First principles calculation of interfacial stability, energy, and elemental diffusional stability of Fe (111)/Al 2 O 3 (0001) interface

First-principles calculation is widely used to study solid-solid interfaces, which provides insights into the atomic and electronic structure of an interface including the interfacial stability and adhesion strength. In general, the interface of the Fe/Al 2 O 3 composite material is hardly wetted, and the aluminum oxide layer is firm and thin. It is difficult to observe the interface via an electron microscope. Thus, the changes at the interface were studied by first-principles calculations. Interfacial stability, energy of the Fe (111) surface, the Al 2 O 3 (0001) surface, and Fe (111)/Al 2 O 3 (0001) interfaces were studied using the first-principles calculation method. The work of adhesion ( W ad ), interface energy ( γ int ), and the electronic structure of Fe (111)/Al 2 O 3 (0001) interfaces were studied. The results indicated that W ad of the O-terminated interface was significantly larger than that of the Al-terminated interface. The O-terminated interface was the most stable interface. Furthermore, the O-terminated interface consisted of strong polar covalent bonds and weak metallic bonds, while the Al-terminated interface primarily consisted of covalent and metallic bonds. Furthermore, the segregation of Al atoms at the interface enhanced the stability of the interface structure, and interfacial bonding ability was increased with the increase in aluminum atoms. Only aluminum atoms diffused through the initial oxide layer forming intermetallic compounds on the iron side. The inclusion of Al 2 O 3 significantly impacts the mechanical properties of steel, such as toughness and fatigue, underscoring that it is important to predict and control the inclusions in steel to obtain desired mechanical properties. The insights obtained from the study described here provide fundamental insights and guidelines into tailoring the steel/aluminum composite interface.


I. INTRODUCTION
Interface between metal and oxide is present in engineering structures that involve dissimilar welding of metals, bimetal casting, thermal spraying, and coatings, which are responsible for promoting efficiency and stability. 1 Fe/Al 2 O 3 is one of the common metaloxide interfaces when the ferrous alloy comes in contact with the aluminum oxide layer present on the surface. The bond strength and toughness of the Fe/Al 2 O 3 interface largely determines the performance and reliability of a number of application systems. The application systems range from conventional large-size structural composites to small-size functional films and coatings to microsize nanoelectronic devices. This includes the interaction of ferroalloys with the solid or semisolid aluminum during welding, diffusion bonding, or the antioxidation process. 2 In general, the optical, magnetic, mechanical, or catalytic properties of metal-ceramic composites are strongly affected by the interface microstructure and chemical composition of the interface. In order to understand chemical bonding, adhesion mechanisms, and other phenomena that are specific to metal-ceramic interfaces, several fundamental studies have been recently carried out on interfaces formed by growing ultrathin metallic layers of different metals on oxide substrates. 3 In particular, an ultrathin iron film grown on an Al 2 O 3 substrate has significant potential in the field of nanoscale magnetism, where the Fe/Al 2 O 3 /Fe system is used as a magnetic tunnel junction (MTJ). Arranz et al. 4 presented a study of growth and electronic properties of Fe ultrathin films deposited on Al 2 O 3 substrates at room temperature using soft X-ray photoelectron spectroscopy (SXPS) and resonant photoemission (RPE). They also analyzed the thermal stability of the interface up to 873 K. Heiba 5 studied the effect of iron doping on structural, optical, and electronic properties of doped alumina. The results showed that the diffused reflectance increased with the increase in wavelength but decreased when the content of Fe doping was increased. The intensity also decreased with the introduction of Fe cations into the Al 2 O 3 lattice. The lattice parameters were increased when some Fe atoms replaced the Al atoms in the Al 2 O 3 matrix because of the difference in their atomic radius.
