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Tougher ceramics through dislocations

Toughness and ductility are desirable properties typically associated with metals, while ceramics are conceived as brittle. Here, the brittleness of some ceramics is revealed to be an artifact of the established synthesis technique. Disentangling the mechanical behaviour of dislocations with dark- field X-ray microscopy and other methods reveals potential to toughen ceramics with dislocations.

In metals, the motion of dislocations, which are one- dimensional lattice defects (Figure 105a), enables deformation on small and large scales. In contrast, the oppositely charged ions in ceramics can make it much more difficult for dislocations to move; they often require temperatures beyond 1000°C to become mobile. In a number of ceramics, however, dislocations can move extremely fast on certain planes, even at room temperature. Single crystals of MgO, LiF or SrTiO3 can all be well deformed [1-3]. Their polycrystals, on the other hand, are all very brittle. If dislocations can move in some ceramics as easily as in metals, why are these ceramics still brittle, and why is there a contrast between single- and polycrystals?

Dark-field X-ray microscopy gives unprecedented insight as a first step to solving this riddle. This new technique, developed at the experimental beamline ID06, is able to visualise individual dislocations [4] in volumes more than five orders of magnitude larger than transmission electron microscopy, as demonstrated in Figure 106. By recording reciprocal space maps for each voxel of 60x60x400 nm3 and with data post-processing, local lattice tilt and strain can be determined for the entire volume with very high local resolution.

Detailed analysis supplemented with transmission electron microscopy and molecular dynamics simulations revealed that neither the mobility nor the multiplication of dislocations are critical. Instead, the difficulty of nucleating dislocations, in combination with a typically very low dislocation density after sintering, is the limiting factor for the ductility and toughness of many ceramics. If no dislocation or dislocation source pre-exists in a certain volume, nucleation is required. However, nucleation requires ≈ 100 times as much stress as moving a dislocation. Because plasticity is required at very local scales, such as at grain boundaries or crack tips, the probability of finding at least one dislocation in the relevant volume is simply too low.

Fortunately, the dislocation density is an engineerable quantity, and tuning it is not an unfeasible undertaking such as changing the bond strength [5]. Therefore, the brittleness of some ceramics appears to be merely an artifact of the established densification technique: sintering. Novel synthesis techniques such as rapid densification [6], e.g., via flash sintering [7] can increase the dislocation density by four to five orders of magnitude. Dislocations then allow local plasticity and inhibit crack propagation as

Fig. 105: a) Sketch of a dislocation with the inserted half-plane in red, slip plane in grey, Burger s vector in green and dislocation line in blue. b) Vickers indentation with 10 g on a surface of a SrTiO3 single crystal in pristine state and with a very high dislocation density of 5*1014 m-2. In the latter case, cracking is completely suppressed.

Fig. 106: Topo-tomography image extracted from a video recorded with dark-field X-ray microscopy of slip bands inside a plastically deformed SrTiO3

single crystal. Dislocations are bundled into shear bands with highly complex structures and are

completely absent elsewhere.