Characterization of duplex coating system (HVOF + PVD) on light alloy substrates

Light metals such as aluminium or magnesium alloys play an important role in many different industrial applications. However, aluminium and especially magnesium alloys show relatively poor resistance to sliding wear, low hardness and load bearing capacity, so that surface performance improvement is recommended, often by PVD processes. This study evaluates the tribological improvement achieved by applying a duplex coating on AW-7022 aluminium alloy or AZ91 magnesium alloy substrates, consisting of a thick coating interlayer, deposited by High Velocity Oxygen Fuel (HVOF), followed by a PVD (TiN, TiAlN) or PE-CVD (DLC) hard top layers. The deposition of thermal sprayed HVOF coatings, as primary layer, leads to improvement of the load bearing capacity of the substrates and allows reducing the tendency of hard thin top layer to cracking and delamination when it is directly deposited on a softer substrate. The number of laps to failure, in the pin-on-disc wear tests, of TiN and TiAlN PVD coatings deposited on the harder interlayer HVOF coating were significantly higher (3000 and 1400 laps, respectively) than the values measured for these coatings deposited directly on the aluminium substrate (140 and 120 laps, respectively). The best combination of properties was obtained with the DLC top layer deposited on the thermally sprayed coatings, with a significant reduction of friction coefficient (< 0.10), which remains almost unchanged even after 40,000 laps in the pin-on-disc wear tests.