The high crack density in the np-Au thin film is due to substrate confinement during deposition volume shrinkage (adapted from Seker et. al., Copyright 2009 MDPI32). Crack-free np-Au thin films are formed by delamination of np-Au films from the Si substrate during deposition (Adapted from Gwak et al., Copyright 2013 Elsevier7.) Figure 2-7 Current/potential behavior in selective sacrificial dissolution.
Introduction
In Chapter 3, the effect of indentation size on nanoindentation in np-Au is investigated in relation to ligament size14. Nanotwinned np-Au is synthesized using optimized conditions in co-spraying the precursor sheet.
Mechanical properties of np-Au
- Effect of relative density
Ligament size effect on the strength of np-Au can be explained by dislocation starvation theory in line with size effect in nanopillar compression. Mechanical properties of Np-Au depending on external microstructures 3.1 Indentation size Effect of Np-Au in sharp tip indentation.
Nanomechanical Modeling
I developed a nanomechanics model to investigate the indentation size effect behavior of np-Au using sharp Berkovich indenter with respect to ligament size. As nanoindentation progresses to the indentation depth of 𝛿ℎ, the circumferential region, 2𝜋(𝑟 + 𝛿𝑟)𝛿ℎ, re-penetrates the np-Au by shear force inducing plastic collapse. The volume of np-Au in contact with the bottom penetrates 𝛿ℎ in the loading direction with the bottom area π𝑟2, that is, the summation of the segment 𝜋(𝑟 + 𝛿𝑟)2− 𝜋𝑟2 from 0 to r with respect to r.
Recent studies showed that np-Au has a plastic constraint factor in the range of 2.65 to 3 similar to solid metals by comparing tensile and compressive tests41, 50-52. Self-similarity in np-Au samples obtained from dealloying and post-heat treatment is an important factor to propose a universal indentation size effect model for np-Au with different ligament structure. Self-similarity in np-Au has been investigated by several researchers using destructive and non-destructive three-dimensional reconstruction, tomography and numerical analysis.
Depending on the change in effective density, the network connectivity of np-Au samples was gradually increased by increasing the ligament size above the ligament size of 150 nm due to volume contraction. In this chapter, the similarity of np-Au is assumed to be consistent during annealing coarsening to simplify the theoretical approach.
Experimental Procedure and Results .1 Experimental Procedure
Relative densities of the np-Au samples were calculated from direct measurements of external volume and weight. 3-3(a)-(d) and typical indentation hardness versus indentation depth for four np-Au samples are shown in Fig.3-3(e)-(h). This is consistent with previous research that np-Au with smaller ligament size has higher indentation hardness12.
To compare trend in indentation size effect for np-Au with different ligament size, normalized hardness (H/H0) versus normalized indentation depth (h/D) for np-Au sample ratio is shown in Fig. To obtain this strength ratio, uniaxial compression tests and pure shear tests were performed for np-Au samples. There is linear scaling relation in four np-Au samples about ligament size with slope of 0.98, where slope is size effect exponent, mcomp in 𝜎 = 𝑙−𝑚𝑐𝑜𝑚𝑝.
Typical pure shear stress to shear strain curves of np-Au samples are shown in Fig. 3-8(a) and (b) give a clue to understand ligament size-related indentation-size effect trend in np-Au.
Ligament Size Effect and Indentation Size Effect Relation
The dislocation starvation model that takes into account the size effect on Au nanopillar compression can be applied to the shear strength of np-Au since dislocations on the ligament surface can travel through the ligament and escape to the free opposite surface of the ligament. Ngo et al., suggested that initial compression yielding of np-Au occurs at a very early stage of loading due to surface-induced prestress and anomalously high elastic compliance through MD66 simulations. They also found that the typical dislocation-based mechanism contributes to the strain hardening of np-Au during compressive deformation while the dislocation-starvation model does not apply to np-Au under compression.
