Speaker
Description
Physical Vapor Deposition have now entered the panel of technologies for fabricating metallic Zn-rich coatings on steel. We have recently demonstrated that General Vapor-Phase Galvanizing can coat complex-shape steel articles after fabrication with a fully alloyed Fe-Zn coating.
Surface activation is a key step of the process because the coating results from the reaction of zinc vapor with steel in a range of temperature between 350°C and 550°C. We have tested several industrial protocols able to clean the surface and stimulate its reactivity. Our results show the importance to produce surface sites providing a sufficient diffusive flux of iron and where zinc atoms can adsorb for nucleating Fe-Zn aggregates. The influence of roughness on reactivity will also be discussed. The other genuine aspect of our process is to maintain an adsorption-desorption equilibrium of zinc atoms to and from the surface so that complex-shape articles can be coated at once without making shadows, nor producing over-thickness. Then, complex-shape articles can be coated at once without requiring a complex handling device into the evaporation chamber. These conditions depart from the one of standard (deposition-condensation) PVD or reactive-PVD (deposition followed by the diffusive reaction), and produce uncommon specific microstructures. We will discuss the surface mechanisms that drive and stabilize the process.
The phase composition of coatings fabricated by GVPD is similar to the one of GA steel. However, coatings produced by GVPD are usually two to three time thicker than the GA coatings. GVPD coatings are especially efficient for the fabrication of high standard duplex coatings. Fabrication in the low range of fabrication temperature (350°C – 390°C) can treat high strength steels without producing a significant tempering of martensite. Coating in vacuum after the fabrication of parts bypass the problem of producing a coating compatible with hot stamping.
Keywords
Gas-phase galvanizing, PVD, alloyed coating, surface mechanisms