Influence of AlN (0001) Surface Reconstructions on the Wettability of

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Influence of AlN(0001) Surface Reconstructions on the Wettability of an Al/AlN System: A First-Principle Study Junhua Cao, Yang Liu and Xiao-Shan Ning * State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100086, China; [email protected] (J.C.); [email protected] (Y.L.) * Correspondence: [email protected] Received: 5 April 2018; Accepted: 9 May 2018; Published: 11 May 2018

 

Abstract: A successful application of a hot dip coating process that coats aluminum (Al) on aluminum nitride (AlN) ceramics, revealed that Al had a perfect wettability to the ceramics under specific circumstances, which was different from previous reports. In order to elucidate the mechanism that controlled the supernormal wetting phenomenon during the dip coating, a first-principle calculation of an Al(111)/AlN(0001) interface, based on the density functional theory (DFT), was employed. The wettability of the Al melt on the AlN(0001) surface, as well as the effect that the surface reconstruction of AlN and the oxygen adsorption had on Al for the adhesion and the wettability of the Al/AlN system, were studied. The results revealed that a LCM (laterally contracted monolayer) reconstruction could improve the adhesion and wettability of the system. Oxygen adsorption on the free surface of Al decreased the contact angle, because the adsorption reduced of the surface tension of Al. A prefect wetting was obtained only after some of the oxygen atoms adsorbed on the free surface of Al. The supernormal wetting phenomenon came from the surface reconstruction of the AlN and the adsorption of oxygen atoms on the Al melt surface. Keywords: surface reconstruction; first-principle; Al/AlN interface; work of adhesion; contact angle

1. Introduction Aluminum nitride (AlN) ceramics possess excellent thermal and dielectric properties, where they have been widely utilized as packaging substrates of a high power semiconductor after surface metallization [1]. Aluminum (Al) is suitable for the metallization, because it has a high electrical conductivity and it is soft, meaning that it does not exert excessive thermal stress on the AlN ceramic substrates after they bond. Al has the potential to improve the thermal shock tolerance property, which is one of the most important properties of the substrate. However, Al is easily oxidized in air, and the oxide film hinders its ability to bond with ceramics. The wettability of the Al melt on the ceramics is also poor, making coating Al onto ceramics extremely difficult [2]. To solve this problem, a low-pressure plasma spraying method [3], molecular beam epitaxy (MBE) [4,5], or magnetron sputtering methods [6] have been used to coat the Al onto the ceramics. Our research group has recently proposed a more convenient and simple hot dip coating method in order to coat Al onto these ceramics [7]. This method allows a 5 µm thick Al film to be coated on the surface of AlN ceramics [8–10]. A perfect wetting is essential for the hot dip coating process, because the liquid that is attached on the solid substrate must spread over the substrate to form a film. Numerous experiment results have revealed that a uniform film is formed on the solid only when the equilibrium contact angle of a liquid on a solid is less than 20◦ , or when the receding contact angle reaches 0◦ [11]. However,

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the equilibrium contact angle of the Al melt on the AlN ceramics, which has been measured by a lot of Materials 2018, 11, FOR PEER REVIEW of the 10 hot research groups, is xhigher than 75◦ under 1073 K [12–14], which is also a typical temperature2 in dip coating experiments. Therefore, a supernormal wetting phenomenon actually occurs in the hot dip a liquid on a solid is less than 20°, or when the receding contact angle reaches 0° [11]. However, the coating of Al on the AlN ceramics. Furthermore, such a supernormal wetting phenomenon was also equilibrium contact angle of the Al melt on the AlN ceramics, which has been measured by a lot of observed in the fabrication of composite materials. Nautiyal et al. [15] used a molten route method to research groups, is higher than 75° under 1073 K [12–14], which is also a typical temperature in the fabricate the coating Al/BNexperiments. (boron nitride) composite materials in the air at 973 K. They observed a layer hot dip Therefore, a supernormal wetting phenomenon actually occursthat in the of thehot AlN had formed on the surface of the BN, as a result of the interface reaction, which led dip coating of Al on the AlN ceramics. Furthermore, such a supernormal wetting phenomenon to a tight was combination withinthe The perfect wetting materials. of Al on AlN is key fabrication also observed theAl. fabrication of composite Nautiyal et to al. the [15]successful used a molten route of methodcomposite to fabricatematerials. the Al/BN (boron nitride) composite materials in the air at 973 K. They observed the Al/BN that experimentally a layer of the AlN confirmed had formed on thethe surface of thecontent, BN, as a result of the interface and reaction, We that oxygen the temperature, the which soak time led to a tight combination with the Al. The perfect wetting of Al on AlN is key to the successful influenced the soundness of the dip-coated Al film [8]. The experimental condition of the hot dip fabrication of the Al/BN composite materials. coating was compared with the contact angle measurement to find the following differences: (1)

(2)

