Development and optimisation of focused ion beam/scanning electron microscopy as a technique to investigate cross sections of organics coatings

A sample of pre-painted metal was investigated using the dual beam system of a focused ion beam (FIB) and scanning electron microscope (SEM). The FIB was used to remove material (known as ‘milling’), clean and ‘polish’ the sample exposing a cross-section of the coating. SEM was then used to investigate and analyse the structure and composition of the coating system. However, preliminary trials showed that in order to have good compositional data the technique needs to be developed and optimised. This paper presents the experimental work that was carried out in order to achieve this. First the milling area was changed from the centre of sample to the edge of the sample. Second, the mill shape needed to be changed from a rectangle to an isosceles trapezoid to allow better detection of the characteristic X-rays for the detector. Finally the tilt and rotation of the stage were changed for further improvement in X-ray detection. Focused ion beam/scanning electron microscope (FIB/SEM) was found to be a useful technique to study the cross-sections of pre-painted metal. Information from secondary and backscatter electrons images can reveal the quality of the coating (for example adhesion to substrate, pigment dispersion, interfacial properties etc.) and also the thickness of the coating causing less damage to the sample compared to other mechanical sectioning techniques. Additionally it offers the ability to look at specific areas of interest such as defects, contamination and corroded areas. Energy dispersive X-ray spectroscopy (EDS) analysis allows mapping of the elements which are shown distributed in the coating and also the quantification of those elements. The results obtained from EDS analysis were representative of the components that were formulated into the pre-painted metal product.