Materials Science & Engineering A 793 (2020) 139859

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Materials Science & Engineering A

journal homepage: http://www.elsevier.com/locate/msea

Microstructure and mechanical properties of the AlON / Ti6Al4V active element brazing joint

Chun Li a, Kaiping Zhang a, Xiaojian Mao b, Xiaoqing Si a, Bo Lan a, Zhan-Guo Liu c, Yongxian Huang a, Junlei Qi a, Jicai Feng a, Jian Cao a,*

a State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China b Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China c School of Materials and Engineering, Harbin Institute of Technology, Harbin, 150001, China

ARTICLE INFO ABSTRACT

Keywords: Joining the transparent ceramic with metal could help to realise their applications. In this paper, the AlON AlON ceramic was successfully bonded to the Ti6Al4V using the active element brazing approach. The microstructure Brazing of the achieved joints is characterised via scanning electron microscope (SEM), electron probe microanalyser Microstructure (EPMA) and transmission electron microscope (TEM), which turned out to be AlON/Ti3(Cu,Al)3O/Ag(s,s) and Cu Mechanical properties (s,s)/Ti4Cu/TiCu/Ti2Cu/Ti6Al4V. The effects of the brazing temperature and the holding time on the micro­ structure and the shear strength of the joint are investigated. It is found that with the increase of the brazing temperature and the holding time, the thickness of the Ti3(Cu, Al)3O, TiCu and Ti2Cu reaction layers increases while the thickness of the Ti4Cu reaction layer declines. The shear strength of the joint first increases with the brazing temperature/holding time and then decreases. The highest shear strength of 78.3 MPa is achieved when � the joint is achieved at 840 C for 10 min. The relationship between the joint microstructure and its mechanical properties is also revealed.

1. Introduction pressing and sintering [4] and vacuum sintering [5]. Spark plasma sin­ tering method [6] and microwave sintering method [7] have been Transparent materials have been widely used in many aspects such as applied to produce transparent yttrium aluminium garnet ceramics and gas-tight optical windows, laser windows and lamp envelopes [1]. The transparent alumina ceramics. Thus transparent ceramics have become commonly used conventional transparent materials include glasses and promising candidates for optical materials. However, transparent ce­ polymers. However, the polymers generally cannot withstand relatively ramics usually need to be attached to metal structures to realise their high mechanical load and the physical and chemical stabilities of the applications [8]. Thus considerable efforts have been carried out on glasses are usually not satisfying [2]. With the development of the joining the transparent ceramics. Tsunekane et al. [9] has utilised ad­ crystal growth technique, single crystals such as sapphires and Y3Al5O12 hesive bonding to join YAG to a copper heat sink to fabricate the pump (YAG) have become alternative candidates for optical materials [3]. light-guide. This method is flexibleand of low cost, but the glues usually However, it could be expensive and challenging to achieve single crystal could not stand high temperature. Diffusion bonding is another widely with large size. Also, due to the shape of the as-grown single crystal is used approach to join transparent ceramics. Dascalu et al. [10] applied often determined by their crystal structure instead of the processing diffusion bonding to manufacture Yb: YAG/YAG microchips, which have conditions, difficultiesarise when the required optical material needs to been successfully used to obtain 90 W and 100 W laser. Yen et al. [11] have a complex shape [2]. Transparent ceramics are polycrystalline also used diffusion bonding to join MgF2 optical ceramic and the materials usually produced using high purity raw materials, which could transmittance of the joint could reach 85.7% of the original ceramic. be fabricated into large specimens. A lot of research about sintering the However, it could be very challenging to carry out the diffusion bonding transparent ceramics has been carried out. MgO transparent ceramics process if the sample got a complex structure. Since laser could penetrate and ZrO2-doped Y2O3 transparent ceramics have been fabricated via hot through the transparent ceramics to reach the interface and generate

