Wetting of MgO, TiO

Abstract

The wettability and interface microstructures of Al-8Si/stainless steel, Al-8Si/MgO, and Al-8Si/TiO₂ systems were investigated by a modified sessile drop method. The formation of interface products of three systems was discussed from the view of thermodynamics. Results show that the interface microstructure of Al-8Si/stainless steel is composed of Fe(Al,Si)₃, Al_7.2Fe_1.8Si and Fe₂Al₅ phases, while that of Al-8Si/MgO and Al-8Si/TiO₂ systems consists mainly of the Al₂O₃ phase with different morphologies and roughness. The wettability results indicate that the Al-8Si/MgO system exhibits a better non-wettability compared with the stainless steel and TiO₂ substrates, whose equilibrium wetting angle is 124°. The wettability differences of the three systems are mainly related to the roughness and properties of interface products. The interface roughness tests show that the interface of Al-8Si/MgO system has the largest roughness of 1.46 μm, which is mainly due to the evaporation of Mg that destroys the morphology of the interface reaction layer during the interface reaction process. Furthermore, the existence of Ti promotes the interface reaction and increases the thickness of interface reaction layer and thus reduces the equilibrium wetting angle of Al-8Si/TiO₂ system.

Keywords

Al-8Si alloy; wettability; 2520 stainless steel; TiO₂; MgO

Recently, Al-Si alloys have been widely used in communication industries owing to their high tensile strengths and toughness, good thermal conductivity, and ease of precision machining. Inclined plate casting, a novel semi-solid rheological slurry preparation technology, can be used to efficiently fabricate Al alloy thin-walled parts with complex size in combination with die casting, especially for the filters of 5G base station^[1]. However, the adhesive problem between Al-Si alloy melts and the inclined plate (stainless steel) is difficult to solve when the semi-solid slurry passes through the inclined plate in practical production (Fig.1). This will directly affect the quality and efficiency of semi-solid slurry preparation.

Fig.1  Schematic diagram of semi-solid metal slurry prepared by shear/vibration coupling and rheological casting (a) and typical Al-8Si alloy metals slurry remained on the inclined plate after casting (b)

It is reported that some ceramic coatings prepared on stainless steel plate can markedly reduce the adhesive phenomenon through the non-wettability between Al melts and ceramics, such as ZrO₂^[2], MgO^[3], ZnO^[4], SiC^[5], Al₂O₃^[6] and TiO₂^[7]. Aside from structure, physical and chemical properties, atmosphere environment (Ar or vacuum), roughness, interfacial reaction products, substrate type (active or inactive metals), and measurement techniques also influence the wettability of ceramics. Generally, the interfacial wetting is mainly characterized by the equilibrium contact angle (θ) according to Young's equation:

$$\mathrm{c}\mathrm{o}\mathrm{s}\theta =\frac{{\sigma }_{\mathrm{s}\mathrm{v}}-{\sigma }_{\mathrm{s}\mathrm{l}}}{{\sigma }_{\mathrm{l}\mathrm{v}}}$$

(1)

where σ_sv, σ_sl and σ_lv are the solid-vapor, solid-liquid and liquid-vapor interfacial tensions, respectively. Wetting of a solid surface by a liquid is achieved when θ<90°; otherwise, it is non-wetting (θ>90°)^[8]. Ueki et al^[2] investigated the wettability of different Al melts and ZrO₂, and pointed out that Al melts are non-wettable with ZrO₂, whose θ value is about 145° at 1173 K. Also, Yang's experimental results suggest that the θ value is 148° at 973 K for the Al melts and ZrO₂ substrate due to the existence of the oxide film of Al melts^[9]. Additionally, Shen et al^[3] reported the wettability of Al melts and single crystal MgO with three different faces of (100), (110) and (111) at 1073–1473 K in an Ar-3%H₂ atmosphere. The experimental results indicate that the wettability of Al melts neither depends on the substrate orientation nor on the temperature. The θ value of the Al/MgO system is possibly between 90° and 105° under the aforementioned conditions owing to the formation of α-Al₂O₃ reaction products. The larger θ value (100°–107°) is also obtained at the interface between Al melts and single crystal ZnO with various orientations after 5–20 min at 1273 K, which is mainly attributed to the non-wetting reaction products of α-Al₂O₃^[4]. An et al^[10] demonstrated that the initial contact angle of the Al melts and SiC is 154° at 1173 K, arising from the presence of oxide film on the Al melt surface, which decreases to 54° at equilibrium as a result of the Al₄C₃ reaction product. Avraham et al^[11] studied that the effects of temperature (973–1273 K) on the wettability of Al melts and TiO₂, and proposed that the system has a good non-wettability at 973 K with the θ of ~150° due to low temperature and non-reactivity at the solid-liquid interface.