When the surface of the oxidized steel is subjected to thermal cycling and other external mechanical stress, the adhesion of the metal-oxide interface is weak, and the oxide layer tends to peel off. [6][7][8] During the welding process, the appearance of brittle intermetallic phases or other undesired constituents induced by the reaction of iron and alumina at the interface can lead to poor adhesion [9][10][11] and can impact the mechanical properties of the weld zone. 11,12 Additionally, the initial oxide layer also affects the diffusion process and final properties of Al 3 Fe/stainless steel laminate composites, which are fabricated by hot pressed iron and aluminum foils. 13 The interface of the Fe/Al 2 O 3 composite material is hardly wetted and is only physically joint to the extent that the cracks are easily formed at the interface. 14 Considering that the aluminum oxide film is very thin, it is difficult to observe via an electron microscope, and the effects of these changes at the interface were largely studied by calculations. 15 First-principles calculation is widely used to study the solid-solid interface, which reveals the atomic and electronic structure of the interface as well as the interfacial stability and adhesion strength.
The first-principles and interface geometry theory can characterize the atomic structure, calculate the energy of the metaloxide system, acquire the work of adhesion and interface energy, and obtain the effect of oxide on the interface adhesion behavior by using first-principles calculation of heat of segregation. Dong et al. 16 used the first-principles calculation method to discuss the effect of additives on Fe (111)/Cr 2 O 3 (0001) interfacial adhesive strength in austenitic stainless steels. They considered that Fe crystallizes as an fcc structure, and the interface was parallel to the cubic (111) and hexagonal (0001) faces where the direction of Fe (111) is parallel to the direction of Cr 2 O 3 (0001). Using the first principles to calculate the adhesion energy and interface bond strength, the key properties of the interface and the effect of the bond on the interface can be further researched from a microscopic perspective. They studied the segregation of different alloying elements X (X = Si, Al, V, Ti, Mo, W, Nb, Y) at the Fe (111)/Cr 2 O 3 (0001) interface and systematically determined the effect of additives on interfacial adhesion using the first-principles method. The results suggested that the dissolved W, Mo, and Nb were easily segregated at the Fe/Cr 2 O 3 interface, which weakened the adhesive strength of the interface through weak-electron effects. Y, Al, Si, Ti, and V were difficult to be segregated at the interface. Zhukovskii et al. 17 performed a study on the Ag/Al 2 O 3 interface for both Al-terminated and O-terminated interfaces ab initio, including the dependence of adhesion energy on the interfacial distance and interface bond strength. It was shown that adhesion on the Al-terminated and O-terminated corundum substrate varied significantly. For the Alterminated interface, they observed smaller adhesion energy, while for the O-terminated interface, there was significantly large binding energy.
However, the Fe/Al 2 O 3 interface is one of the most widely used interfaces in engineering and is rarely studied. Zhang 18 studied the effect of active elements (HF and Y) on the adhesive behavior of the Fe (110)/Al 2 O 3 (0001) interface by using the first-principles method. In the calculations, the surface stability and the number of surface convergence layers were not elucidated. Instead, they directly used (110) the surface of Fe as the stability interface for calculation. In general, the crystal structure of ferrous alloys such as austenitic stainless steel is fcc with (111) as the close-packed plane. Thus, it is reasonable to select the cubic (111) surface of fcc-Fe and the (0001) surface of hexagonal Al 2 O 3 as the test surface to study the stability of the surface and the number of convergent layers on the surface.
In the study described here, we systematically studied the properties of the Fe/Al 2 O 3 interface by using first-principles calculation. The interface was considered parallel to the cubic (111) and hexagonal (0001) facets, where the [110] direction in Fe (111) is parallel to [1010]. The bulk and surface properties of fcc-Fe and α-Al 2 O 3 are described via simulation of interfacial interactions and the analysis of simulation results. The interfacial energy, electronic structure, and bond of the Fe/Al 2 O 3 interface were calculated for obtaining a stable interfacial state. The stabilities of aluminum atoms at the interface of Fe/Al 2 O 3 and Al/Al 2 O 3 were calculated based on the stable interfacial model and was experimentally confirmed.