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

https://doi.org/10.1016/j.msea.2020.139859 Received 29 April 2020; Received in revised form 27 June 2020; Accepted 30 June 2020 Available online 15 July 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved. C. Li et al. Materials Science & Engineering A 793 (2020) 139859

heat, laser bonding could be applied to join transparent materials. holding time on the microstructure and the mechanical properties of the Richter [12] used this method to join a series of transparent ceramics AlON/metal brazing joint to achieve the optimised brazing parameters. and glasses such as zerodur and fused silica and the residual stress Ti6Al4V alloy has shown excellent mechanical properties and induced by the laser welding is modelled. But to realise the laser direct favourable corrosion resistance. Joining it with AlON ceramic could help bonding of transparent materials, sophisticated facilities such as short to realise the potential of both materials. Thus, the objectives of this pulse laser is required. Brazing is another feasible approach to realise the paper are to realise the brazing between the AlON ceramic with the joining between transparent ceramics and metals and it is suitable to Ti6Al4V alloy, to characterise the typical microstructure of the joint achieve the bonding between materials with complex shape. Gambaro using a series of methods including SEM, Energy-dispersive X-ray et al. [13] applied Ag–Cu–Ti as the brazing filler to join YAG ceramics spectroscopy (EDS), TEM and X-ray diffraction (XRD), to demonstrate and reaction products are characterised. Zhu et al. [14] bonded MgAl2O4 the effect of the brazing temperature and the holding time on the ceramic via vacuum brazing and the shear strength of the achieved joint microstructure and the strength of the joint and to explore the rela­ could reach 61.3 MPa. Transparent alumina is another commonly used tionship between the microstructure and the strength of the joint. optical ceramic and Liu et al. [15] has applied active brazing to join it with TiAl alloy. McCauley et al. [16] reviewed the development history 2. Experimental of AlON ceramic and stated that aluminium oxynitride (AlON) is a kind of cubic spinel ceramic which has favourable optical properties, AlON was fabricated by Shanghai institute of ceramics. The fabri­ outstanding mechanical properties and excellent corrosion resistance. cation process has been described elsewhere [26]. The starting AlON Thus it has shown great potential in a wide range of industrial appli­ powders were ball milled with 0.1 wt% Y2O3–La2O3 additives and cold cations. Surprisingly, there is little published data on the joining of the isostatic pressed under a pressure of 200 MPa. The sintering temperature � � AlON ceramics. Liu et al. [17] achieved the bonding between AlON and ranged from 1880 C to 1950 C and the maximum flexural strength of BN-Si3N4 via active element brazing and the highest shear strength the AlON substrates could reach ~280 MPa. Before joining, the ceramics reached 94 MPa. The research on the joining between AlON and metal were carefully cut into 5 mm ✕ 5 mm ✕ 5 mm substrates by the diamond has been seldom carried out. Thus it is important to investigate on the cutting wheel and the Ti6Al4V substrate was electrical discharge brazing between AlON ceramic and metal to realise the application of machined into 10 mm ✕ 5 mm ✕ 3 mm pieces. Since the Ti6Al4V sub­ the AlON ceramic. strate could offer the active Ti element, Ag–28Cu (wt%) eutectic brazing However, brazing ceramic with metals can be challenging. Ali et al. filler was selected, which was received in foil form with a thickness of [18] indicated that the ceramics are chemically stable, which makes it 100 μm and subsequently cut into 5 mm ✕ 5 mm sections to fitthe size of difficult to react with most of the metal elements and be wetted by the the AlON substrates. Prior to the joining process, both substrates were metal brazing filler. Active element brazing, which incorporates active ground to 1 μm finishand the substrates and the brazing fillerwere put elements, e.g., Ti, Zr and Hf into the brazing filler to react with the in acetone and ultrasonically cleaned for ~10 min. ceramic and thus promotes the brazing filler wettability on ceramics. During brazing, the brazing filler was inserted between the two Nagatsuka et al. [19] investigated the effect of Ti content on the joint substrates, as shown in Fig. 1a). The brazing process was established in a À between sialon ceramic and WC-Co alloy. It