Effects of transition element additions on the mechanical and electronic structure properties of Al (111)/6H-SiC (0001) interface: A first principles study

A first principles study Changqing Wang [1, , Weiguang Chen , Jingpei Xie [1]* 1. Collaborative Innovation Center of Nonferrous Metals of Henan Province, College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471023, China 2. Department of Mathematics and Physics, Luoyang Institute of Science and Technology, Luoyang 471023, China 3. School of Physics and Electronic Engineering, Zhengzhou Normal University, Zhengzhou, Henan 450044, China


Introduction
Due to their good physical and chemical properties, SiC particle reinforced aluminum matrix composites have been widely used in aerospace, automobile and other industries [1][2][3]. As a bridge between SiC particle and Al matrix, the SiC/Al interface structure plays an important role in the properties of the composites [4]. In order to improve the interfacial wettability and adhesion of Al/SiC interfaces, elements additive in Al matrix has become one of the most extensively used techniques to fabricate composites with excellent performance [5].
In the experiment, many researchers have studied the wetting and bonding behavior of the interface between Al and SiC [6][7]. By the sessile drop technique in high vacuum, Laurent et.al. have researched the wetting kinetics in the A1-SiC system [8]. They found that the addition of Sn can improve wetting of the A1-SiC interface owing to the decrease in surface tension of Al, while Cu additions deteriorate wetting due to the decrease in A1 interactions with the SiC. The wettability of Al-SiC system can be enhanced by adding a small amount of Mg in Al matrix [9]. Moreover, the experimental results showed that Cu, Si and Mg all can reduce the amount of Al4C3 formed on the interface to varying degrees and improve the Al-SiC interfacial reaction [10][11]. The role of the Si addition in molten Al on the wetting was presumably attributed to its segregation at the interface and the formation of strong chemical bonds with the SiC surface [11].
In recent decades, the first-principles calculation based on density functional theory (DFT) has become one of the most extensively powerful tool to study the metal-ceramic interface information at atomic and even electronic levels [12][13][14][15]. It can accurately estimate atomic and electronic structures at the interface and the influence of alloying element on the stability of the interface [16][17][18]. The results show that strong covalent bonds can be formed at the metal-ceramic interface and the bonding strength of the interface can be improved by adding alloying elements to the metal matrix. In earlier years, the Al-SiC interfaces have been investigated by quantum chemistry methods [19] and ab-initio calculations [20,21]. Recently, the structural and mechanical properties of the Al(111)/6H-SiC(0001) [22][23][24] and Al (100)/6H-SiC(0001) [25] interfaces have been researched by first-principles method.
All these studies suggest that the strong bonding of SiC / Al interface is attributed to the formation of covalent bonds. Apart from these, effects of alloying element additions on interfacial adhesion properties of Al(111)/4H-SiC(0001) [26] and Al(111)/3C-SiC(111) [27] interfaces have been studied by the first-principles method.
However, a systematic theoretical study on effects of transition metal additives on the Al(111)/6H-SiC(0001) interfacial properties have been rarely reported.
In this paper, first-principles calculations were performed to investigate effects of twenty transition element additions on the mechanical and electronic structure properties of Al(111)/6H-SiC (0001)

Details of Calculation Methods
In this work, first-principles calculations were carried out by the Vienna ab-initio simulation package (VASP) code [28,29]. Total energy and electronic structure calculations were performed with the projector augmented-wave (PAW) [30,31] method. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) [32] approach was used to describe the exchange correlation functional. The cut-off energy value of wave functions was set to 600 eV. The energy calculations were conducted in the first irreducible Brillouin zone with a Г-centered 15 15  1 Monkhorst-Pack (MP) grid [33]. The convergence criteria for electron and ion relaxation are 10 -5 and 10 -4 eV, respectively. Meanwhile, for interface calculations, the force tolerance of each atom was set to 10 -2 eV/Å. According to previous studies [19][20][21][22][23]27], the binding energy of the Al(111)/6H-SiC(0001) is the highest when the C (Si) atom is directly above the Al atom. Therefore, we only study this configuration here. As shown in Figure 1 supercell is relaxed to release the internal stress. One of the interfacial Al atoms is replaced by a transition metal atom X. In this way, the interface doping concentration is 25%, while the bulk doping is only 3.57%.