II. THEORETICAL CALCULATION METHOD
The calculations were performed with the Cambridge Serial Total Energy Package (CASTEP) code. The first-principles calculation, which is based on plane wave expansion technology, was used in the reciprocal space. The exchange-correlation interactions in the present calculations considered the Perdew-Burke-Ernzerhof (PBE) function in the generalized gradient approximation (GGA) of the plane wave pseudopotential method. 19,20 Ultrasoft pseudopotential was used to calculate the interactions between the ionic nucleus and valence electrons. 21 In order to further optimize the structure, the Broyden-Fletcher-Goldfarb-Shannon (BFGS) algorithm was used for geometry optimization. 22 Furthermore, 3s 2 3p 6 3d 6 4s 2 for Fe, 3s 2 3p 1 for Al, and 2s 2 2p 4 for O were selected as the valence electrons of the atoms. The value of cut-off energy was set at 380 eV, and the value of k-point was set to 10 × 10 × 10 for a bulk. For a slab, a 10 × 10 × 1 k-point mesh was employed, and the cut-off energy was selected as 400 eV for all the slabs.
A vacuum layer of 15 Å is selected for each surface and interface structure to eliminate the interactions between the surface atoms. The BFGS convergence parameters are set in the calculations as follows: the total energy and the maximum force tolerance is set to 1.0 × 10 −5 eV/atom and 0.03 eV/Å, respectively, the stress on   Table I, with deviation between simulation results and experimental data. A good agreement between the calculated values and the experimental data and other calculation results in the fcc-Fe and Al 2 O 3 crystal structure verifies the suitability of the selected method for calculating the interface.
The surface energy of compounds depends on the characteristics of terminating atoms. Al 2 O 3 has three different terminations, such as O-terminated (O-rich), Al-terminated (stoichiometric), and Al 2 -terminated (Al-rich), as schematically shown in Fig. 1. Considering that Al 2 O 3 (0001) is a polar surface, the chemical potential of Al and O atoms should be considered during the calculation of surface energy (σ). The surface energy can be obtained by using the following equation: 28,29 where E slab is the total energy of the fully relaxed surface model, A is the surface area of the slab, N Al and N O are the number of Al and O atoms in the slabs, respectively, and μ slab Al and μ slab O denote the chemical potential for Al and O, respectively. The terms PV and TS can be neglected at 0 K and atmospheric pressure.
In fact, the Al 2 O 3 (0001) slab and bulk Al 2 O 3 could reach equilibrium when the surface structure is fully relaxed. Therefore, the chemical potential of the Al 2 O 3 (0001) slab is equal to the chemical potential of the bulk and is defined by where μ bulk Al 2 O 3 is the total energy of bulk Al 2 O 3 , and μ bulk Al and μ bulk O represent the single atomic energy. According to Eqs. (1)-(3), the following equation is obtained: Considering the stability of the compound at equilibrium, it cannot spontaneously decompose into a single substance. The chemical potential of each component is less than the chemical potential of its corresponding bulk and is described by the following inequalities:  (6), the range of Al chemical potential is given by The heat of the formation of bulk Al 2 O 3 is obtained from 30,31 ΔrH where ∆rH(MxNy) and Etot(MxNy) are the formation enthalpies of the formation and the total energy of Al 2 O 3 , respectively, E bulk (M) is the cohesive energy of Al, and E bulk (N) is the cohesive energy of O.
