| Authors | Title | Location | Year |
| M.Dao, N.Chollacoop, K.J.VanVliet, T.A.Venkatesh and S.Suresh | "Computational modeling of the forward and reverse problems in instrumented indentation" | Acta Materialia, 49 [19], 3899 - 3918. | 2001 |
| Abstract: | A depth-sensing indentation apparatus fitted with a sharp indenter generally induces large strains (in excess of 5%) in a wide variety of materials. In this study, large deformation finite element analyses of indentation were carried out for 76 different combinations of elasto-plastic properties that encompass a wide range of parameters commonly found in pure and alloyed engineering metals....Using dimensional analysis, a new set of dimensionless functions was constructed to characterize instrumented sharp indentation....Forward and reverse analyses were thus established; the forward algorithms allow for the calculation of indentation response for a given set of elasto-plastic properties, whereas the reverse algorithms enable extraction of elasto-plastic properties for a given set of indentation data....Computational results were compared with experiments for two materials. | ||
| A. Gouldstone, K.J. VanVliet, and S. Suresh | "Experimental simulation of defect nucleation during nanoindentation" | Nature, 411, 656. | 2001 |
| Abstract: | Nanoindentation, i.e.,the quasistatic normal penetration of a surface to depths on the order of nanometers, is simulated using the Bragg bubble raft model in which a close-packed array of soap bubbles represents the equilibrium positions ofatoms within a crystalline solid. Systematic experiments reveal that homogeneous dislocation nucleation occurs within the crystal when its surface roughness is comparable to indenter radius, and that thedepth of the nucleation site below the surface scales with contact half-width. These in-situobservations appear to provide a fundamental justification for defect nucleation in nanoindented face-centered (FCC) crystals. | ||
| S. Suresh | "Graded materials for resistance to contact deformation and damage" | Science, 292, 2447. | 2001 |
| Abstract: | The mechanical response of materials with spatial gradients in composition and structure is of considerable interest in disciplines as diverse as tribology, geology, optoelectronics, biomechanics, fracture mechanics and nanotechnology. The damage and failure resistance of surfaces to normal and sliding contact or impact can be changed substantially through such gradients. This review assesses the current understanding of the resistance of graded materials to contact deformation and damage, and outlines future research directions and possible applications for graded materials. | ||
| S. Veprek, A.S. Argon | "Mechanical properties of superhard nanocomposites" | Surface and Coatings Technology, 146-147, 175-182. | 2001 |
| Abstract: | Novel superhard nanocomposites preared acording to generic design concept [Thin Solid Films 268, (1995) 64], which is based on the formation of the appropriate nanostrutre due to strong segregation and spinodal decomposition, show an unusual combination of mechanical properties, such as high intrinsic (i.e. not falsified by a large compressive stress) Vickers microhardness from 40 to > 100 GPa, high elastic recovery (up to > 90%), high resistence against crack formation even at a large strain of > 10 % and high thermal stability. We shall show that these properties can be relatively easily understood on the basis of conventional fracture mechanics scaled down to dimensions of a few nanometers small nanocrystals and nanocracks, in combination with a low concentration of possible flaws introduced into the material during its preparation. The latter is a consequence of the 'self-organization' of the system due to the thermodynamically driven formation of the stable nanostructure. | ||
| J.-J. Kim, Y. Choi, S. Suresh, A.S. Argon | "Nanocrystallization during nanoindentation of a bulk amorphous metal alloy at room temperature " | Science, 295, 654-657. | 2002 |
| Abstract: | It is known that nanocrystallites can form in shear bands produced during severe bending or high-energy ball milling of thin ribbons of a metallic glass. We present direct experimental evidence that highly confined and controlled local contact at the ultrafine scale in the form of quasi-static nanoindentation of a bulk glassy metal alloy at room temperature can also cause nanocrystallization. Atomic force microscopy and transmission electron microscopy show that nanocrystallites nucleate in and around shear bands produced near indents and that they are the same as crystallites formed during annealing without deformation at 783 kelvin. Analogous to results from recent experiments with glassy polymers, our results are reasoned to be a consequence of flow dilatation inside the bands and of the attendant, radically enhanced, atomic diffusional mobility inside actively deforming shear bands. | ||
| A.S. Argon, S. Veprek | "A consistent rationale for the superior strength and ultra-hardness of ceramic nano-composite coatings" | MRS Symposium Paper, 697, 3-8. | 2002 |
| Abstract: | Nano-structured composite ceramic coatings such as TiN with Si3N4 or Ti Si2 prepared by various forms of plasma assisted CVD, composed of crystalline components of equiaxed TiN of several nm diameter, surrounded by amorphous Si3N4 intercrystalline layers of roughly 0.2 volume fraction have exhibited hardnesses in the range of 70-100 GPa-quite commensurate with polycrystalline diamond layers, and thermal stability up to 1000C. Preliminary considerations indicate that such ultra-hardness, uninfluenced by the usual artifacts of nano-indentations are not governed by processes of crystal plasticity in the crystalline component but by the characteristic flow mechanisms of the often topologically continuous amorphous component exhibiting "liquid-like" behavior in the constrained spaces between the crystalline components. | ||
