Reactive air brazing of YSZ ceramic with novel Al2O3 nanoparticles reinforced Ag-CuO-Al2O3 composite filler: Microstructure and joint properties

JMADE-02419; No of Pages 9 Materials and Design xxx (2016) xxx–xxx

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Reactive air brazing of YSZ ceramic with novel Al2O3 nanoparticles reinforced Ag-CuO-Al2O3 composite filler: Microstructure and joint properties Xiaoqing Si, Jian Cao ⁎, Xiaoguo Song, Yang Qu, Jicai Feng State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Al2O3 nanoparticles reinforced Ag-CuOAl2O3 composite filler was successfully employed to reactive air braze YSZ ceramic. • The thermal expansion coefficient of Ag-CuO-Al2O3 composite filler can be tailored. • Sufficient low leak rate (3.2 ± 0.9 × 10− 10 Pa m3 s− 1) guaranteed the strict sealing requirements. • YSZ joints possessed a better ability to resist deformation with a maximum shear strength of 60 MPa.

a r t i c l e

i n f o

Article history: Received 27 July 2016 Received in revised form 25 September 2016 Accepted 25 October 2016 Available online xxxx Keywords: Reactive air brazing Yttria-stabilized zirconia Composite filler Microstructure Mechanical properties

a b s t r a c t The YSZ ceramic coated with a copper layer (~5 μm) was successfully reactive air brazed using novel Al2O3 nanoparticles (NPs) reinforced Ag-CuO-Al2O3 composite filler. The effects of Al2O3 NPs content on the interfacial microstructure and joint properties were investigated. When the content of Al2O3 NPs was 8 wt.%, YSZ joints obtained fine interfacial microstructure and optimal mechanical properties. The coefficient of thermal expansion of composite filler was calculated to be 16.26 × 10−6 K−1, presenting a decline of 12.1% compared to that of AgCuO base filler. The hardness and modulus values of braze joint slightly increased to 2.6 GPa and 164 GPa, respectively, possessing a better ability to resist deformation. Consequently, the YSZ joint possessed a maximum shear strength of 60 MPa, which was 25 MPa (~71%) higher than that of the joints brazed using Ag-CuO base filler. The leakage rate of YSZ-tube/AgCuO-8Al2O3/YSZ-block samples was measured to be 3.2 ± 0.9 × 10−10 Pa m3 s−1, which met the leakage rate requirements for vacuum tight components (less than 10−9 Pa m3 s−1). It implied that this novel method was an effective way to obtain hermetic YSZ joints. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Yttria-stabilized zirconia ceramics (YSZ) demonstrate a series of fascinating properties, such as excellent fracture toughness, high ionic

⁎ Corresponding author. E-mail address: [email protected] (J. Cao).

conductivity and chemical stability at elevated temperature [1–3]. These properties are beneficial in numerous applications ranging from gas turbines to solid oxide fuel cells and chemical sensors [4–7]. What limits their potential benefits, however, is the current inability to economically manufacture large or complex-shaped ceramic components that exhibit reliable performance. Ceramic joining technologies possess great potential in manufacturing large or complex-shaped ceramic components [8–12].

http://dx.doi.org/10.1016/j.matdes.2016.10.062 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