We experimentally confirmed that the oxygen content, the temperature, and the soak time influenced the soundness of theindip-coated film [8]. Thedip experimental condition of the hot dip that The AlN ceramics are soaked an Al meltAlduring a hot coating experiment, which means coating was compared with the contact angle measurement to find the following differences: they are in an Al-rich and N-poor environment. This differs from the contact angle measurement,

where AlN ceramics typically maintained high environment. (1) Thethe AlN ceramics are are soaked in an Al melt duringina ahot dipvacuum coating experiment, which means that they are in an Al-rich and N-poor environment. This differs from the contact angle There are dozens of parts per million (ppm) of oxygen in the atmosphere of the dip coting measurement, where the in AlN ceramics arewhich typically maintained in than a highthe vacuum environment. equipment that were used experiment, is much higher residual oxygen that is (2) There arecontact dozens angle of parts per million (ppm) oxygen environment. in the atmosphere of the dip coting found in the measurement’s highofvacuum equipment that were used in experiment, which is much higher than the residual oxygen that is

found in the contact angle measurement’s high vacuum environment. Previous reports have demonstrated that an Al-terminated AlN(0001) surface would reconstruct in an Al-richPrevious and N-poor environment [16,17]. Lee et al. [16] has observed several ofreconstruct reconstruction reports have demonstrated that an Al-terminated AlN(0001) surfacetypes would ◦ RHEED (reflection high-energy electron diffraction) from 700–800 several C MBE AlN(0001) in an Al-rich and N-poor environment [16,17]. Leepatterns et al. [16] has aobserved types of surface, including RHEED 2 × 2, 1(reflection × 3, 2 ×high-energy 6, and so on. They studied the structures and the°Cstability reconstruction electron diffraction) patterns from a 700–800 MBE of AlN(0001) surface, including 2 × 2, 1 × 3, 2 using × 6, andfirst-principle so on. They studied the structures the stability several AlN(0001) surface reconstructions calculations. Theyand found that a 2 × 2 of several AlN(0001) surface reconstructions using first-principle calculations. They found that a 2 ×at the structure of N adatom at the H3 site is stable in N-rich conditions, a 2× Al adatom √  of Al √ 2 structure 2 structure of N adatom at the H 3 site is stable in N-rich conditions, a 2 × 2 structure of ◦ adatom T4 site becomes stable as the Al chemical potential increases, and a 3 × 3 R30 structureatof the the T4 site becomes stable as the Al chemical potential increases, and a √3 × √3 R30° structure of Al laterally contracted monolayer (LCM, see Figure 1) is more stable in Al-rich conditions. Similar the Al laterally contracted monolayer (LCM, see Figure 1) is more stable in Al-rich conditions. Similar results have been obtained by other researchers [17–19]. In the hot dip coating experiment, an LCM results have been obtained by other researchers [17–19]. In the hot dip coating experiment, an LCM reconstruction willwill alsoalso form onon thethe AlN(0001) becauseofofthe thesimilar similar Al-rich environment. reconstruction form AlN(0001)surface, surface, because Al-rich environment.

Figure 1. Top views of the LCM aluminum nitrate (AlN)(0001)/aluminum (Al)(111) interface models.

Figure 1. Top views of the LCM aluminum nitrate (AlN)(0001)/aluminum (Al)(111) interface models. The red balls represent the Al atoms of the AlN(0001) slab, the blue balls represent N atoms, and the The red balls represent ofthe thelaterally AlN(0001) slab, the blue balls represent N atoms, and the green balls representthe theAl Al atoms atoms of contracted Al monolayer. green balls represent the Al atoms of the laterally contracted Al monolayer. The surface reconstructions could influence the wettability of the Al on the AlN ceramics. Liu et 2O3(0001) surface by a first-principle calculation, al. [20] studiedreconstructions the wettability of the Al melt on the Al The surface could influence the wettability of the Al on the AlN ceramics. and found that the Al on the Al2O3(0001) × R ± 9° reconstruction surface had a better Liu et al. [20] studied the wettability of the Al√31 melt√31 on the Al2 O3 (0001) surface by a first-principle  AlN(0001)/Al(111) √ the interface wettability than that on an Al2O3(0001) 1 × 1 surface. In√this work, ◦ reconstruction surface calculation, andas found thatforthe on the Al2 Ocalculations. 31 × 31that R the ± 9surface 3 (0001) was chosen a model theAl first-principle The impact reconstruction had aand better wettability than that Alwettability 1× In this work, experiment condition hadon onan the the1 surface. system was detailed inthe the AlN(0001)/Al(111) calculation, in 2 O3 (0001) of interface chosen as model for the first-principle calculations. The that the surface orderwas to illuminate theatheory of supernormal wetting in the hot dip coating of impact Al on AlN ceramics.

reconstruction and experiment condition had on the wettability of the system was detailed in the calculation, in order to illuminate the theory of supernormal wetting in the hot dip coating of Al on AlN ceramics.