is found that Ti tends to vacuum furnace with a vacuum better than 1 ✕ 10 2 Pa. The heating � � diffuse to the sialon side and react with the ceramic. The reaction rate was 15 C/min, while the cooling was fixed at 5 C/min to inhibit product layer becomes thicker with the increasing Ti content in the the possible cracking of the joint. The eutactic point of the Ag–28Cu � brazing filler.Kim et al. [20] selected AgCuZr as the brazing fillerto join brazing filleris 779 C. To ensure the flowabilityof the brazing filler,the � alumina with Ni–Cr steel and Zr is found to react with alumina to form a initial brazing temperature was set as 790 C. The specimens were then � � � ZrO2 layer next to the ceramic. Loehman et al. [21] added the active held at various brazing temperatures (790 C, 820 C, 840 C and 860 � element Hf into the Ag substrate as the brazing filler to bond alumina, C) for 1 min, 3 min, 10 min and 15 min to investigate the effect of the and HfO2 is found adjacent to the ceramic. The above discussion could brazing parameters on the microstructure and mechanical properties of indicate that the active elements in the brazing filler tend to diffuse the joint. towards the ceramic and form a reaction layer, which plays a crucial role After the brazing procedure, the samples were mounted in resin, in realising the joining. However, it is found that these reaction products cross sectioned by the diamond cutting wheel, ground and polished to are usually brittle phases and it can be detrimental to the joint reliability 500 nm finishfor microstructure observation. The microstructure of the if the reaction layers are too thick [22]. The brazing temperature and the joint is characterised by SEM (Quanta 200FEG, FEI, USA) with an EDS holding time are the two important parameters for the brazing process, and a thin layer of gold (~20 nm thick) was applied on the surface of the and it is believed to be closely related to the growth of the reaction layer. samples before observation to improve the conductivity of the sample. Thus a considerable amount of research has been carried out to inves­ The X-ray mapping of the element distribution in the joint is achieved tigate the effect of the brazing temperature and the holding temperature via EPMA (JXA-8230, JEOL, Japan). The phases in the ceramic sub­ on the microstructure and the mechanical properties of the ceram­ strates are characterised by XRD (D8 Advanced, Bruker, Germany). Thin ic/metal brazing joint. Klotz et al. [23] found that the thickness of the sections of AlON/brazing fillerinterface for TEM analysis was prepared reaction layer (TiC) between the CuSnTi brazing fillerand the Diamond using focused ion beam (FIB, HELIOS NanoLab 600i, FEI, USA), which grows linearly with the increase of the brazing temperature. Wang et al. consists of a lift-out procedure to transfer the section to a copper grid. [24] also draw similar conclusions that the thickness of the reaction Then the thickness of the section was reduced to less than 100 nm by layer between the cBN and Co-based brazing filler rise with the FIB. The achieved TEM sample was observed using TEM (Talos F200x, increasing temperature and the bonding strength of the first increases FEI, USA). This microscope is fitted with an EDS system to analyse the and then decreases with the growth of the brazing temperature. Ali et al. element in the sample and also a high angle annular dark field(HAADF) [18] also demonstrated that the two reaction layers between alumina detector to perform the atomic number contrast imaging. The shear and AgCuTi show different growth behaviour as the brazing temperature strength of the joint was measured via Instron 1186 universal mechan­ rises. The thickness of the TiO layer increases sharply with the increase ical testing machining with a loading rate of 0.5 mm/min. For each of the brazing temperature while the thickness of the Ti3Cu3O layer first measurement, at least 3 specimens were tested to ensure the reliability increases and then remains almost constant as the brazing temperature of the measurement results. The schematic of the shear test is depicted in rises. Jiang et al. [25] found that the shear strength of the Fig. 1b). WC-Co/carbon steel braszing joint rises and then drops with the increase of the holding time. From the above discussion, it can be seen that it is necessary to investigate the effect of brazing temperature and the

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Fig. 1. The schematics showing a) the structure of the joint, b) the geometry of the shear test.

