Structure, bonding, and adhesion of MoSi$_{2}$/Fe and SiO$_{2}$/MoSi$_{2}$ from first principles

The high-melting-point compound MoSi$_{2}$ is a promising candidate for a high temperature coating on iron steels and refractory metals. A coherent silica scale that forms leads to the high temperature oxidation and corrosion resistance of MoSi$_{2}$. Using periodic density functional theory techniques we examine the adhesion strength, interfacial geometry, and bonding characteristics of MoSi$_{2}$/Fe and SiO$_{2}$/MoSi$_{2}$ interfaces. We predict that MoSi$_{2}$/Fe interfaces have intrinsic adhesion energies of $\sim $3.85 J/m$^{2}$, significantly stronger than the adhesion between iron and other ceramic coating materials such as ZrC and TiC. We find that the bonding at the interface is local, with covalent character exhibited between Fe-Si and Fe-Mo across the interfaces. Experiment shows that amorphous (a) silica forms on MoSi$_{2}$. We use $\beta $-cristobalite to model a-silica, since they have similar local structure. We find that Si-O covalent bonding dominates the interfacial adhesion of SiO$_{2}$/MoSi$_{2}$, yielding strong adhesion energy of 5.75 J/m$^{2}$. These very high interfacial adhesion energies suggest that MoSi$_{2}$ indeed should be a quite thermally stable coating for steels.