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2. Method Method 2. 2.1. Calculation Method 2.1. Calculation Method The calculations in this work were based on the Density Functional Theory (DFT) and The calculations in this work were based on the Density Functional Theory (DFT) and used the used the Vienna Ab initio Simulation Package [21]. A plane-wave basis set was used for the Vienna Ab initio Simulation Package [21]. A plane-wave basis set was used for the expansion of the expansion of the single-particle Kohn–Sham wave functions. The generalized gradient approximation single-particle Kohn–Sham wave functions. The generalized gradient approximation of the Perdew– of the Perdew–Burke–Ernzerhof (GGA-PBE) [22] was employed to so as to approximate the Burke–Ernzerhof (GGA-PBE) [22] was employed to so as to approximate the exchange-correlation exchange-correlation energy. The interaction between the ions and electrons was described by the energy. The interaction between the ions and electrons was described by the projector-augmented projector-augmented wave (PAW) method [23], which has been shown to be in good agreement with wave (PAW) method [23], which has been shown to be in good agreement with other pseudo other pseudo potentials and exchange-correlation functions [16,18,19]. potentials and exchange-correlation functions [16,18,19]. Our calculation method was validated by the calculation of the lattice constants of thee bulk Al Our calculation method was validated by the calculation of the lattice constants of thee bulk Al and AlN. The calculated lattice constants of Al were a = b = c = 4.049 Å, which were in good agreement and AlN. The calculated lattice constants of Al were a = b = c = 4.049 Å, which were in good agreement with the experimental value of 4.03 Å [24], and a calculation value of 4.039 Å, which was previously with the experimental value of 4.03 Å [24], and a calculation value of 4.039 Å, which was previously obtained [25]. The calculation results were a = b = 3.112 Å, c/a = 1.602 for the lattice constants of AlN, obtained [25]. The calculation results were a = b = 3.112 Å, c/a = 1.602 for the lattice constants of AlN, which was in good agreement with the experimental value (a = b = 3.11 Å, c/a = 1.602 [26]) and the which was in good agreement with the experimental value (a = b = 3.11 Å, c/a = 1.602 [26]) and the previous calculation results [19]. previous calculation results [19]. Montesa et al. [27] observed that the Al-Si alloy/AlN interface by HRTEM (high Montesa et al. [27] observed that the Al-Si alloy/AlN interface by HRTEM (high resolution resolution transmission electron microscope) and found an orientation relationship of transmission electron microscope) and found an orientation relationship of Al(111)h110i||AlN(0001)h1120i. The Al/AlN interface models were based on these findings. Al(111)〈110〉||AlN(0001)〈1120〉. The Al/AlN interface models were based on these findings. Six Al Six Al layers of Al(111), and three Al–N bilayers of AlN(0001) were contained in each interface model. layers of Al(111), and three Al–N bilayers of AlN(0001) were contained in each interface model. Both Both of the Al terminated and N terminated conditions of the AlN slabs were taken into consideration. of the Al terminated and N terminated conditions of the AlN slabs were taken into consideration. Three different models were established, which were dependent on the matchup between the Al(111) Three different models were established, which were dependent on the matchup between the Al(111) slab and the AlN(0001) slab. The outmost Al atom layer of the Al(111) slab was sited on top of the slab and the AlN(0001) slab. The outmost Al atom layer of the Al(111) slab was sited on top of the surface atom (type A), subsurface atom (type B), and a hollow position (type C) of the AlN(0001) surface atom (type A), subsurface atom (type B), and a hollow position (type C) of the AlN(0001) slab, slab, see Figures 2 and 3. In the LCM reconstructed interface model (Figure 4a), the AlN(0001) slab see Figures 2 and 3. In the LCM reconstructed interface model (Figure 4a), the AlN(0001) slab contained four Al–N bilayers and the Al(111) slab contained five Al layers. A laterally contracted contained four Al–N bilayers and the Al(111) slab contained five Al layers. A laterally contracted monolayer of Al atoms was added on top of the AlN slab. The slight mismatch (about 7%) between monolayer of Al atoms was added on top of the AlN slab. The slight mismatch (about 7%) between the Al(111) slab and the AlN(0001) slab was deemed to be absorbed through the distortion of the soft the Al(111) slab and the AlN(0001) slab was deemed to be absorbed through the distortion of the soft Al. A 10 Å thick vacuum slab was added on the top and bottom of the models, in order to prevent Al. A 10 Å thick vacuum slab was added on the top and bottom of the models, in order to prevent interactions between the slabs and their mirror images. A 9 × 9 × 1 k-points mesh was chosen for all interactions between the slabs and their mirror images. A 9 × 9 × 1 k-points mesh was chosen for all of the models, so as to ensure both accuracy and efficiency. of the models, so as to ensure both accuracy and efficiency.

Figure 2. Side views of the Al-terminated AlN(0001)/Al(111) interface models before structure Figure 2. Side views of the Al-terminated AlN(0001)/Al(111) interface models before structure optimization. The red balls represent the Al atoms and the blue balls represent the N atoms. (a) Aloptimization. The red balls represent the Al atoms and the blue balls represent the N atoms. (a) Al-TA ; TA; (b) Al-TB; and (c) Al-TC. (b) Al-TB ; and (c) Al-TC .