3. Results between the AlON ceramic and the brazing filler.It can be seen that Al mainly distributes inside the AlON substrate and the segregation of Ti The microstructure of the AlON/Ti6Al4V brazing joint obtained at can be observed at the interface between AlON and the brazing seam. � 840 C for 10 min is shown in Fig. 2. It can be seen in Fig. 2a) that the Also, Cu segregation could be observed in this reaction layer. The ma­ joining between AlON ceramic and Ti6Al4V has been realised using the jority of the Cu element and Ag element mainly distributes inside the AgCu brazing filler and no defects such as cracks can be observed. brazing seam and forms a typical eutectic microstructure. Fig. 3b) shows the magnified image of the interface between the AlON EDS analysis is carried out to investigate the content of phase A-H ceramic and the brazing filler. It can be seen that a uniform reaction inside the joint shown in Fig. 2 and the results are listed in Table 1. layer A with a thickness of ~1 μm has formed adjacent to the AlON As presented in Fig. 1 and Table 1, the bright phase mainly consists of ceramic. silver and a small amount of copper, which turns out to be the Ag solid As demonstrated in Fig. 2a), the filler alloy mainly consists of solution (Ag (s,s)). The relatively dark phases adjacent to the reaction eutectic microstructure which is made of bright substrate phase E and layer and inside the eutectic microstructure mainly contains Cu, which dark phase C. Also some large circular phases D and small column phases could be inferred to be the copper solid solution (Cu (s,s)). Three reac­ B with dark contrast can be found inside the brazing seam. Next to the tion layers are presented next to the Ti6Al4V substrate, which is mainly Ti6Al4V substrate, three reaction layers (F, G and H) can be observed. comprised of Ti and Cu. As shown in Table 1, the atomic ratio between Ti The X-ray mapping of various elements achieved using EPMA are shown and Cu in the reaction layer H next to the Ti6Al4V substrate is about 2:1, in Fig. 3a–f) to illustrate the element distribution across the interface which is inferred to be Ti2Cu. The Ti–Cu atomic ratio of the F and G

� Fig. 2. a) The SEM image of an AlON/Ti6Al4V brazing joint obtained at 840 C for 10 min using AgCu as the brazing filler, b) the magnified image showing the microstructure of the interface region between AlON and the brazing filler.

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� Fig. 3. a) The BSE image showing the microstructure of the AlON/brazing fillerinterface region of the joint achieved at 840 C for 10 min, the element distribution measured via EPMA of b) Ti, c) Al, d) Ag, e) O and f) Cu.

reaction layers are 4:1 and 1:1 and thus, these two reaction layers are the Table 1 Ti4Cu phase and the TiCu phase. The reaction layer adjacent to the AlON The EDS analysis results of the various phases in the joint shown in Fig. 2 (at%). ceramic mainly consists of Ti, Cu, Al and O and the atomic ratio between position Al O Ag Cu Ti Possible phase Ti and Cu þ Al is around 1. However, the reaction layer adjacent to the

A 22.85 6.88 5.3 19.7 45.27 Ti3(Cu,Al)3O AlON ceramic is only about 1 μm thick, which is usually smaller than the B 3.97 – 1.81 91.99 2.22 Cu (s,s) diameter of the SEM electron beam interaction volume in samples. Thus – – C 3.57 1.89 94.55 Cu (s,s) to detail characterise this reaction layer, which plays a key role in the D 13.97 – 0.81 62.17 23.04 TiCu4 E – – 86.5 12.55 0.96 Ag(s,s) bonding between the ceramic and the brazing filler, TEM analysis was F 13.04 – 1.19 62.08 23.69 TiCu4 applied. Fig. 4 shows the observed microstructure at the AlON/brazing G – – 2.09 51.35 46.56 TiCu filler interface region via TEM. – – H 1.13 34.15 64.71 Ti2Cu From Fig. 4a), it can be seen that the thickness of the reaction layer between the ceramic and the brazing filler is relatively uniform. To