The range of variation of μ slab Al − μ bulk Al is known. Thus, the surface energy can be obtained using Eq. (4) and has a certain variation. However, when the surface energy is determined in a certain direction of the crystal, it should be constant. In order to deal with this situation, the calculation of the chemical potential of each component in the compound is solved as follows: There is a compound composed of 1 mol of regular solution and T = 0 K; the chemical potential of Al and O are given as follows: Interaction energy is defined as , where z is the coordination number, Na is the Avogadro constant, and u Al-Al , u O-O , and u Al-O represent the bond energies of three chemical bonds, Al-Al, O-O, and Al-O, respectively. From thermodynamics, the narrowly defined normal solutions and the system's free energy calculation expression are as follows: where X Al and X O are the atomic fractions of Al and O components, respectively, and 0 G Al and 0 G O are the Gibbs free energies of Al and O components, respectively. Based on these normal solution energies, the chemical potential of the aluminum and oxygen components in alumina are calculated as −62.05 eV and −435.93 eV, respectively. The ideal work of adhesion (W ad ) is used to describe the bond strength of interface atoms, which is reversible work. It can be calculated by the following equation: 32,33 where E total Fe/Al 2 O 3 and A are the total energy and interface area of the Fe/Al 2 O 3 interface, respectively, E slab Fe represents the total energy of Fe (111) with five layers, and E slab Al 2 O 3 represents the total energy of Al 2 O 3 (0001) with 15 layers.
The interface energy (γint) is an important parameter to evaluate the stability of the interface; γint can be defined as 33,34 where σFe and σ Al 2 O 3 are the surface energies of the Fe (111) and Al 2 O 3 (0001) slabs, respectively, and W ad is the work of adhesion of the Fe/Al 2 O 3 interface.

III. RESULTS AND DISCUSSION
A. Bulk calculations Figure 2 shows the calculated density of states (DOS) of Fe and Al 2 O 3 , and the black vertical dash lines of the DOS represents the Fermi level. From Fig. 2(a), it can be noted that the bond peak is between −5 and 2.5 eV and the metal bond strength between Fe atoms is mainly generated by d-orbital electrons. From Fig. 2 (Table II) were calculated, and the results show that the surface energy is well converged by a five layer thick slab. Interlayer relaxation of the Fe (111) surface as a function of slab thickness is listed in Table III, where Δij represents the increased or decreased percentage of layer spacing compared with the bulk material, and n represents the number of layers. 33,36 It can be concluded from Table III that for 9-layer and 11-layer interlayer tests, the interlayer relaxation between the first layer and the second layer dose not converge, but the rest are convergent, and the calculation of the surface energy indicates that when n ≥ 5, results are convergent such that the five layered Fe (111) surface was selected for further calculations.
Substituting the chemical potentials given in Eq. (4), the obtained surface energy of O-terminated, Al-terminated, and Al 2terminated structures are shown in Table IV. The Al-terminated structure with a lower surface energy is the most stable in the system, while the Al 2 -terminated structure with the highest value is unstable. For determining the suitable number of atomic layers in the Fe and Al 2 O 3 slabs, the interlayer relation was continually measured with identical layer spacing calculation, which is given in Sec. III B. Table V Fig. 3. From the above results of surface energy, it can be seen that for the Al 2 O 3 (0001) surface, the Al-terminated structure is most stable and easy in forming a stable interface. When the Al 2 O 3 surface is Al-terminated, there are many different combinations of atoms on both sides of the interface. The interface formed by the O-terminated structure is also shown in Fig. 3(d).

Work of adhesion and interface stability
Table VI shows the calculated W ad and interfacial optimal separation (d 0 ) of different Fe/Al 2 O 3 interfaces. For the Fe (111)/Al 2 O 3 (0001) interface, the average W ad from the three models (top, bridge, and center) of the Al-terminated interface is 2.40 J/m 2 , and γint is 1.82 J/m 2 . The W ad of the O-terminated interface is greater than that of the Al-terminated interface, and γint of the O-terminated interface is less than that of the Al-terminated interface, indicating that the O-terminated interface is more stable. However, the O-terminated interface needs to be formed from the O-terminated surface, which is unstable because of its higher surface energy, as calculated in Sec. III B. Comparing W ad and γint of the three models of the Al-terminated surface, the bridge model with a higher W ad and lower γint is relatively stable.

