Shear bands and the evolving microstructure in a drying colloidal film studied with scanning µ-SAXS

Scientific Reports, Aug 2018

Shear localisation in thin bands is an important process involved in the plastic deformation of materials subject to stress. This process is often sensitive to the sample microstructure (amorphous/crystalline). Here we show using the scanning µ-SAXS technique, how these different microstructures influence the plastic deformations in a drying colloidal film. In crystalline samples, the presence of an ordering transition at the compaction front was directly identified through the development of a six-fold symmetry in the scattering pattern in 20 wt% samples. It is shown that plastic deformations in individual groups of particles during the compaction process can be tracked and measured in real time. Higher concentration suspensions were found to result in amorphous structures. The transition between crystalline and amorphous microstructures with initial particle concentration was also found to correlate with the appearance of shear bands. Through 2D spatial mapping of the local film structure, the presence of shear bands in the films was directly related to the microscale spatial variations in strain magnitude and compression direction. Our measurements also showed that shear bands lead to a reduction in the local particle volume fraction ~1–2%, indicating significant dilatancy.

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Shear bands and the evolving microstructure in a drying colloidal film studied with scanning µ-SAXS

Abstract Shear localisation in thin bands is an important process involved in the plastic deformation of materials subject to stress. This process is often sensitive to the sample microstructure (amorphous/crystalline). Here we show using the scanning µ-SAXS technique, how these different microstructures influence the plastic deformations in a drying colloidal film. In crystalline samples, the presence of an ordering transition at the compaction front was directly identified through the development of a six-fold symmetry in the scattering pattern in 20 wt% samples. It is shown that plastic deformations in individual groups of particles during the compaction process can be tracked and measured in real time. Higher concentration suspensions were found to result in amorphous structures. The transition between crystalline and amorphous microstructures with initial particle concentration was also found to correlate with the appearance of shear bands. Through 2D spatial mapping of the local film structure, the presence of shear bands in the films was directly related to the microscale spatial variations in strain magnitude and compression direction. Our measurements also showed that shear bands lead to a reduction in the local particle volume fraction ~1–2%, indicating significant dilatancy. Introduction Relating the plastic response of materials under an applied stress to their microstructure (crystalline, amorphous) is a problem of fundamental importance in a range of areas of materials science including granular1, complex fluids2 and metal alloys/glasses3,4. Under a uniaxial stress, shear can become localised within thin bands at ~45° to the direction of compression/tension. These shear bands can significantly modify a material’s properties, however they are still poorly understood. Whilst shear bands can form in crystalline materials, in bulk metallic glasses/alloys, shear localisation and the concomitant effects on material plasticity are believed to be strongly influenced by an amorphous microstructure3,4. Recently, it was shown that a drying colloidal film can also exhibit shear bands5,6,7. A colloidal film is created at the edge of a thin layer of drying suspension by evaporation, which causes a lateral flow of particles. This process is fundamental to the production of ceramic coatings, paint films and photonic crystals. As particles join the film from the suspension, they may undergo a transition from liquid-like to crystalline order8. However, it is also possible that drying induced stresses which result in compaction of the film may lead to an irreversibly aggregated and disordered network8. It is known that the final microstructure can be influenced by a variety of factors such as the rate of film formation9, particle charge10 and particle polydispersity11,12. Following this initial solidification into a film the particles undergo compaction (see Fig. 1a) leading to plastic deformations of the underlying microstructure7,8. However, with conventional techniques it is difficult to relate changes in film microstructure to much larger scale instabilities, such as shear bands. Figure 1 Scanning µ-SAXS setup. (a) Different concentration of AS-40 Silica Ludox were dried horizontally in cells constructed from 2 sheets of Mica with an 180 µm spacer. The microfocus beam (diameter 1 µm) probes a small region of the sample, resulting in a scattering pattern of the local microstructure. Scanning the sample through the beam, spatial variations in the local micro structure can be studied. (b) An optical microscope image of a drying sample showing the growth of shear bands which follow the growth of the film. Where the film first solidifies from the liquid suspension it gradually undergoes compaction. The sample can be scanned through the beam such that (1) single lines are scanned repeatedly or (2) a 2D box is measured. Full size image Films of drying colloidal suspension have been widely studied using Small Angle X-ray Scattering (SAXS)7,13,14,15,16. Using this technique changes in the mean volume fraction13 and strain7 as the drying front moves can be inferred by collecting SAXS patterns from the relatively small region exposed to the incident beam. However, this region is still large compared to many of the structural features of interest (eg shear bands, cracks). Recently, Schroer et al.17 also performed SAXS measurements on dry films containing two different sizes of particle. Using a very small beam diameter (~400 nm) they were able to spatially map the phase separation of the two types of particle. A few innovative optical microscopy studies have also made spatially resolved measurements of deformations near cracks18 and shear bands19,20. The direction of maximum compression was shown to vary locally by ~ ± 5° near shear bands in a colloidal film. However, spatially resolved measurements of the strain and volume fraction variations, which are used for example in theories of shear instabili (...truncated)


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Bin Yang, Nathan D. Smith, Andreas Johannes, Manfred Burghammer, Mike I. Smith. Shear bands and the evolving microstructure in a drying colloidal film studied with scanning µ-SAXS, Scientific Reports, 2018, Issue: 8, DOI: 10.1038/s41598-018-31405-6