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The glass or glass-ceramics bonding is a feasible technique for joining YSZ ceramics [13–15]. The glass composition design can tailor many properties, such as flow characteristics, thermal expansion behavior and crystallization kinetics. However, glass-based seals have poor resistance to thermal cycling and other static and dynamic forces due to the inherent brittleness of the glass sealant materials [16]. The glass structure determines the performances of glass-based seals, but it is very complex and not fully understood yet [17]. Besides, active metal brazing (AMB) is considered as one of most reliable methods for joining YSZ ceramics [18–20]. In this technique, the matrix braze (based on Ag, Cu, Pd, etc.) contains active metals like Ti, Zr or Cr, which is employed to actively modify the ceramic surface so that the remainder of the filler may wet the surface. To prevent the active elements from oxidation during brazing, a high vacuum or inert gas atmosphere is required. Thus the wider application of this method is limited. Moreover, active elements will be oxidized above 500 °C, which will result in a rapid deterioration in the strength and hermeticity of ceramic joint. To overcome these issues, a new ceramic brazing technique, referred to as reactive air brazing (RAB), has been developed [21–23]. The RAB can be performed in air, and the brazing temperature is usually above 900 °C. The braze system is composed of a relatively inert element (Ag, Au or Pt) and an oxide (CuO, V2O5 or WO3) [24,25]. Therefore, the obtained joints possess inherently superior high-temperature oxidation resistance [26]. This process utilizes an oxide component to reactively modify the ceramic surface so that the newly formed surface is readily wetted by the molten filler. Until now, one particular noblemetal/oxide system suitable for RAB is the Ag-CuOx system, which has been employed to RAB join alumina [27], zirconia [28] and conducting membranes [29]. Whereas two concerns in their use for joining YSZ ceramics arise: (i) the high thermal stress is readily caused by the large CTE mismatch between the Ag-CuOx filler and YSZ substrate; (ii) the braze joint is prone to mechanical failure due to the poor strength of Ag-CuOx filler, especially its unsatisfactory high-temperature strength [30]. Above concerns may be alleviated by adding fine reinforcements (particles or fibers) with a low CTE in the base braze [31,32]. So far, some researchers have carried out the studies on the composite fillers in the RAB field [33–35]. But the reinforcements added in the composite fillers are all micro-scale in size, which easily leads to the poor flowability and the formation of voids and cracks. Kim et al. [35] have employed alumina microparticle (~1.6 μm) reinforced Ag-CuO composite filler to RAB join Al2O3 ceramics. It revealed that the distribution of alumina reinforcement was uneven, which was not conducive to maximize the potential of reinforcement in improving joint properties. It is reported that nanoparticle reinforced composite fillers can alleviate these issues in the vacuum brazing [36,37]. To the best knowledge of authors, there is no report about nanoparticle reinforced composite fillers in the RAB field. Therefore, Al2O3 nanoparticles (NPs) were selected to develop a novel Ag-CuO-Al2O3 composite filler. Al2O3 particles have a tendency to react with CuO to form CuAl2O4 phase, which guarantees the compatibility between reinforcement and filler matrix [26,27]. In addition, a copper layer has been coated on the YSZ faying surface because a higher CuO content in the region adjacent to ceramic substrate can effectively improve the wetting of Ag-CuOx filler [25–29]. In the present work, novel Al2O3 NPs reinforced Ag-CuO-Al2O3 composite fillers were fabricated to reactive air braze YSZ with a copper coating. By adjusting Al2O3 NPs content, the evolution of interfacial microstructure was investigated in detail. The mechanical properties of YSZ joints were comprehensively measured by shear-strength test, fracture analysis, nanoindentation and hermeticity test. 2. Experimental procedures Yttria-stabilized zirconia ceramics (YSZ, 3 mol% yttria) used in this study was provided by Shanghai Unite Technology Co. Ltd. Shanghai, China. The density and three point bending strength of