Clean Al (111)/6H-SiC (0001) interfaces
The atomic and electronic structures of Al (111) / 6H-SiC (0001) interface have been given in detail our in previous studies [22,23]. In order to compare with the results of the following doping interfaces, we further study the interface charge transfer. Figure 2 shows the charge density difference of the Al (111) / 6H-SiC (0001) interfaces. The charge density difference is defined as Moreover, the length of C-Al bond is about 1.99 Å, which is much smaller than that of Si-Al bond about 2.53 Å. The shorter the bond length is, the greater the binding energy is, and there should be more charge transfer. The adhesion energy of C-terminated interface is about 3.90 J/m 2 , which is indeed larger than that of Si-terminated interface, 2.93 J/m 2 . The interfacial adhesion energy is defined as the energy required to form the interface per unit area. It is expressed by the formula:  Detailed analysis of atomic charges may help to understand bonding properties such as bond strength. In this paper, we will focus on the atomic net charge distributions according to Bader analysis [34,35]. The Bader charge difference of each atom in the interface system is defined as

Al-X (111)/6H-SiC (0001) interfaces
The effects of alloying elements on the mechanical properties of the interface have been systematically studied by replacing an Al atom with a transition metal atom.
For all 3d and 4d transition families, a total of 20 transition metal elements are considered in this work. The adhesion energy is a very important mechanical parameter to describe the interface bonding characteristics. Similar to the clean interface, it can be defined as  For the C-terminated interface, only Co element doped in the Al matrix can significantly improve the interfacial adhesion energy. It is mainly because the bond strength of C-Co is greater than that of C-Al. However, for the Si-terminated interface, many elements, such as Mn, Fe, Co, Ni, Cu, Zn 3d transition elements and Tc, Ru, Rh, Pd, Ag 4d transition elements, can improve the interfacial adhesion energy. It is mainly due to the greater bonding strength of these elements with Si than that of Al and Si. It can be concluded that the introduction of transition metal elements into Al matrix is mainly to improve the binding energy of the Si-terminated interface. The same conclusion is obtained for Cu doped at the Al(111)/4H-SiC(0001) interface [26] and Mg doped at the Al(111)/3C-SiC(111) interface [27]. That is to say the doping of Cu and Mg into the Al matrix can increase the bonding of the Si-terminated interface, but decrease the binding of the C-terminated interface.  Al-X(111)/SiC(0001) interface, respectively. X stands for a doping element. As can be seen from Figure 4, the bond length at the C-terminated interface, whether C-Al or C-X, is shorter than that at the Si-terminated interface, just like that at the clean interface. It shows again that the bonding strength of C-terminated interface is higher than that of Si-terminated interface. For all transition elements X, the length of C-X bond is longer than that of C-Al bond at the C-terminated interface. The same is true for the Si-terminated interface. This is mainly because the atomic radius of the transition metal element X is longer than that of Al. Due to the introduction of transition elements, such as Mn, Fe, Co, Ni, Cu, Zn, Tc, Ru, Rh, Pd, Ag, the length of Si-Al bond at the Si-terminated Al-X(111)/6H-SiC(0001) interface is shorter than that of clean Al (111)/6H-SiC(0001) interface. The shorter the bond length, the stronger the bond. In order to facilitate researchers to access the relevant data, all results of bond lengths and adhesion energies at the Al-X(111)/6H-SiC (0001) interface are summarized in Table 1 and Table 2.     Especially, when a Co atom is doped into the C-terminated Al(111) /SiC (0001) interface, it obtain electrons not only from the C atom, but also from the Al atom. In this way, the introduction of Co atoms promotes the formation of not only strong Co-C bonds, but also stronger Al-C bonds at the interface. Therefore, the adhesion energy of the C-terminated Al-Co (111) / SiC (0001) interface is higher than that of the clean C-terminated Al (111)  Si-terminated Al(111)/SiC(0001) interface is more prone to existing. Therefore, adding transition metal elements into SiC particle reinforced aluminum matrix composites is mainly used to improve the adhesion energy of Si-terminated interface, and then improve the mechanical properties of the composites.

Conclusions
Effects of 20 transition elements doping on the interfacial adhesion and electronic structure of Al (111) / 6H-SiC (0001) have been studied by first principles calculations in this paper. The main conclusions are summarized as follows: (1) For the clean Al (111) / 6H-SiC (0001) interface, covalent bonds are formed at both C-terminated and Si-terminated interfaces. According to Bader's charge analysis, there is more charge transfer between C and Al at the C-terminated interface, which leads to higher adhesion energy.
(2) For the C-terminated Al (111) / 6H-SiC (0001) interface, the adhesion energy of the interface can be improved only when Co is doped at the interface. The strength of covalent bond between transition metal atom and C atom is weaker than that of C-Al bond. This may be attributed to the larger atomic radius of transition metal atoms.
(3) For the Si-terminated Al (111) / 6H-SiC (0001) interface, when Mn, Fe, Co, Ni, Cu, Zn, Tc, Ru, Rh, Pd or Ag is doped at the interface, the adhesion energy of the interface can be improved. It is mainly due to the formation of stronger Si-X bonds at the interface. The doped transition metal atom not only forms a strong covalent bond with the Si atom, but also promote more charge transfer between Al atoms and Si atoms, forming stronger Si-Al bonds.