Enhanced surface properties of aluminum by PVD-TiN coating combined with cathodic cage plasma nitriding

Aluminum and its alloys are widely employed in aerospace and automobile industries. However, they exhibit poor surface properties, which considerably reduce their applicability. The titanium nitride (TiN) coatings deposited by physical vapor deposition (PVD) are extensively used on aluminum due to their excellent surface features. Unfortunately, TiN coatings contain pores, voids, and pinholes; as well as they get oxidized during machining, which significantly reduces their surface hardness. In this work, we have investigated a hybrid coating combining TiN (deposited by PVD) with cathodic cage plasma nitriding (CCPN), in alternating sequence, to improve the surface properties of aluminum. The results show that post-CCPN treatment of TiN coating attains higher surface hardness, better wear resistance and excellent film quality due to combined diffusion of iron, nitrogen, and substrate aluminum in TiN. Also, the problem of pores has been eliminated. The results are contradictory to the conventional routine in duplex coatings, where pre-plasma nitriding is typically used.