YSZ were 5.95 g cm− 3 and 780 MPa, respectively [38]. The YSZ was machined to blocks with two different dimensions (5 × 5 × 5 mm and 5 × 8 × 17 mm). Prior to evaporation coating, all of the faying surfaces were polished by 1 μm diamond paste, and then were ultrasonically cleaned in acetone for 10 min. Subsequently, a copper layer was prepared on the faying surfaces of YSZ by evaporation coating. Finally, a copper coating with a thickness of ~5 μm was evenly deposited on the faying surface. Al2O3 NPs (0–12 wt.%) with a mean particle size of ~ 10 nm were added into the Ag-CuO (Ag-8 mol%CuO) base filler, and then the mixture was milled for 4 h using QM-SB planetary ball mill to prepare AgCuO-Al2O3 composite fillers (AgCuOc). All the filler materials were purchased from Beijing Dk Nano technology Co. Ltd. Beijing, China. Eventually, five kinds of AgCuOc fillers were prepared with the addition of Al2O3 NPs: 0 wt.%, 2 wt.%, 4 wt.%, 8 wt.% and 12 wt.%. The as-milled composite filler was pressed into sheets with a thickness of ~100 μm using the powder tableting machine (PT-15T, LEAO, China). The AgCuOc filler was well-placed between the brazing couples, and a normal load of 1 kPa was applied on it to maintain a proper contact. Reactive air brazing was conducted in a static air muffle furnace. The assembly was heated to 1050 °C, held at the temperature for 30 min, and slowly cooled down to room temperature without active cooling. The interfacial microstructure was characterized employing scanning electron microscope (SEM, Helios Nanolab600i, FEI, USA) equipped with energy dispersive spectrometer (EDS). Further analyses of reaction products at the YSZ/braze interface were observed by a transmission electron microscopy (TEM, Tecnai G2 F30, FEI, USA) equipped with EDS. Meanwhile, the crystal structures of reaction products were characterized by selected area diffraction patterns (SADPs) of TEM and EDS. The thinning process of specimens for TEM was performed by the focused ion beam (FIB, Helios Nanolab600i, FEI, USA). The shear test was carried out at room temperature using a universal testing machine (Instron 5569, Instron, USA) with a constant loading rate of 0.5 mm min−1. In order to obtain the accuracy of shear strength, the average shear strength was calculated using five joint shear results at least. Moreover, fracture analyses were conducted by X-ray diffraction spectrometer (XRD, D8-Advance, Bruker, Germany) with Cu-Kα radiation to identify the interfacial phases accurately. And the differential scanning calorimeter (DSC 200F3, Netzsch, Germany) was performed to determine the melting process of composite fillers. The composite filler was heated from 20 °C to 1050 °C at a heating rate of 2 °C min−1. The test was carried out in the dry simulated air at a flow rate of 50 ml min−1. The simulated air is composed of pure N2 and O2 with a volume ratio of 4:1. The alumina crucibles used in the DSC test were 99.9 wt.% alumina with a small amount of SiO2, CaO and Na2O. Furthermore, the hardness and elastic modus across the joints were characterized with a nanoindenter to investigate the performance of the reaction products in the joint. A peak force of 5.0 mN was used for all reaction products. Furthermore, a hermeticity test was performed with a helium leakage detector (ASM 192T, Adixen, France) to detect whether this method meet the sealing requirements. 3. Results and discussion 3.1. Characterization of AgCuOc fillers Fig. 1(a) and (b) show the typical morphologies of as-prepared AgCuOc fillers (8 wt.% and 12 wt.% Al2O3). It appeared that Al2O3 NPs uniformly distributed on the surfaces of Ag or CuO particles. However, it was noted that Al2O3 NPs significantly aggregated with the increase in Al2O3 NPs content. X-ray diffraction analysis was carried out on the AgCuO-8Al2O3 filler to verify whether metallurgical reactions occurred during the ball milling process. The XRD pattern in Fig. 1(c) illustrates that there were no diffraction peaks of new phases, indicating that no metallurgical reactions occurred during ball milling process. Fig. 1(d) suggests that the DSC curves for all of the AgCuOc fillers existed two endothermic peaks: one at the eutectic temperature (~943 °C) and

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Fig. 1. Microstructure of AgCuOc composite fillers: (a) 8 wt.% Al2O3 and (b) 12 wt.% Al2O3. (c) XRD pattern of AgCuO-8Al2O3 filler. (d) DSC curves of AgCuOc composite fillers.

the other one at the monotectic temperature (~967 °C). It indicated that the Al2O3 NPs addition had no effect on the melting process of composite fillers. 3.2. Typical microstructure of YSZ joint brazed with 4-AgCuOc filler Fig. 2 shows the typical interfacial microstructure of the YSZ joint brazed using AgCuO-8Al2O3 filler. It could be seen that no pores or micro-cracks existed in the braze joint and the width of the braze joint was ~120 μm. The details of the interfacial microstructure can be seen in the enlarged images of Fig. 2. As shown in the left enlarged image, the YSZ substrate (marked as A) was tightly bonded with a dark-grey phase (marked as C) and white matrix phase (marked as D). Some light-grey phases (marked as B) separated in the region adjacent to YSZ substrate. According to the difference in contrast, it was seen that the braze joint contained four kinds of phases in the right enlarged image (marked as E, F, G and H). The major elements at each spot in Fig. 2 detected by EDS are shown in Table 1. The light-grey phases (B) had a similar composition with the YSZ substrate (A). Thus, they were regarded as ZrO 2 particles detached from the ceramic surface. The YSZ faying surface might de damaged by the molten filler in a high-temperature oxidation