Electronic structure and bond
To further explain the bond characteristics of the Fe (111)/Al 2 O 3 (0001) interface, the density of states (DOS) and partial density of states (PDOS) of the Al-terminated (top, bridge, and center) and O-terminated interface were calculated, and results are shown in Figs. 4(a)-4(d). Among them, the black solid line represents the summation state, the blue solid line represents s-states, the red solid line represents p-states, the green solid line represents dstates, and the black dotted line denotes the Fermi level. For the four different interfaces, it can be seen from the figure that there are significant peaks at the Fermi level. This indicates that these interfaces have certain metallic properties. In addition, the s, p, and d orbitals contribute to the total density to some extent, but p and d orbital contributions are more significant. This reveals that the hybridization between p and d orbitals determines the adhesion strength and stability of interfaces. The DOS values at the Fermi level of the top, bridge, center, and O-terminated interfaces are 38.66, 35.48, 35.67, and 35.18 electron/eV, respectively. The stability of the interface increases with the decrease in the DOS value of the Fermi level. Therefore, the O-terminated interface has the best stability among the four kinds of models, and the bridge model is relatively stable in three Al-terminated models, which is consistent with the above  In order to understand the influence of the interface on the atoms near the interface, the Al-terminated (bridge) interface and the O-terminated interface system model was used as an example to analyze the PDOS in different layers of the interface system model. It can be seen from Fig. 5 that the density of atomic states in the first layer of the interface is relatively different from the atoms in the matrix. For the Al-terminated (bridge) interface, interfacial atoms are significantly different from the interior layers. Considering the hybridization between the interfacial Fed orbit and Al-sp orbit, one obvious peak in the range of −4.5 to 1.25 eV was observed, which indicated that the interface bond had a covalent characteristic [ Fig. 5(a)]. In addition, the iron atoms at the interface have large DOS values near the Fermi level, which revealed the formation of the metallic bond. In summary, there is a mixture of covalent bonds and metallic bonds in the Al-terminated (bridge) interface. For the O-terminated interface [ Fig. 5(b)], there is obvious orbital hybridization between the interfacial Fe-3d,4s,4p and O-2p states in the range of −10 eV to 1.25 eV, which indicates that a strong covalent bond is obtained at the interface. These observations suggest the formation of strong covalent bonds (p-d hybridization) in the O-terminated interface, which also explains the calculated largest W ad of the O-terminated interface in Sec. III C 2.
To further compare the different interface bonding between the Al-terminated and O-terminated interfaces, the charge density distributions and the charge density differences are calculated and shown in Figs. 6 and 7, respectively. It can be seen that chemical bonds with different strengths are formed between interfacial Fe, Al, and O atoms in the interface model. As shown in Fig. 6(a), a weak covalent/metallic bond is formed in the Alterminated (bridge) interface, while as shown in Fig. 6(b), there is a charge accumulation region between iron and aluminum atoms at the interface, indicating that a covalent bond is formed at the O-terminated interface. Figure 7(a) shows that the Al-terminated (bridge) interface has localized features at the interface. A charge depletion region exists on the Al side and extends to the Fe atoms of the Fe side, implying that these two atoms form a metal bond at the interface. As shown in Fig. 7(b), the charge depletion region exists in the interfacial Fe atom, and this region has  Table VIII also summarizes the Mulliken bond population analysis results of the interface systems. The overlap population of the Fe-O bond in the Al-terminated (bridge) interface is 0.06, which proves that a covalent bond is formed at the interface. The bond overlap population of the Fe-Al bond is 0.16, which indicates that the metallic bond exists between interfacial Fe atoms and Al atoms. The bond overlap population of the Fe-Al bond is −0.10. From the molecular orbital theory point of view, it can be explained as the electrostatic repulsion between a charged Fe atom and Al atom, which suggests a formation of a metallic bond between Fe-Al. However, for the Fe-O band in the Oterminated interface, the bond overlap population is 0.31 confirms the covalent bond formed at the interface, which corresponds to the results from the density of states and charge density difference analysis.