| A.S. Argon, S. Veprek | "Towards the understanding of mechanical properties of super- and ultrahard nanocomposites" | Journal of Vacuum Science and Technology B, 20 [2], 650-664. | 2002 |
| Abstract: | This article presents an attempt to explain in a simple way, understandable to a broad spectrum of readers, the unusual combination of the mechanical properties of the recently developed new class of superhard nanocomposites, such as high hardness which significantly exceeds that of the rule of mixtures, enhancement of the elastic modulus as measured by the indentation technique, very high elastic recovery which is observed upon the indentation and the absence of crack formation even under elastic deformation corresponding to a strain of more than 10%. Future experimental work is suggested which should bring further progress towards the understanding of these materials. | ||
| E.S. Ahn, N.J. Gleason, A. Nakahira, J.Y. Ying | "Nanostructure processing of hydroxyapatite-based bioceramics" | Nano Letters, 1 [3], 149-153. | 2001 |
| Abstract: | Nanostructure processing was applied to derive hydroxyapatite and hydroxyapatite-zirconia bioceramics with ultrafine microstructures and significantly improved mechanical properties for orthopedic and dental implant applications. Despite its attractive bioactivity, hydroxyapatite (HAP) has been limited in applications due to the poor processability and mechanical strength of the conventional material. Through nanostructure processing, high-strength HAP has been obtained by pressure-assisted sintering. To further toughen the HAP matrix, nanocrystalline yttria-stabilized zirconia (YSZ) dispersoids have been introduced during HAP precipitation. The nanostructured HAP and HAP-YSZ composites demonstrated excellent chemical and microstructural uniformity and mechanical properties, compared to conventional coarse-grained systems. | ||
| K.J. VanVliet and S. Suresh | "Simulations of cyclic normal indentation of crystal surfaces using the bubble-raft model" | Philosophical Magazine A, 82 [10], 1993-2001. | 2002 |
| Abstract: | The evolution of contact-induced deformation on the nanoscopic scale is of considerable interest in terms of both the scientific understanding of defect nucleation and the practical concern of contact damage resistance of a wide range of surfaces in engineering applications. Currently, experimental tools such as nanoindentation, atomic force microscopy, and atomic-resolution transmission electron microscopy allow quantification of nanoscale deformation and damage induced by contact at surfaces. However, none of these methods allows for in-situ visualization of atomic-level deformation during contact loading. Recently, we have employed the Bragg-Nye bubble raft to study in-situ the conditions governing defect nucleation in fcc crystals subjected to nanoindentation. Although there are inherent limitations to this two-dimensional model, we have found useful parallels to the mechanisms of homogeneous defect nucleation and deformation in three-dimensional fcc crystals. Such observations have the potential to guide computational models based on molecular dynamics. In this paper, we compare the characteristics of defect nucleation and slip step formation under monotonic and cyclic normal indentation using the Bragg-Nye model. We identify the atomic-level surface roughening process arising from homogeneous and heterogeneous defect nucleation and cyclic slip under repeated indentation loading. These findings provide insights into the atomic level mechanisms of cyclic slip and surface roughening during contact fatigue. | ||
| J. Li, K.J. VanVliet, T. Zhu, S. Yip, and S. Suresh | "Atomistic mechanisms governing elastic limit and incipient plasticity in crystals" | Nature, 418 [18 July 2002], 307-310. | 2002 |
| Abstract: | Nanometre-scale contact experiments and simulations demonstrate the potential to probe incipient plasticity--the onset of permanent deformation--in crystals. Such studies also point to the need for an understanding of the mechanisms governing defect nucleation in a broad range of fields and applications. Here we present a fundamental framework for describing incipient plasticity that combines results of atomistic and finite-element modelling, theoretical concepts of structural stability at finite strain, and experimental analysis. We quantify two key features of the nucleation and subsequent evolution of defects. A position-sensitive criterion based on elastic stability determines the location and character of homogeneously nucleated defects. We validate this stability criterion at both the atomistic and continuum levels. We then propose a detailed interpretation of the experimentally observed sequence of displacement bursts to elucidate the role of secondary defect sources operating locally at stress levels considerably smaller than the ideal strength required for homogeneous nucleation. These findings provide self-consistent explanation of the discontinuous elastic-plastic response in nanoindentation measurements, and a guide to fundamental studies across many disciplines that seek to quantify and predict the initiation and early stages of plasticity. | ||
| K.J. VanVliet, J. Li, T. Zhu, S. Yip, and S. Suresh | "Quantifying the early stages of plasticity through nanoscale experiments and simulations" | Physical Review B, 67 [10], 10415-1-10415-15. | 2003 |