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Figure 3. Side views of the N-terminated AlN(0001)/Al(111) interface models before structure Figure 3. Side views of the N-terminated AlN(0001)/Al(111) interface models before structure optimization. red of balls represent the Al atoms and the blue balls represent the N atoms. N-TA; Figure 3. Side The views the N-terminated AlN(0001)/Al(111) interface models before(a)structure optimization. The red balls represent the Al atoms and the blue balls represent the N atoms. (a) N-TA ; (c) red N-Tballs C (b) N-TB; and optimization. The represent the Al atoms and the blue balls represent the N atoms. (a) N-TA; (b) N-TB ; and (c) N-TC . (b) N-TB; and (c) N-TC

Figure 4. Side views of the LCM AlN(0001)/Al(111) interface models. The red balls represent the Al atoms and blue balls represent N atoms. (a) Before structure optimization and (b) after structure Figure of of thethe LCM AlN(0001)/Al(111) interface models. The red balls the Al optimization. Figure 4. 4. Side Sideviews views LCM AlN(0001)/Al(111) interface models. The redrepresent balls represent

atoms blue balls represent N atoms.N(a) Before(a) structure optimization and (b) after the Al and atoms and blue balls represent atoms. Before structure optimization and structure (b) after structure optimization.

2.2.optimization. Work of Adhesion and Contact Angle

The contact angle (θ) is usually used to describe the wettability, which is defined by the Young’s 2.2.Equation, Work of Adhesion and Contact Angle as follows: 2.2. Work of Adhesion and Contact Angle The contact angle (θ) is usually used to wettability, which is defined by the Young’s (1) σsvdescribe = σsl + σlvthe · cos θ The contact angle (θ) is usually used to describe the wettability, which is defined by the Young’s Equation, as follows: Equation, follows: where σas sv , σsl , and σlv are the surface or interface energy of the solid-vapor, the solid-liquid, and the =sv =σslσsl++ σ(W σlvlv·adcos ·)cos θ ) important, which is defined as the(1) (1) (also sv σadhesion liquid-vapor interfaces. The work of σthe is θ work that is required to separate an interface into two free surfaces without considering the elastic and σσlvlvare arethe thesurface surfaceor orinterface interfaceenergy energyof of the the solid-vapor, solid-vapor, the the solid-liquid, and the where σσsv sv, σσslsl,, and deformation and diffusion [28]: liquid-vapor liquid-vapor interfaces. interfaces. The The work work of of the the adhesion adhesion (W (Wadad) )isis also also important, important, which which isis defined defined as as the the (2) Wad into = σsv two + σlv free − σslsurfaces without considering work work that is required to separate an interface considering the the elastic elastic deformation diffusion [28]: deformation ad is linked with θ by Young–Dupré ’s Equation, as follows: The Wand Wad = σsv + σlv − σsl (2) Wad == σσsv·[1+ +σlvcos(θ)] − σsl (3)(2) W ad lv The W Wadadisislinked linkedwith withθθ by by Young–Dupré’s Equation, as as follows: follows: ’s Equation, The Strictly, the contact angle, Young–Dupré which is calculated by the equation, should be a receding one, in consideration of the definition of Wad. WadW can =beσ calculated by the following equations: (3) + cos(θ)] Wad =ad σlv lv· ·[1 cos(θ )] (3) [1 + tot tot tot Wad = (Eslab Al + Eslab AlN − Einterface )/A (4) in Strictly, the contact angle, which is calculated by the equation, should be a receding one, Strictly, the contact angle, which is calculated by the equation, should be a receding one, consideration of the definition of Wad. Wad can be calculated by the following equations: in consideration of the definition of Wad . Wad can be calculated by the following equations: tot tot tot Wad = (Eslab (4) Al + Eslab AlN − Einterface )/A

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 tot tot ( E tot slab( Al ) + Eslab( ALN ) − Einter f ace /A

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tot tot tot tot tot and EEtot are and EEinter is the where EEslab arethe thetotal totalenergy energy of of the the Al Al slab and the AlN slab, and interface slab(Al slab (AlN f ace Al ) slab ALN ) total energy of the interface model. A is the area of the interface.

3. Results Results and and Discussion Discussion 3. 3.1. Al-Terminated Al-Terminated AlN(0001)/Al(111) 3.1. AlN(0001)/Al(111) Interface Interface Models Models The optimized optimized structures structuresof ofthe theAl-terminated Al-terminatedAlN(0001)/Al(111) AlN(0001)/Al(111) interface models shown The interface models areare shown in in Figure 5. Compared with the original models (Figure 2), fewer position changes near the interface Figure 5. Compared with the original models (Figure 2), fewer position changes near the interface were were found found throughout throughout the the three three models, models, indicating indicating aa favorable favorable stability. stability.

Figure 5. Side views of the Al-terminated AlN(0001)/Al(111) interface models after the structure Figure 5. Side views of the Al-terminated AlN(0001)/Al(111) interface models after the structure optimization. The red balls represent the Al atoms and the blue balls represent the N atoms. (a) Aloptimization. The red balls represent the Al atoms and the blue balls represent the N atoms. (a) Al-TA ; TA; (b) Al-TB; and (c) Al-TC. (b) Al-TB ; and (c) Al-TC .

