Investigation of Fe3Al-based PVD/HVOF duplex coatings to protect stainless steel from sliding wear against alumina

In this work, the sliding wear properties of several duplex (thin-on-thick) coating systems used to protect stainless steels were evaluated under different loading conditions. The coating systems consisted of a 304-stainless steel (SS304) substrate or the same steel previously coated with a layer of iron aluminide (Fe3Al) based material using the high velocity oxy-fuel (HVOF) technique, onto which a thin film of CrN or DLC (diamond-like carbon) was deposited using the physical vapor deposition (PVD). The mechanical properties of coatings were investigated by indentation, and their wear properties were evaluated using a pin-on-disc tribo-system with alumina as counterpart material in ambient conditions under applied loads between 5 N and 15 N. Wear mechanisms of coatings were assessed by examining the morphology of top surfaces and cross-sections of wear tracks, and by analyzing the counterpart material using scanning electron microscopy, energy dispersive and Raman spectroscopies. Results show that the addition of a top coat significantly enhances the wear resistance of the SS304 substrate and of the HVOF coatings due to the excellent tribological properties of DLC and CrN films. Moreover, the PVD/HVOF/SS304 coating systems exhibit excellent wear properties when compared to PVD/SS304 coatings alone due to the high load bearing capacity of the HVOF layers. The wear resistance of such systems increases with the hardness of the HVOF layer. The DLC/HVOF/SS304 duplex coatings show the best tribo-mechanical performance due to their very low coefficients of friction and good bond strengths between layers. On the contrary, the relatively poor tribological performance of the CrN/HVOF/SS304 duplex coatings can be attributed to the brittle nature of the CrN film. The main degradation mechanisms for the DLC-based duplex coatings were adhesive wear and plastic deformation, while in the case of the CrN-based duplex system, brittle fractures and oxidation wear were predominant. In both coating systems, abrasive wear has also contributed to the material loss.