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Present paper deals with modelling, production and characterization of HVOF PVD (WC-Co and TiN or CrN, respectively) duplex coatings deposited on Ti-6Al-4V substrate for application in automotive industry. As a preliminary analysis, an analytical study of the contact stress distribution under spherical indenter in both coated and uncoated systems, and a finite element evaluation of residual stresses were performed: results showed that the presence of an interlayer with intermediate hardness and stiffness (such as the WC-Co coating) plays a fundamental role in the improvement of the load carrying capacity. Starting from the results of simulations, morphological and compositional characterization of the coatings were performed using SEM and AFM techniques. Mechanical properties were investigated by micro indentation techniques and composite hardness modelling; toughness of the system was qualitatively analysed by Rockwell C indentation. Wear rate of the coatings was measured by an implemented rotating wheel method. Results show higher superficial composite hardness, toughness, adhesion and lower wear rate, in comparison with the simple monolayer system. (c) 2005 Elsevier B.V. All rights reserved.

Titanium and its alloys are extensively used in aerospace and mechanical applications because of their high specific strength and fracture toughness. On the other hand, titanium alloys often show poor resistance to sliding wear, low hardness and load bearing capacity, so that surface performance improvement is often recommended. Present work deals with design, production and characterization of a duplex coating system for Ti6Al4V components (i.e. crankshafts, piston rings, connecting rods) consisting of an high velocity Oxygen fuel (HVOF) WC-Co interlayer and a Ti/TiN multilayer (two layer pairs) deposited by cathodic arc evaporation physical vapour deposition (CAE-PVD). 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). After the choice of the optimal PVD coating configuration, the influence of HVOF WC-Co substrate roughness on PVD adhesion on cylindrical components was analysed, with the aim of optimising the cost to performance ratio of the coating system. Morphological properties of the produced coatings (thickness, roughness, grain size, interfaces) were measured by means of Scanning Electron (SEM) and Focused Ion Beam (FIB) microscopy techniques, while mechanical properties were investigated using Rockwell C adhesion test, micro-scratch, microand nano-indentation techniques. The use of a multilayer Ti/TiN PVD coating, which thicknesses and Ti distribution were suggested by Finite Element Modelling, lead to a significant increase (about 60%) in adhesion to the HVOF coating, compared to monolayer TiN, without reduction in superficial hardness. Furthermore, the maximum allowed HVOF coating surface roughness (Ra) for an optimal adhesion of the PVD coating has been evaluated. INTRODUCTION An excellent combination of high specific strength, fracture toughness, corrosion resistance and thermal stability makes Titanium and its alloys particularly suitable candidates for biomedical, aerospace and extreme mechanical applications. Titanium alloys do however exhibit some disadvantages: low resistance to sliding wear; low hardness; low load bearing capacity; and poor adhesion with respect to coatings. * Corresponding author: marco.sebastiani@stm.uniroma3.it Plasma nitriding and PVD coating are commercial methods for improving properties in those applications which involve high contact stresses and severe sliding wear. However, a very thin, hard layer on a Titanium alloy substrate (even if hardened by plasma nitriding) cannot lead to a mechanically improved structure in terms of load bearing capacity: differences in both coating and substrate hardness and stiffness do not provide a good distribution of contact stresses, while the presence of localised cracks under contact or tribological loads can generate galvanic corrosion between substrate and the PVD coating. An optimised duplex coating may contain a harder and also stiffer (compared to the nitrided layer) interlayer, which provides a better distribution of contact stresses, avoiding plastic deformation of the substrate and brittle failure of the coating. Bemporad et. Al. showed in a recent work that an effective configuration in terms of enhanced load bearing capacity on Ti6Al4V substrates can be achieved by a duplex coating system, consisting of an HVOF thermally sprayed WC-Co thick interlayer and a cathodic arc evaporation (CAE) PVD (TiN or CrN) thin coating. Adhesion can be enhanced by deposition of multilayered PVD metal/ceramic systems: as an example, several studies showed that the interposition of a Ti interlayer with a thickness of 0.5–1.5 μm accommodates the PVD TiN internal stresses and allows thicker composite coatings to be produced, with significant improvements in toughness, adhesion, impact resistance and corrosion resistance; however, the presence of a relatively thick Ti buffer layer usually involves a significant reduction in hardness and wear resistance of the PVD coating, especially in the case of a hard PVD coating on a soft Ti-alloy substrate. It is therefore clear that a deeper optimisation of coating procedures for Ti alloy based components is required, with respect to their surface mechanical performances and corrosion resistance. The idea for the present research activity was therefore to develop and optimise a coating system for Ti6Al4V components, consisting of a Ti/TiN (two layer pairs) CAE-PVD coating on a HVOF WC-Co thick interlayer, with the aim of increasing adhesion and load bearing capacity of the PVD coating. All experimental work consisted of three different steps: (1) design optimisation procedure of the PVD coating (parametric study of the influence of the Ti buffer layer position and thickness on interfacial stress peak, based on finite element simulation of residual stress); (2) produced coatings morphological, microstructural and mechanical characterisation, (3) study of the influence of the HVOF surface roughness on PVD coating adhesion and final coating procedure application on crankshafts for high performance engines. COATING DESIGN AND OPTIMISATION Starting from results of a previous experimental work by the same authors, coating optimisation consisted of a Finite Element based design of the Ti/TiN (two layer pairs) PVD coating, based on residual stress evaluation for several configurations of its multilayered structure. The objective of this numerical study was to determine the influence of the Ti buffer layer position and thickness on the interfacial residual stress field due to deposition processes. Delamination failure of PVD coatings is generally caused by high interfacial residual stress fields resulting from the deposition process parameters and from poor adhesion. Residual stresses in PVD coatings arise from the contribution and interaction of two main factors: (A) thermal stress (σth), arising from differences in thermal expansion coefficient between coating and substrate during final cooling from deposition to room temperature, and (B) intrinsic stress (σi), which occur as a consequence of deposition parameters induced crystallographic growth orientation; according to models available in literature, they can be expressed as a function of the ionic and atomic flux arriving at the substrate during deposition, and of the energy distribution of the bombarding ionic species. Usually the resulting residual stress field is approximately calculated as follows: th i tot σ σ σ + = (1) CAE-PVD coatings always show high in-plane compressive intrinsic stress, which necessarily involves high normal to surface tensile stress. The use of ductile Titanium interlayers in multilayer Ti/TiN coatings can be effective for the residual stresses relaxation, in order to improve coating-substrate interfacial adhesion and impact wear resistance, but the presence of a Ti interlayer involves also a reduction in mechanical properties of the coating, such as hardness and sliding and abrasive wear resistance. In the case of HVOF coatings, residual stresses also arise from two sources: the quenching stress (σq), due to the instantaneous cooling of impacting droplets, and thermal stress, due to differential thermal contraction. Quenching stresses can be evaluated by the following equation