A composite structural high-temperature-resistant adhesive based on in-situ grown mullite whiskers

Materials Today Communications 23 (2020) 100944

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A composite structural high-temperature-resistant adhesive based on in-situ grown mullite whiskers

T

Zhaojie Fenga, Mingchao Wanga,*, Ruoyun Lua, Wence Xub, Ting Zhanga, Tong Weia, Jingfang Zhanga, Yunlong Liaoc,* a

College of Science, Civil Aviation University of China, Tianjin 300300, PR China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, PR China c Center for Aircraft Fire and Emergency, Civil Aviation University of China, Tianjin 300300, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Mullite whiskers In situ grow High-temperature-resistant adhesive Composite structure Bonding performance Reinforcing mechanism

The effect of whisker growing catalyzed by different amounts of AlF3 on the various properties of an epoxy/ silicone resin-based high-temperature-resistant adhesive was comprehensively investigated. Although the addition of AlF3 promoted the oxidation reactions, accelerated the premature formation of mullite whiskers, increased the production of SiC ceramic, and reduced the thermal expansion of bonding layer, the excessive use of AlF3 induced many more large holes in the bonding layer, caused the structural heterogeneity, and made mullite whiskers overgrow, thus finally led to the weaker mechanical properties of adhesives. The best dosage of AlF3 in this work was 3 % (mass fraction), a uniform composite structure that whisker weaved adhesive frame kept steady from 1100 °C to 1500 °C, and an impressive bonding strength of 63 MPa was achieved at 1100 °C, 80 % higher than that of the blank sample. Under the synergistic effect of whisker repair, whisker bridging, whisker drawing and crack deflection, a multi-stage fracture phenomenon happened during the shear test, and the fracture displacement of 1100 °C-calcined adhesive with 3 % AlF3 increased by 61 %, compared with that of the black sample.

1. Introduction Developed from the ancient time of natural pastes to the modern various advanced adhesives, they had always been an integral part of our life, which bring great conveniences to our lives and work [1]. Similarly, some special high-temperature-resistant adhesives are also very essential to engineering application fields. Nowadays, more and more fields are facing with extreme high-temperature conditions, such as aerospace, aviation, navigation, and nuclear industry, etc., the hightemperature-resistant adhesives play essential roles in not only the component fabrication but also the assembly, sealing and repair of components or systems [2,3]. Till now, many kinds of special hightemperature-resistant adhesives have been reported for bonding engineering hot-ends of different materials, such as carbon/carbon composites [4,5], SiC [6,7], Si3N4 [8], mullite [9], Al2O3 [10], and even some superalloys [11]. Compared with other connection methods (welding, brazing, solid diffusion, mechanical joint, etc.), adhesive bonding is the fastest, simplest, and cheapest. It is characterized with once bonding at low temperature, directly used in practice, or even without any pre-treatment, which was very suitable for on-site ⁎

operation [3]. In general, two types of adhesives have been developed, including pre-ceramic polymer-based adhesive and inorganic ceramic-based adhesive [12]. The former type has attracted much attention due to its high bonding strength, composition adjustability, structural designability, and high interfacial reactivity. It was usually prepared from polymers as matrix, metallic powders as volume compensators, oxides or carbides as fortifiers, and glass powders as structural improvers [6,10,12]. According to the interface reaction requirement and end product matching, many special adhesives of different components have been produced to cater for different materials and products. Wherein, the “GRABER” based on phenolic resin as matrix was no doubt one of the famous adhesives, which has been successfully applied to repair reinforced C/C composites for thermal protection of aircraft served in NASA [13]. Besides, Wang et al. [14] used B4C to modify D4Vi-mixed methyl dichlorosilane, and developed an excellent hightemperature-resistant adhesive whose bonding strength was up to 50 MPa after heating at 1200 °C. Luan et al. [15] developed a polyborosilazane-based adhesive which still provided 15 MPa for bond Al2O3 at 1500 °C due to the formation of Al2O3·B2O3. However, the

Corresponding authors. E-mail addresses: [email protected] (M. Wang), [email protected] (Y. Liao).

https://doi.org/10.1016/j.mtcomm.2020.100944 Received 31 October 2019; Received in revised form 5 January 2020; Accepted 17 January 2020 Available online 18 January 2020 2352-4928/ © 2020 Elsevier Ltd. All rights reserved.