The calculated work of the adhesion (Wad) and contact angle (θ) throughout the three interface Theiscalculated work1.of the adhesion (Wad ) and contact anglethe (θ) values throughout threetension interface models listed in Table There were minor differences between of thethe surface of models is listed in Table 1. There were minor differences between the values of the surface tension of the Al melt at 1073 K by different researchers [29–31], so the calculated contact angle was a bit the Al melt at 1073 K by different researchers [29–31], so the calculated contact angle was a bit scattered. scattered. The temperature, 1073 K, was approximately the same as in the dip coating. The results The temperature, was approximately the same as models in the dip coating. The results demonstrated demonstrated that1073 the K, contact angle values of the three were higher than 83°, which agreed ◦ that the contact angle values of the three models were higher than 83 , which agreed well with the well with the results of many sessile drop experiments [12–14]. In the work of Kumamoto et al. [32], 2W results of many sessile drop experiments [12–14]. In the work of Kumamoto et al. [32], a 2.46 J/m 2 ad a 2.46 J/m Wad was obtained in an Al/AlN interface model, which was similar to the Al-TA model that was in in an Al/AlN interface model, which was similar to the Al-TA angle, model that wasapparently presented was obtained presented this work. However, such a W ad led to a 0° contact which in this work. However, such a Wresults a 0◦ contact angle, k-points which apparently the ad led to disagreed with the experiments [12–14]. The smaller mesh thatdisagreed was usedwith in their experiments results [12–14]. The smaller k-points mesh that was used in their calculation, 3 × 3 × 1, calculation, 3 × 3 × 1, might have been responsible for the deviation. might have been responsible for the deviation. Table 1. Calculated work of the adhesion (Wad) and contact angles (θ) of the aluminum (Al)Table 1. Calculated work of the(AlN)(0001)/Al(111) adhesion (Wad ) andinterface contact angles (θ)atof1073 the aluminum (Al)-terminated terminated aluminum nitride models K. aluminum nitride (AlN)(0001)/Al(111) interface models at 1073 K. Contact Angle θ (°) Interface Work of Adhesion Model Wad (J/m2) γ Al = 1.143 J/m2 [29] γAl = 1.160 γAl = 1.122 J/m2 [31] Contact AngleJ/m θ (2◦[30] ) Work of Adhesion Interface Al-TA 1.262 ) 84 85 83 W ad (J/m Model γAl = 1.143 J/m2 [29] γAl = 1.160 J/m2 [30] γAl = 1.122 J/m2 [31] Al-TB 0.96 99 100 98 Al-T 1.26 8485 85 86 83 84 Al-TAC 1.24 Al-TB 0.96 99 100 98 Al-TC 1.24 85 86 84

3.2. N-Terminated AlN(0001)/Al(111) Interface Models

The optimized structures of the Al-terminated AlN(0001)/Al(111) interface models are shown in Figure 6. Compared with the original models (Figure 3), the distance between the Al(111) slab and the AlN(0001) slab was shortened throughout the three models. The outmost aluminum atoms of the Al(111) slab in the N-TB model was close to the hollow position, which was similar to the N-TC model.

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3.2. N-Terminated AlN(0001)/Al(111) Interface Models The optimized structures of the Al-terminated AlN(0001)/Al(111) interface models are shown in Figure 6. Compared with the original models (Figure 3), the distance between the Al(111) slab and the AlN(0001) slab was shortened throughout the three models. The outmost aluminum atoms of the Al(111) slab in the N-TB model was close to the hollow position, which was similar to the N-TC model. Materials 2018, 11, x FOR PEER REVIEW

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Figure 6. Side views of the N-terminated AlN(0001)/Al(111) interface models after the structure Figure 6. Side views of the N-terminated AlN(0001)/Al(111) interface models after the structure optimization. The red balls represent the Al atoms and the blue balls represent the N atoms. (a) N-TA; optimization. The red balls represent the Al atoms and the blue balls represent the N atoms. (a) N-TA ; (b) N-TB; and (c) N-TC. (b) N-TB ; and (c) N-TC.

The calculated work of the adhesion (Wad) and contact angle (θ) at 1073 K throughout the three The calculated work of the adhesion angle (θ) atresults 1073 Kofthroughout the three ad ) and contact interface models were calculated and are(W displayed in Table 2. The the N-TB and N-TC interface models were calculated and are displayed in Table 2. The results of the N-T and N-T C models models were similar. The Wad was 0.82 J/m2 and the contact angles were aboutB 106°~107°, which 2 and the contact angles were about 106◦ ~107◦ , which dissatisfied were similar.the Therequirement Wad was 0.82 dissatisfied forJ/m the hot dip coating experiments. The calculated results of the N-TA the requirement for theThe hot dip calculated results of the N-Tthan were A model model were different. Wad coating was upexperiments. to 2.40 J/m2, The which was significantly larger the other 2 , which was significantly larger than the other models. As a different. The W was up to 2.40 J/m ad the contact angle of the Al melt on the N-TA surface reached 0°, revealing a good models. As a result, result, the contact angle of the Al melt on the N-TA surface reached 0◦ , revealing a good wettability. wettability. Table 2. Calculated work of adhesion (Wad ) and contact angles (θ) of the N-terminated AlN (0001)/Al Table 2. Calculated work of adhesion (Wad) and contact angles (θ) of the N-terminated AlN (0001)/Al (111) interface models at 1073 K. (111) interface models at 1073 K.

