WEB Improving sample preparation of multicomponent materials for scanning electron microscopy
The bonding of steel and aluminum alloys is of great importance for future light-weight applications. However, those materials are very difficult to join due different thermal expansion coefficients and formation of oxide layers. To overcome this problem, an additional AlSi layer deposited by a PVD process is introduced between the steel and the aluminum as an adhesive layer.
For the thorough examination of such bulk multilayer materials and cross-sections by means of scanning electron microscopy (SEM), optimal sample preparation is essential. A perfect quality of the sample surface is mandatory in particular for electron backscattered diffraction (EBSD) where structural defects below the surface and the grain size strongly influence the pattern quality.
However, this sometimes proves to be difficult and is often time-consuming independent of the preparation routine chosen (mechanical, chemical, FIB milling). Particular difficulties in the mechanical preparation of multicomponent materials with two or more phases with different bonding and thus mechanical characteristics are preferential removal causing corrugations, smearing of soft materials like metals or scratching of the surface if a hard phase produces particles during grinding. In this contribution, we point out obstacles during preparation of such multicomponent materials.
During a first cutting step, it is important not to impair the sample surface to be examined by the different cutting machines. Special care must be taken with such multicomponent materials in order not to damage the prevailing bond. In the multilayer Fe/AlSi/Al sample already standard cutting with a disc saw introduced delamination cracks at the interfaces. Afterwards, when preparing the specimen by means of grinding processes, it is already important that the initial grain size of the abrasive paper is not chosen too coarse as otherwise defects far below the surface are introduced, e.g. completely impeding analysis by EBSD. At the end of manual polishing, the surface quality might be improved even more by using an automated ultrasonic polishing step. It is pointed out that choosing the right abrasive component (Al2O3, SiC, Diamond, etc.) has a large impact in this final process.