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tank on the basis of mass ratio of 2.25: 1.25: 0.3: 0.7 and 0-0.75, and the stuffing mix was ball milled for 2 h under a revolving speed of 500 rpm. The epoxy modified silicone resin was synthesized in the three-necked flask in a water bath of 80 °C. In order to avoid the excessive evaporation of isopropanol, the reaction process was equipped with a cooling condenser pipe. Epoxy resin, silicone resin and isopropyl alcohol were mixed based on the mass ratio of 1: 4: 5. After ∼2 h of reaction, the adhesive matrix solution with a low viscosity about 1500 mPa·s was filled in the ball mill tank according to the solid to solution ratio of 2: 3, and then the mixed solution was ground completely by low-speed ball milling at 100 rpm for 6 h. In this work, four kinds of adhesives with different addition of AlF3 (0 wt.%, 3 %, 6 %, 9 % based on the entire solid content) were produced to detect the effect of the amount of AlF3 on the physicochemical property and mechanical property of adhesive. Before the bonding process, a few drops of silane coupling agent KH560 was added in adhesive and stirred even manually. The specific bonding, calcining and shear test processes are shown in Fig. 1. The asachieved adhesive was brushed on the substrates, then they were bonded to each other to form the joints with double-notched structure (bonding area: 12 mm × 10 mm), and the corresponding fixture for the shear test was the same in Ref. [12]. These as-bonded joints were placed in an ambient atmosphere for curing overnight, and then they were heat-treated in an airtight furnace for 1 h at each different given temperatures. The apparent shear test at room temperature (RT) was performed by CMT4304 universal testing machine at a speed of 0.5 mm/ min. The peak load and the load-displacement curves were collected for analyzing mechanical property, and the rupture surfaces were used for macro-/microstructure observation and reinforcing mechanism analysis. The composition evolution of different adhesives with temperature increasing was identified by X-ray diffraction machine (XRD, Max2500, Kα1 = 1.5406 Å). The structure evolution of different adhesives and the reinforcing mechanism of whiskers were investigated by field emission scanning electron microscopy (FESEM, S-4800). The micromorphology and atomic composition of mullite whiskers were analyzed with transmission electron microscopy (TEM, Philips Tecnai F20) equipped with energy-dispersive X-ray spectroscopy (EDS). Besides, the thermal stability of different adhesives from RT to 1500 °C was tested by thermogravitmetry differential scanning calorimetry (TG-DSC, STA 449C) at a heating rate of 10 °C/min. The thermal expansivity of different adhesives calcined at 1100 °C was also compared by using thermal mechanical analysis equipment (TMA, NETZSCH DIL 402C, RT1000 °C). In addition, porosity which is closely related to the strength of the adhesive was also tested in this work. As the bonding layer is very thin, it is tough to directly test the density and porosity of adhesive applied in the joints, so some slightly larger piece of adhesive bulks were set as substitutes and tested by using 3H-2000TD1 Automatic True Density Analyzer based on the gas method. The specific process of the porosity test was shown in Fig. S1.

connection strength provided by adhesion was still much lower than other processes. Yuan et al. [16] used nickel-based brazing filler to braze ZrB2 ceramic, and the shear strength of ZrB2/Ni/ZrB2 system was as high as 59.7 MPa. Li et al. [17] employed a solid state diffusion method to bond SiC, and the shear strength of SiC joints was up to 123.5 MPa. Moreover, the adhesive after calcination presented great brittleness as the main phases are ceramics [12]. Therefore, it is urgent to strengthen and toughen the pre-ceramic polymer-based adhesive for further application. The most popular and traditional reinforcing way was directly adding nano- or micro- phases in the adhesive, such as whiskers and carbon tubes [18,19]. However, the result was less lucrative than it hoped. Agglomeration always happened to these small size phases under the action of electrostatic forces, intermolecular forces, and hydrophobic effects, and it is very tough to disperse them uniformly in the adhesive. Secondly, using them would increase the cost of adhesive. Nevertheless, the main constraint is that the reinforcing result is not apparent. Luo et al. [20,21] directly added short carbon fiber to reinforce silicone resin based adhesive, and the bonding strength was only increased by 17–27 %; Zielecki et al. [22] dispersed 1 wt.% multiwall carbon nanotubes to reinforce an epoxy adhesive, the fatigue strength was only enhanced by 13 %. Compared with the traditional reinforcing methods, whisker in situ growing technology shows better reinforcing effect, which has been widely used to produce high toughness ceramic or refractory. Pei et al. [23] used iron as catalyst to pyrolysis polycarbenes at high temperature, and successfully grew silicon carbide whiskers on the surface of carbon fiber in carbon fiber/silicon carbide ceramics, which increased the bending property of ceramics by 92 %. Chu et al. [24] in situ synthesized SiC whiskers by using chemical vapor deposition and carbon package technologies in SiC-Si ceramic coating, and the fracture toughness was increased by 98 %. Lu et al. [25] reinforced Al2O3-based ceramic mold by in situ growing mullite whiskers, and made its bending strength increase by 104 %. Moreover, the in situ reinforcing technology also had other advantages, like uniform whisker distribution and low cost, as it just needs to mix appropriate amount growth sources and catalysts into the substrate uniformly in advance. However, it has been seldom used to reinforce adhesives. Considering that much oxygen-containing molecules contained in these high-temperature-resistant adhesives, such as SiO2 and Al2O3, it is possible to in situ synthesize mullite whiskers by using AlF3 as catalyst. Actually, the amount of AlF3 has a big impact on the structure, and AlF3 is also used to produce porous ceramics based on its high-temperature corrosion [26]. As the bonding layer is very thin about 100∼200 μm [4,12,20], the excessive pores would be no doubt bad for bonding performance, so the addition amount of AlF3 has to be carefully controlled. In this work, silicon carbide was chosen as the adherend, and the effect of amount of AlF3 as well as temperature on the mechanical properties, composition evolution, structure evolution, thermal properties, and porosity of an epoxy/silicone-resin based adhesive was investigated systematically. When the amount of AlF3 was 3 wt.%, the maximum strength of 63 MPa was achieved at 1100 °C, and the adhesive displayed better damage tolerance or toughness based on the fabric-like composite structure. Besides, the reinforcing mechanism of in situ growing whiskers was also discussed in detail.