atmosphere. As a result, a small amount of YSZ particles were peeled off from the damaged YSZ surface with the flow of molten filler. The EDS results of spot C suggest the presence of Cu and O with a molar ratio of 1:1, which could be regarded as CuO-rich phase. The white matrix phases (D and E) mainly contained Ag, and these phases can be attributed to Ag-rich phase. The rod-like black phase (F) can be attributed to Al2 O 3 phase based on the analyses of elemental content. It was concluded that Al 2 O 3 NPs agglomerated, transforming into micro-sized particles. The darkgrey phase (G) also can be considered as CuO-rich phase due to the similar composition with CuO-rich phase (C) at the interface. According to the pseudobinary CuO-Al 2 O 3 phase diagram [39], it was summarized that CuAl 2 O 4 could be formed through the reaction of CuO (l) + Al 2 O 3(s) → CuAl 2 O 4(s) at the maximum temperature. The light-black phase (H) mainly contained Cu, Al and O with the molar ratio of 1:2:4, which can therefore be regarded as CuAl2O 4 phase. In order to further analyze the reaction mechanism between AgCuO8Al2O3 filler and YSZ ceramic, TEM observations were conducted and the results were discussed below. Fig. 3(a) and (b) illustrate the bright-field images (BFIs) of 4-AgCuOc filler/YSZ interface. It could be seen that CuO and Ag both directly contact with ZrO2 substrates, and a fine interface formed. There was no new reaction phase at the interface, which indicated that RAB process was completely different from AMB process. AMB usually employed active elements to chemically reduce the ceramic surface, which leaded to the formation of reaction layers Table 1 Chemical compositions and possible phases of each spot marked in Fig. 2 (at.%).

Fig. 2. Typical interfacial microstructure of YSZ joint brazed using AgCuO-8Al2O3 filler.

Spots

Zr

Al

Cu

O

Ag

Possible phase

A B C D E F G H

32.4 30.5 1.2 1.7 0.6 – – –

– – – – 1.8 36.5 1.5 28.3

– 0.5 48.2 3.0 2.6 0.5 47.3 14.4

67.6 67.7 45.9 4.1 4.9 60.6 46.8 54.8

– 1.3 4.7 91.2 90.1 2.4 4.4 2.5

ZrO2 ZrO2 CuO-rich Ag-rich Ag-rich Al2O3 CuO-rich CuAl2O4

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Fig. 3. TEM bright field images of the AgCuO-8Al2O3/YSZ interface: (a) CuO/ZrO2 interface and (b) Ag/ZrO2 interface. (c) and (d) SADPs of CuO and ZrO2 at the CuO/ZrO2 interface. (e) and (f) SADPs of Ag and ZrO2 at the Ag/ZrO2 interface.

fraction patterns (SADPs) of the CuO with zone axis of [121] and ZrO2

c = 5.132 Å and β = 99.506°) and tetragonal ZrO2 (PDF#50-1089, a = 3.598 Å and c = 5.152 Å), respectively. Fig. 3(e) and (f) present the

along with the zone axis of [331] at the CuO/ZrO2 interface, corresponding with the monoclinic CuO (PDF#48-1548, a = 4.688 Å, b = 3.423 Å,

SADPs of Ag with zone axis of [011] and ZrO2 along with the zone axis of [010] at the Ag/ZrO2 interface, corresponding with the cubic Ag

at the interface [38,40]. Fig. 3(c) and (d) display the selected area dif-

Fig. 4. Microstructure formation physical model: (a) the initial morphology after joint assembly, (b) the oxidation of copper coating and the melting of filler metal, (c) the holding stage, (d) the precipitation of CuO and (e) the final morphology of as-brazed joint.

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Fig. 5. Interfacial microstructure of YSZ joints brazed using different AgCuOc fillers: (a) 0 wt.% Al2O3, (b) 2 wt.% Al2O3, (c) 4 wt.% Al2O3 and (d) 12 wt.% Al2O3.