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� Fig. 4. a) The HAADF image showing the microstructure of the AlON/brazing fillerinterface region of the joint achieved at 840 C for 10 min and the inserted figure shows the bright field TEM image of the reaction layer adjacent to the AlON substrate, b) The HRTEM image of the AlON substrate and the inserted figure is the corresponding FFT pattern, c) the diffraction pattern of the reaction layer adjacent to the AlON substrate, the distribution of d) Ag, e) Ti, f) Cu, g) O and h) Al, i) the EDS spectrum measured at position 1 and 2 shown in Fig. 4a) and the corresponding element contents.

� investigate the element distribution around the interface, EDS mapping microstructure with the joint obtained at 840 C, which is the eutectic is carried out and the results of which are demonstrated in Fig. 4d–i). As Ag(s,s)þCu(s,s) microstructure. Fig. 5b) is the magnifiedmicrostructure shown in these figures, Ag mainly distributes in the brazing filler. The of the ceramic/brazing fillerinterface, from which it can be seen that the reaction layer is mainly composed of Ti, Cu and a small amount of O and thickness of the Ti3(Cu, Al)3O reaction layer is thinned to about 0.5 μm Al. The HRTEM images of the AlON ceramic with the corresponding fast as the brazing temperature drops. The morphologies of the three reac­ Fourier transform (FFT) patterns and the diffraction pattern of the re­ tion layers on the Ti6Al4V substrate sides also change when the brazing � action layer are depicted in Fig. 4b) and c). From the indexing of the FFT temperature is dropped to 790 C, as demonstrated in Fig. 5c). The most patterns and the diffraction pattern, together with the previous EDS significantdifference is that the thickness of the TiCu4 layer increases to � analysis results, it can be confirmedthat the reaction layer is the Ti3(Cu, about 10 μm. As the brazing temperature rises to 820 C, the micro­ Al)3O phase and the substrate is the AlON. From the above discussion, structure of the obtained joint is shown in Fig. 5d) to f). It can be seen the typical microstructure of the joint is identified to be AlON/Ti3(Cu, that the joint microstructure has not been significantlyaltered compared � � Al)3O/Ag(s,s)þCu(s,s)/Ti4Cu/TiCu/Ti2Cu/Ti6Al4V. to the joint obtained at 790 C and 840 C. The middle part of the The brazing temperature is a vital parameter for the brazing process, brazing seam is also mainly composed of Ag(s,s) þ Cu(s,s) eutectic which is closely related to the microstructure of the joint and thus microstructure. The thickness of the Ti3(Cu,Al)3O reaction layer be­ affecting the reliability of the joint. For this reason, the effect of the comes thicker (about 0.8 μm thick), as depicted in Fig. 5e). For the re­ brazing temperature on the microstructure of the AlON/Ti6Al4V joint is action layers next to the Ti6Al4V substrate, the thickness of the TiCu4 � investigated. Fig. 5 shows the observed microstructure of the joint layer drops compared to that of the joint obtained at 790 C. The � � achieved at 790 C–860 C for 10 min. thickness of the TiCu and Ti2Cu reaction layers is found to increase with As illustrated in Fig. 5a), the joining between AlON and Ti6Al4V can the rising brazing temperature. When the brazing temperature is further � � be achieved at 790 C. The middle of the brazing seam shows a similar increased to 860 C, the microstructure of the joint changes

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� � � Fig. 5. a), d) and g) show the SEM images of an AlON/Ti6Al4V brazing joint obtained at 790 C, 820 C and 860 C for 10 min, b), e) and h) show the magnified image of the interface between the AlON substrate and the brazing filler, c), f) and i) show the magnified image of the interface between the brazing filler and the Ti6Al4V substrate.