3. Results and discussion 3.1. The effect of AlF3 on mechanical properties As seen from Fig. 2, the changing trend of bonding strength with temperature increasing was similar for different adhesives, which exhibited a regular of increase first and then decrease afterward. The bonding strength values of them were almost same in the temperature range from RT to 700 °C, indicating that AlF3 had not come into effect when the temperature was not above 700 °C. The difference of their bonding strength started to grow from 900 °C, which not only declared that AlF3 had already involved in the reaction, but also illustrated that its amount greatly influenced the mechanical property of adhesives. After heating at 900 °C, the more AlF3 the adhesive contained, the higher the bonding effect it had. And the bonding strength value of

2. Experimental process Silicon carbide ceramic substrate with dimensions of 40 mm × 10 mm × 5 mm and 20 mm × 10 mm × 5 mm was provided by Xiangtan Mingchuang Noval Materials Company. Its basic physical parameters are shown in Table 1. All chemicals for adhesive synthesis were used directly without further treatment, and their expatiation is presented in Supporting Information. The chemicals of silicon, aluminum, boron carbide, low-temperature glass powder and aluminum fluoride were added in the ball mill 2

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Table 1 Basic physical parameters of silicon carbide ceramic. Reaction sintering sources

Density (g/cm3)

Porosity (%)

Bending Strength (MPa)

Compressive Strength (MPa)

Roughness of Surface (μm)

Si:C 1:2

3.05 ± 0.01

1.5

≥350

≥1200

0.08–0.17

Fig. 1. The preparation process of joints and the specific shear test.

displacement of curve, which meant that the adhesive with 3 % AlF3 had better damage resistance property. Besides, the slope of the top half of red curve in Fig. 2b was the smallest, which also indicated the growth of whisker initiated by 3 % AlF3 would increase the toughness of adhesive to some extent. However, when the content of AlF3 was too much, it made the toughness of adhesive worse, as seen from Fig. 2b, the stiffness (displacement) for adhesive with 9 % AlF3 was higher (shorter) than that of the blank sample. On the whole, the optimal addition amount of AlF3 was 3 %. In addition, the typical loading-displacement curves for 3 % AlF3added adhesive after calcination at different temperatures were compared in Fig. 2c. Its curves were almost straight at 500 °C and 700 °C, while some small fluctuations began to appear on the curve of 900 °C, and the slope also became lower, which should be related to the growth of whiskers. And then the slope of curves gradually increased with temperature increasing from 900 °C to 1500 °C. Moreover, the changing trend of damage tolerance of adhesive was consistent with the law of strength changing with temperature, the best damage resistance was achieved at 1100 °C, while the worst one was gotten at 1500 °C. All these above mechanical results were tied up with the evolution of composition and structure of different adhesives with temperature increasing, which was explained in detail in the following parts.

adhesives with 9 %, 6 %, 3 % and 0 % AlF3, at 900 °C was 36.7 MPa, 32.3 MPa, 28.4 MPa and 25 MPa, respectively. However, the bonding strength of adhesive with 9 % AlF3 had become the worst when the temperature reached 1100 °C, which was even 5.7 % lower than that of the blank sample under the same condition. Simultaneously, the adhesive with 3 % AlF3 obtained the maximum strength of 63 MPa at 1100 °C, 80 % higher than the blank sample, and the second one was 6 % AlF3-added adhesive with a strength of 47 MPa. Subsequently, their bonding strength gradually declined as temperature increased from 1100 °C to 1500 °C, and their order based on bonding strength at 1300 °C kept the same with that at 1100 °C. The strength value of adhesive with 9 % AlF3 at 1300 °C was about 20 % lower than that of the blank sample. After heating at 1500 °C, only adhesive with 3 % AlF3 presented better mechanical strength than the blank sample, and the bonding strength of adhesives with 6 % and 9 % AlF3 was 13.5 % and 36.4 % lower than that of the blank sample. On the other hand, the damage tolerance or toughness of different adhesives was also analyzed through comparing the typical loadingdisplacement curves of their bonded joints after heating at 1100 °C, as shown in Fig. 2b. The curve for the blank sample was almost straight, while the multi-stage fracture phenomenon occurred for adhesives with the addition of AlF3 whose curves showed up serration, especially for 3 % and 6 % AlF3-added adhesives. The serration phenomenon was mainly due to the toughening mechanism of growing whiskers during the shearing process, and this result also proved that whiskers had already grown in adhesives with AlF3 at 1100 °C. As seen from the red curve in Fig. 2b, when the load reached its limit, it did not immediately drop to zero but gradually decreased, thus led to a longer fracture

3.2. The effect of AlF3 on composition evolution As AlF3 had not come into effect when the temperature was below 900 °C, the composition evolution of adhesives with different contents of AlF3 was almost the same with that of the blank sample which had

Fig. 2. (a) The bonding strength results of adhesives with different contents of AlF3 after treatment at different temperatures; (b) The typical loading-displacement curves for different adhesives after treatment at 1100 °C; (c) The typical loading-displacement curves for adhesive with 3 % AlF3 after calcination at different temperatures. 3

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Fig. 3. XRD spectra of adhesives with different contents of AlF3 after calcination at 900 °C (a), 1100 °C (b), 1300 °C (c) and 1500 °C (d).

analysis, the development of various ceramics (mullite, Al2O3 and SiO2) took responsibility for the enhancement of bonding performance of most adhesives from 700 °C to 1100 °C. However, the composition evolution was hard to explain the decrease of bonding effect from 1100 °C to 1500 °C as well as the differences of bonding effect among these adhesives, but could be well illustrated by the structure evolution.

