Epitaxial growth of aluminum borate whiskers and simultaneous joining of alumina ceramics via a sol–gel method

Abstract

Aluminum borate whiskers were epitaxially grown on alumina by a sol–gel method. The morphology of the whiskers was investigated via scanning electron microscopy, X-ray diffraction and transmission electron microscopy. As the holding time and growth temperature increased, the length of the whiskers first increased and then decreased. The whiskers (Al₄B₂O₉ and Al₁₈B₄O₃₃) were 5–20 μm in length and hundreds of nanometres in diameter under all growth parameters. In addition, the whiskers synthesized by the sol–gel method were successfully used to join alumina in air. SEM images showed that the whiskers grew across the seam, and the width of the joint was consistently less than 10 μm. The shear test results showed that the highest strength of the joints reached 84.5 MPa at a bonding temperature of 950 °C maintained for 8 h. Fracturing consistently occurred within the network structure in the seam.

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Introduction

Alumina is a type of ceramic widely used in aerospace, power electronics, chemical, and mechanical fields that has high strength, high abrasion resistance, high-temperature resistance, insulation properties, and corrosion resistance.^1–4 It is difficult to manufacture alumina into complex-shaped products due to the high hardness and brittleness of the material.⁵ Techniques to successfully join alumina ceramics are necessary to extend the applications of the material. Brazing and diffusion bonding can be used to join alumina ceramics, but the joints suffer from poor high-temperature strength, poor toughness, and high residual stress.^6–11 In addition, the application of these methods is limited due to the necessary vacuum or inert gas atmosphere during processing and the high cost and low efficiency of such complicated processes.

The aluminum borate whisker is a single-crystal fibre with high strength (8 GPa), good thermostability, and low thermal expansion coefficient (4.2 × 10⁻⁶ °C⁻¹).^12–15 It is widely used in fabricating whisker-reinforced composites, such as poly composites,^15,16 metal matrix composites,^17,18 ceramic matrix composites,^19,20 and oxidation-resistant composites.¹⁷ Additionally, it is reported that aluminum borate whiskers can improve the mechanical performance of brazed joints.^21,22 Moreover, the growth of aluminum borate whiskers can be performed in air.²³ Therefore, the aluminum borate whiskers were selected for joining Al₂O₃ due to their unique characteristics. Previous studies have shown that B₂O₃ powder can be used as an interlayer to achieve the joining of Al₂O₃ by whiskers,^24–26 but because B₂O₃ powder is in the solid state at room temperature, Al₂O₃ cannot be compressed closely during the installation process, which leads to large widths in the joints and reduces the quantity of whiskers that can connect two sides Al₂O₃. To solve this problem, applicative high-temperature fixture or more complicated installation processes must be adopted.²⁶ In addition, if the surfaces to be bonded are not horizontal or have complex shapes, it is almost impossible to ensure the uniform distribution of B₂O₃ powder during installation process, even B₂O₃ powder cannot be held somewhere. This will lead to mechanical heterogeneity of the joints and will greatly limit the application of bonding bulks by whiskers. It is necessary to modify the existing method to expand the application range of bonding bulks by whiskers and to improve the performance of the resultant joints.

There are many advantages to the sol–gel method, such as low-temperature processing, low cost, easy operation, and an even mixture of raw materials.^27–31 Because the product is aquogel, it can be flattened to less than 10 μm and used as the interlayer during the assembly process. Furthermore, it can be smeared everywhere evenly, regardless of the orientations and shapes of the surfaces to be bonded. The materials can be mixed evenly and content-controlled accurately because the sol–gel reaction process occurs in a solution environment.^31–33 Accordingly, the sol–gel method has the unique potential advantage of improving the morphology of whiskers and the quality of joints by adding a co-solvent to the raw materials.^34,35

In this study, the epitaxial growth of aluminum borate whiskers and the simultaneous joining of alumina ceramics were performed by sol–gel method. The microstructure of the joints was studied in detail, the mechanical properties were tested, and the bonding mechanism was analysed, as discussed below.