| Abstract: | Nucleation and kinetics of defects at the atomic scale provide the most fundamental information about the mechanical response of materials and surfaces. Recent advances in experimental and computational analyses allow us to study this phenomenon in the context of nanoindentation and localized mechanical probing of surfaces. Here, we present an analytical formulation of the elastic limit that predicts the location and slip character of a homogeneously nucleated defect in crystalline metals, and extend this formulation to the atomic scale in the form of an energy-based, local elastic stability criterion, termed the lambda criterion. We demonstrate that this approach can be incorporated efficiently into computational methods such as molecular dynamics and finite-element models. Furthermore, we validate and calibrate the lambda criterion directly through nanoindentation experiments and two-dimensional experimental analogs such as the bubble raft model. We outline explicitly a compact and efficient application of the lambda criterion within the context of a nonlinear, interatomic potential finite-element model (IPFEM). Further, we report three-dimensional molecular dynamics simulations in several face-centered cubic systems that elucidate the transition from the initiation to the early stages of plasticity during nanoindentation of metals, as characterized by homogeneous and heterogeneous nucleation of up to hundreds of dislocations. Correlation of these simulations with direct observations from nanoindentation experiments provides atomistic insights into the early stages of plasticity. | ||
| S. Veprek, S. Mukherjee, P. Karvankova, H.-D. Mannling, J.L. He, K. Moto, J. Prochazka, and A.S. Argon | "Limits to the strength of super- and ultrahard nanocomposite coatings" | Journal of Vacuum Science and Technology A, 21 [3], 532-544. | 2003 |
| Abstract: | Hertzian analysis of the nonlinear elastic response upon unloading provides analytical solutions that were used to verify if the hardness values measured on the super- and ultrahard coatings are self-consistent. The analytical solutions were also used to estimate the tensile strength of the coatings. The highest tensile stress occurs at the periphery of the contact between the coating and the indenter and, in the case of ultrahard coatings, it can reach values in the range of tens of Gpa, thus giving an estimate of their tensile strength. The results show that the tensile strength of the superhard nanocomposites reaches an appreciable fraction of the ideal cohesive strength that is predicted on the basis of the universal binding energy relation. The data are compared with finite element computer modeling in order to obtain a deeper insight into the complex problems. Reliable values of the hardness can be obtained if coatings of a thickness greater than 8 um are used and the load-independent values are measured at sufficiently large indentation depths of greater than 0.3 um. | ||
| Y. Choi and S. Suresh | "Nanoindentation of patterned metal lines on a Si substrate" | Scripta Materialia, 48, 249-254. | 2003 |
| Abstract: | A systematic experimental study of nanoindentation on unidirectionally patterned aluminum lines on a Si substrate has been conducted. The results show that the observed increase in indentation compliance with decreasing line width cannot be rationalized on the basis of continuum considerations. A simple mechanistic model based on discrete dislocations is suggested. | ||
| N. Chollacoop, M. Dao, and S. Suresh | "Depth-sensing instrumented indentation with dual sharp indenters" | Acta Materialia, in press. | 2003 |
| Abstract: | A methodology for interpreting instrumented sharp indentation with dual sharp indenters with different tip apex angles is presented by recourse to computational modeling within the context of finite element analysis. The forward problem predicts an indentation response from a given set of elasto-plastic properties, whereas the reverse analysis seeks to extract elasto-plastic properties from depth-sensing indentation response by developing algorithms derived from computational simulations. The present study also focuses on the uniqueness of the reverse algorithm and its sensitivity to variations in the measured indentation data in comparison with the single indentation analysis on Vickers/Berkovich tip (Dao et al. Acta Mater 49 (2001) 3899). Finite element computations were carried out for 76 different combinations of elasto-plastic properties representing common engineering metals for each tip geometry. Young's modulus was varied from 10 to 210 GPa, yield strength from 30 to 3000 MPa, and strain hardening exponent from 0 to 0.5, while the Poisson's ratio was fixed at 0.3. Using dimensional analysis, additional closed-form dimensionless functions were constructed to relate indentation response to elasto-plastic properties for different indenter tip geometries (i.e. 50 degree, 60 degree and 80 degree cones). The representative plastic strain, as defined in Dao et al. (Acta Mater 49 (2001) 3899), was constructed as a function of tip geometry in the range of 50 degrees and 80 degrees. Incorporating the results from the 60 degree tip to single indenter algorithms, the improved forward and reverse algorithms for dual indentation can be established. This dual indenter reverse algorithm provides a unique solution of the reduced Young's modulus, the hardness and two representative stresses (measured at two corresponding representative strains), which establish the basis for constructing power-law plastic material response. Comprehensive sensitivity analyses showed much improvement of the dual indenter algorithms over the single indenter results. Experimental verifications of these dual indenter algorithms were carried out using a 60 degree half-angle cone tip (or a 60 degree cone equivalent 3-sided pyramid tip) and a standard Berkovich indenter tip for two materials: 6061-T6511 and 7075-T651 aluminum alloys. Possible extensions of the present results to studies involving mulitple indenters are also suggested. | ||
