Mechanical and microstructural properties of post-treated Zn4Al sprayed coatings using twin wire arc spraying

Zn-Al coatings are widely as corrosion protection of metal frames and structures. These coatings are mostly subjected to environmental and static stresses. In order to improve the behavior of Zn-Al coatings in a collective stress environment, post-treatment is required. Machine hammer peening (MHP) is an innovative and promising technique to improve the hardness and surface quality of bulk materials. The presented work deals with the effect of the MHP on the microstructure, hardness, induced residual stresses, and the phase distribution at the near-surface of Zn4Al coatings. The roughness of the post-treated coatings was reduced from almost Rz = 60 µm in the as-sprayed condition to 33 µm after MHP treatment. Coating densification was also revealed in the near-surface zone, reaching a depth of 50 µm. The measurement of hardness profile across the post-treated coatings shows a slight increase in the hardness at the densified zone. The residual stresses were surprisingly compressive at the as-sprayed condition. This could be related to the low melting point of Zn4Al, so that the impacting in-flight particles have a higher kinetic effect then thermal. The compressive residual stresses increase in the post-treated coatings due to the mechanical work applied on the coating surface using MHP.