been reported in Ref. [27]. For more effective comparison, the composition difference of adhesives from 900 °C to 1500 °C was most concerned in this work. It should be noted that every XRD spectrum in Fig. 3 was obtained under the same conditions, including test parameters (8KV, 40 mA, 0.02°/s, etc.), adhesive amount (standard tank) and particle size. As seen from Fig. 3a, the metallic additives such as Al and Si had not been completely oxidized after heating at 900 °C. The peak intensity of Al (Al2O3) gradually reduced (enhanced) with the content of AlF3 increasing, which implied that AlF3 facilitated the oxidization reaction. Meanwhile, the crystal peaks of mullite had been obvious on the spectrum of adhesive with 9 % AlF3, while they were just enough to be identified on the spectrum of adhesive with 6 % AlF3, and no peak of mullite appeared on the spectrum of adhesive with 3 % AlF3 and the blank sample. This phenomenon fully illustrated that the addition of AlF3 would promote the formation of mullite. Actually, the silicon source for mullite at this moment was mainly SiO2 decomposed from glass (its other decomposition products: Sn and SnO2). After heating at 1100 °C, the peaks of Al for all adhesives disappeared (accompanied by the peak increase of Al2O3), and the peak intensity of Si also declined compared with that at 900 °C, indicating that Al had been completely oxidized into Al2O3, and the oxidization of Si was also further strengthened. Besides, the peak intensity of mullite for AlF3-added adhesives was increased at 1100 °C (still no mullite formed in the blank sample), and the content of mullite in 9 % AlF3added adhesive was still the highest, while that in 3 % AlF3-added adhesive was the least. Afterward, except for that the peak of Al2O3 gradually decreased with temperature further increasing, the change of peaks of mullite and Si at 1300 °C and 1500 °C was similar with that at 1100 °C. Till now, an important rule would be concluded for the mullite formation in these adhesives: the content of mullite increased with both temperature and AlF3 amount increasing. In addition, because the heattreatment was performed in anaerobic environment, SiC started to appear at 1300 °C and its content also increased at 1500 °C (check peaks at ∼36°, 42° and 60° degrees in Fig. 3c and d). Moreover, the more AlF3 was added, the more SiC would be generated in the adhesive. The reason was that AlF3 accelerated the oxidization of Si and Al, and the increased AlF3 would consume more oxygen-containing molecules and provide a more favorable environment for SiC. According to the above

3.3. The effect of AlF3 on structure evolution Based on the above mechanical and XRD results, the catalyst started working at 900 °C, and the amount of AlF3 influenced the structural evolution of adhesive at higher temperatures. It should be noted that no whisker was grown in adhesive without AlF3 addition during the heating process (see Figs. 4a and S2). In general, the more aluminum fluoride was added, the more whiskers with larger sizes were formed, and the poorer the denseness of the adhesive. After heating at 900 °C, some tiny whiskers with a length of 3−4 μm had been formed in the adhesive with 9 % AlF3 (see Fig. 4d), while there were just roots of whiskers started to appear in the adhesive with 6 % AlF3 (see Fig. 4c). Accordingly, only whisker growing points (see Fig. 4b) could be observed in the adhesive with 3 % AlF3, and these fine white particles distributed uniformly in the adhesive matrix. As well known, the growing of whiskers by using AlF3 as catalyst was based on V-S mechanism, so more and more pores would be generated in the adhesive with the content increasing of AlF3, which was demonstrated by the structure changing trend shown in Fig. 4. Actually, the addition of AlF3 had not impacted the macro-structure of bonding layer at 900 °C yet, no apparent large hole was formed in the bonding layer, and the fracture surfaces of these adhesives were relatively flat (see Fig. S3). However, the excess of AlF3 caused the barbaric growth of whiskers in some local areas of adhesives from 1100 °C to 1500 °C, which led to the inhomogeneous structure of adhesives with 6 % and 9 % AlF3. Judged from the section morphology shown in Figs. 5–7, whatever the heating temperature was, no large hole was found in the adhesive with 3 % AlF3, while some large holes appeared in the adhesives with 6 % and 9 % AlF3. Apparently, the size of these holes grew as the content of AlF3 increased. Took adhesives heated at 1100 °C as an example, most holes in the adhesive with 6 % AlF3 were of size between 50 μm 4

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Fig. 4. SEM images of adhesives calcined at 900 °C with different contents of AlF3 (a) 0 %, (b) 3 %, (c) 6 %, (d) 9 %.

and 120 μm, only a few holes reached ∼250 μm, while some vast holes with size over 500 um formed in the adhesive with 9 % AlF3. The introduction of these large holes would definitely cause the compactness decrease and structural heterogeneity of the bonding layer. As sources of stress, the more and larger of these holes were formed in adhesive, the weaker its mechanical performance would be, which well explained that the increase of the amount of AlF3 led to the decline of bonding

performance in Fig. 2a. Essentially, the difference of adhesives in their macro-structure was still caused by whisker growing under different conditions. It could also be concluded easily by comparing the SEM images that the optimal dosage of AlF3 was 3 %, which was in accordance with the results of the mechanical test. As seen from Fig. 5, when the content of AlF3 was 3 %, no apparent hole formed in bonding layer during the

Fig. 5. Low magnification SEM images of fractured surfaces for adhesive with 3 % AlF3 at different temperatures and the appointed micro-photography images (1100 °C: A, a; 1300 °C: B, b; 1500 °C: C, c). 5

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Fig. 6. Low magnification SEM images of fractured surfaces for adhesive with 6 % AlF3 at different temperatures and the appointed micro-photography images located in different areas (1100 °C: A, a, α; 1300 °C: B, b, β; 1500 °C: C, c, γ).