(PDF#04-0783, a = 4.086 Å) and tetragonal ZrO2 (PDF#50-1089, a = 3.598 Å and c = 5.152 Å), respectively. Based on the above analysis, a concept physical model was established to analyze the microstructure formation mechanism, which is schematically illustrated in Fig. 4. The formation process of RAB joint was divided into the following five stages: (a) The initial morphology after joint assembly. Shown in Fig. 4(a) is a cross-sectional micrograph of the YSZ/AgCuOc assembly. At the interface of YSZ, a continuous copper coating was attached to the original faying surface. As for the AgCuOc filler, Al2O3 NPs were homogeneously distributed in it. (b) The oxidation of copper coating and the melting of filler metal. According to the pseudobinary CuO-Ag phase diagram [41], it was determined that a silver-rich liquid phase (abbreviated as L2) would gradually form in the AgCuOc filler when temperature exceeds 940 °C. With further slow heating, another small amount of liquid phase that was rich in copper oxide (abbreviated as L1) could form in the AgCuOc filler. However, these two kinds of liquid phases were immiscible, and therefore they would have a tendency to segregate. It was noted that Al2 O3 NPs did not melt during RAB, which could readily move in the molten filler. The high specific surface area might force the agglomeration of Al2 O3 NPs. And they gradually sintered together to form large-size Al2O3 particles, as shown in Fig. 4(b). In addition, the copper coating on the surface of YSZ ceramic was oxidized to form CuO during heating, and then CuO gradually dissolved into the molten filler. Due to the good compatibility between L1 and CuO layer, the L1 mainly distributed in the region adjacent to the CuO layer. (c) The holding stage. With the dissolution of CuO, the content of CuO in the L 1 would gradually increase, which significantly enhanced the wettability of AgCuOc filler system on the YSZ surface. Finally, the CuO completely dissolved into the molten filler and the region of L1 also expanded towards the central region of molten filler. The Al 2 O 3 particles cannot dissolve in the molten filler at the brazing temperature due to its high melting point of 2054 °C. The CuO-rich liquid

phase could be enriched on the surface of Al 2 O 3 particles due to the good compatibility between them. Therefore, the reaction of CuO (l) + Al 2 O 3(s) → CuAl 2 O 4(s) was likely to occur at the maximum temperature during holding stage. Thus, a small amount of CuAl 2 O 4 particles finally formed in the regions containing CuO and Al2O3. This process also guaranteed the good combination between Al 2 O 3 particles and Ag-CuO base filler, which avoided the formation of pores and cracks in the braze joint. Fig. 4(c) displays that a small amount of YSZ particles could detach from the ceramic surface and separate in the region adjacent to the ceramic substrate. Meanwhile, Al2O 3 particles continued to grow under the effect of sintering.

(d) The precipitation of CuO. Due to the lower interfacial energy between L1 and oxides (YSZ ceramics or Al2O3 particles), the L1 preferentially migrated to and wetted the YSZ ceramic or Al2O3 particle surfaces. Upon cooling to the monotectic temperature, the pseudobinary CuO-Ag phase diagram indicated that the CuO began to precipitate from the liquid, nucleating along the YSZ boundary the Al2O3 particle surface, as shown in Fig. 4(d). (e) The final morphology of as-brazed joint. With the further cooling, the CuO phase continued to precipitate. As it did, the L2 was enriched with silver. At the eutectic temperature, the CuO and Ag would simultaneously nucleate from the remaining liquid. Eventually, a nearly continuous CuO layer formed along the interface of the YSZ ceramic, as shown in Fig. 4(e).

3.3. Effects of Al2O3 NPs content on the interfacial microstructure of YSZ joints Fig. 5 illustrates the interfacial microstructure of YSZ joints brazed using AgCuOc fillers containing different Al2O3 NPs content. It was