significantly. Even though the microstructure in the middle of the It can be seen from Fig. 7a–c) that when the holding time is 1 min, the brazing seam is still mainly composed of the Ag(s,s) þ Cu(s,s) eutectic brazing seam mainly consists of the Ag(s,s) and Cu(s,s). Similar to the microstructure, a crack can be found inside the AlON substrate about 50 joint achieved with a holding time of 5 min, a reaction layer of about 0.8 μm away from the ceramic/brazing fillerinterface, as shown in Fig. 5g). μm thickness can be found adjacent to the AlON ceramic. The TiCu4 The reason for its formation will be discussed later. From Fig. 5h), it can reaction layer could also be observed on the Ti6Al4V side, as indicated be observed that the thickness of the Ti3(Cu,Al)3O reaction layer is in Fig. 7c). When the holding time is increased to 3 min (Fig. 7d)), the further increased to about 2 μm due to the increased brazing tempera­ microstructure of the joint did not change significantly. As shown in ture. As can be seen in Fig. 5i), the thickness of the TiCu4 layer further Fig. 7e), the reaction layer next to the AlON ceramic becomes a little decreases. The thickness of the other reaction layers adjacent to the thicker and the TiCu4 reaction layer thickness decrease slightly Ti6Al4V substrate (TiCu and Ti2Cu) grows. (Fig. 7f)). As the holding time is further expended to 15 min, the brazing It can be seen that the brazing temperature could affect the joint seam still mainly consists of the Ag(s,s) and Cu(s,s), which is demon­ microstructure, which is closely related to the mechanical properties of strated in Fig. 7g). It can be seen from Fig. 7h) that the reaction layer the joint. Thus the effect of the brazing temperature on the shear between the AlON ceramic and the brazing fillergrows thicker. For the strength of the AlON/Ti6Al4V brazing joint is researched and the Ti6Al4V side, as depicted in Fig. 7i), the TiCu4 reaction layer disappears. measured result is shown in Fig. 6. The effect of the holding time on the shear strength of the joint is shown It can be seen that the bonding of the joints obtained at a lower in Fig. 8. temperature is relatively weak, the shear strength of which is less than When the holding time is short (1 min), the shear strength of the joint � 30 MPa. As the brazing temperature rises to 840 C, the shear strength of is relatively low (~13.8 MPa). The shear strength of the joint firstrises the joint increases to 78.3 MPa. With the further increase of the joining with the increasing of the holding time and then drops to only 3.5 MPa temperature, the shear strength of the joint decreases sharply to 17.8 when the holding time reaches 15 min. The highest shear strength of MPa. 78.3 MPa is achieved when the holding time is 10 min. The holding time is another important parameter for brazing and thus the effect of the holding time on the microstructure and mechanical 4. Discussion properties of the AlON/Ti6Al4V joint is investigated. Fig. 7 demon­ strates the microstructure of the AlON/Ti6Al4V joints achieved at 840 As shown in Figs. 2 and 3, Ti can be found in the reaction layer � C for 1 min, 3 min and 15 min. adjacent to AlON ceramic. Since the brazing fillerused in this research is

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� � � � Fig. 6. The shear strength of the AlON/Ti6Al4V brazing joints achieved at 790 C, 820 C, 840 C and 860 C for 10 min.