why the damage tolerance of 1500 °C-calcined adhesive was even lower than that of 900 °C-calcined adhesive. For adhesive with 6 % AlF3 calcined at 1100 °C, two kinds of microstructure were observed, one structure that the adhesive matrix mixed with a few short whiskers (∼10 μm) was located outside of hole, and the other structure of longer whiskers cluster was located in the hole (see Fig. 6). When the content of AlF3 reached a certain value, the gaseous AlF3 would be concentrated in some localized areas and severely eroded adhesive matrix to form large holes, and then the intermediate products (AlOF (g) and SiF4 (g)) reacted with each other to form a mass of whiskers in the holes. As seen from Fig. 6α, the whisker growing points had been detached from the adhesive matrix, so the hole had a negative effect on bonding strength. In contrast, the areas possessed an appropriate amount of AlF3 would obtain uniform composite structure as shown in Fig. 6a. This was the reason why the bonding strength of adhesive with 6 % AlF3 was lower than that of adhesive with 3 % AlF3. Actually, the influence of temperature on the macro-structure was much lower than that on micro- structure. Once the macro-structure was determined at 1100 °C, it was difficult to change the macroscopic morphology at higher temperatures further. Judging from Fig. 6A–C,

whole heating process, and although the fracture surfaces at different temperatures showed varying degrees of disrupted folds, the whiskers had always been distributed evenly throughout the adhesive since the moment of whisker grew at 1100 °C. It should be noted that no matter where was chosen for high magnification observation, the microstructure shown in Fig. 5a–c would always be respectively observed. A large number of tiny, delicate and straight whiskers densely distributed in the framework of adhesive with 3 % AlF3 after heating at 1100 °C, and now their length was about 3−5 μm, the diameter was just 50 μm (see Fig. 8a). The porous structure that was filled with many in situ grown whiskers presented high mechanical properties, which was mainly ascribed to the synergistic effect from the whisker overlap joint and high connection between whisker and adhesive. Also, under the action of whisker toughening, the adhesive displayed high damage tolerance property (see Fig. 2c). After calcination at 1300 °C, the whiskers were obviously grown up, the diameter was about ∼200 μm (see Fig. 5d), and there were even some curved nanowires distributed in Fig. 5b. When the temperature reached 1500 °C, most whiskers had been grown into rods and were stacked together, as shown in Fig. 5c. Because the weak interface area decreased, this stacked structure no doubt led to higher stiffness and weaker toughness. This was the reason 6

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Fig. 7. Low magnification SEM images of fractured surfaces for adhesive with 9 % AlF3 at different temperatures and the appointed micro-photography images located in different areas (1100 °C: A, a, α; 1300 °C: B, b, β; 1500 °C: C, c, γ).

Fig. 8a shows the tip of a whisker grown at 1100 °C, no droplet formed on the tip and the diameter gradually diminished from stem to tip, which indicated that the formation of mullite followed V-S grow mechanism. The SAED (selected area electron diffraction) pattern in Fig. 8a demonstrated that the component of whisker was mullite (3Al2O3·2SiO2). Moreover, as seen from the high-resolution crystal plane diffraction fringe of whisker in Fig. 8b, its crystal lattice spacing was measured to be 0.2843 nm, 0.2572 nm and 0.5858 nm, which respectively represented the {001}, {111} and {110} lattice planes of mullite. The lattice plane of {001} was perpendicular to the whisker edge, indicating that the whisker grew along with the direction of [001]. Besides, the atomic number ratio of Al/Si was about 3.2 as analyzed by EDS in Fig. 8c, which further proved that the whisker contained the stoichiometric mullite. What is more, the whisker grew at 1300 °C was also chosen for TEM observation to better understand the change of whisker size with temperature increasing. A small section of whisker grown at 1300 °C away from the tip was shown in Fig. 8d. Compared with Fig. 8a, the diameter of whisker at 1300 °C was about four times larger than that at 1100 °C. No stacking fault formed in the direction perpendicular to its growth orientation, and the surface was very smooth. Besides, the high angle annular dark field scanning

there was little difference in the size and number of holes at different temperatures, and the big difference was the surface gloss which had to do with the micro-photography. The whiskers out of holes grew up from slender-like at 1100 °C (Fig. 6α) to needle-like at 1300 °C (Fig. 6β) and then to rod-like at 1500 °C (Fig. 6γ), and their distribution accordingly changed from loosely overlapped to compactly stacked. Meanwhile, the loosely clustered whiskers in the hole also turned into stacked rod-like whiskers (see Fig. 6a–c). This kind of structure development for adhesive with 6 % AlF3 finally led to the gradual decline of its bonding performance with temperature increasing from 1100 °C to 1500 °C. The structure evolution of adhesive with 9 % AlF3 was similar to that of adhesive with 6 % AlF3. However, the former structural changes were more dramatic, as shown in Fig. 7, the needle-like whiskers had already grown up at 1100 °C, and finally transformed into scattered columnar-like whiskers at 1500 °C. Moreover, the larger holes in the former adhesive were about two sizes of those in the later adhesive. In these cases, the bonding effect of adhesive with 9 % AlF3 from 1100 °C to 1500 °C was lower than that of adhesive with 6 % AlF3. In order to better figure out the physicochemical property of in situ grow whiskers, TEM technique was applied to analyze whiskers grown in the adhesive with 3 % AlF3 after heating at different temperatures. 7

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Fig. 8. Typical TEM images of in situ grow whisker chosen from 1100 °C-calcined adhesive with 3 % AlF3: (a) low magnification view; (b) high magnification view; (c) EDS spectrum of whisker; (d) Low magnified TEM image of in situ grow whisker chosen from 1300 °C-calcined adhesive with 3 % AlF3, and its HAADF-STEM images for elements of Al (e), Si (f) and O (g).

