NEWS & VIEWS

nature materials | VOL 5 | APRIL 2006 | www.nature.com/naturematerials 253

SUBRA SURESHis at the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA. e-mail: ssuresh@mit.edu

Nanoindentation is much used to probe the mechanical response of surfaces, thin fi lms and bulk materials. In this method, a ‘sharp’ or

blunt diamond tip is pressed against a surface. Th e resulting force and depth of penetration are monitored at resolutions of micronewtons and nanometres, respectively, and used to infer the mechanical properties of the indented material. Nanoindentation also provides a means to introduce the line defects known as dislocations into the crystal structure of a material, allowing defect nucleation and atomic-scale mechanisms of strength and failure to be studied. In a recent issue of Nature, Schall et al.1 examine defect nucleation in real time during indentation of colloidal crystals made of silica spheres. Th ese crystals serve as physical analogues for face-centred-cubic (f.c.c.) atomic crystals, such as copper or gold1,2. Despite the complexity in interpreting results, this method could off er insights into some of the morphological and dynamic details that we need to correlate atomic-scale events with macroscopic material properties.

Th e response of crystals to microindentation looks like a continuous rise in the plot of force, P, against indentation depth, h. During nanoindentation, however, early-stage nucleation of dislocations can result in discrete jumps and abrupt discontinuities in the P–h curve3–5 (Fig. 1). Th ese correspond to the moments when, at a fi xed load, a sudden displacement jump takes place or when a sudden drop in load occurs at a fi xed displacement. During homogeneous nucleation — that is, when nucleation of dislocations occurs through breaking of the atomic bonds of an initially perfect crystal — the critical shear stress needed to activate the fi rst such ‘pop-in’ event in f.c.c. crystals generally correlates with the theoretical shear strength4,5. Th e P–h response extracted from nanoindentation thus provides insights into the origins of defect nucleation in initially defect-free crystals.

Th ese studies led to a search for techniques whereby quantitative nanoindentation could be combined with real-time imaging of dislocations.

Consequently, several ‘atomic-scale’ visualization methods have been pursued in recent years. Th ey include: indentation of monocrystalline6 or polycrystalline soap bubble raft s7, mesoscale self-assembly of soft polymeric objects8, and depth-sensing nanoindentation inside a transmission electron microscope (TEM)9. As indicated by the comparison in Table 1, the method developed by Schall et al.1 to image the nucleation and thermal fl uctuation of crystal defects during indentation greatly adds to the collective capabilities of these earlier defect visualization tools.

In the colloidal crystal method1,2, a rectangular block of a defect-free crystal, tens of micrometres along its edges, is obtained by slowly sedimenting silica microparticles onto a patterned substrate. Th e crystal is then indented with a sewing needle whose tip creates a strain fi eld (the area aff ected by the indentation load in which the crystal structure becomes distorted) below the surface. Laser diff raction microscopy2 is used to obtain a real-time image of the dislocation loops (the circular regions inside which the crystal is distorted).

Pushing a sewing needle into a colloidal crystal may seem a crude experiment. On the contrary, combined with laser diffraction microscopy and confocal microscopy, it provides a valuable analogy to nanoindentation and promises a deeper understanding of the mechanical response of crystalline materials.

CRYSTAL DEFORMATION

Colloid model for atoms

P

ah

P

h

ba

Figure 1 Signs of weakness. a, Schematic representation of dislocation nucleation in an initially defect-free crystal. The location of subsurface defect nucleation is marked by the red star. The indentation load P, and penetration depth h, or equivalently the contact radius a, along with the location of defect nucleation, provide valuable information about mechanical properties of crystalline materials. b, During a load-controlled indentation test, a plot of P against h for an initially perfect crystal shows a sudden displacement burst (known as a ‘pop-in’) during homogeneous defect nucleation.

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NEWS & VIEWS

254 nature materials | VOL 5 | APRIL 2006 | www.nature.com/naturematerials

Th ese defects appear in the subsurface region as dark lines on a light background because of the diff erence in the scattering of the laser beam between the perfect and defective spots in the crystal. Confocal microscopy is then used to image the individual colloidal particles1,2 and to map out the strain fi eld produced by the indentation. A noteworthy advantage of the colloidal crystal system is that these dislocation loops and microscopic strain fi elds can be obtained along with the detection of thermal fl uctuations of the colloidal particles over reasonable timescales. Such quantitative results off er new information about indentation tests that could be used to develop predictive models of defect nucleation10.