the AgCu brazing fillerwithout ant Ti content, it can be inferred that the temperature increases. This could be explained by the increased diffu­ Ti element found in the reaction layer adjacent to AlON comes from the sion rate of the Ti element towards the AlON substrate induced by the Ti element in the Ti6Al4V substrate, which is dissolved into the brazing higher brazing temperature. Also, with the rising of the brazing tem­ filler and diffuses towards the ceramic. perature, the thickness of the TiCu4 layer drops, which might be an It needs to be noted in Fig. 4a) that there seem to be two different indication that the diffusion rates of the elements are improved. While contrasts of Ti and Cu element presented in the reaction layer. However, for the other reaction layers on the Ti6Al4V side, the thickness of them as shown the inserted figurein Fig. 4a), only one layer of column grains increases with the brazing temperature. This phenomenon could be can be observed in the reaction layer and thus it can be inferred that this explained by the higher element diffusion ability and faster reaction contrast difference might be caused by the uneven thickness in the between Ti and Cu. sample. In Fig. 4a), a small piece of AlON ceramic can be found on the Fig. 6 shows that when the brazing temperature is low, the strength top of the reaction layer. Since machining speeds of the AlON ceramic of the joint is relatively low (less than 30 MPa). This is probably due to and the reaction layer by FIB can be different, the finalthickness of the the reason that at relatively low temperature, the reaction between the prepared TEM sample with and without the ceramic on top could also be ceramic and the brazing filler is not sufficient and the reaction layer is different. In addition, the EDS analysis results in Fig. 4i) illustrates that relatively thin, resulting in a weak bonding between the ceramic and the the contents measured in the positions with different contrasts in the brazing filler. As illustrated in Fig. 6, with the increase of the joining reaction layer are very similar, which helps to ensure that the element in temperature and reaction between the ceramic and the brazing filler the reaction layer is generally uniform and the different contrast becomes sufficient and the strength of the joint is improved. However, � observed in the EDS mapping might be caused by the uneven thickness when brazing temperature is further raised to 860 C, the strength of the of the sample. joint declines significantly. The coefficient of thermal expansion (CTE) As depicted in Fig. 5a), the reaction layer between AlON and the of the AlON ceramic, Ti3(Cu,Al)3O reaction layer, the AgCu brazing filler � À À brazing filler in the joint achieved at 790 C is thin. This could be and the Ti6Al4V substrate is about 7.8 ✕ 10 6/K [28], 15.1 ✕ 10 6/K, À À explained by the reason that at a lower temperature, the diffusion of the 18.5 ✕ 10 6/K and 9 ✕ 10 6/K [29]. It can be seen that the CTE of the active element Ti from the Ti6Al4V substrate is suppressed, resulting in reaction layer and the brazing filler is about one time and two times less Ti element arriving at the AlON/brazing fillerinterface to react with higher than that of the AlON ceramic. Thus during the cooling process of the ceramic. In addition, at a lower temperature, the reaction between brazing, residual stress could be generated due to the mismatch in CTE the AlON substrate and the brazing filler slows down, which also con­ between different phases. Ag (s,s) and Cu(s,s) have been reported to be tributes to the decline of the Ti3(Cu,Al)3O reaction layer thickness. It is ductile phases and the residual stress in the joint could be partially noted that the TiCu4 reaction layers on the Ti6Al4V substrate side in the released by its plastic deformation [30]. However, Ti3(Cu,Al)3O has � joint obtained at 790 C is thicker compared to the joint brazed at 840 minimal deformation ability, the residual stress induced by the CTE � C. Lin et al. [27] reported that the elements in the TiCu4 phase is mismatch between the ceramic and the reaction layer could not be believed to diffuse across the brazing seam and to form the reaction released. With the increase of the brazing temperature, the reaction layer next to the ceramic. At relatively low temperature, the diffusion layer thickness improves, and the strain energy induced by the residual ability of the elements is weak, resulting in a thicker TiCu4 layer and a stress stored in the reaction layer also increases. This could lead to the thinner Ti3(Cu,Al)3O layer. The thickness of the TiCu and Ti2Cu layer cracking of the ceramic substrate observed in Fig. 5g), resulting in becomes thinner, which could be expected from the slower diffusion rate relatively poor shear strength for the joint achieved at high temperature. of the elements and the suppressed chemical reaction at decreased As shown in Fig. 7, similar to the variation of the brazing tempera­ brazing temperature. It can be seen from Fig. 5 that the reaction layer ture, the reaction layer on the ceramic side becomes thicker with the adjacent to the AlON ceramic becomes thicker as the brazing increase of the holding time. This could also be explained by the further

