Structural, electronic and adhesion characteristics of zinc/silica interfaces: ab initio study on zinc/β-cristobalite

Abstract

The weak interaction between zinc and silica is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels. With the goal of identifying its microscopic origin, we report an extensive ab initio study on the structural, electronic, and adhesion characteristics of a variety of model zinc/β-cristobalite interfaces, representative for different oxidation conditions. We show that the weakness of the zinc-silica interaction at non polar interfaces is driven by the presence of surface siloxane rings. These latter are drastically detrimental to interface adhesion when intact and their breaking is impeded by a large energy barrier. Conversely, the characteristics of polar interfaces are principally driven by the capacity of zinc to screen the surface compensating charges and to form O-Zn bonds. This screening is especially efficient in an oxygen-rich environment where the substrate-induced partial oxidation of the zinc deposit produces a considerable enhancement of interface adhesion. The identified microscopic mechanisms of interface interactions furnish precious guidelines towards practical attempts to improve adhesion. In particular, processes which enable breaking the surface siloxane rings are expected to noticeably reinforce the interaction at non-polar interfaces.

Theoretical study of metal/silica interfaces: Ti, Fe, Cr and Ni on β-cristobalite

“…Beyond the existing studies treating interfaces of different silica polymorphs with Cu, Ni, Al, Ag, and Pt, [25][26][27][28][29][30][31][32][33] we have recently analyzed zinc/β -cristobalite interfaces. 34 We have shown that the characteristics of the interface are very sensitive to the silica surface composition and structure. The stoichiometric terminations show a reconstruction which saturates all dangling bonds by forming two-membered siloxane rings, which have also been identified on dehydroxylated amorphous silica surfaces.…”

Optimization of Anticorrosive Zinc Coatings: Tuning the Adhesion of Zinc/Silica Contact by Interfacial Ternary Oxide Formation

“…We recall that the zinc/silica interface is very weak, with a separation energy of 0.5 J m −2 or lower, due to the reconstruction of the silicate surface and the formation of weakly interacting siloxane rings. 10 If compared to the calculated separation energy in bulk zinc (1.05 J m −2 ), this clearly shows that the poor performance of zinc coatings to the silica-covered steels is due to the interfacial cleavage at the weaklyinteracting metal/oxide interface. Our present computational results show that zinc/silicate interfaces systematically show much better adhesion characteristics, with E sep twice or more larger compared to the zinc/silica one (see Tab. 3).…”

“…10 ), and thus somewhat smaller than the breaking energy within bulk zinc. However, we remind that the SiO 2rich silicate surfaces and the associated interfaces with zinc are thermodynamically less sta- It is also worthwhile to compare zinc adhesion at silicate surfaces with that at oxygen-rich silica surfaces which can be obtained by silica pre-hydroxylation followed by interface dehydrogenation 10,11 . In this latter case, a larger density of Zn-O bonds is formed than at the silicate/zinc interfaces (24.8 bonds/nm 2 , to be compared with 6.2 at the Zn 2 O 2 silicate/zinc interface), which translates in a very strong interfacial adhesion.…”

“…10,11 However, the twice lower density of such bonds at the zinc/silicate interface, makes its separation energy (E sep ∼ 1.0 J m −2 ) roughly twice smaller compared to the corresponding non-reconstructed zinc/silica interface (E sep ≥ 1.8 J m −2 , Ref. 10 ), and thus somewhat smaller than the breaking energy within bulk zinc. However, we remind that the SiO 2rich silicate surfaces and the associated interfaces with zinc are thermodynamically less sta- It is also worthwhile to compare zinc adhesion at silicate surfaces with that at oxygen-rich silica surfaces which can be obtained by silica pre-hydroxylation followed by interface dehydrogenation 10,11 .…”

“…In particular dispersive interactions account for the majority of the adhesion energy between zinc and reconstructed silica surfaces. 10 In Tab. 1 we show that also the cohesive and surface energies of zinc, zinc oxide, and silica are somewhat improved by inclusion of van der Waals interactions.…”