Th e forces required to nucleate dislocation loops in the colloidal crystal1 are orders of magnitude smaller than those needed for creating homogeneous dislocations in atomic crystals3–5. As a result, quantitative estimates of P against h cannot be obtained from the colloidal crystal model. Although thermal fl uctuations reported for colloidal crystals are similar to those in atomic crystals, the bond energies between the colloidal particles are vastly diff erent from those in atomic crystals. Despite these limitations, the defect nucleation criteria emerging from this work, in conjunction with the fl exibility to observe thermal fl uctuations, provide opportunities to examine dislocation dynamics. For example, extensions of this work could explore how dislocation generation occurs beyond the fi rst ‘pop-in’ associated with

the homogeneous nucleation of dislocations4,5 and how thermal fl uctuations and defect interactions continue to infl uence multiple ‘pop-in’ events (Fig. 1) observed in nanoindentation tests on f.c.c. crystals. Such experiments would also provide insights for the development of atomistic computational simulations of nanoindentation and for formulating theories of defect nucleation and dynamics in crystalline solids10. Th e colloidal system could also off er a physical model to examine how dislocation nucleation at grain boundaries and grain-boundary sliding might combine or compete in infl uencing the strength of nanocrystalline metals7. Such a model might also provide mechanistic information as to why the hardness and strength in nanocrystalline metals are so much more sensitive to the rate of loading than in microcrystalline metals.

REFERENCES1. Schall, P., Cohen, I., Weitz, D. A. & Spaepen, F. Nature 440, 319–323 (2006).2. Schall, P., Cohen, I., Weitz, D. A. & Spaepen, F. Science 305, 1944–1948 (2004).3. Gerberich, W. W., Nelson, J. C., Lilleodden, E. T., Anderson, F. & Wyrobek, J. T.

Acta Mater. 44, 3585–3598 (1996).4. Suresh, S., Nieh, T.-G. & Choi, B. W. Scripta Mater. 41, 951–957 (1999).5. Gouldstone, A., Koh, H. J., Zeng, K. Y., Giannakopoulos, A. E. & Suresh, S. Acta

Mater. 48, 2277–2295 (2000).6. Gouldstone, A., Van Vliet, K. J. & Suresh, S. Nature 411, 656 (2001).7. Van Vliet, K. J., Tsikata, S. & Suresh, S. Appl. Phys. Lett. 83, 1441–1443 (2003).8. Th alladi, V. R., Schwartz, A., Phend, J. N., Hutchinson, J. W. & Whitesides, G. M.

J. Am. Chem. Soc. 124, 9912–9917 (2002).9. Minor, A. M., Lilleodden, E. T., Stach, E. A. & Morris, J. W. Jr J. Mater. Res. 19,

176–182 (2004).10. Li, J., Van Vliet, K. J., Zhu, T., Yip, S. & Suresh, S. Nature 418, 307–310 (2002).

Table 1 Comparison of salient features of the colloidal crystal model with other techniques used for visualization of indentation.

Characteristics Soap bubble raft model6 Self-assembly model8 In situ indentation in TEM9 Colloidal crystal model1,2

Basic features Close-packed planes of soap bubbles simulate atomic planes.

Soft objects interact by capillarity to self-assemble into 2D structure.

A thin fi lm is deposited on a wedge-shaped Si substrate and indented inside a TEM.

Thin fi lms of 3D colloidal crystals patterned on a substrate are indented with a sewing needle.

Scaling analogy A 1-mm bubble represents a 0.3-nm atom and the indenter has equivalent tip radius of a few nanometres.

Soft millimetric objects simulate strength and directionality of inter-atomic bonds; indented by round tip probe.

200-nm polycrystalline thin fi lms are indented normal to the electron beam with a diamond tip.

Microparticles represent atoms, needle tip represents indenter. The rectangular crystal is tens of nanometres along its edges and the tip radius is several nanometres.

Control of initial structure Possible to create initially defect-free crystals, different crystal orientations, amorphous rafts, ‘atomic-scale’ roughness at indented surface6 and nanostructured polycrystals7.

Initially defect-free assembly of few hundred atoms, but diffi cult to design grain structure and surface roughness.

Diffi cult to produce single crystal and initially defect-free grains, or to control crystallographic orientation, grain size and texture.

Possible to create initially defect-free crystals and study polycrystals; mismatch in defect nucleation energies with those of atomic systems.

Quantitative results from nanoindentation

Possible to measure indenter contact radius during defect formation, and location of dislocation nucleation through video imaging.

Video imaging provides information about fracture initiation. Little fl exibility to probe dislocation nucleation.

Provides P–h data, but insuffi cient resolution to identify nucleated defects and details of dislocation interactions.

Indentation-induced defects, strain fi elds, defect nucleation sites and thermal fl uctuations are imaged through laser diffraction microscopy and confocal microscopy.

Physical model constraints and attributes

Raft thickness is at most a few atomic planes, but planar arrays are large compared with bubble and indenter size.

Physical model is limited to a few hundred atoms.

Range of thickness is limited for the thin-fi lm specimen with respect to the indenter tip radius.

Substrate can infl uence results and indenter tip radius is a large fraction of crystal size; possible to study dislocation dynamics.

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