Subsurface cracking during indentation on hybrid coatings on polycarbonate

Abstract

Subsurface cracking after nano-and Vickers indentation on transparent silica-(meth)acrylate hybrid coatings deposited on a polycarbonate substrate was observed for the first time. It appeared that after initiation at the interface, these cracks propagated toward the surface and in radial directions as the indentation load increased. It was found that, for chemically identical coatings, the thicker the coating, the higher load necessary to initiate these cracks. Subsurface cracks formed during nanoindentation occur at lower loads than the normally observed surface cracks and are accompanied by a very small change in the slope of the loading curve.

Cross-Linked Epoxy Composites Filled with Intrinsically Conductive Phthalocyanine Nanocrystals. Influence of Filler Amount, Layer Thickness, and Cross-Linkers Used on the Percolation Threshold and Conductivity Level of the Composites

“…The low φ c found for our cross-linked Phthalcon 11/epoxy composites can be explained by the fractal nature of our primary particle and secondary particle aggregates. It is well-known that both can lower considerably the φ c of conductive nanofiller polymer matrix composites. ,,,, This can be understood as follows. The percolating network can be considered as a network build of N spherical building blocks or blobs with radii R inside a composite volume V .…”

“…Unfortunately, the factors which account for such a low φ c are complicated and partly unknown. It has been reported that the properties of the fillers, such as size, aspect ratio, and surface properties, the properties 1,2 of the matrix including viscosity and crystallinity, , the interfacial energy between the particles and the matrix, ,, and the quality of the nanoparticle dispersion may influence the percolation threshold. ,, However, currently only a few models exist which take into account some of these factors and by which some of the experimental results found could be explained. ,,− …”

“…The morphology of the composite after processing is also important. ,,,, This morphology may be a very complex arrangement of particle aggregates and networks with different size and shape distributions. In some cases these aggregates/networks exhibit a self-similarity when the scale of observation is changed (fractal network structures). ,,, Another problem is that the networks may be distributed in an inhomogeneous way through the matrix …”