Fig. 9. The effect of AlF3 on thermostability (a: TG, b: DSC), porosity (c) and thermomechanical properties (d).

was mainly due to the decomposing of polymer, and the following mass increase was because of the oxidization of various fillers. However, there were turning points that appeared on three TG curves at about 850 °C that their growth rate gained slower. The more content of AlF3 in adhesive, the slower the mass increasing rate it had, and now the separation of three curves started to increase. Especially for adhesive with 9 % AlF3, there was even a platform that appeared on its TG curve from 1000 °C to 1100 °C. Based on Ref. [28], the reason for the above phenomenon was mainly the vaporization of AlF3. What is more, the TG curves of adhesives with 6 and 9 % AlF3 declined again at about 1140 °C, and the drop rate for the later one was as high as 1.8 %, which

transmission electron microscopy (HAADF-STEM) images in Fig. 8e–f show the mappings for different elemental distributions. All these mapping images demonstrated that their distribution was uniform. 3.4. The effect of AlF3 on other physicochemical properties It was clearly seen from Fig. 9a that the TG curves of different adhesives almost covered with each other before 600 °C, and their tracks were still very close from 600 °C to 800 °C, so as the DSC curves in Fig. 9b. The result also illustrated that the addition of AlF3 had not come into effect in this temperature range. The mass loss before 600 °C 8

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Fig. 10. Several reinforcing mechanisms of in situ grow whiskers for adhesive with 3 % AlF3: (a) crack repair viewed from the outer surface of 1100 °C-calcined adhesive; (b) hole bridging viewed from the fracture surface of 1300 °Ccalcined adhesive; (c) the whisker broken after the stress propagation; (d) the whisker drawing; (e1) the crack propagation through 1100 °C-calcined adhesive; (e2) a single whisker against crack propagation; (f) a sectional view of the fracture surface.

3.5. The reinforcing mechanism of whisker growing

was ascribed to the loss of more intermediate gaseous materials derived from the excessive corrosion of AlF3 to adhesive matrix. And the sharp decline on its DSC curve also illustrated the intensity of the endothermic reaction. Besides, the porosity evolution with temperature increasing of different adhesives was detected by density tester and concluded in Fig. 9c. Apparently, the more AlF3 added, the larger porosity the adhesive had. And no matter what the content of AlF3 it was, the porosity of adhesive firstly increased and then decreased with the increase of temperature. The above changing trend was well consistent with SEM results. From 900 °C to 1100 °C, the increase of porosity was attributed to the erosion of adhesive by AlF3, and many large holes generated in adhesive with 9 % AlF3, so its porosity was about twice as much as that of adhesive with 3 % AlF3 at 1100 °C; while the decrease of porosity from 1100 °C to 1500 °C was due to not only the growth of whiskers but also the further oxidization of additives (such as Si). In addition, the variation trend of density was opposite to that of porosity. Took adhesive with 3 % AlF3 as an example, its density firstly decreased from 2.3 g/cm3 at 900 °C to 2.01 g/cm3 at 1100 °C, and then steadily increased to 2.14 g/cm3 at 1500 °C. In addition, the thermal expansion coefficient (TEC) test was also used to analyze the effect of AlF3 on the thermal mechanical property of adhesives. The TEC curves of 1100 °C-calcined adhesives (3 %, 6 % and 9 % AlF3) were collected from RT to 1000 °C. Factually, as the order of these three adhesives based on the porosity followed: 9 % > 6 % > 3 %, so their order was reversed in terms of TEC, as shown in Fig. 9d. Besides, the corresponding temperature of the inflection points (from the fast rising stage to the slow rising stage) of these adhesives was about 650 °C, which was a little bit higher than other ceramic materials. For 1100 °C-calcined adhesive with 3 % AlF3, its TEC sharply increased from zero at RT to ∼11.4 × 10−6 at 700 °C, and then slowly increased to ∼13.8 × 10-6 at 1000 °C. As the TEC between adhesive and SiC was kind of mismatch, it supposed that their interfacial connection might be weak. However, as described in our previous work, the positive interfacial reaction during the heating process effectively relieved the mismatch between adhesive and substrate and kept the connection strong. Meanwhile, the addition of AlF3 did not influence the interfacial reaction much, and the fracture of bonded joints still happened in adhesive instead of the interface. As seen from the physical photos of ruptured joints in Fig. S4, the most fractured area of these ruptured joints was covered with adhesive.

A better damage tolerance or toughness of adhesive was achieved after in situ whiskers growing, and the relevant reinforcing mechanisms for adhesive with 3 % AlF3 were summarized in Fig. 10. When the load stress pass through the adhesive, cracks preferentially extended along with the weak parts of adhesive, such as the inherent cracks and holes formed during the heating process. As the V-S mechanism-based whiskers were prone to grow in these pre-existing spaces, so these weak cracks and pores obtained repair through whisker bridging. As seen from Fig. 10a, a certain number of whiskers grew and filled in a large crack, and the shape looks like a zipper between two pieces of clothes. Because the mechanical property of most whiskers is better than bulk ceramics, the repair for crack by using whisker growth would effectively improve the strength of adhesive [29]. Also, the whisker bridging (or overlap connection) between the hole in Fig. 10b also reduced its status as a stress concentration source. In addition, the whiskers were also grown in the pores created by erosion, as seen from Fig. 5a and b, thus formed a composite structure where whiskers were tightly coupled to the matrix. When the load stress extended to these areas, two possible cases happened: whisker rupture (very rare) or stress deflection (more often). If the bridged whisker was not enough to stand the stress, the whisker consumed load energy by breaking itself to reduce the destruction of adhesive, while the stress deflected to the sides of whiskers when they could bear enough load, and thus leading to the whisker drawing and crack deflection. As seen from Fig. 10c, there were several whiskers bridged on a hole in the direction of stress propagation (the red arrow), the whisker at position one bear most of the force and the fracture was clearly visible. Its right part was still tightly attached to the matrix, and its left root was slightly cracked. In this case, the transmitted stress was greatly attenuated, and the whisker at position two was able to bear residual stress and kept intact. An approximate whisker drawing happened to its left root, which indicated that the stress propagation had changed direction. After preventing twice stress propagation, the whiskers at position three had not been affected. The above phenomenon was sufficient to illustrate that partial stress would be decreased step by step after layers of obstacles of whiskers. The inset TEM graph of Fig. 10c gives a clear demonstration of whisker fracture and the abrupt fracture surface was obviously caused by external forces. To be honest, the whisker fracture was rare to be observed during our test, 9