Experimental

Hot-pressed, sintered Al₂O₃ substrates (>99.5%, Shanghai Unite Technology Co., Ltd., Shanghai China) were cut into different bulks by a diamond cutting machine: 10 mm × 10 mm × 5 mm for the microstructural investigation and 8 mm × 8 mm × 5 mm and 20 mm × 10 mm × 5 mm for the shear strength testing. The analysis reagents boric acid (H₃BO₃, 99.5%), manganous acetate (Mn(CH₃COO)₂·4H₂O, 99.0%), citric acid (C₆H₈O₇·H₂O, 99.5%), ammonium hydroxide (NH₃·H₂O, 28 wt%), and deionized water used in this study were made by Sinopharm Chemical Reagent Co., Ltd. Boric acid, manganous acetate, and citric acid were weighed with an electronic balance (precision was 10⁻³ g) and dissolved in deionized water at room temperature, and the pH was adjusted to approximately 6 with ammonium hydroxide. Then, the solution was warmed with a heated magnetic stirrer (90-2, Xinbao Electronic Instrument Co., Ltd, Jiangsu China) and held at 80 °C for 4 h to obtain the sol. The heating continued until the sol transformed to aquogel with a suitable viscosity. The Al₂O₃ bulks were polished by 1 μm diamond paste and were then ultrasonically cleaned in an acetone bath. The obtained gel was smeared onto the polished surfaces of the bulk Al₂O₃ or between the two bulks evenly. The samples were subsequently heated to 850–1250 °C in a muffle furnace at a rate of 10 °C min⁻¹, and after holding for 4–10 h, the products were furnace-cooled to room temperature.

The viscosity of the gel was tested via rotational rheometer (AR 2000ex, TA Instruments-Waters LLC) and the morphology of the whisker-coated Al₂O₃ and the bond seam was examined via scanning electron microscopy (SEM, FEI Helions Nanolab 600i). The phase composition of the whisker-coated surface was identified via low-angle X-ray diffraction (XRD, Bruker D8 ADVANCE) with copper Kα radiation under an accelerating voltage of 40 kV. The joint samples were sectioned perpendicular to the bond seam and then subjected to SEM for microstructural observation. To further investigate the microstructural features of the whiskers grown in the joint, transmission electron microscopy (TEM, FEI TECNAI G² F30) and high-resolution transmission electron microscopy (HRTEM) analyses were conducted. Film samples were extracted from the cross-section of the rough surface using a focused ion beam instrument (FIB, FEI Helions Nanolab 600i). The shear strength was tested on an electron universal test machine (Instron-5596) at a loading rate of 0.2 mm min⁻¹. Five samples were tested for each experimental parameter to calculate the average value of the joint strength and the standard deviation. The fracture morphologies were investigated via SEM.

Results and discussion

Effect of the raw material ratio on the gel state

The effects of test temperature on the state of the gel with different molar ratio of H₃BO₃ to Mn(CH₃COO)₂ were analysed to ensure the homogeneous coating of the gel on the surface of the Al₂O₃ substrates. The state of the gel was quantified by viscosity to characterize the effects intuitively.³⁶ Fig. 1 shows the viscosity of gels with different molar ratios of H₃BO₃ to Mn(CH₃COO)₂; as the molar ratio increased, the viscosity of the gel first decreased slowly and then decreased sharply to a low point when the ratio exceeded 12 : 1. We also tested the changes in viscosity due to changes in temperature that occur during the smearing process and found that when the molar ratio of H₃BO₃ to Mn(CH₃COO)₂ was less than 12 : 1, the viscosity of the gel increased slowly and smoothly as the temperature decreased. However, when the ratio reached 15 : 1, the viscosity of the gel increased greatly with the decrease of temperature. When the temperature decreased to 65 °C from 70 °C, the viscosity of the gel increased to 16 000 Pa·s from 600 Pa s. The excess H₃BO₃/NH₄HB₄O₇ in the gel with high molar ratio of H₃BO₃ to Mn(CH₃COO)₂ separated from the liquid during the cooling process due to the decreased solubility of H₃BO₃/NH₄HB₄O₇ in the solution. At high temperatures, the system existed as a solution of Mn(CH₃COO)₂ and excess H₃BO₃ (low viscosity). When the temperature decreased, large amounts of H₃BO₃/NH₄HB₄O₇ crystals separated from the liquid, and the system turned to a solid–liquid mixture (high viscosity). The gel in this state was not suitable for smearing evenly and sufficiently thinly on Al₂O₃; therefore, the molar ratio of H₃BO₃ to Mn(CH₃COO)₂ was controlled within 12 : 1 in all subsequent experiments.

Fig. 1 Viscosity of the gel with different molar ratios of H₃BO₃ to Mn(CH₃COO)₂ at different temperatures.