As mentioned above, the wetting of ceramics by molten pure aluminum was extensively studied^[2–11]. However, the wet-tability of ceramics by molten Al-Si alloys was seldom reported. Therefore, in this study, the wetting of three sub-strates (stainless steel, TiO₂ and MgO) with Al-8Si melts was investigated at 1173 K using a modified sessile drop method.

1 Experiment

1.1 Material preparation

The stainless steel (2520), MgO, and TiO₂ polycrystalline ceramics (substrate material) were machined into the samples with a size of 30 mm×30 mm×6 mm and polished to a surface roughness of 1–2 μm. The experimental Al-8Si alloy (deposition material on stainless steel and ceramics) was used to observe the wetting and spreading behavior, which was prepared with pure Al (99.999%) and Al-30%Si master alloy in a graphite crucible put on an electric resistance furnace at 973 K. The melt was stirred and kept at 953 K for 20 min and poured into the preheated cylindrical iron mold at 473 K. The mould was 150 mm in height and 50 mm in diameter. The substrates and Al-8Si alloy were ultrasonically cleaned before placed in the chamber.

1.2 Wetting test

A wetting test was carried out on a sessile drop equipment that contained a high vacuum system, tube furnace, data acquisition and processing system and drop shape analysis software in the computer terminal, and the details can be referred in Ref.[12–13]. Al-8Si alloy sample with a mass of 0.03 g was firstly heated to 573 K at 15 K/min, and then heated to 1173 K at 20 K/min after holding for 10 min in a vacuum of 7×10^-4 Pa. When the temperature remained stable under a vacuum of 5×10^-4 Pa, the alloy sample was directly dropped through an open alumina tube and deposited on three substrates. The variation of the whole wetting process was recorded by an infrared camera at 15 s intervals. The contact angle and drop base radius of all samples were analyzed from the profiles using Surface Meter Elements Analysis software. When the wetting experiments ended, the whole chamber was cooled under a flowing Ar (~99.999%) atmosphere at a rate of 15 K/min to the room temperature.

1.3 Interface microstructure characterization

The morphology, composition, and interface microstructure of a cross-section of Al-8Si/MgO, Al-8Si/TiO₂ and Al-8Si/stainless steel systems were characterized by optical microscopy (OM, LSM800) and scanning electron microscopy (SEM, Quanta Feg 450) equipped with an energy dispersing spectrometer (EDS). For SEM observation, all samples were roughly and finely ground with 400#, 600#, 800# and 1000# sandpapers and mechanically polished, and then etched for ~30 s in Keller's reagent for 13 s for corrosion. The samples were immersed into a NaOH aqueous solution to remove the solidified Al-8Si alloy for the subsequent roughness analysis. The roughness of three substrates was measured by confocal laser scanning microscope (CLSM).

2 Results and Discussion

2.1 Wetting behavior

Fig.2a shows the variation of the contact angle (θ) of Al-8Si alloy melts on stainless steel, MgO, and TiO₂ with time at 1173 K. It can be seen that the θ value of MgO and TiO₂ substrates slowly decreases, but that of stainless steel substrates rapidly drops at less than 500 s. Conversely, the contact angle of three substrates is close to a constant value, that is, the equilibrium contact angle (ECA) at 30 min. The initial contact angle of the Al-8Si alloy melts on MgO, TiO₂ and stainless steel is similar, i.e. 143°, 144° and 137°, respec-tively. The presence of the oxide film on Al-8Si alloy melts leads to a high initial contact angle. The process of eliminating the oxide film on the surface of the alloy melts is accompanied by the formation of vapor-phase AlO₂ and interface products, which improve the wettability of the interface. The corresponding reaction equation at the interface of the melts is follows:

Fig.2  Variations of contact angle (a) and normalized contact radius R₀/R_d (b) with time for Al-8Si alloy melts on MgO, TiO₂, and stainless steel at 1173 K

$$\mathrm{A}{\mathrm{l}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}+\mathrm{A}\mathrm{l}(\mathrm{l})\to \mathrm{A}{\mathrm{l}}_{\mathrm{2}}\mathrm{O}(\mathrm{g})$$

(2)

When the interface starts to react, the equilibrium contact angle depends on the characteristics of the reaction product. As shown in Fig.3, at higher temperatures (1173 K), the liquid drops of three systems show different spreading behavior, and the final equilibrium contact angles reach 29° (stainless steel), 96° (TiO₂) and 124° (MgO). This trend can be reflected by the variation of normalized contact radius of Al-8Si alloy melts on three substrates with time at 1173 K, as shown in Fig.2b.

Fig.3  Contact angles of Al-8Si alloy melts on stainless steel, TiO₂ and MgO at 0 s (a₁–a₃), 900 s (b₁–b₃), and 1800 s (c₁–c₃)

2.2 Interfacial microstructures

SEM images and EDS mappings of interface reaction layer of Al-8Si alloy melts on stainless steel are shown in Fig.4. According to EDS results (Table 1), the interface reaction products of Al-8Si/stainless steel are primarily composed of Fe(Al,Si)₃, Al_7.2Fe_1.8Si and Fe₂Al₅ phase, as reported by Liu et al^[14]. It is known that Al-8Si/stainless steel is a typical reactive wetting system which inevitably leads to the formation of intermetallic compounds. Actually, the formation order of these interface products determines the distribution of the reaction layer. It can be calculated that the interfacial free energy (∆G⁰) of Fe(Al,Si)₃, Al_7.2Fe_1.8Si and Fe₂Al₅ are –83.4, –184.4 and –138.4 kJ/mol at 1173 K, respectively, according to the following equations:

$$\mathrm{\Delta }{G}_{\mathrm{F}\mathrm{e}\mathrm{A}{\mathrm{l}}_{\mathrm{3}}}^{\mathrm{0}}=-\mathrm{142770}+\mathrm{50.58}T$$

(3)

$$\mathrm{\Delta }{G}_{\mathrm{F}{\mathrm{e}}_{\mathrm{2}}\mathrm{A}{\mathrm{l}}_{\mathrm{5}}}^{\mathrm{0}}=-\mathrm{253971}+\mathrm{98.52}T$$

(4)

$$\mathrm{\Delta }{G}_{\mathrm{A}{\mathrm{l}}_{\mathrm{7.2}}\mathrm{F}{\mathrm{e}}_{\mathrm{1.8}}\mathrm{S}\mathrm{i}}^{\mathrm{0}}=-\mathrm{295355}+\mathrm{94.59}T$$

(5)

Fig.4  SEM images (a) and EDS mappings of element Si (b), Al (c), and Fe (d) at interface of Al-8Si/stainless steel

Table 1  EDS results of each point marked in Fig.4a (at%)

Point	Al	Fe	Si	Cr	Ni	Phase
1	68.6	13.8	7.3	8.6	1.6	Al7.2Fe1.8Si
2	59.1	20.3	5.8	10.0	4.7	Fe(Al,Si)3
3	43.4	26.1	6.7	12.5	11.4	Fe2Al5
4	0.6	52.0	1.6	28.4	17.3	α-Fe

It can be seen that the ΔG⁰ value of Al_7.2Fe_1.8Si phase is more negative than that of other phases, which indicates that the phase should precipitate preferentially at the interface of Al-8Si/stainless steel. The similar interface microstructure was also reported for Al alloy/stainless steel joint prepared by tungsten inert gas welding-brazing^[15].

The interface microstructure and EDS mappings of the Al-8Si/TiO₂ is displayed in Fig.5. The smooth surface reaction layer is observed at the interface, as seen in Fig.5a. EDS results in Table 2 indicate that the interface reaction products are mainly composed of Al₂O₃ phase that contains a small amount of Ti. It is reported that the interface reaction of the Al/TiO₂ system is relatively complex at high temperatures^[7]. Generally, three reactions will occur at the Al/TiO₂ interface as follows:

$$\mathrm{4}\mathrm{A}\mathrm{l}+\mathrm{3}{\mathrm{O}}_{\mathrm{2}}\to \mathrm{2}\mathrm{A}{\mathrm{l}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$$ (Al surface oxidation)

