Ab initio study of decohesion properties in oxide/metal systems

Several studies of the decohesion properties of various oxide/metal systems have been performed recently by ab initio calculations. However, the use of different computational methods, which involve diverse approximations, energy functionals, or calculation conditions, makes the identification of general trends difficult. In the present work, a broad range of interfaces between an ionic oxide (${\text{Al}}_2{\text{O}}_3$, ${\text{ZrO}}_2$, ${\text{HfO}}_2$, and MgO) and a metal [either transition metal (TM) or Na], has been investigated systematically in order to find correlations among the work of separation (Wsep) and the intrinsic properties of the interface, such as the crystal structure, the strain conditions, or the electronic properties of both constituents. Our main result is that the calculated Wsep adjusts very accurately to a parabolic dependence on the summed surface energies of the metal and the oxide, regardless of the oxide and metal components, crystal lattices, interface orientations, and atomic terminations. Furthermore, Wsep is mostly determined by the surface energies although for interfaces involving nonpolar oxide surfaces the contribution of the interfacial energy is not negligible. The strongest adhesion is found for interfaces formed by polar surfaces and bcc TM, e.g., the Wsep of ${\text{ZrO}}_2{\left(001\right)}_{\text{O}}/\text{TM}$ interfaces changes almost by a factor of 2 depending on whether the TM has bcc or fcc structure. In addition, a correlation between the strain conditions of the equilibrium interface structure and the adhesion properties has been obtained. Finally, in order to predict metal/oxide systems whose mechanical properties are reinforced by the plastic deformation of the metal, we examine the expected behavior of the system beyond the elastic regime in the light of the calculated adherence at the interface. The comparison with the scarcely available experimental data provides good agreement for both the Wsep and the qualitative prediction of mechanical reinforcement.

Figure 1

Top view of the $c{\text{-HfO}}_2/\text{metal}$ or $c{\text{-ZrO}}_2/\text{metal}$ interfaces modeled for different metals and relative crystallographic orientations showing the metal-oxygen interface bonds. Only the Zr, O, and metal planes closer to the interfaces are represented. The relative crystal orientations of the ceramic and the metal are specified for each case.

Figure 2

(Color online) Same as Fig. 1 for the ${\text{Al}}_2{\text{O}}_3{\left(0001\right)}_{\text{O}}/\text{metal}\left(111\right)$ and MgO/metal systems.

Figure 3

Relation between the values of Wsep and $\Delta A_{\text{MET}}$ for the ST and NST interfaces formed by ${\text{ZrO}}_2$ and ${\text{HfO}}_2$ with different metals. The continuous lines correspond to a linear fit of the data. Notice the scale of the vertical axis on the right is twice smaller.

Figure 4

Wsep versus the bulk metal BOP for the NST ${\text{ZrO}}_2{\left(001\right)}_{\text{O}}$ interfaces with different (001) metal structures, empty and full circles correspond to fcc and bcc structures, respectively.

Figure 5

Wsep vs $⟨\sigma ⟩=⟨{\sigma }_{\text{MET}}⟩+⟨{\sigma }_{M_x{\text{O}}_y}⟩$ for all calculated interfaces. The continuous line represents a quadratic fit to the data, circles (triangles down) corresponds to ST interfaces with misfit of less (more) than 10%, and squares (triangles up) to NST interfaces with misfit of less (more) than 10%.

Figure 6

Same as Fig. 5 for the polar interfaces formed only by ${\text{HfO}}_2$ or ${\text{ZrO}}_2$ ceramics. Triangles (circles) correspond to less (more) than 10% misfit between cell constituents.

Figure 7

Wsep vs $⟨\gamma /\sigma ⟩$ for all calculated interfaces. Crosses correspond to NST interfaces formed by O-terminated ${\text{Al}}_2{\text{O}}_3$ and bcc metals with lattice mismatch close to 10%. Inverted triangles correspond to ST ${\text{ZrO}}_2/\text{metal}$ interfaces with more than 10% mismatch. Triangles correspond to NST ${\text{ZrO}}_2\left(001\right)/\text{metal}$ interfaces with more than 10% mismatch. NST ${\text{ZrO}}_2\left(001\right)$ and ${\text{HfO}}_2\left(001\right)$ interfaces with fcc metals and less than 10% mismatch are represented by circles. Squares correspond to ST ${\text{ZrO}}_2/\text{metals}$ with less than 10% mismatch. And rhombus indicate ST interfaces constituted by MgO with Ag or Na and mismatch close to 10%.