Quantitive EDX-analysis of PVD hard coatings deposited on sharp edges
Hard metal nitride coatings, used to coat cutting tools to give them excellent wear and oxidation resistance, are often deposited by ion assisted PVD because of the relatively low deposition temperature. However during the coating deposition, focusing of ions around the biased cutting edge occurs. This can have significant impact on the properties and performance of the coating (Jehn et al 1997), and since the edge area is crucial for the cutting applications, it is very important to analyse this problem in detail. To fully characterise the properties of coatings deposited on sharp edges, transmission electron microscopy (TEM) would be ideal. However the sample preparation of electron transparent cross-sectional samples from the cutting edge is very difficult and time consuming. We have therefore started by using scanning electron microscopy (SEM), where the sample preparation is extremely easy compared to TEM, to get as much information as possible in a relatively short time. The SEM combined with energy-dispersive X-ray spectroscopy (EDX) allowed us to analyse the composition of the curved coatings and establish its relationship to the geometrical variables of the edge. However, the quantification of the results from SEM+EDX is complicated because of the large excitation volume relative to the coating thickness. ZAF analysis, taking into account the atomic number, absorption and fluorescence corrections, has to be applied. Additional complications can arise close to an interface between two different materials. This problem was found to be very important in our study when measuring the composition of the coating near the substrate-coating interface.