Interface Interface Model Model N-TA N-TA N-TB N-T N-TCB N-TC

Contact Angle Work of Adhesion Contact Angle θ (◦θ) (°) Work of Adhesion 2) 2 [29] 2 [30] 2 [31] 2 ad (J/m W γ Al = 1.143 J/m γ Al = 1.160 J/m = 1.122 2 [31] W ad (J/m ) γAl = 1.143 J/m2 [29] γAl = 1.160 J/m2 [30] γAl γ=Al1.122 J/mJ/m 2.40 0 0 0 2.40 0 0 0 0.82 107 107 106 0.82 107 107 106 0.82 107 107 106 0.82 107 107 106

The distance between the bottommost Al atoms of the Al(111) slab and the topmost N atoms of The distance between theÅ,bottommost Al atomsoptimization. of the Al(111)This slab was and close the topmost N atoms of of the the AlN(0001) slab was 1.92 after the structure to the distance the AlN(0001) slab was 1.92 Å, after the structure optimization. This was close to the distance of the nearest nearest Al and N layers of 1.90 Å. According to the electron density schematic diagram of the N-TA Al and N layers of 1.90 Å. According the electron density near schematic diagram was of the N-TA model model (110) surface (Figure 7), the to electron distribution the interface similar to the (110) surface (Figure 7), the electron distribution near the interface was similar to the AlN(0001) slab. AlN(0001) slab. These two results illustrated that the stable chemical bonds that were formed between These two results illustrated that the stable chemical bonds that were formed between the terminated the terminated N atoms of the AlN(0001) and the Al atoms of the Al(111) slab in the interface, which N atoms of the AlN(0001) andthe theWAl atoms of the Al(111) slab in the interface, which had subsequently had subsequently increased ad. This was beyond the scope of the work of the adhesion, however increased the W . This was beyond the scope of the work of the adhesion, however a reconstruction a reconstructionad surfaced on the N-terminated AlN(0001) surface. During the hot dip coating surfaced on the N-terminated AlN(0001) surface. During the hot dip coating experiments, the N-T A experiments, the N-TA structure was more stable and more likely to appear, which suggested that the structure was more stable and more likely to appear, which suggested that the N-terminal AlN(0001) N-terminal AlN(0001) surface chemically reacted with the Al melt so as to bond with a layer of Al surface chemically with the melt so as toAlN(0001) bond withsurface a layertended of Al atoms. atoms. In an Al-richreacted environment, theAlN-terminated to bond In theanAlAl-rich atoms and transformed into an Al-terminated surface.

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environment, the N-terminated AlN(0001) surface tended to bond the Al atoms and transformed into Materials 2018, 11, x FOR PEER REVIEW an Al-terminated surface.

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Figure Figure 7. 7. Electron Electron density density schematic schematic diagram diagram of of the the optimized optimized N-T N-TAA model model (110) (110) surface. surface.

3.3. LCM Reconstructed AlN(0001)/Al(111) 3.3. LCM Reconstructed AlN(0001)/Al(111) After the structure optimization (Figure 4b), a few position changes were found, and the laterally After the structure optimization (Figure 4b), a few position changes were found, and the laterally contracted Al monolayer showed some motion along a vertical axis. The calculated work of the contracted Al monolayer showed some motion along a vertical axis. The calculated work of the adhesion (Wad) and contact angle (θ) at 1073 K of the LCM interface models were calculated and listed adhesion (Wad ) and contact angle (θ) at 1073 K of the LCM interface models were calculated and listed in Table 3. The Wad showed an evident increase when compared with the original Al-terminated in Table 3. The Wad showed an evident increase when compared with the original Al-terminated AlN(0001)/Al(111) interface models, which demonstrated that the combination of the AlN(0001) slab AlN(0001)/Al(111) interface models, which demonstrated that the combination of the AlN(0001) slab and the Al(111) slab was tighter. The calculated contact angle of the Al melt on the LCM AlN(0001) and the Al(111) slab was tighter. The calculated contact angle of the Al melt on the LCM AlN(0001) surface had decreased to 35°~41°. The LCM reconstruction improved the wettability, but the contact surface had decreased to 35◦ ~41◦ . The LCM reconstruction improved the wettability, but the contact angle was still far from the 0°◦ that the hot dip coating experiment required. angle was still far from the 0 that the hot dip coating experiment required. Table 3. Calculated work of the adhesion (Wad) and contact angles (θ) of the LCM AlN(0001)/Al(111) Table 3. Calculated work of the adhesion (Wad ) and contact angles (θ) of the LCM AlN(0001)/Al(111) interface interface models models at at 1073 1073 K. K. Interface Model Interface Model LCM LCM