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Fig. 11. The growth procedure of mullite whiskers in adhesive.

good, which was different from the pull-out phenomenon that is common in the traditional toughen way by adding whiskers. In fact, the pull-out phenomenon was rare in adhesives toughened by in situ growing whiskers, as these whisker roots were usually strong. On the other hand, the whisker in situ growing also often caused crack deflection. Fig. 10e1 compares the crack propagation in different areas. The propagation path of the crack was straight in the area with few whiskers, while the path was curved in the area with dense whiskers. A single whisker against crack propagation was shown in Fig. 10e2, the extension direction was changed by ∼60°. Under the comprehensive action of the above situations, the fracture surface was usually cragged (see Fig. 10f), which made a strong case that the whisker in situ growth effectively increased the fracture surface area and the fracture energy.

Table 2 The comparison of bonding performance among different adhesives. Reference

Maximum Bonding strength (MPa)

Treatment temperature

This work Ref. [6] Ref. [7] Ref. [12] Ref. [14] Ref. [30] Ref. [31]

63 MPa 26.2MPa 9 MPa 26.3MPa 50.8 MPa 13 MPa 9 MPa

1100 °C 800 °C 1100 °C 1100 °C 1200 °C 1300 °C 1300 °C

Note: silicon carbide as the bonded substrate.

declaring that most mullite whiskers had high strength. In contrast, the whisker drawing in Fig. 10d and the crack deflection in Fig. 10e were two common phenomena. As seen from Fig. 10d, there was residual adhesive attached to the terminal end of whiskers (whisker bridging before fracture), indicated that these whiskers endured the tension during the loading process. Moreover, the whisker drawing also implied the connection between whiskers and adhesive matrix (whisker pull-out means weaker) was

3.6. The growth mechanism of mullite whiskers Based on the structure and composition evolution of adhesives, the specific formation of mullite whiskers based on V-S growth mechanism is described in Fig. 11, and the main reactions may be as follows [29]: 2AlF3 (s) + O2 (g) = 2AlOF (g) + 4 F (g) 10

(1)

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2Al2O3 (s) + 4 F (g) = 4AlOF (g) + O2 (g)

(2)

Declaration of Competing Interest

2SiO2 (s) + 8 F (g) = 2 SiF4 (g) + 2 O2 (g)

(3)

7 6AlOF(g) + 2SiF4 (g) + O2 (g) = 3Al2O3 ·2SiO2 (s) + 14F(g) 2

(4)

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

A certain number of holes or cracks are produced in the resin-based adhesive with the increase of temperature, which provide space for whisker growth. AlF3 located on the surface of adhesive matrix preferentially reacted with oxygen based on the reaction (1), and generated the gaseous AlOF (g) and F (g). Then the released F (g) attacks Al2O3 and SiO2 exposed to the space, to form AlOF (g) and SiF4 (g), respectively, according to reactions (2) and (3). As the gaseous AlOF (g) and SiF4 (g) have high vapour pressure at higher temperatures, they can easily reach supersaturation in the enclosed spaces, and deposit to the surfaces of pores and cracks. Meanwhile, they react with oxygen based on reaction (4), and then solid mullite begins to precipitate out and condenses together to form whisker growing points at first. As the above reactions processed, the matrix of adhesive is eroded harder, and leads to the holes and cracks became larger. The gaseous AlOF (g) and SiF4 (g) continue reacting and precipitating out mullite upon the growing points to form thorn-like whisker roots. After that, rapid crystal growth occurs along the direction of whisker roots, and whisker shapes are formed. Finally, the mullite whiskers bridge on the pores and cracks of adhesive, effectively modifies the weak defects of adhesive and improves the mechanical properties.

