RESEARCH NEWS

April 200512

The dentin-enamel junction (DEJ) in

human teeth is an exception to the

general materials rule that an interface

is one of the most susceptible sites for

fracture. Researchers at Lawrence

Berkeley National Laboratory (LBNL),

and the Universities of California at

Berkeley and San Francisco are

seeking to understand this fracture-

resistant system to learn a lesson

from nature [Imbeni et al., Nat. Mater.

(2005) 4 (3), 229]

“In general, when you look at implants

of any sort, the interface is always the

weakest point. If you have a hip implant

the first place it will fail is where it’s

bonded to the bone,” says Robert

Ritchie of LBNL. “Nature does it

brilliantly. By understanding the DEJ

we can solve this interface problem.”

The study found that cracks arrest in

the tough inner layer of the tooth,

dentin, after penetrating the harder

outer enamel layer and the DEJ.

Within 10 µm of crack propagation in

the dentin adjacent to the DEJ, a

series of uncracked ligament bridges

develop. These bridges decrease the

stress or driving force behind the

primary crack, causing it to stop

growing. This fracture resistance is

believed to originate from a gradual

microstructural change between the

dentin and the enamel. The

researchers quantified the DEJ

toughness as 5-10 times higher than

the adjacent enamel and 75% lower

than the adjoining dentin using

interface impingement, scanning

electron microscopy, and Vickers

hardness testing.

The group is applying the findings to

design composites for tooth

restoration. “If you are trying to do a

restoration of the tooth, you could try

to mimic nature and this could be a

very good way to go,” says Ritchie.

Patrick Cain

Learning fromtough teeth BIOMATERIALS

Researchers at the Universidad Autónoma deMadrid in Spain have watched a phase transition ina layer of Pb atoms on a Si surface as it happensby using a scanning tunneling microscope (STM)[Brihuega et al., Phys. Rev. Lett. (2005) 94,046101].The team, led by José M. Gómez-Rodríguez, studieda reversible phase transition in a 1/3 monolayer ofPb on a Si(111) surface. Here, a flat arrangementof Pb atoms with a surface periodicity of (3 x 3) atlow temperature changes to a corrugated (√3 x √3)structure at room temperature. The group’s homebuilt ultrahigh-vacuum STM canremain fixed on the same surface area while varyingthe temperature from 40-200 K. This allows asingle surface region 20 x 20 nm2 in size to be

tracked with atomic resolution in real space andreal time as the temperature is slowly raised. Theresearchers used this ability to observe the phasetransition as it evolved in a defect-free region of Pbatoms. A transition temperature of 86 ± 2 K wasfound, and fitted parameters describing thetransition are in good agreement with thoseexpected for this type of phase transition. Where point defects existed in the sample, thesewere found to locally stabilize the low-temperaturephase. However, the researchers were able toreject these defects as being the driving force forthe phase transition, as had been suggested insome previous reports on the related Sn/Ge(111)system.Jonathan Wood

Watching a phase transition atom by atomCHARACTERIZATION

Dislocations in intermetallics in atomic detailCHARACTERIZATION

The subangstrom resolution of an aberration-corrected scanning transmission electronmicroscope (STEM) has enabled researchersto observe dislocations in an intermetalliccompound directly for the first time[Chisholm et al., Science (2005) 307, 701].This allowed the team from BrownUniversity, Oak Ridge National Laboratory,and UES to confirm the mechanism ofdislocation motion in these materials.Many intermetallics can withstand hightemperatures, making them desirable for theaerospace, defense, and energy industries.However, they are often hopelessly brittle atroom temperature. Plastic deformation isprimarily facilitated by dislocation motion, so

the easier it is to move such defects, theless brittle the material is. “The motion of dislocations in response to anapplied stress is dependent on the atomicarrangement in the vicinity of thedislocation,” explains Sharvan Kumar ofBrown University. “Understanding the localatomic arrangement is a first step inunderstanding dislocation motion andperhaps modifying it favorably.”Deformation in metals and alloys, whichpossess simple atomic arrangements, is wellunderstood. Here, dislocations glide betweenwell-separated slip planes. In more complexcrystal structures, this may not always bepossible. Laves phases, the most commonclass of intermetallics, are one such case. Z-contrast imaging in the aberration-corrected STEM was able to reveal theatomic structure of the Laves phase alloyCr2Hf. Stacking faults and dislocation corescould be observed in atomic detail. Theresults confirm a mechanism for dislocationmotion that was first proposed in the1950s. Here, dislocation motion occursthrough the coordinated movement of atomsin two adjacent planes, or ‘synchroshear’.“The next step is to think about ways tolubricate the synchroshear process byalloying additions so that slip can beenhanced and some much needed plasticdeformability provided,” says Kumar.Jonathan Wood

Z-contrast STEM image of the Laves phase structure of Cr2Hf:

Cr atomic columns are red and Hf atomic columns are yellow.

(Reprinted with permission. © 2005 AAAS.)