Contact Angle θ (°) Work of Adhesion θ (◦2)[30] ad (J/m2) γAl = 1.143 J/m2 [29]Contact γAl =Angle 1.160 J/m γAl = 1.122 J/m2 [31] Work of W Adhesion 2 2 2 W ad (J/m 2.04) 41 [30] 35 2 [31] γAl = 1.143 J/m38 [29] γAl = 1.160 J/m γAl = 1.122 J/m 2.04

38

3.4. Influence of Oxygen Adsorption on Free Surface of Al Melt

41

35

3.4. Influence of Oxygen on Free Surface Al Melt During the hot dipAdsorption coating experiment, thereofwere dozens of ppm of oxygen in the atmosphere of the dip coating equipment. Garcia-Cordovilla et al. [31] had observed a decrease in the surface During the hot dip coating experiment, there were dozens of ppm of oxygen in the atmosphere of tension of the Al melt as the oxygen (O) atoms were adsorbed onto the surface of the melt. The the dip coating equipment. Garcia-Cordovilla et al. [31] had observed a decrease in the surface tension previous experiment measured a surface tension value of 0.869 J/m2 at 1073 K as a monolayer of the of the Al melt as the oxygen (O) atoms were adsorbed onto the surface of the melt. The previous O atoms that were adsorbed. According to Young–Dupré ’s Equation, the contact angle would have experiment measured a surface tension value of 0.869 J/m2 at 1073 K as a monolayer of the O declined with the decrease in the surface tension of the Al melt. We calculated the contact angle as atoms that were adsorbed. According to Young–Dupré’s Equation, the contact angle would have the surface tension changing from 0.869 J/m2 to 1.160 J/m2, and results were shown in Figure 8. declined with the decrease in the surface tension of the Al melt. We calculated the contact angle Throughout the three original Al-terminated AlN(0001) surfaces, the contact angle of the Al melt on as the surface tension changing from 0.869 J/m2 to 1.160 J/m2 , and results were shown in Figure 8. the Al-TA, Al-TB, and Al-TC surfaces decreased to 63°, 84°, and 65° when the γAl had decreased to 0.869 Throughout the three original Al-terminated AlN(0001) surfaces, the contact angle of the Al melt on J/m2 . The contact angle of the Al melt on the LCM surface decreased to 0° when the γAl had decreased the Al-TA , Al-TB , and Al-TC surfaces decreased to 63◦ , 84◦ , and 65◦ when the γAl had decreased to 2. A perfect wetting was obtained after a partial adsorption of the O atoms. The to 1.02 J/m 0.869 J/m2 . The contact angle of the Al melt on the LCM surface decreased to 0◦ when the γAl had supernormal wetting phenomenon that was observed in the hot dip coating likely came from the decreased to 1.02 J/m2 . A perfect wetting was obtained after a partial adsorption of the O atoms. surface reconstruction of the AlN and the adsorption of the O atoms on the free surface of the Al film, which was attached on the AlN.

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The supernormal wetting phenomenon that was observed in the hot dip coating likely came from the surface reconstruction of the AlN and the adsorption of the O atoms on the free surface of the Al film, which2018, was11, attached on REVIEW the AlN. Materials x FOR PEER 8 of 10

Figure 8. 8. Diagram of of thethe calculated contact angle of of thethe molten AlAl onon thethe Al-terminated and thethe LCM Figure Diagram calculated contact angle molten Al-terminated and LCM AlN(0001) surface at 1073 K, as a function of surface tension of Al. AlN(0001) surface at 1073 K, as a function of surface tension of Al.

