Tribological studies on PVD/HVOF duplex coatings on Ti6Al4V substrate

In the present work, tribological performances of duplex coatings (Ti/TiN PVD and WC-Co HVOF) on Ti6Al4V substrates were investigated by analytical modelling and experimental assessment of mechanical/tribological properties. An analytical study of the contact stress distribution under a spherical indenter was performed, considering changes in stress distribution after adoption of the WC-Co intermediate coating as a load support layer. Experimental activities consisted of micro- and nanoindentation testing, HRC adhesion test and ball-on-disc sliding wear test (Alumina counterpart) under dry atmosphere. Morphology of the worn surface and layer failure mechanisms (i.e. wear mechanisms, coating surface and sub-surface delamination, crack propagation modes, role of interfaces) were then analysed by FIB-SEM techniques. Results showed that the use of a WC-Co interlayer with intermediate hardness and stiffness provides a more effective distribution of contact strain, with a significant increase of load carrying capacity and wear resistance of samples. A correlation between results of simulations, H/E and H3/E2 ratios and wear resistance of coatings is finally proposed.