Modelling, production and characterisation of duplex coatings (HVOF and PVD) on Ti-6Al-4V substrate for specific mechanical applications

Titanium and its alloys are extensively used in aerospace and mechanical application because of their high specific strength and high fracture toughness. On the other hand, titanium alloys often show low hardness, very low load bearing capacity and poor resistance to sliding wear, so that surface properties improvement is in many cases recommended, often by PVD processes.Present work deals with design, production and experimental characterisation of a duplex coating for Ti-6Al-4V components, consisting of a thick WC-Co interlayer deposited by High Velocity Oxygen Fuel (HVOF), followed by a Ti/TiN multilayer (two layer pairs, including the Ti bond layer) deposited by Cathodic Arc Evaporation (CAE) PVD.Before deposition, a preliminary coating design was carried out, based on finite element simulation of residual stress fields on the PVD coating for a range of configurations of its multilayered structure (Ti buffer layer position and thickness).Morphological properties of the produced coatings (thickness, grain size, surface defect size distribution, roughness) were measured by means of Digital Optical (DOM), Scanning Electron (SEM), Atomic Force (AFM) and Focused Ion Beam (FIB) microscopy techniques.Coatings mechanical properties were investigated by micro-scratch testing, Rockwell C adhesion test, nano-indentation techniques, Vickers/Knoop micro-hardness testing and composite hardness modelling.Results showed that the use of a CAE-PVD multilayer Ti/TiN top layer, whose thicknesses and Ti distribution were suggested by finite element modelling optimisation, leads to a significant increase (45%) in adhesion of PVD coating to the HVOF layer and load bearing capacity of the coated system, compared to monolayered TiN, without reduction in superficial hardness and load bearing capacity.