4. Conclusions 4. Conclusions In order to characterize the supernormal wetting phenomenon that occurred in the hot dip In order to characterize the supernormal wetting phenomenon that occurred in the hot dip coating coating of the Al on the AlN ceramics, the wettability of the Al melt on the AlN(0001) surface was of the Al on the AlN ceramics, the wettability of the Al melt on the AlN(0001) surface was studied by a studied by a first-principle study. A surface reconstruction of the Al-terminated AlN(0001) surface first-principle study. A surface reconstruction of the Al-terminated AlN(0001) surface and the oxygen and the oxygen adsorption were taken into consideration. The conclusions were as follows: adsorption were taken into consideration. The conclusions were as follows: (1) The Wad of the Al-terminated AlN(0001)/Al(111) was within the limits of 0.96 J/m2~1.26 J/m2, (1) The W ad of the Al-terminated AlN(0001)/Al(111) was within the limits of 0.96 J/m2 ~1.26 J/m2 , where the calculated contact angle of the Al melt on the Al-terminated AlN(0001) surface was where the calculated contact angle of the Al melt on the Al-terminated AlN(0001) surface was beyond 83°, which agreed well with the sessile drop experiments. beyond 83◦ , which agreed well with the sessile drop experiments. (2) The Wad of the N-TB and N-TC types N-terminated AlN(0001)/Al(111) interface was 0.82 J/m2, (2) which The W AlN(0001)/Al(111) interface 0.82 J/m2 , B and N-Tcalculated C types N-terminated ad of the resulted inN-T a 106°~107° contact angle of the Al melt on the N-Twas B and N-TC ◦ ◦ which resulted inThe a 106 angle of the Al melt on the was N-TB2.40 andJ/m N-T 2, C surfaces, at 1073 K. Wad~107 of thecalculated N-TA typecontact of the AlN(0001)/Al(111) interface 2 surfaces, at 1073 K. The W of the N-T type of the AlN(0001)/Al(111) interface was 2.40 J/m A ad which resulted in a 0° calculated contact angle of the Al melt on the N-TA surface at 1073 K. This , which resulted in achemical 0◦ calculated angle ofbetween the Al melt on the N-TANsurface was caused by the bondscontact that formed the terminated atoms aton1073 the K. This was caused by the chemical bonds that formed between the terminated N atoms on the AlN(0001) surface and the Al atoms of the Al melt. The results demonstrated that, in an Al-rich AlN(0001) surface and the Al atoms of the Al melt. The results demonstrated that, in an Al-rich environment, the N-terminated AlN surface tended to bond the Al atoms and reconstructed, environment, surface tended to bond the Al atoms and reconstructed, which resulted inthe an N-terminated Al-terminatedAlN surface structure. which resulted in an Al-terminated surface structure. (3) The LCM reconstruction improved the Al and AlN bonding in the Al-terminated AlN(0001) (3) surface. The LCM reconstruction improved the Al and AlN bonding in the Al-terminated AlN(0001) The W ad of the LCM AlN(0001)/Al(111) interface was 2.04 J/m2, which was higher than surface. The W of the LCM AlN(0001)/Al(111) interfaceThe waswettability 2.04 J/m2 ,of which was than 2 2 the 0.96 J/m ~1.26adJ/m of the dis-reconstructed interface. the Al onhigher the AlN 2 2 theimproved 0.96 J/m and ~1.26 of the dis-reconstructed The wettability of the Al on the AlN was theJ/m contact angle of the Al on theinterface. AlN(0001) surface decreased from 83°~100° ◦ ~100◦ was improved and the contact angle of the Al on the AlN(0001) surface decreased from 83 to 35°~41°. ◦ ◦ to adsorption 35 ~41 . of the oxygen atoms on the free surface of the Al melt further improved the (4) The (4) wettability The adsorption of the on the free thethe Al non-reconstructed melt further improved of the Al. Theoxygen contact atoms angle between the surface Al meltofand Althe wettability of the Al. The contact angle between the Al melt and theone non-reconstructed terminated AlN(0001) surface decreased from 83°~100° to 63°~84°. When monolayer of Al-terminated AlN(0001) 83◦melt, ~100◦the to surface 63◦ ~84◦tension . When one monolayer oxygen atoms was adsorbedsurface on thedecreased surface offrom the Al decreased fromof 1.122~1.160 J/m2 to 0.869 J/m2. The partial adsorption of the oxygen atoms decreased the contact angle to 0° when the surface tension decreased to 1.02 J/m2 within the LCM reconstruction. (5) The supernormal wetting phenomenon in the dip coating of the Al on AlN ceramics originated from the surface reconstruction of the AlN ceramics under an Al-excess condition when they were immersed in molten Al. A quick adsorption occurred for the oxygen atoms on the free

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oxygen atoms was adsorbed on the surface of the Al melt, the surface tension decreased from 1.122~1.160 J/m2 to 0.869 J/m2 . The partial adsorption of the oxygen atoms decreased the contact angle to 0◦ when the surface tension decreased to 1.02 J/m2 within the LCM reconstruction. The supernormal wetting phenomenon in the dip coating of the Al on AlN ceramics originated from the surface reconstruction of the AlN ceramics under an Al-excess condition when they were immersed in molten Al. A quick adsorption occurred for the oxygen atoms on the free surface of liquid Al, which were attached on the AlN ceramics, as they withdrew from the Al melt. A thorough immersion and a proper amount of oxygen in the atmosphere were key for the hot dip coating.

Author Contributions: Conceptualization, Y.L. and X.-S.N.; Methodology, J.C., Y.L. and X.-S.N.; Software, J.C. and Y.L.; Validation, J.C. and X.-S.N.; Formal Analysis, Y.L. and X.-S.N.; Investigation, J.C. and X.-S.N.; Resources, J.C. and Y.L.; Data Curation, J.C. and Y.L.; Writing-Original Draft Preparation, J.C.; Writing-Review & Editing, J.C. and X.-S.N.; Visualization, J.C.; Supervision, X.-S.N.; Project Administration, X.-S.N.; Funding Acquisition, X.-S.N. Funding: This research was funded by the National Natural Science Foundation of China grant number 51472136. Acknowledgments: The authors acknowledge with thanks the Supercomputing Center of Chinese Academy of Sciences for providing computational resources. Conflicts of Interest: The authors declare no conflict of interest.

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