Chemical and Electrochemical characterization of hybrid PVD + ALD hard coatings on tool steel

In the last 30 years, thin hard coatings were intensively investigated due to their high-performance characteristics: high wear resistance, low friction coefficient and good thermal stability. Performances were furthermore improved using multilayer configurations, although often limited by feasibility or cost considerations. One of the main problems to overcome was the low corrosion protection that these coatings assure to the tool steel substrates on which they are applied. To assure high performances and corrosion protection it is possible to employ a completely new approach, suitable either as a pre-treatment of the substrate or as top coating, by using materials of different nature in the form of nano-layers. Monolayer and multilayer coatings obtained by PVD (Physical Vapor Deposition, using reactive arc evaporation) and by PVD + ALD (Atomic Layer Deposition) were studied and compared. Both in-depth composition and corrosion resistance properties were investigated using SEM–EDXS, GDOES (Glow Discharge Optical Emission Spectrometry) and polarization curves in a 0.2 M NaCl solution. The coatings studied were a TiAlN/TiN bi-layer and a TiCN monolayer obtained by PVD and an Al2O3 nano-layer obtained by ALD.