Deposition of duplex Al2O3/TiN coatings on aluminum alloys for tribological applications using a combined microplasma oxidation (MPO) and arc ion plating (AIP)

Microplasma oxidation (MPO) has recently been studied as a cost-effective plasma electrolytic process to provide thick and hard ceramic coatings with excellent surface load-bearing capacity on aluminum alloys. However, for sliding wear applications, such ceramic coatings often exhibit relatively high friction coefficients against many counterface materials. Although coatings deposited by physical vapour deposition (PVD) techniques such as TiN coatings are well known for providing surfaces with a high hardness, in practice they often exhibit poor performance under mechanical loading, since the coatings are usually too thin to protect the substrate from the contact conditions. In this paper, these challenges were overcome by a duplex process of microplasma oxidation and arc ion plating (AIP), in which an alumina layer Al₂O₃ was deposited on an Al alloy substrate (using MPO as a pre-treatment process) for load support, and a TiN hard coatings were deposited (using AIP) on top of the Al₂O₃ layer for low friction coefficient. Microhardness measurements, pin-on-disc sliding wear tests, and antiwear tests using a Timken tester were performed to evaluate the mechanical and tribological properties. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc test. The research demonstrates that a hard and uniform TiN coating, with good adhesion and a low coefficient of friction, can successfully be deposited on top of an alumina intermediate layer to provide excellent load support. The investigations indicate that a duplex combination of MPO coating and TiN PVD coating represents a promising technique for surface modification of Al alloys for heavy surface load bearing application.