The stabilities of aluminum with different occupation
The calculated stable interface of Al 2 O 3 and Fe is the Oterminated Fe (111)/Al 2 O 3 (0001) interface which can be used to study the stability with a different occupation behavior of the aluminum atom at the Fe/Al 2 O 3 interface from a microscopic point of view.
Based on Fe/Al diffusion during the welding process, a model of the diffusion welding interface is built. The model consists of Al along with the initial Al 2 O 3 layer on the Al surface in contact with Fe, as shown Fig. 8 The effect of different sites of aluminum atoms on the interfacial bond ability can be described by the interface ideal adhesion energy. 40 The equation of the ideal adhesion energy W ad of the interface is consistent with the equation given above. In addition, the effect of aluminum atom occupation on the interfacial properties can also be expressed as heat of segregation. The equation for heat of segregation is as follows: 41  Table IX lists the calculated heat of segregation and work of adhesion of four configurations with different sites and coverage of aluminum atoms at the Fe/Al 2 O 3 interface. By comparing the work of adhesion, it is noted that when an Al atom is exited at the Fe/Al 2 O 3 interface, the model had the lowest work of adhesion (1.98 J/m 2 ), which means that the state is the initial state of the Al segregation process. Furthermore, when the aluminum atom is inside the Fe cubic structure [as shown in Fig. 9 To reveal the impact of aluminum occupation sites and concentration on the interfacial properties, the work of adhesion is illustrated in Fig. 10. It is interesting to note that for an aluminum atom, no matter which site the aluminum occupies, the adhesion of the interface is improved compared to the clean interface. In particular, the bonding ability of the interface can be increased as the interfacial work of adhesion is increased to 0.23 J/m 2 when an aluminum atom takes the place of an iron atom at the interface. Compared with the clean interface of (0%) Al, it was  found that the interfacial binding ability of Fe/Al 2 O 3 was greatly improved with increase in aluminum content, and when the aluminum atom content reached 100%, the interface structure was most stable.
In order to reflect the effect of aluminum content on structural stability, the interface density of states for different aluminum contents at 0%, 50%, and 100% are plotted in Fig. 11. From Fig. 11(a), it can be seen that for 0% aluminum atoms at interface model more stable. When the aluminum atom content is increased to 100%, as shown in Fig. 11(c), the highest peak value is reduced to 33.15 electrons/eV compared to the structural density of states with a 50% aluminum atom content, implying that the interface structure becomes more stable with an increased number of aluminum atoms. The positive value of interfacial segregation represents segregation from the substrate to the interface (and vice versa). Comparing the segregation energies of identical elements at different positions of the interface, the segregation path of the element can be determined. From the previously calculated values of heat of segregation, the aluminum atoms near the Fe/Al 2 O 3 interface will pass through the Fe/Al 2 O 3 interface into the Fe layer. Next, the segregation value of aluminum atoms at the Al/Al 2 O 3 interface was calculated to be 1.75 eV, while the value of the Fe atom at the Fe/Al 2 O 3 interface was only 0.48 (Table X). This suggests that it is almost impossible for Fe atoms and aluminum atoms to segregate from the substrate to the interface. In summary, from first principles calculation, the aluminum atoms will pass through the initial oxide layer into the iron layer.
