Erosion resistance of the nanostructured Cr/CrN multilayer coatings on Ti6Al4V alloy

Abstract

The aim of the present work was to investigate erosion properties of the nanostructured Cr/CrN multilayer coatings on titanium Ti6Al4V alloy. Seven types of multilayer (Cr/CrN)×8 coatings were deposited by PVD vacuum arc method. The coatings had the same total thickness and number of the constituent layers but differed in the thickness ratio of Cr and CrN constituent layers (QCr/CrN parameter). The erosion test was performed using sandblasting equipment. Small (40–80 μm), spherical silica particles were used as an erodent, which hit the surface at angle of 90°. The SEM and STEM observation were focused on the cracking behaviour as well as on the microstructure changes in the substrate material and ductile Cr layers.The investigations demonstrate that the applied surface treatment significantly improves the erosion resistance of the titanium alloy. The best erosion resistance was achieved for the Cr/CrN with the thickness ratio of 0.81. Increase, as well as decrease of the QCr/CrN parameter resulted in decrease of erosion properties. Observations performed on cross-sections through the erosion zones revealed the small cracks, which were observed mainly in the CrN layers. The STEM observations showed also microstructure changes in the Cr layers and refinement of grain size in substrate material due to the accommodation of plastic deformation. Microscopic examination revealed also the mechanisms of the effective cracks blocking at the interfaces between the layers as well as in the microstructure of the ductile Cr layers.