| K.S. Kumar, S. Suresh, M.F. Chisholm, J.A. Horton, and P. Wang | "Deformation of electrodeposited nanocrystalline nickel" | Acta Materialia, 51, 387-405. | 2003 |
| Abstract: | The mechanisms of deformation and damage evolution in electrodeposited, fully dense, nanocrystalline Ni with an average grain size of ~30 nm and a narrow grain size distribution were investigated by recourse to (i) tensile tests performed in situ in the transmission electron microscope and (ii) microscopic observations made at high resolution following ex situ deformation induced by compression, rolling and nanoindentation. Particular attention was also devoted to the characterization of the structure in grain interiors and in the vicinity of grain boundaries at Angstrom-level resolution in the as-deposited material and following compression, and to the real-time video-imaging of the evolution of dislocation activity and damage during deformation; these images are presented in this paper and in the web sites provided as supplementary material to this paper. These observations clearly reveal that dislocation-mediated plasticity plays a dominant role in the deformation of nanocrystalline Ni examined in this study. Fracture surface examination confirms dimpled rupture with the scale of the dimples being several times larger than the grain size. Dislocation emission at grain boundaries together with intragranular slip and unaccommodated grain boundary sliding facilitate the nucleation of voids at boundaries and triple junctions. Individual monocrystal ligaments, formed by the growth/linking of these voids, undergo extensive local plasticity to the extent that many of them neck down to a chisel point. These voids as well as those that may have existed prior to deformation can act as nucleation sites for dimples leading to fracture that does not occur preferentially along grain boundaries. The transmission electron microscopy observations of in situ and ex situ deformed specimens are synthesized to formulate a mechanistic framework that provides new insights into the mechanisms of flow and fracture in nanostructured metals. | ||
| K.J. Van Vliet, J. Li, T. Zhu, S. Yip, and S. Suresh | "Quantifying the early stages of plasticity through nanoscale experiments and simulations" | Physical Review B, 67 [10], 104105-1-104105-15. | 2003 |
| Abstract: | Nucleation and kinetics of defects at the atomic scale provide the most fundamental information about the mechanical response of materials and surfaces. Recent advances in the experimental and computational analyses allow us to study this phenomenon in the context of nanoindentation and localized mechanical probing of surfaces. Here, we present an analytical formulation of the elastic limit that predicts the location and slip character of a homogeneously nucleated defect in crystalline metals, and extend this formulation to the atomic scale in the form of an energy-based, local elastic stability criterion, termed the lambda criterion. We demonstrate that this approach can be incorporated efficiently into computational methods such as molecular dynamics and finite-element models. Furthermore, we validate and calibrate the lambda criterion directly through nanoindentation experiments and two-dimensional experimental analogs such as the bubble raft model. We outline explicitly a compact and efficient application of the lambda criterion within the context of a nonlinear, interatomic potential finite-element model (IPFEM). Further, we report three-dimensional molecular dynamics simulations in several face-centered cubic systems that elucidate the transition from the initiation to the early stages of plasticity during nanoindentation of metals, as characterized by homgeneous and heterogeneous nucleation of up to hundreds of dislocations. Correlation of these simulations with direct observations from nanoindentation experiments provides atomistic insights into the early stages of plasticity. | ||
| C.A. Schuh, A.S. Argon, T.G. Nieh, and J. Wadsworth | "The transition from localized to homogeneous plasticity during nanoindentation of an amorphous metal" | Philosophical Magazine, 83 [22], 2585-2597. | 2003 |
| Abstract: | Using nanoindentation, we examine the fundamental nature of plasticity in a bulk amorphous metal. We find that the mechanics of plasticity depend strongly on the indentation loading rate, with low rates promoting discretization of plasticity into rapid bursts. For sufficiently slow indentations, we find that plastic deformation becomes completely discretized in a series of isolated yielding events. As the loading rate is increased, a transition from discrete to continuous yielding is observed. These results are fundamentally different from the classical expectations for metallic glasses, in which the transition from discrete to continuous yielding occurs upon a decrease in the deformation rate. The present experimental results are analysed with reference to the theoretical ideal-plastic strain field beneath an indenter and rationalized on the basis of mechanistic models of glass plasticity. | ||
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