The primary advantage of molecular dynamics (MD) is that it gives a direct view of dynamic conformation of molecular structures, which elucidates the relationship between molecular structure and diffusion. In order to verify the dynamic stability, we conducted a preliminary study by finding the migration path and transition state of the atoms in the crystal lattice. Also, using molecular dynamics, we can obtain the lowest energy position and diffusion path of atoms in the crystal lattice to determine the most stable position of atoms. We selected a stable interface model after optimizing Al 2 O 3 bulk and Fe bulk through the cutting surface. The approach and parameters that were set for the calculation work are as follows: first, the time step to 2 fs was set, and the system was allowed to relax by 5000 steps so that it reaches the lowest energy state. After the energy is minimized, the simulation of diffusion was carried out. Simulating the diffusion at a temperature of 927 K by NVT (constant number of atoms, volume, and temperature), the time step was 1.0 fs, the number of steps was 500 000, and the dynamics time was 500 ps. We recorded the configuration of the system and the change in the position of the labeled yellow aluminum atoms. Figure 12 is a simulated cross-section perpendicular to the diffusion interface at a temperature of 927 K, at different times of dynamics, to dynamically describe the diffusion of atoms. As shown in Fig. 12, the red circle represents the position of atomic diffusion, yellow circle represents the labeled atom, and the direction of the arrow represents the direction of atomic diffusion. From the cross-sectional view, we can see that atomic diffusion occurs, and aluminum atoms diffuse into the iron atom layer. Thus, it is reasonable to believe that Al atoms will move   13 where the effect of the initial Al 2 O 3 oxide layer on interdiffusion of Al with pure Fe was studied. The microstructural evolution of intermetallic compound layer nucleation and growth at the Al/Fe diffusion couple was studied by chemical analysis via EDS and crystallography by EBSD. It was observed that aluminum diffuses through the initial oxide layer (Al 2 O 3 ) to form a thin (1.5 μm) Al 13 Fe 2 phase (87 at. % Al and 13 at. % Fe by EDS analysis). The intermetallic compound nucleated and grew on the Fe side across the interface rather than on the Al side, and Al diffused through the Al/Al 2 O 3 /Fe interface, whereas Fe was not able to diffuse to the Al side of the Al 2 O 3 initial oxide layer. Interdiffusion of Al and Fe started with the fragmentation of the oxide layer, which allows the intermetallic compound to grow more rapidly, as observed experimentally. Moreover, Al atoms diffused across the Al 2 O 3 /metal interface and were also observed experimentally, 42,43 where the presence of the initial oxide layer hinders the diffusion of other metal atoms (Ti or Ni) through the layer. In summary, the calculation results are supported by previous experimental studies and are useful in describing the impact of the initial oxide layer of Al 2 O 3 on the nucleation and interdiffusion in Al/Fe diffusion bonded systems.

IV. CONCLUSIONS
First-principles calculations were carried out to study the interfacial properties of the Fe/A1 2 O 3 interface. Four Fe (111)/Al 2 O 3 (0001) models with different terminations, Al-terminated (with different stacking sites, namely, top, bridge, and center) and Oterminated interfaces, were adopted. The work of adhesion (W ad ), interface energy (γint), and partial density of states (PDOS) were calculated. The interfacial atomic configuration, adhesion strength, and nature of bonding are discussed. The conclusions are as follows: (1) The Fe (111) slab with 5 layers and the Al 2 O 3 (0001) slab with 15 layers can represent bulklike interior characteristics. The Al-terminated surface is more stable. 18 electron/eV, respectively. The lower is the DOS value at the Fermi level, and the superior is the stability of the interface. Therefore, the O-terminated interface had the best stability among the four kinds of models, and the bridge model was relatively stable among the three Al-terminated models, which is consistent with the analysis.
(4) The O-terminated interface consisted of a combination of strong polar covalent bonds and weak metallic bonds and exhibited the strongest interfacial interaction, while the Al-terminated interfacial bond was primarily covalent and metallic and exhibited a relatively weaker adhesion strength. (5) With segregation of Al atoms at the interface, the stability of the interface structure was enhanced, and with the increase of aluminum atoms, the interfacial bond ability was increased. From the experimental and computational aspect, only aluminum atoms diffused through the initial oxide layer forming intermetallic compounds on the iron side.