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clearly observed that the Al2O3 NPs content had a great influence on the interfacial microstructure of YSZ joins. When Al2O3 NPs was not added into the brazing filler, the molten brazing filler exhibited excellent flowability, which promoted the dissolution of CuO that formed by the oxidation of copper coating on the YSZ substrate. Therefore, a small amount of CuO matrix phases precipitated on the YSZ/braze interface, as seen in Fig. 5(a). Extending from the interface towards the braze joint was predominantly silver with minor amount of CuO. Although there was a huge CTE mismatch between silver (~19 × 10−6 K−1) and YSZ (~9.75 × 10−6 K−1) [42], the Ag-rich braze joint could plastically deform to sustain the thermal stress, eventually forming a defect-free joint. Fig. 5(b) displays that the addition of Al2O3 NPs have a great impact on the morphology of YSZ joint. It was seen that the braze joint was decorated by a certain amount of Al2O3 particles that had experienced a volume growth during RAB. Due to a copper layer has been pre-deposited on the YSZ substrates, the CuO-rich liquid was readily formed in the region adjacent to the YSZ substrates. With the dissolution of CuO in the liquid filler, the CuO-rich liquid had a tendency to flow into central region of the composite filler due to its good compatibility with Al2O3. However, the excessive undissolved Al2O3 reinforcements would also deteriorate the fluidity of the liquid filler due to its high melting point of 2054 °C. The similar phenomena have been reported by other research groups [33,43]. Fig. 5(c) shows that further increasing the Al2O3 NPs content had little impact on the amount of CuO at the YSZ interface. Therefore, it was concluded that the flow of CuOrich liquid had been seriously hindered. Fig. 5(d) displayed that a large number of voids or cracks were formed when excessive Al2O3 NPs (12 wt.%) were added. The cracks might be shrinkage cracks during solidification. The volume change in the braze joint had to be compensated by the flow of liquid filler or plastic deformation of the solid. Both could be reduced by adding Al2O3 NPs. So shrinkage cracks readily formed when excessive Al2O3 NPs were added. The shrinkage cracks might also be caused by the non-uniform distribution of Al2O3 particles. Besides, it was reported that the interactions between CuO and Al2O3 particles guaranteed the compatibility between Ag-CuO base filler and Al2O3 reinforcements [26,27]. As the consumption of CuO by Al2O3 became serious when excessive Al2O3 NPs were added, leaving less CuO to react with Al2O3 reinforcements. So the molten filler could not well wet the Al2O3 particles, which could also contribute to the voids or cracks. 3.4. Effects of Al2O3 NPs content on the mechanical properties of YSZ joints The shear strength of YSZ joints brazed at 1050 °C for 30 min is plotted as a function of Al2O3 NPs content in Fig. 6. The results illustrated

Fig. 6. Effect of Al2O3 NPs content on the shear strength of YSZ joints.

that Al2O3 NPs content had a great effect on the shear strength of YSZ joints. Despite the specimen brazed using the AgCuO-0Al2O3 filler exhibited a defect-free joint, as shown in Fig. 5(a), the strength of predominantly silver joint was too poor to bear a high shear force, displaying only 35 MPa. Adding Al2O3 NPs into the composite filler could not only reduce the CTE of brazing seam, but also significantly enhanced its strength. Accordingly, the shear strength of YSZ joints obviously improved with increasing the Al2O3 NPs content. The CuO phase had a lower CTE of 9.3 × 10−6 K−1, which was close to the CTE of YSZ. Thus, the stress concentration caused by the CTE mismatch between YSZ and brazing seam could be further released. When the Al2O3 NPs content was 8 wt.%, the YSZ joints possessed a maximum shear strength of 60 MPa, which could be mainly attributed to the good CTE match, the improvement of brazing seam strength and the fine interfacial microstructure. However, it was noted that the addition of Al2O3 NPs also weakened the wettability of composite filler. Although the copper coating on the YSZ surface could relieve this problem to some extent, excess addition of Al2O3 NPs would lead to the excessive aggregation of Al2O3 NPs and the crack formation, as seen in Fig. 5(d). So when Al2O3 NPs content was 12 wt.%, the shear strength dropped dramatically. The Ag-8 mol%CuO base filler has a large CTE of ~18.5 × 10−6 K−1 [44], which is much larger than the CTE of YSZ (~ 9.75 × 10− 6 K− 1) [42]. Thus, there will be a great residual stress in the braze joint if the YSZ was directly brazed by the Ag-CuO filler. Researches on the particle reinforced composite materials have noted that adding particles with a low CTE in the base material can effectively reduce the CTE of composite materials [45]. Accordingly, adding Al2O3 NPs with a low CTE of 6.5 × 10−6 K− 1 in the Ag-CuO base filler is an effective way to reduce the CTE of AgCuOc filler. The CTE of AgCuOc filler can be deduced by the following formula, as reported in Refs. [46–48]:  X  X αfV fÞ Vf þ α ¼ α m 1−

ð1Þ

where, α is the CTE of AgCuOc filler, αm is the CTE of Ag-CuO base filler, αf is the CTE of Al2O3 NPs and Vf is the volume fraction of Al2O3 NPs. According to the Eq. (1), the CTE of AgCuOc fillers with different Al2O3 NPs content was calculated, and the result is shown in the Fig. 7. It was seen that the CTE of AgCuOc filler gradually reduced with increasing Al2O3 NPs. When the addition of Al2O3 NPs was 8 wt.%, the CTE of AgCuOc filler reduced to 16.26 × 10− 6 K− 1, exhibiting a decline of 12.1% compared to the CTE of Ag-CuO filler. It could be concluded that adding Al2O3 NPs in the Ag-CuO base filler was an effective way to reduce the CTE of AgCuOc filler, thereby significantly lowering the residual stress in the braze joint. Although the calculation may be different with any future experimental test results, it still has a certain reasonability to

Fig. 7. Effect of Al2O3 NPs content on the CTE of AgCuOc fillers.

