The interplay of crack hopping, delamination and interface failure in drying nanoparticle films

Abstract

Films formed through the drying of nanoparticle suspensions release the build-up of strain through a variety of different mechanisms including shear banding, crack formation and delamination. Here we show that important connections exist between these different phenomena: delamination depends on the dynamics of crack hopping, which in turn is influenced by the presence of shear bands. We also show that delamination does not occur uniformly across the film. As cracks hop they locally initiate the delamination of the film which warps with a timescale much longer than that associated with the hopping of cracks. The motion of a small region of the delamination front, where the shear component of interfacial crack propagation is believed to be enhanced, results in the deposition of a complex zig-zag pattern on the supporting substrate.

Figure 1. Time lapse images taken using an inverted microscope in reflection mode.

The image sequence covers a period of 10 minutes. In image (d) the distinct regions of the drying process described in the text are labelled. The dark zig-zag pattern appears first at the delamination front (bright fringes) resulting from the adhesion of a thin layer of the film to the substrate. The cracks hop intermittently resulting in a breaking of the symmetry following the hopping of a crack. The delamination pattern can be seen to move towards the trailing crack tip, changing direction when this trailing crack hops further forward. Once the delamination front has moved to a new position it exerts a force on the crack, causing it to widen where the delamination front becomes pinned. These regions are circled in each image.

Figure 2. Deposits left behind on the substrate after delamination of a silica film.

Two types of surface structure are observed. A pattern which matches the dark zig-zag line observed in the drying films, and deposits which match the location of the cracks in the film. The deposits near the positions of the cracks were not uniform in width. The wider sections arise from the features in Fig. 1 which are formed as the crack slowly widens under the influence of shear stresses in the film.

Figure 3. The influence of shearbands during crack hopping.

The horizontal position of a crack tip () as it propagates through a drying film of 47 nm PS colloids with time. The position of all the shearbands present on the crack trajectory are shown as horizontal dashed lines. The crack tips are observed to hop between different positions that correlate well with the position of shear bands in the sample. Other inhomogeneities in the film may also stop the crack tip but the majority of crack hops finish at the location of shear bands.

Figure 4. Relationship between the positions of the crack tips and the delamination front.

Top and bottom panels show the same film but track the horizontal position of different features that are highlighted in the inset of the bottom panel. (a) The compaction front () moves smoothly as the film dries. In contrast, a crack tip () hops in discrete jumps. The film next to the crack tip () delaminates in response to crack propagation but over a longer timescale. The inset shows a close up of one particular crack hop and delamination event. (b) The delamination of both edges of the piece of film (,) and the position of the delamination pattern at the point of delamination ().

Figure 5. Deposition of the delamination pattern.

(a) Schematic illustrating how the apex of the delamination front becomes compressed and moves in response to crack propagation and delamination of the adjacent film. As the apex moves towards the trailing crack tip more of the film can delaminate. The DP forms a zig-zag pattern as a result of the fact that it tracks the alternating motion associated with crack hopping in the film. (b) The width of the delamination pattern scales linearly with the film thickness. Inset) the y motion of the delamination pattern () correlates well with changes in the width of the deposited pattern ().