The control of adhesion at metal/oxide interfaces is of key importance in modern applications, whenever three-dimensional metal clusters or two-dimensional metal overlayers are to be synthesized on an oxide support. By focusing on the zinc/alumina system, we address here one of the long-standing issues in this context, which is the poor wetting of wide bandgap oxides by noble and post-transition metals. It has recently been recognized to have detrimental industrial consequences for the adhesion of anti-corrosive zinc coatings to new high strength steels grades. We have combined photoemission, thermal desorption and plasmonics with atomistic simulation to describe the energetics of zinc deposits on dry and hydroxylated α-Al 2 O 3 (0001) surfaces. Both experimental and computational results show that an activated reaction of the metal with the OHcovered surface, followed by hydrogen desorption, produces dispersed interfacial moieties involving both oxidized Zn species and undercoordinated oxygen ions, that lead to a significant improvement of adsorption/adhesion characteristics on the hydroxylated surface. In particular, the key role of interfacial undercoordinated anions, remnants of the hydroxylation layer, is highlighted for the first time, pointing to a general mechanism by which surface hydroxylation appears as a promising route towards a systematic improvement of wide band gap oxide wetting by metals. 1
Currently, boron steel with Al-Si coating experiences a rapid growth in the anti-intrusion applications in the car body due to its superior mechanical properties after hot stamping. However, the final microstructure can be sensitive to delayed fracture if the product is exposed to a critical combination of diffusible hydrogen content, stresses and other metallurgical factors. As the metallurgical parameters and stresses are usually defined, the proper control of the diffusible hydrogen content is the key parameter to improve the safety aspect of the product. However, this content is quite difficult to determine. In this paper, the parameters governing the absorption and desorption of diffusible hydrogen in aluminized boron steels is investigated. The present research shows that the dew point and the austenitizing holding time have a bigger influence on the diffusible hydrogen content than the austenitizing temperature. Simultaneously, four-point bending test, which is simple and representative of the stress field that may be encountered in car bodies, is used to determine the acceptable limit of the diffusible hydrogen amount. Using this test, a delayed fracture map is proposed, which can be used as a guideline to determine the safe process areas. The study reveals that fast cooling rates or the sheared edges lead to lower the critical diffusible hydrogen content. Nevertheless, under the standard industrial operating conditions, the materials remain safe. Finally, an e-coating process that is applied to the sample surface induces an efficient degassing that provides an additional safety margin.KEY WORDS: aluminizing boron steel; delayed fracture; hot stamping; diffusible hydrogen; bending test; thermal desorption analysis.
Zinc-based coatings are widely used for protection against corrosion of steel-sheet products in the automotive industry. The objective of the present article is to investigate the deformation modes at work in three different microstructures of a thin (8 m) zinc coating on an interstitial-free steel substrate under tension, plane-strain tension, and expansion loading. Damage mechanisms are addressed in a companion article. The plastic slip and twinning activity in the zinc grains of an untempered cold-rolled coating (labeled NSK), a tempered cold-rolled coating (labeled SK), and a recrystallized coating are compared with the response of the corresponding bulk low-alloyed zinc material. The in-plane grain size in the NSK and SK coatings ranges from 300 to 600 m, vs about 30 m in the recrystallized coating and bulk material. The coatings exhibit a strong crystallographic texture, with the c-axis generally normal to the sheet plane. Basal slip is shown to be the main deformation mechanism in bulk zinc and the recrystallized coating, whereas pyramidal 2 slip and mechanical twinning are found to be major modes in the NSK and SK coatings. These results, obtained from an extensive, quantitative slip-line analysis combined with electron backscattered diffraction (EBSD) measurements, are explained by the constraining effect of the substrate. This effect is successfully modeled using a simple Taylor-like polycrystalline approach. The recrystallized coating behaves much like the bulk material. The interpretation of this grain-size effect between the NSK and SK coating, on the one hand, and the recrystallized coating, on the other hand, requires a full three-dimensional finite-element analysis of the multicrystalline coating provided in this work. The simulations show that strong strain gradients can develop in the recrystallized coating from the interface to the surface, which is not the case in the NSK and SK coatings.
The weak interaction between zinc and alumina is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels. In this context, we report a theoretical study on the eect of realistic multi-component metal buers on the adhesion strength of a model-alumina(0001)jzinc interface. Relying on results of ab initio calculations on relevant individual oxidejoxide, oxidejmetal, and metaljmetal interfaces (separation and interface energies), we determine by Monte Carlo simulations the thermodynamically preferred sequence of components in a multicomponent buer, as a function of buer composition and oxygen conditions. We nd that stainless steel buers considerably enhance the overall strength of the aluminajzinc interface. Most importantly, we show that a partial oxidation of multi-component buers, which is unavoidable under realistic conditions, does not degrade their performance. This advantageous property relies on the separation of metal and oxide components in the buer and on the resulting suppression of weakly interacting oxidejzinc, and moderately strong aluminajmetal interfaces. More generally, owing to the possibility of selective oxidation and component segregation, multi-component buers appear as promising solutions for adhesion improvement at weakly interacting metaljoxide interfaces.
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