(6)

$$\mathrm{4}\mathrm{A}\mathrm{l}+\mathrm{A}{\mathrm{l}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}\to \mathrm{3}\mathrm{A}{\mathrm{l}}_{\mathrm{2}}\mathrm{O}$$ (Al surface deoxidation)

(7)

$$\mathrm{A}\mathrm{l}+\mathrm{T}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}\to \mathrm{A}{\mathrm{l}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}+\mathrm{T}\mathrm{i}\mathrm{O}{}_{\mathrm{2}-x}{}^{}$$

(8)

$$\mathrm{A}\mathrm{l}+\mathrm{T}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}\to \mathrm{A}{\mathrm{l}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}+{[\mathrm{T}\mathrm{i}]}_{\mathrm{A}\mathrm{l}}$$

(9)

Fig.5  SEM images (a) and EDS mappings of element Al (b), O (c), Ti (d), and Si (e) at interface of Al-8Si/TiO₂

Table 2  EDS results of each point marked in Fig.5a (at%)

Point	Al	Si	O	Ti	Phase
5	84.06	11.73	3.84	0.39	α-Al
6	41.09	0.29	57.78	0.84	Al2O3
7	1.97	0.1	61.25	36.68	TiO2

At 1173 K^[16], the value of Gibbs free energy for Al₂O₃ (–1295.046 kJ/mol) is lower than that of Al₂O (–231.473 kJ/mol), AlTi (–64.378 kJ/mol), TiO (–427.976 kJ/mol) and AlTi₃ (–108.491 kJ/mol), and thus the Al₂O₃ phase is easier to form at the interface. Avraham et al^[11] also confirmed that the interface reaction product of Al/TiO₂ is mainly Al₂O₃ at 1273 K. Fig.6 shows SEM images and EDS mappings of reaction layer at Al-8Si/MgO interface. The reaction product is Al₂O₃ with a rough morphology based on the EDS results in Table 3. Due to different experimental conditions, there are two kinds of interface reaction products for the Al/MgO system at present. One is Al₂O₃^[3] and the other one is MgAl₂O₄^[17]. Actually, MgAl₂O₄ is easy to form at high interface reaction temperatures (>1300 K), which will further hinder the oxidation of Al. In the present work, only Al₂O₃ is generated at low interface reaction temperature (1173 K), and the corresponding interface reaction is following^[18]:

$$\mathrm{2}\mathrm{A}\mathrm{l}+\mathrm{3}\mathrm{M}\mathrm{g}\mathrm{O}\to \mathrm{A}{\mathrm{l}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}+\mathrm{3}\mathrm{M}\mathrm{g}\uparrow$$

(10)

Fig.6  SEM images (a) and EDS mappings of element Al (b) O (c), Mg (d), and Si (e) at interface of Al-8Si/MgO

Table 3  EDS results of each point marked in Fig.6a (at%)

Point	Al	Si	O	Mg	Phase
8	89.8	3.4	6.7	0	α-Al
9	60.4	0.2	39.4	0	Al2O3
10	0.3	0	46.2	53.5	MgO

2.3 Discussion

The wettability of substrate is mainly related to the inter-facial properties. Among the factors determining wettability, the interfacial reaction product is not negligible^[19–21]. In our work, different interface products between Al-8Si melts and substrates during reaction determine various wetting proper-ties. When the interface products are the same, the roughness of the interface products and the active elements affect the wettability.

In the Al-8Si/stainless steel system, the loose interfacial reaction layer consists of Al_7.2Fe_1.8Si, Fe(Al,Si)₃ and Fe₂Al₅ phase at the interface (Fig.4). It is obvious that these reaction products tend to grow simultaneously inside the droplet and substrate, and form a dissolution-diffusion channel at the interface owing to the large solid solubility of Fe in Al-Si melts. So, the diffusion of Fe and other elements in the triple phase line is enhanced, which also promotes the extension of triple phase line region and finally improves the wetting properties of Al-8Si/stainless steel system^[22].

