A new experimental method for studying the cracking behaviour of PVD multilayer coatings

One way to improve the cracking resistance of PVD hard coatings is to prepare them in the form of a multilayer structure, where micro-cracks tend to branch and deflect at the interface between two alternating layers of different materials. However, it is still not clear how a layer structure influences the cracking resistance. One of the main reasons for the shortage of data in this field is the lack of appropriate experimental methods to prepare the samples and to observe the micro-cracks that appear through the multilayer structure. Due to the extremely small layer thicknesses, a SEM analysis on a perpendicular sample cross-section is too coarse, while a cross-sectional TEM analysis is time consuming and, therefore, expensive. In this study a new experimental method for observating the propagation of micro-cracks in different multilayer structures is presented. It is based on microscopic observations of the micro-cracks on the cross-section of a ball crater over a scratch track. The first step of this test is to prepare a scratch on the sample using a diamond stylus. Next, a ball crater must be ground over the scratch track at the position where the first cracks are observed in the scratch track. Along the edges of the scratch track the low-angle cross-section of the multilayer structure can be observed. On such a cross-section we could clearly see how the micro-cracks propagate from the coating surface through all the layers that compose the multilayer structure. For the inspection of the micro-cracks we used optical as well as SEM microscopy. The applicability of this new technique was tested on a 2.6 μm-thick TiN/CrN multilayer structure with 6, 8, 16 and 32 layers and on a 2.4 μm-thick CrN/TiAlN multilayer structure with 8 layers. The coatings were prepared by reactive sputtering in a Sputron (Balzers) plasma–beam sputtering apparatus. The coating adhesion was characterized with a conventional scratch tester (CSEM Revetest). For the ball–crater preparation we used a steel ball with a diameter of 20 mm and a diamond paste with a grain size 0.25 μm. The ball craters were prepared on the scratch track at the positions that corresponded to the different scratching loads. We discovered that the individual layer structure has a strong influence on the crack propagation as well as on the plastic deformation of multilayer coatings.