Growth defect density in PVD hard coatings prepared by different deposition techniques

Abstract

Growth defects are imperfections which form during coating growth. In this work stylus profilometer was used to measure the defect surface density of a given coating surface on smaller areas (typically 2mm2) and larger areas (several tens of mm2). From a large number of defect density measurements on samples from industrial production batches, we performed statistical analysis in order to evaluate the influence of deposition parameters on the defect surface density. We analyzed various PVD hard coatings (TiN, TiAlN, CrN, TiAlN/a-CN, nanostructured AlTiN/TiN and TiAlSiN/TiSiN/TiAlN layers) prepared by different PVD deposition techniques (thermionic arc evaporation, magnetron sputtering) at various deposition conditions. The surface morphology of the coated substrates was examined by field emission scanning electron microscope in combination with focused ion beam and 3D stylus profilometer. We found that the defect density on various samples in the same batch scatters a lot. There can also be a substantial difference (up to 50% in the peak density) on the two faces of the same 3-fold rotated sample. This suggests that the formation of growth defects is sporadic and spatially localized. For this reason a statistical approach for presentation of defect density was used. We found that the defect density depends on sample position, deposition time, type of coating material, and batching material. However, there is no significant difference in peak density distributions for 1-, 2- and 3-fold rotated samples prepared in the same batch. The substrate material type does not have an unambiguous influence.The influence of steel inclusions on coating growth was also analyzed. In order to understand the effects of different inclusions and other irregularities the substrate surface morphology was followed from the cleaning to ion etching and deposition. By transformation of the local (sample-based) coordinate system to the current instrumental coordinate system we were able to locate any defect and followed it through sample preparation. Shallow craters and voids were observed at positions of MnS inclusions, while the growth of TiAlN/CrN nanolayer coating on SiO2 inclusions was coherent.