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Fig. 8. Fractographies of YSZ joints brazed using different AgCuOc fillers: (a) 0 wt.% Al2O3, (b) 2 wt.% Al2O3, (c) 8 wt.% Al2O3 and (d) 12 wt.% Al2O3.

reflect the changes in the CTE of composite filler according to the analysis of Refs [46–48]. Besides, it is noted that the stiffness mismatch between YSZ and composite fillers also contributes to the cooling stress [49]. In order to better understand failure mechanism of these joints, SEM and EDS analyses were conducted on their fracture surfaces. Both sides of the fractures were detected. The results showed that the fractures were almost same on both sides. There was no need to present both sides of fractures. Only one representative side of the fractures was displayed in the Fig. 8. As seen in Fig. 8(a), the sample brazed with AgCuO-0Al2O3 filler showed ductile fracture through the continuous silver phase, which proved once again that the poor strength of silver phase was the major factor of limiting the performance of YSZ joints. Fig. 8(b) displays that the sample brazed with AgCuO-2Al 2 O 3 filler also demonstrated a typical ductile fracture mode, and in this case a certain amount of Al2O3 particles appeared on the fracture surface. The addition of Al2O3 NPs played an important role in enhancing the strength of brazing seam and hindering the crack propagation. The sample brazed with AgCuO-8Al2O3 filler suggested that cracks occurred along the ductile silver phase and the matrix Al2O3 phase, as shown in Fig. 8(c). The rough appearance of matrix Al2O3 phase was the evidence that increasing the Al2O3 NPs content could significantly enhance the advantages of matrix Al2O3 in hindering the crack propagation. Observations of the de-bonded samples revealed that fracture in all tested joints mainly occurred in central region of the braze joint, but scarcely extended to the YSZ/braze interface. It was indicated that the bonding strength at the interface was sufficient to prevent the crack propagation. With the higher Al2O3 NPs content (Fig. 8(d)), fracture took place predominantly within the matrix Al2O3 phase. It was noted that the Al2O3 matrix was not well wrapped by the base filler, and clear cracks presented on the fracture. It confirmed that the Al2O3 particles could not be well wetted by the molten filler during brazing. Finally, plenty of cracks or voids formed within the braze joint during cooling, as shown in Fig. 5(d). Therefore, these joints possessed the poor shear strength. It could be concluded that the good compatibility between Ag-CuO base filler and Al2O3 NPs reinforcements was a prerequisite for optimizing the joint properties.

The XRD test was performed on the fracture surface of YSZ joints brazed using AgCuO-8Al2O3 filler. To eliminate the interference of strong silver peaks, the fracture was first cleaned by a nitric acid solution. As shown in Fig. 9, Ag, Al2O3, CuO, ZrO2 and CuAl2O4 were detected. So the above analyses about interfacial phases were convinced. Fig. 10(a) shows the hardness (H) and elastic modulus (E) of the brazing seam of YSZ joints fabricated by different AgCuOc fillers. Followed by the nano-indentation test, SEM was used to identify the locations of indents within the brazing seam. It was found that CuO and Al2O3 phases within different joints both exhibited similar H and E values, i.e. 5.9 GPa/150 GPa and 6.3 GPa/162 GPa, respectively. The H and E values of brazing seam both slightly increased from 1.4 GPa/146 GPa to 2.6 GPa/164 GPa, along with adding Al2O3 NPs. It was noted that the variation of hardness and modulus might be not statistically significant based on the error bars. This phenomenon might be due to the small increase and the measuring error. The average values claimed that both the hardness and modulus increased with the addition of alumina particles, which were consistent with the results reported by other

Fig. 9. XRD patterns taken from the fracture surface of YSZ joint brazed using AgCuO8Al2O3 filler.

