)
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Plastic deformation of gallium arsenide micropillars under uniaxial compression at room temperature
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:4, s. 043123-
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Tidskriftsartikel (refereegranskat)abstract
- The authors have experimentally investigated the compressive strength of GaAs pillars with a diameter of 1 µm by uniaxial compression tests. The tests were performed at room temperature and, contrary to macroscopic tests, the micropillars were found to exhibit ductile plasticity comparable to that of metal single crystal micropillars. The yield stress was 1.8±0.4 GPa and, for one pillar that was more closely examined, a total deformation of 24% was observed. In the diffraction patterns from transmission electron microscopy studies of this pillar, a high density of twins was observed.
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| 8. |
- Östlund, Fredrik, et al.
(författare)
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Brittle-to-Ductile Transition in Uniaxial Compression of Silicon Pillars at Room Temperature
- 2009
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 19:15, s. 2439-2444
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Tidskriftsartikel (refereegranskat)abstract
- Robust nanostructures for future devices will depend increasingly on their reliability. While great strides have been achieved for precisely evaluating electronic, magnetic, photonic, elasticity and strength properties, the same levels for fracture resistance have been lacking. Additionally, one of the self-limiting features of materials by computational design is the knowledge that the atomistic potential is an appropriate one. A key property in establishing both of these goals is an experimentally-determined effective surface energy or the work per unit fracture area. The difficulty with this property, which depends on extended defects such as dislocations, is measuring it accurately at the sub micrometer scale in this Full Paper the discovery of an interesting size effect in compression tests on silicon pillars with sub-micrometer diameter is presented: in uniaxial compression tests, pillars having a diameter exceeding a critical value develop cracks, whereas smaller pillars show ductility comparable to that of metals. The critical diameter is between 310 and 400 nm. To explain this transition a model based on dislocation shielding is proposed. For the first time, a quantitative method for evaluating the fracture toughness of such nanostructures is developed. This leads to the ability to propose plausible mechanisms for dislocation-mediated fracture behaviour in such small volumes.
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| 9. |
- Östlund, Fredrik, et al.
(författare)
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Ductile-brittle transition in micropillar compression of GaAs at room temperature
- 2011
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Ingår i: Philosophical Magazine. - : Informa UK Limited. - 1478-6435 .- 1478-6443. ; 91:7-9, s. 1190-1199
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Tidskriftsartikel (refereegranskat)abstract
- Experiments have been carried out on how compressive failure of < 100 > axis GaAs micropillars at room temperature is influenced by their diameter. Slip was observed in all micropillars, often on intersecting slip planes. Cracks could nucleate at these intersections and then grow axially in the sample, with bursts of crack growth. However, GaAs micropillars with diameters less than approximately 1 mu m did not split, nor was splitting observed where slip occurred on only one plane. The conditions under which such splitting can occur have been estimated by modifying an existing analysis. This predicts a ductile-brittle transition at a micropillar diameter of approximately 1 mu m, consistent with experimental observations.
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