Np-Au has irregular geometry in terms of curvature, ligament thickness, and orientation of ligaments, unlike nano- and micropillars made by FIB milling and lithography. Structural irregularities of np-Au illustrate unique deformation behavior that stress can be concentrated on the thinnest parts, possibly the necks of ligaments, and that the existence or density of initial dislocation in volumes of necks is more important for yielding than for overall initial dislocation density . But it is difficult to define k precisely in the case of real indentation on np-Au.
Following my definition of k, it stands to reason that k would correlate with the depth at which shear stress is applied at the indentation boundary between the np-Au surface and the nanoindenter. Therefore, the origin of indentation size effect in np-Au is attributed to a combination of ligament size-dependent 𝜏/𝜎 and k shown in Fig.
Indentation Size Effect and Indentation Size Effect Relation in Spherical Tip Indentation
- Experimental Results and Nanomechanical Modeling
- Relation between Indentation Size Effect Trend of Sharp Tip and Spherical Tip
In other words, np-Au shows similar tendency to indentation size effect in spherical indentation on solid materials70. Combination of equations through Eq. (3-8) to Eq. 3-12) suggests that hardness depends on the indenter radius normalized by the unit cell size of np-Au, R/D, in the spherical indenter on np-Au49. But the contact angle of spherical indentation was assigned to be 8.16°, which is much smaller than the contact angle of Berkovich indentation of 19.7°.
Thus, the aberration due to the contact position of the tip does not give a significant difference in the hardness values obtained with the spherical indenter, as shown in the figure. Thus, strain hardening can occur during spherical indentation by changing the representative strain as shown in the figure. reported the indentation size effect in a spherical indentation with different indentation tip radius by adjusting the indentation depth to have the same representative strain, meaning that the a/R value is the same for all indentations70. -12), the indentation size effect for a spherical indentation is related to the constant KR caused by the shear work during the indentation, and the value of KR is not zero.
This means that indentation work caused by shear force is the primary mechanism of the indentation size effect during spherical indentation of np-Au. The indentation work by shear force for Berkovich indentation and spherical indentation is 𝑤𝑠ℎ𝑒𝑎𝑟= 𝜏 ⋅ 2𝜋ℎ(𝑡𝑎𝑛 𝜃)−1⋅ 𝑷 𝑷 𝐅 ℎ𝑒𝑎𝑟= 𝜏 ⋅ 2𝜋𝑅 𝑠𝑖𝑛 𝜃 ⋅ 𝐷 ⋅ 𝑘𝑅𝐷 .
Conclusion
Mechanical properties of Np-Au depending on internal microstructures 4.1 Effect of grain size and initial dislocation density on deformation of Np-Au.
Mechanical Properties of Np-Au Depending on Internal Microstructures 1 Effect of Grain Size and Initial Dislocation Density on Deformation of Np-Au
Fabrication of Nanocrystalline and Pre-strained Np-Au
Same as in the previous chapter, np-Au samples were prepared by free-corrosion dealloying of precursor Au-Ag alloys (Ag72Au28 in at.%), which were prepared from Au (99.99%) and Ag (99. 99%) pellets by melting at 1100 °C and homogenized for 72 hours at 850 °C under nitrogen atmosphere. From this homogenized state, three different 'annealed', 'prestressed' and 'ball milled' precursor alloys were prepared. Ball-milled alloys were cut into approximately 1 mm thick disk and carefully polished on both sides with a 0.25 μm diamond suspension.
Np-Au samples were examined using field emission scanning electron microscopy (FE-SEM, FEI NovaNano 230) for imaging, energy dispersive X-ray spectroscopy (EDS) to measure the amount of Ag retained, and electron backscatter diffraction (EBSD, TSL - OIM) to obtain an inverted pole figure. Nanoindentation and bending tests were performed on np-Au samples to see the effect of internal microstructure on mechanical responses under different stress conditions. Nanoindentations (KLA Nanoindenter G200) were performed on precursor alloys and np-Au samples with a Berkovich indenter using continuous stiffness measurement (CSM) in XP module with force capacity 500 mN for the precursor alloys and dynamic contact module II (DCM II) with force capacity 30 mN for np-Au samples.