Characterization of the whisker-coated Al₂O₃ surface

Fig. 2 presents SEM images of the morphology of the whisker-coated surfaces on Al₂O₃ fabricated at 950 °C for 4 h (typical parameters were obtained through experimentation and literature review²⁶). The molar ratios of H₃BO₃ to Mn(CH₃COO)₂ corresponding to Fig. 2(a)–(d) were 3 : 1, 6 : 1, 9 : 1, and 12 : 1. Fig. 2(a) shows whiskers with an average size of 100 nm in width and 10 μm in length that tended to cluster when the molar ratio of H₃BO₃ to Mn(CH₃COO)₂ was 3 : 1. As the molar ratio of H₃BO₃ to Mn(CH₃COO)₂ increased, both the length and the diameter of the whiskers increased and the clustering phenomenon disappeared, as shown in Fig. 2(b)–(d). Fig. 2(d) shows longer and larger whiskers, with an average size of 200 nm in width and 20 μm in length, which were obtained at a large molar ratio of H₃BO₃ to Mn(CH₃COO)₂.

Fig. 2 SEM images of aluminum borate whiskers synthesized at 950 °C for 4 h by gels with a molar ratio of H₃BO₃ to Mn(CH₃COO)₂ of (a) 3 : 1, (b) 6 : 1, (c) 9 : 1 and (d) 12 : 1.

These observations suggest that the morphology of the whiskers grown on Al₂O₃ can be manipulated according the molar ratio of H₃BO₃ to Mn(CH₃COO)₂. Whiskers synthesized at a large molar ratio of H₃BO₃ to Mn(CH₃COO)₂ had larger size and are thus better suited to bonding Al₂O₃. Mn_xO_y decomposed from Mn(CH₃COO)₂ was not the reactant but the catalyst/co-solvent in the whisker-growth reaction. Small amounts of Mn_xO_y were enough for the entire growth process. On the one hand, excess Mn_xO_y meant insufficient reactants (B₂O₃); on the other hand, excessive Mn_xO_y could remain in the solid state during the growth process and cause negative effects on whisker growth. Our conclusion regarding the effects of the H₃BO₃ to Mn(CH₃COO)₂ molar ratio on the morphology of the gel obtained at 950 °C can therefore be generalized to all growth temperatures, so the molar ratio of H₃BO₃ to Mn(CH₃COO)₂ was set to 12 : 1 for all subsequent experiments.

Fig. 2(d) and 3 show SEM images of the products obtained at different temperatures after holding for 4 h. The whiskers synthesized at 850 °C had a relatively small size of 100 nm in width and 10 μm in length. When the holding temperature was 950 °C (Fig. 2(d)), longer and larger whiskers, with an average size of 200 nm in width and 20 μm in length, were observed, and the angle between the whiskers and the Al₂O₃ surface was consistently greater than 60°. Further increase of the growth temperature resulted in a decrease in the average length of the whiskers and a rougher surface of the whiskers, as shown in Fig. 3(b). With a growth temperature of 1150 °C, nearly no whiskers were observed on the Al₂O₃ surface (Fig. 3(c)).

Fig. 3 SEM images of aluminum borate whiskers synthesized at (a) 850 °C, (b) 1050 °C, and (c) 1150 °C for 4 h.

Next, XRD diffractograms (Fig. 4) of the products were analysed to identify the phase structures of the whiskers. The results showed that different types of whiskers were obtained under different calcination temperatures. At 950 °C, most of the diffraction peaks could be indexed to the orthorhombic phase of Al₄B₂O₉, with lattice parameters of a = 1.475 nm, b = 1.510 nm, and c = 0.560 nm. When the heating temperature increased to 1050 °C, a large portion of the diffraction peaks could be indexed to the orthorhombic phase of Al₁₈B₄O₃₃, with lattice parameters of a = 0.768 nm, b = 1.501 nm, and c = 0.566 nm.

Fig. 4 XRD spectrum of aluminum borate whiskers formed on the surface of Al₂O₃ at 950 °C and 1050 °C.

Joining of Al₂O₃ by epitaxially grown whiskers

Fig. 5 shows SEM images of the interfacial microstructure of the joints formed by whiskers at different bonding temperatures. When the bonding temperature was 850 °C, most of the obtained whiskers were too short to connect the two Al₂O₃ bulks (Fig. 5(a)). As the bonding temperature increased, the quantity of the whiskers that could connect the two Al₂O₃ bulks increased due to the increase in average whisker length. Whiskers with an average diameter of 500 nm formed a network structure in the joint, as shown in Fig. 5(b) and (c). When the bonding temperature was greater than 1050 °C, the length of the whiskers decreased (Fig. 5(d)) leading to deterioration of the density of the joints. Fig. 5(b) and 6 show SEM images of the interfacial microstructure of the joints obtained at 950 °C with different holding times. The whiskers first became longer as the holding time increased; most whiskers were long enough to connect the two Al₂O₃ bulks when the bonding temperature was held for 6 h at 950 °C (Fig. 6(a)). The whiskers became thicker as they became unable to grow in the axial direction with further increase in holding time, as shown in Fig. 6(b). Increasing the holding time to 10 h increased the average diameter of the whiskers to 1 μm or more (Fig. 6(c)).

