Inspired by the catechol and amine-rich adhesive proteins of mussels, polydopamine (pDA) has become one of the most widely employed methods for functionalizing material surfaces, powered in part by the versatility and simplicity of pDA film deposition that takes place spontaneously on objects immersed in an alkaline aqueous solution of dopamine monomer. Despite the widespread adoption of pDA as a multifunctional coating for surface modification, it exhibits poor mechanical performance. Attempts to modify the physical properties of pDA by incorporation of oxidizing agents, cross-linkers, or carbonization of the films at ultrahigh temperatures have been reported; however, improving mechanical properties with mild post-treatments without sacrificing the functionality and versatility of pDA remains a challenge. Here, we demonstrate thermal annealing at a moderate temperature (130 °C) as a facile route to enhance mechanical robustness of pDA coatings. Chemical spectroscopy, X-ray scattering, molecular force spectroscopy, and bulk mechanical analyses indicate that monomeric and oligomeric species undergo further polymerization during thermal annealing, leading to fundamental changes in molecular and bulk mechanical behavior of pDA. Considerable improvements in scratch resistance were noted in terms of both penetration depth (32% decrease) and residual depth (74% decrease) for the annealed pDA coating, indicating the enhanced ability of the annealed coating to resist mechanical deformations. Thermal annealing resulted in significant enhancement in the intermolecular and cohesive interactions between the chains in the pDA structure, attributed to cross-linking and increased entanglements, preventing desorption and detachment of the chains from the coating. Importantly, improvements in pDA mechanical performance through thermal annealing did not compromise the ability of pDA to support secondary coating reactions as evidenced by electroless deposition of a metal film adlayer on annealed pDA.
Large, freestanding membranes with remarkably high elastic modulus (>10 GPa) have been fabricated through the self-assembly of ligand-stabilized inorganic nanocrystals, even though these nanocrystals are connected only by soft organic ligands (e.g., dodecanethiol or DNA) that are not cross-linked or entangled. Recent developments in the synthesis of polymer-grafted nanocrystals have greatly expanded the library of accessible superlattice architectures, which allows superlattice mechanical behavior to be linked to specific structural features. Here, colloidal self-assembly is used to organize polystyrene-grafted Au nanocrystals at a fluid interface to form ordered solids with sub-10-nm periodic features. Thin-film buckling and nanoindentation are used to evaluate the mechanical behavior of polymer-grafted nanocrystal superlattices while exploring the role of polymer structural conformation, nanocrystal packing, and superlattice dimensions. Superlattices containing 3-20 vol % Au are found to have an elastic modulus of ∼6-19 GPa, and hardness of ∼120-170 MPa. We find that rapidly self-assembled superlattices have the highest elastic modulus, despite containing significant structural defects. Polymer extension, interdigitation, and grafting density are determined to be critical parameters that govern superlattice elastic and plastic deformation.elasticity | buckling | nanocomposite | thin film | nanoindentation N anocrystal superlattices are ordered arrays of ligand-stabilized colloidal nanocrystals with unique thermal (1, 2), optical (3, 4), and electronic (5, 6) properties due to the nanoscale dimensions and periodic spacing of the inorganic crystals. Nanocrystal superlattices also exhibit superior mechanical performance: superlattice elastic modulus has been shown to rival that of lightweight structural composites (>10 GPa), and superlattice membranes are capable of withstanding repeated indents to large displacements (7)(8)(9). This is all the more remarkable because mechanical cohesion in the superlattices is attributed to van der Waals interactions between ligands on neighboring nanocrystals (10, 11), which are weak enough that the ligands are liquid at room temperature when not attached to nanocrystals. Superlattice strength and stiffness can be further elevated to values that are unprecedented for polymer nanocomposites by cross-linking the organic ligands that coat the nanocrystals (12). The unusual combination of physical properties in nanocrystal superlattices presents intriguing opportunities to use these materials as mechanically actuated optoelectronic sensors (13,14), lightweight solar sails (15), and ultrathin barriers and coatings (16,17), but warrants the development of a thorough understanding of the mechanical behavior of nanocrystal superlattices. In particular, the roles of nanocrystal packing geometry, ligand structural conformation, and ligand-ligand and ligand-nanocrystal interactions must be clarified to design multifunctional, self-assembled polymer nanocomposites with improved mechanical ...
The domain structure of short-range order is directly observed in an aged Ti-6Al alloy.
In this work, we develop and demonstrate novel protocols based on spherical nanoindentation and orientation image mapping (OIM) for quantifying the local increases in slip resistances in the individual grains of a deformed (or strain hardened) polycrystalline sample. These new protocols utilize the recently developed data analyses methods for extracting indentation stress-strain (ISS) curves in conjunction with the measurements of the local crystal orientations at the indentation sites using the OIM technique. The proposed protocols involve two main steps. In the first step, spherical nanoindentation measurements are conducted on fully annealed samples of the material of interest to map out the functional dependence of the indentation yield strength (Y ind ) on the crystal lattice orientation in the annealed condition. In the second step, spherical nanoindentation and OIM measurements are conducted on the deformed samples of the same material and are analyzed rigorously to reliably estimate the increase in the local slip resistance at the indentation sites. The function established in the first step is utilized in the second step to properly account for the influence of the local crystal orientation on the measured Y ind in the deformed sample. This novel measurement and data analysis protocol is demonstrated in this paper on polycrystalline samples of high purity aluminum. From this study, it was noted that the influence of the crystal lattice orientation on the measured Y ind in Al crystals can be as high as 40%, with the lowest values corresponding to the [100] (cube) orientation and highest values corresponding to the [111] orientation. The measurements on the deformed samples showed a significant variation in the strain hardening rates in the individual grains of the polycrystalline sample. A positive correlation was observed between the percentage increase in the local slip resistance and the value of the Taylor factor computed for the local crystal orientation at the indentation site subjected to the macroscale imposed deformation.
IntroductionMesoscale interfaces in materials, including grain boundaries, play a significant role in determining the mechanical properties of materials. The Hall-Petch effect [1,2], discovered over sixty years ago, provides a robust but empirical relationship between the macroscopic yield strength and average grain size in a material. However, there is still very little understanding about the precise role of grain boundaries during plastic deformation and there is no consensus on the correct physics-based model for predicting this effect [3,4]. Moreover, the Hall-Petch effect, captures only the homogenized effect of grain boundaries on the macroscale mechanical properties of metallic materials. In other words, this simple model does not pay attention to the difference in the behavior of the different type of boundaries. Various criteria have been put forth to describe the behavior of grain boundaries based on grain boundary character [5-7], interfacial structure (CSL boundaries) [7-9], grain boundary energy (special boundaries) [10,11], and slip transfer rules [12][13][14]. However, these dependencies are not yet established quantitatively. An accurate understanding of the role of grain boundaries is critical for the development of robust physics-based crystal plasticity models [15-18] to accurately predict material behavior during and following plastic deformation of polycrystalline materials.Lack of rigorous protocols for reliably capturing the mechanical response at the submicron length scales of interest constitutes the foremost technical gap in efforts aimed at advancing the fundamental understanding of the role of grain boundaries during plastic deformation. Approaches such as in-situ testing of micro-pillars [19][20][21][22] have an inherent drawback in that these methods require sophisticated equipment and are effort intensive, and therefore cannot be used to test a large number of interfaces in a cost and time efficient manner.Alternatively, instrumented indentation testing (both nano and micro) [23][24][25][26], when combined with improved data analyses protocols provides a high throughput approach suitable for addressing this challenge. The use of nanoindentation testing for characterizing grain boundary regions has been reviewed in Ref. [27].
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