Please cite this article as: X. Si, et al., Reactive air brazing of YSZ ceramic with novel Al2O3 nanoparticles reinforced Ag-CuO-Al2O3 composite filler: Microstructure and jo..., Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.10.062

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Fig. 10. (a) Hardness of elastic modulus of the reaction phases. (b) Typical load versus depth curves.

groups [50,51]. It could be deduced that a higher E of brazing seam possessed a better ability to resist deformation for joints, thereby gradually improving the shear strength of YSZ joints with increasing Al2O3 NPs. Fig.10(b) illustrates the typical load-displacement curves of indents, displaying the elastic and plastic behaviors of reaction phases. Upon unloading, the brazing seam of YSZ joint brazed using 1-AgCuOc filler recovered 32 nm of the total 350 nm, corresponding to an elastic recovery of 9.1%. Additionally, the elastic recovery of brazing seam experienced a continuous increase to an elastic recovery of 12.4% with the gradual addition of Al2O3 NPs, approximately an improvement of 36.3% compared with the elastic recovery for 1-AgCuOc filler. Moreover, the elastic recovery was 26.4% for CuO, 31.5% for Al2O3 and 48.7% for YSZ. Therefore, the deformation behavior of CuO, Al2O3 and YSZ was found to be both elastic and plastic, while the deformation of brazing seam was primarily plastic even when the addition of Al2O3 NPs increased from 0 wt.% to 8 wt.%. Although the plastic deformation capability of brazing seam decreased due to the addition of Al2O3 NPs, it still retained enough capability to absorb the residual stress in the brazing seam. Fig. 11(a) showed the schematic diagram of hermetic test in which the YSZ ceramic tube with an inner diameter of 5.8 mm was reactive air brazed with a YSZ block by the AgCuO-8Al2O3 filler. If pores or tiny cracks existed in the brazing seam, the helium gas would penetrate through these defects, which would lead to an increase in the gas leakage rates. Fig. 11(b) displays the hermetic test sample of YSZ-tube/ AgCuO-8Al2O3 filler/YSZ-block, revealing a favorable joint. Five samples were tested to average the experimental data to guarantee the accuracy of leakage rate. It was reported that the leakage rate should be less than 10−9 Pa m3 s−1 for vacuum tight components [42]. The leakage rate for

the YSZ tube/4-AgCuOc filler/YSZ block sample was measured to be 3.2 ± 0.9 × 10− 10 Pa m3 s−1, meeting the requirements of vacuum tight components, which implied that this novel method was an effective way to obtain hermetic YSZ joints. 4. Conclusions The YSZ ceramic coated with a copper layer (~5 μm) was successfully reactive air brazed using novel Al2O3 NPs reinforced AgCuOc fillers. The interfacial microstructure and mechanical properties of the joints were investigated in detail. Primary conclusions were summarized as follows: All of the AgCuOc fillers existed two endothermic peaks: one at the eutectic temperature (~943 °C) and the other at the monotectic temperature (~967 °C). TEM analyses indicated that the YSZ substrate was directly bonded with CuO or Ag phases, and no new phase was found at the interface. The Al2O3 NPs content had a great influence on the interfacial microstructure of YSZ joins. Adding Al2O3 NPs was an effectively way to reduce the CTE of AgCuOc fillers. When the content of Al2O3 NPs was 8 wt.%, the CTE of AgCuOc filler was calculated to be 16.26 × 10− 6 K−1, a decline of 12.1% compared with that of Ag-CuO base filler. The H and E values of braze joint slightly increased to 2.6 GPa/164 GPa when the Al2O3 NPs content was 8 wt.%. The deformation behavior of braze joint was primarily plastic. When the Al2O3 NPs content was 8 wt.%, the YSZ joint possessed a maximum shear strength of 60 MPa, which could be mainly attributed to the good CTE match, the improvement of braze joint strength and fine interfacial microstructure. The leakage rate of the YSZ-tube/AgCuO-8Al2O3 filler/YSZ-block sample was measured to be

Fig. 11. (a) Schematic of hermetic test sample. (b) The hermetic test sample of YSZ-tube/AgCuO-8Al2O3 filler/YSZ-block.

Please cite this article as: X. Si, et al., Reactive air brazing of YSZ ceramic with novel Al2O3 nanoparticles reinforced Ag-CuO-Al2O3 composite filler: Microstructure and jo..., Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.10.062

X. Si et al. / Materials and Design xxx (2016) xxx–xxx

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Please cite this article as: X. Si, et al., Reactive air brazing of YSZ ceramic with novel Al2O3 nanoparticles reinforced Ag-CuO-Al2O3 composite filler: Microstructure and jo..., Materials and Design (2016), http://dx.doi.org/10.1016/j.matdes.2016.10.062