Fig. 5 SEM images of the morphologies of Al₂O₃ joints fabricated at (a) 850 °C, (b) 950 °C, (c) 1050 °C, and (d) 1150 °C for 4 h.

Fig. 6 SEM images of the morphologies of Al₂O₃ joints fabricated at 950 °C for (a) 6 h, (b) 8 h, and (c) 10 h.

Mechanical properties of the joints

Shear strength tests were then performed at room temperature to investigate the mechanical properties of the joints formed by the whiskers. Fig. 7(a) shows the effect of the bonding temperature on the shear strength of the joints of the Al₂O₃ bulks with the gel (H₃BO₃ : Mn(CH₃COO)₂ = 12 : 1) interlayer that were calcined for 4 h in air. The shear strength first increased as the bonding temperature increased until it reached a maximum value of 53.1 MPa at 1050 °C. Above this temperature, any further increase in temperature caused the shear strength to rapidly decrease. These results were then fitted to the interfacial microstructures in Fig. 5. Based on the results of the shear strength obtained at different temperatures and the morphology of the whisker-coated Al₂O₃ surfaces, the bonding temperature was considered optimal at 950 °C and 1050 °C in further tests. Shear strength tests of the products obtained at different holding times were carried out and are shown in Fig. 7(b). The results indicated that when the bonding temperature was 950 °C, the shear strength of the joints first increased as the holding time increased until reaching the maximum value (84.5 MPa, corresponding to the interfacial morphology shown in Fig. 6(b)) at 8 h. While the maximum strength of the joint was 54.1 MPa (holding for 6 h) when the bonding temperature was 1050 °C. In general, when the low bonding temperature and short holding time were employed, the whiskers started to grow epitaxially, but the length was too short to reach the opposite side of the Al₂O₃ substrate to be joined, which led to low joint strength. Proper increase of the bonding temperature and holding time increased the length and diameter of whiskers and resulted in the increase of the quantity of whiskers to connect two sides of Al₂O₃ substrate. This contributed to the increase of the bonding strength of the joints. However, when the temperature increased to 1050 °C, Al₄B₂O₉ was transformed into Al₁₈B₄O₃₃, which was observed to have worse performance in bonding Al₂O₃ in our previous work²⁶ and others studies.²⁵ In addition, over-holding might cause malignant growth of whiskers and lead to the decrease of joint strength. Due to the comprehensive effect of these factors, the joints achieved the maximum strength at 950 °C for 8 h.

Fig. 7 Shear strength of joints obtained (a) at different temperatures for 4 h and (b) at 950 °C and 1050 °C with different holding times.

Fig. 8 shows SEM images of the morphology of the fracture surfaces, where it is clear that fractures consistently occurred on the whiskers regardless of the technological parameters. Compared with whiskers grown on only Al₂O₃ surfaces, whiskers grown in the joints had smoother surfaces and sharper edges and corners. The diameters of whiskers grown in the joint were also more uniform in the axial direction.

Fig. 8 SEM images of the morphology of the fracture surfaces obtained at (a and b) 950 °C and (c and d) 1100 °C.

Detailed TEM examinations were conducted to further identify the microstructures of the whiskers grown in the joints. Fig. 9(a) and (d) show TEM images of an individual whisker obtained at 950 °C and 1100 °C, respectively. Selected area electron diffraction (SAED) acquired in the [22̄1]-axis zone (Fig. 9(b)) indicated that the whisker obtained at 950 °C was Al₄B₂O₉, while the SAED acquired in the [21̄0]-axis zone (Fig. 9(e)) revealed that the whisker obtained at 1100 °C was Al₁₈B₄O₃₃. In addition, based on the SAED results, both types of whiskers had single-crystal structure. The HRTEM images presented in Fig. 9(c) and (f) suggested that the [1̄02] and [001] crystallographic directions are the possible growth directions of individual Al₄B₂O₉ and Al₁₈B₄O₃₃ whiskers, respectively.

Fig. 9 TEM characterization of an individual whisker obtained from the fractured joint bonded at (a–c) 950 °C and (d–f) 1100 °C.

The relationship between Al₄B₂O₉ and Al₂O₃ was investigated by TEM and HRTEM, as shown in Fig. 10(a) and (b), respectively. The results were similar to our previous work of synthesizing Al₄B₂O₉ whiskers on Al₂O₃ surfaces via B₂O₃ powder.³⁷ The (220) planes of Al₄B₂O₉ and the (101̄0) planes of Al₂O₃ were observed, and the Al₄B₂O₉/Al₂O₃ interface was a good semi-coherent interface with low strain energy.