Acknowledgements This work is supported by the National Natural Science Foundation of China Projects: 51802343, 51672187, 51772326 and 11604348, Natural Science Foundation of Tianjin City (19JCQNJC02300) and Fundamental Research Funds for the Central Universities (3122018L008). Yunlong Liao gratefully acknowledges financial support in China by Tianjin Natural Science Foundation (18JCYBJC43900) Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mtcomm.2020. 100944. References [1] R.F. Hu, J. Zhao, Y.H. Wang, Z.X. Li, J.P. Zheng, Highly stretchable, self-healing, recyclable and interfacial adhesion gel: preparation, characterization and applications, Chem. Eng. J. 360 (2019) 334–341. [2] W. Robert, Messler Jr., Adhesives, cements, mortars, and the bonding process, in: W. Robert, MesslerJr. (Eds.), Joining of Materials and Structures, ButterworthHeinemann, London, 2004, pp. 227–283. [3] J.A. Fernie, R.A.L. Drew, K.M. Knowles, Joining of engineering ceramics, Int. Mater. Rev. 54 (2009) 283–331. [4] M. Koyama, H. Hatta, H. Fukuda, Effect of temperature and layer thickness on these strengths of carbon bonding for carbon/carbon composites, Carbon 43 (2005) 171–177. [5] M.C. Wang, J.C. Liu, A.R. Guo, L. Zhao, G. Tian, S.B. Shen, S. Liu, Preparation and performance of the room-temperature-cured heat-resistant phosphate adhesive for C/C composites bonding, Int. J. Appl. Ceram. Technol. 12 (2015) 837–845. [6] X.Z. Wang, J. Wang, H. Wang, Preparation of high-temperature organic adhesives and their performance for joining SiC ceramic, Ceram. Int. 39 (2013) 1365–1370. [7] M. Salvo, S. Rizzo, V. Casalegno, M. Ferraris, Shear and bending strength of SiC/SiC joined by a modified commercial adhesive, Int. J. Appl. Ceram. Technol. 9 (2012) 778–785. [8] L.B. Sun, C.F. Liu, J. Zhang, J. Fang, Joining pre-oxidized dense Si3N4 to porous Si3N4 with β-spodumene based glass-ceramic interlayer, Appl. Surf. Sci. 481 (2019) 515–523. [9] M.C. Wang, X. Tao, X.Q. Xu, R. Miao, H.Y. Du, J.C. Liu, A.R. Guo, High-temperature bonding performance of modified heat-resistant adhesive for ceramic connection, J. Alloys Compd. 663 (2016) 82–85. [10] Y. Qin, Z.L. Rao, Z.X. Huang, H. Zhang, F.Z. Wang, Preparation and performance of ceramizable heat-resistant organic adhesive for joining Al2O3 ceramics, Int. J. Adhes. Adhes. 55 (2014) 132–138. [11] M.C. Wang, X. Dong, Z.P. Li, R.Y. Lu, J.C. Liu, A.R. Guo, T. Wei, M.R. Du, The connection and repair of Ni-based superalloys by a simple heat-resistant adhesion technique, J. Alloys Compd. 791 (2019) 1146–1151. [12] M.C. Wang, X. Dong, X. Tao, M.M. Liu, J.C. Liu, H.Y. Du, A.R. Guo, Joining of various engineering ceramics and composites by a modified pre-ceramic polymer for high-temperature application, J. Eur. Ceram. Soc. 35 (2015) 4083–4097. [13] M. Singh, R. Asthana, Advanced joining and integration technologies for ceramic matrix composite systems, in: W. Krenkel (Ed.), Ceramic Matrix Composites: Fiber Reinforced Ceramics and Their Applications, John Wiley & Sons, New York, 2008, pp. 303–326. [14] X.Z. Wang, J. Wang, H. Wang, Synthesis of a novel preceramic polymer (V-PMS) and its performance in heat-resistant organic adhesives for joining SiC ceramic, J. Eur. Ceram. Soc. 32 (2012) 3415–3422. [15] X.G. Luan, J.Q. Wang, Y. Zou, L.F. Cheng, A novel high temperature adhesive for bonding Al2O3 ceramic, Mater. Sci. Eng. A-struct. 651 (2016) 517–523. [16] B. Yuan, G.J. Zhang, Microstructure and shear strength of self-joined ZrB2 and ZrB2SiC with pure Ni, Scripta. Mater. 64 (2011) 17–20. [17] H.X. Li, Z.Q. Wang, Z.H. Zhong, Tailoring the interfacial microstructure and mechanical strength of SiC ceramic joints using joining temperature and interlayer thickness, Mater. Charact. 142 (2018) 470–477. [18] S.S. Han, Q.S. Meng, S. Araby, T.Q. Liu, M. Demiral, Mechanical and electrical properties of graphene and carbon nanotube reinforced epoxy adhesives: experimental and numerical analysis, Compos. Part A-Appl. S. 120 (2019) 116–126. [19] S. Rahmanian, A.R. Suraya, M.A. Shazed, R. Zahari, E.S. Zainudin, Mechanical characterization of epoxy composite with multiscale reinforcements: carbon nanotubes and short carbon fibers, Mater. Design 60 (2014) 34–40. [20] Y.F. Zhang, R.Y. Luo, J.S. Zhang, Q. Xiang, The reinforcing mechanism of carbon fiber in composite adhesive for bonding carbon/carbon composites, J. Mater.

4. Conclusion The effect of whisker growing catalyzed by different amounts of AlF3 on the properties of epoxy modified silicone resin-based hightemperature-resistant adhesive was comprehensively investigated, including its mechanical properties, composition evolution, structure evolution, thermal properties and the porosity. The results showed that the effective temperature range in which AlF3 worked was 900 °C to 1500 °C. The addition of AlF3 promoted the oxidation reaction, accelerated the premature formation of mullite whiskers, increased the production of SiC ceramic, and reduced the thermal expansion of the bonding layer. However, the extra addition of AlF3 yet induced many more large holes in the bonding layer, caused the structural heterogeneity, and made mullite whiskers overgrow, thus finally led to the weaker mechanical properties of adhesives. The best dosage of AlF3 in this work was 3 % (mass fraction), no apparent large hole was formed in 3 % AlF3-added adhesive, and a uniform composite structure that whisker weaved adhesive frame kept steady from 1100 °C to 1500 °C. In this case, the bonding strength of 3 % AlF3-added adhesive achieved impressive values of 63 MPa at 1100 °C and 47 MPa at 1300 °C, 80 % and 56.7 % respectively higher than those of the blank sample under same conditions. Besides, under the synergistic effect of whisker repair, whisker bridging, whisker drawing and crack deflection, a multi-stage fracture phenomenon happened during the shear test, and the fracture displacement of 1100 °C-calcined adhesive with 3 % AlF3 increased by 61 %, compared with that of the black sample, which made a strong case that the in situ whisker-grown adhesives had a better damage tolerance and toughness. In addition, the bonding performance of the mullite whisker-grown adhesive for silicon carbide was apparently better than most of the other reported adhesives, and the comparison between different adhesives are listed in Table 2. CRediT authorship contribution statement Zhaojie Feng: Data curation, Writing - original draft. Mingchao Wang: Writing - review & editing, Funding acquisition. Ruoyun Lu: Formal analysis. Wence Xu: Formal analysis. Ting Zhang: Software. Tong Wei: Visualization. Jingfang Zhang: Formal analysis. Yunlong Liao: Supervision. 11

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