Grain sizes were 238 μm for the annealed precursor alloy, 266 μm for the prestrained one and 206 nm for ball milled one, respectively. On the other hand, the nearly identical ligament sizes for three np-Au samples imply that changes in initial dislocation density and grain size in a precursor alloy are unlikely to alter the nanoporous structure obtained by dealloying.
Nanoindentation on Np-Au Samples
The relative density of np-Au samples in this chapter was 28%, measured by direct weighing. The relative density of np-Au could be a primary factor causing the ligament deformation behavior. Figures 4-4(d) and (e) show typical plastic collapse features due to dislocation slip in ligaments of annealed and prestressed np-Au samples.
4-7(a) shows typical bending stress-strain curves for three np-Au samples obtained from three-point bending test. Lower flexural strength of ball-milled np-Au suggests that high density of grain boundaries may weaken the flexural strength of np-Au. Li and Sieradzki measured fracture strength and deformation of np-Au in three-point bending tests58.
This broken feature clearly illustrates the much lower bending strength and less dispersion for the ball-milled np-Au. This could also be a possible weakening mechanism of grain boundaries in spherical np-Au.
Strengthening Effect of Nanotwinned Structure in Ligament of Np-Au
- Fabrication and Microstructure Evolution of Nanotwinned Np-Au 15 µm-thick Ag 70 Au 30 precursor foils with high density of growth twins were deposited by co-
- Deformation Behavior during In-Situ Tensile Test
- Bimodal Distribution for UTS of Nanotwinned-normal Np-Au
- Relation between UTSs and Relative Density for Np-Au
4-10(f)–(h) show that the high-density twin boundaries exist over the entire volume of the nanowound np-Au samples. 4-12(d), the UTS values of nanowind-prone np-Au and np-Au with rare twins exhibit a unimodal distribution. The UTS of nanomined-normal np-Au corresponding to the high-UTS group is MPa.
In the case of the nanointerlaced normal np-Au sample, all defects were caused by transgranular fractures rather than intergranular fractures. But for np-Au with rare twins, the phenomenon of “the stiffer the stronger” is not observed as shown in the figure. .
The UTS of nanotwinned-inclined np-Au, which was MPa, was not significantly greater than that for np-Au with rare twins. This ligament strengthening mechanism could be a reason for the high UTS of nanotwinned-normal np-Au.
Conclusion
Conclusion
Scythe, R.; Mathur, A.; Fritz, G.; Lee, J.; Erlebacher, J., Platinum-plated nanoporous gold: an efficient electrocatalyst with low Pt loading for PEM fuel cells. M; Baeumer, M., Nanoporous gold catalysts for selective gas-phase oxidative coupling of methanol at low temperature. Kertis, F.; Snyder, J.; Govada, L.; Khurshid, S.; Chayen, N.; Erlebacher, J., Structure/processing relationships in the fabrication of nanoporous gold.
Liu, L.-Z.; Yes, X.-L.; Jin, H.-J., Interpretation of anomalous low strength and low stiffness of nanoporous gold: quantification of network connectivity. J; Spolenak, R.; Arzt, E., Dealloying of Au-Ag thin films with a compositional gradient: influence on the morphology of nanoporous Au. Lu, Q.; You, Z.; Huang, X.; Hansen, N.; Lu, L., Dependence of dislocation structure on orientation and slip systems in highly oriented nanoconnected Cu.
Curriculum Vitae
Eun-Ji Gwak
Education
Fellowships and Awards
Research Experience and Skills
Synthesis and characterization of np-Au with various microstructures
Investigation and modeling of deformation mechanism for nanoindentation on np-Au
Metallic glass thin films in electronic applications
Publications (6 publications as first author)
Patents
Conference Presentations (as presenter, selected)
Oral presentation) “Nano-winning nanoporous gold with improved tensile strength”, E.J. Oral presentation) “Mechanical behavior of nanotwinned nanoporous gold”, E.J. Kim, 2nd International Symposium on Nanoporous Materials by Alloy Corrosion (Bostalmeer, Germany, September 2016). Oral presentation) “Enhancement of mechanisms in nanoporous gold”, E.J.
Professional Experience
