Influence of roughness and grinding direction on the thickness and adhesion of sol-gel coatings deposited by dip-coating on AZ31 magnesium substrates. A Landau–Levich equation revision

Abstract

AZ31 magnesium alloys with four different roughness values and two different grinding directions were coated with sol-gel silica coatings to assess the influence of the initial surface conditions of the substrates on the final coating thickness. Sol-gels were prepared from two silicon alkoxide precursors and deposited on the surface of the magnesium substrates by the dip-coating method. Roughness tester and scanning electron microscopy (SEM) were used to assess the thickness of the silica coatings and the presence of defects in the coatings. Shear stress tests were developed to study the adhesion of the coatings. An analysis of variance was carried out to determine the implication of the substrate roughness and the direction of the grinding lines in the final sol-gel coating thickness and its adhesion on the substrate. The results show that the roughness has a significant influence both on the thickness and on the adhesion of the coatings. Thus, thicker coatings were obtained on substrates with higher roughness values, going from 2.18 μm to 1.42 μm for substrate roughness values of 1.47 μm and 0.27 μm respectively, in the case of horizontal grinding lines, and from 2.01 μm to 1.37 μm for substrate roughness values of 1.47 μm and 0.27 μm respectively, in the case of vertical grinding lines. However, the grinding direction has no significant influence on the thickness nor the adhesion, but it has a clear influence on the formation of defects in the coatings. Finally, the inclusion of the r parameter in the Landau-Levich equation is proposed to adjust it, taking into account the roughness of the substrates intended to be coated and the evaporation of solvent and water from the coating during the dip-coating.