Fig. 10 TEM examinations of interface between the whisker-coated rough surface and the Al₂O₃ substrate. (a) TEM image of the Al₄B₂O₉/Al₂O₃ interface. (b) Corresponding HRTEM image in (a) showing the relationships between Al₄B₂O₉ and Al₂O₃.

Possible joining mechanism

Based on previous work²⁶ and the observations in this study discussed above, the process of joining Al₂O₃ could be divided into the reactions of the gel and the core reaction of whisker growth. Dehydration, decomposition, and combustion reactions of the gel proceeded first. At 100 °C, the residual water and the NH₄⁺ used to adjust the pH of the solution volatilized into air and the aquogel turned to xerogel. H₃BO₃ decomposed into B₂O₃ and water at approximately 200 °C, and once the temperature reached 300 °C, organics in the gel burned into CO₂ and H₂O. When those reactions finished, the gel had turned to a mixture of B₂O₃ and Mn_xO_y. As the temperature continued to increase, the core reaction of whisker growth, which can be described as xAl₂O₃ + yB₂O₃(l) → (Al₂O₃)_x(B₂O₃)_y, proceeded. The reactions were as follows:

NH₃·H₂O → NH₃ (gas) + H₂O (gas) (1)

4H₃BO₃ → 2B₂O₃ (solid) + 6H₂O (gas) (2)

Mn(CH₃COO)₂ + O₂ → Mn_xO_y (solid) + H₂O (gas) + CO₂ (gas) (3)

B₂O₃ (solid) → B₂O₃ (liquid) (4)

2Al₂O₃ + B₂O₃ → Al₄B₂O₉ (solid) (5)

9Al₂O₃ + 2B₂O₃ → Al₁₈B₄O₃₃ (solid) (6)

The mechanism of aluminum borate whisker growth could be explained according to solution–liquid–solid (S–L–S) theory. When the temperature reached 450 °C, B₂O₃ turned into a viscous liquid and spread out on the Al₂O₃ surface. As the temperature continued to increased, Al₂O₃ particles began to dissolve in the B₂O₃ (liquid), while the core reaction of whisker growth occurred. When (Al₂O₃)_x(B₂O₃)_y in the liquid solution became supersaturated, nucleation of Al₄B₂O₉ occurred and then grew into one-dimensional and single-crystal fibres along a specific direction, forming aluminum borate whiskers. Al³⁺ was derived from alumina ceramics only under the experimental conditions, so (Al₂O₃)_x(B₂O₃)_y near the ceramic was supersaturated first, which explains why the whiskers consistently nucleated on the surface of the Al₂O₃ and grew epitaxially. As the holding time increased, more raw material was transported to the top of the whiskers, allowing them to grow sustainably. At elevated calcination temperatures, the reaction and transportation were accelerated further and the whiskers grew even longer. When the top of the whisker contacted the opposite Al₂O₃, it was able to form a geometrical lattice, matching the surface of the Al₂O₃ and blending effectively with the alumina ceramic, completing the joining of the Al₂O₃ by the whiskers.

Conclusion

In the present study, Al₂O₃ ceramics were bonded by alumina borate whiskers via sol–gel method. The most notable conclusions can be summarized as follows.

(1) The whisker size increased as the molar ratio of H₃BO₃ to Mn(CH₃COO)₂ increased, but a molar ratio greater than 12 : 1 made for inconvenient gel-smearing. The ratio of 12 : 1 was the most appropriate molar ratio of H₃BO₃ to Mn(CH₃COO)₂ to ensure the appropriate morphology of the gel-fabricated alumina borate whiskers.

(2) The length, diameter, and density of the whiskers grown on the surface of Al₂O₃ first increased and then decreased as the calcination temperature increased. The XRD results showed that below 950 °C, most whiskers were Al₄B₂O₉ and above 1050 °C, most whiskers were Al₁₈B₄O₃₃.

(3) The bonding temperature and holding time were shown to influence the bonding quality. Joints obtained at 950 °C and held for 8 h had the optimum shear strength (84.5 MPa), where the width of the joint was less than 10 μm, both types of whiskers had single-crystal structures, and the preferential growth orientation of Al₄B₂O₉ was [1̄02], while that of Al₁₈B₄O₃₃ was [001]. The Al₄B₂O₉/Al₂O₃ interface was a good semi-coherent interface with low strain energy.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under Grant No. 51275133 and U1537206. This work is also supported by the Fundamental Research Funds for the Central University (Grant No. HIT.BERTIII.201508).

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