Laser-acoustic, thermal and mechanical methods for investigations of bond lines

Abstract The curing of epoxy adhesives on metal adherends produces interphases near the substrate surface. The determination of the interphase thickness in small joints will be complicated due to the influence of the adherend. To exclude this effect from the characterization of the interphase, resin–metal-composites were created by spin-coating process of different resin layer thicknesses on the aluminium surface. Before the tests, the coated aluminium was cured. The thickness of the cross-linked epoxy layer ranges between 2 and 150 μm. These coated sheet metals were characterized by a new laser-acoustic method (LAwave) and by micro-thermal analysis (μTA). Both methods are non-destructive. LAwave is based on the measurement of laser-induced surface waves. It enables the Young's modulus of thin films to be determined. The μTA was used for studying the interphase formed composites based on aluminium and epoxy resin. It is a thermo-analytical technique that combines the principles of scanning probe microscopy with thermal analysis. This allows samples to be spatially scanned in terms of both topography and thermal conductivity. It is possible to perform localized thermal analysis experiments on discrete small regions with high resolution. The results were compared with results of mechanical (nanoindentation) investigations. We could detect an interphase of some micrometers (about 25 μm) between the aluminium surface and the epoxy-bulk phase.