Compared with Al-8Si/stainless steel, the dense Al₂O₃ reaction layer forms at the interface of the Al-8Si/MgO and Al-8Si/TiO₂ systems, which prevents the atomic diffusion in the triple phase line and results in a large equilibrium contact angle. Although interface products Al₂O₃ appears at the interface of both systems, their roughness is different, which is the main factor affecting wetting properties of Al-8Si/MgO and Al-8Si/TiO₂ system. For example, Qi et al^[23] found that the average contact angle is significantly increased by 16.4° when R_a value increases from 0.092 μm to 0.834 μm for Pb/Al₂O₃ system at 973 K. So, it can be seen that the surface roughness of the interface is one of the key factors affecting the wettability of Al₂O₃. Fig.7 shows the roughness of interface products of Al-8Si/stainless steel, Al-8Si/MgO and Al-8Si/TiO₂ systems. It can be found that the interface roughness (1.46 μm) of Al-8Si/MgO system is an order of magnitude higher than that of Al-8Si/stainless steel (0.573 µm) and Al-8Si/TiO₂ (0.528 µm). The large interface roughness of Al-8Si/MgO mainly arises from the formation of loose macro-pores due to the evaporation of Mg during the interface reaction (Fig.6). It should be emphasized that the roughness is proportional to the surface area of the interface reaction layer. This relationship can be reflected by a schematic diagram in Fig.8, which indicates the variation of surface area with smooth or rough interface. Furthermore, the surface free energy (G) and surface area (A) satisfy the following linear relation:

Fig.7  CLSM images of 2520 stainless steel (a), TiO₂ (b), and MgO (c) substrate surface

Fig.8  Schematic diagrams showing the wetting of liquid on rough (a) and smooth (b) substrates

$$\mathrm{d}G=\sigma \mathrm{d}A$$

(11)

where σ is a constant. Accordingly, it can be concluded that the large interface roughness directly leads to an increase in surface free energy (γ_sv) of interface reaction layer. Based on the modifier formula of Berthelot rule^[24], the γ_sv values of each system can be calculated through Eq.(12) using equilibrium wetting angle (θ), where θ is the contact angle for the Al-8Si/ceramic system, and β is the constant (β=1.247×10^-4 m²/mJ), γ_lv is the surface free energy of the Al-8Si alloy melts, and γ_sv is the surface free energy of the substrate.

$$\mathrm{c}\mathrm{o}\mathrm{s}\theta =-\mathrm{1}+\mathrm{2}\sqrt[]{\frac{{\gamma }_{\mathrm{s}\mathrm{v}}}{{\gamma }_{\mathrm{l}\mathrm{v}}}}\mathrm{e}\mathrm{x}\mathrm{p}[-\beta ({\gamma }_{\mathrm{l}\mathrm{v}}-{\gamma }_{\mathrm{s}\mathrm{v}})]$$

(12)

So, the γ_sv values of MgO and TiO₂ substrates are 926 and 805 mJ/m², respectively. This also confirms that the large equilibrium contact angle corresponds to high surface energy. Thus, it can be concluded the Al-8Si/MgO system has better non-wetting properties than Al-8Si/TO₂ system. In addition, Ti is a reactive element for Al-8Si/TiO₂ system, which can improve the interface wettability by decreasing liquid/solid interfacial free energy^[25]. Also, the existence of Ti can promote the interface reaction and increase the thickness of interface reaction layer (Fig.5). In contrast, the interface reaction accelerates the exchange of substances near the triphase line and promotes the migration of the triphase line, resulting in a lower ECA of TiO₂/Al-8Si system. This is also another factor leading to the good wettability of Al-8Si/TiO₂ compared with Al-8Si/MgO.

3 Conclusions

1) Interface microstructure of Al-8Si/stainless steel is composed of Fe(Al,Si)₃, Al_7.2Fe_1.8Si and Fe₂Al₅ phase, while that of Al-8Si/MgO and Al-8Si/TiO₂ systems consists mainly of Al₂O₃ phase.

2) The wettability results of three systems indicate that the Al-8Si/MgO system exhibits a better non-wettability com-pared with the Al-8Si/stainless steel and Al-8Si/TiO₂ systems, whose equilibrium contact angle (124°) is higher than that of the stainless steel (29°) and TiO₂ (96°) substrate.

3) The wettability differences of the three systems are mainly related to the roughness and properties of interface products. The interface roughness tests indicate that the Al-8Si/MgO system has the largest roughness of 1.46 μm compared with Al-8Si/stainless steel and Al-8Si/TiO₂ systems. The larger interface roughness increases the surface free energy of interface reaction layer and improves the non-wetting property of the system.