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� Fig. 7. a), d) and g) show the SEM images of an AlON/Ti6Al4V brazing joint obtained at 840 C for 1 min, 3 min and 15 min b), e) and h) show the magnifiedimage of the interface between the AlON substrate and the brazing filler, c), f) and i) show the magnified image of the interface between the brazing filler and the Ti6Al4V substrate.

� Fig. 8. The shear strength of the AlON/Ti6Al4V brazing joints achieved at 840 C for 1 min, 3 min, 10 min and 15 min.

8 C. Li et al. Materials Science & Engineering A 793 (2020) 139859 reaction between the ceramic and the brazing filleras the holding time review & editing. increases. And the thinning of the TiCu4 reaction layer is also due to the diffusion of the elements in TiCu4 towards the ceramic to form the References Ti3(Cu,Al)3O layer. It can be observed in Fig. 8 that the trend of the shear strength [1] L.B. Kong, Y. Huang, W. Que, T. Zhang, S. Li, J. Zhang, Z. Dong, D. Tang, Transparent Ceramics, Springer, 2015. evolution with and the increase of the holding time is comparable to that [2] S. Wang, J. Zhang, D. Luo, F. Gu, D. Tang, Z. Dong, G.E. Tan, W. Que, T. Zhang, with the increase of the brazing temperature. This phenomenon could S. Li, Transparent ceramics: processing, materials and applications, Prog. Solid also be explained in a similar way to that of the variation of the brazing State Chem. 41 (1–2) (2013) 20–54. [3] Y.H. Ko, J.S. Yu, Highly transparent sapphire micro-grating structures with large temperature. When the holding time is short, the bonding between the diffuse light scattering, Optic Express 19 (16) (2011) 15574–15583. ceramic and the brazing filleris relatively weak. With the increase of the [4] Y. Fang, D. Agrawal, G. Skandan, M. Jain, Fabrication of translucent MgO ceramics holding time, the reaction between the ceramic and the brazing filler using nanopowders, Mater. Lett. 58 (5) (2004) 551–554. becomes sufficient and the strength of the joint is improved. However, [5] L. Jin, G. Zhou, S. Shimai, J. Zhang, S. Wang, ZrO2-doped Y2O3 transparent ceramics via slip casting and vacuum sintering, J. Eur. Ceram. Soc. 30 (10) (2010) when the holding time is further increased, the Ti3(Cu,Al)3O layer be­ 2139–2143. comes thicker, which results in the increase of the strain energy of the [6] N. Frage, S. Kalabukhov, N. Sverdlov, V. Ezersky, M.P. 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A reaction layer can be observed between the AlON [10] T. Dascalu, N. Pavel, T. Taira, 90 W continuous-wave diode edge-pumped ceramic and the brazing seam which is determined to be the Ti3(Cu, microchip composite Yb: Y 3 Al 5 O 12 laser, Appl, Phys. Lett. 83 (20) (2003) 4086–4088. Al)3O phase via SEM and TEM. Three reaction layers: Ti4Cu, TiCu and [11] T. Yen, Y. Chang, D. Yu, F. Yen, D. Tsai, I.-N. Lin, Diffusion bonding of MgF2 Ti2Cu can be found adjacent to the Ti6Al4V substrate. The thickness of optical ceramics, Mater. Sci. Eng. A 147 (1) (1991) 121–128. the Ti3(Cu,Al)3O, TiCu and Ti2Cu reaction layer is found to increase with [12] S. Richter, Direct Laser Bonding of Transparent Materials Using Ultrashort Laser the rising brazing temperature/holding time while the thickness of the Pulses at High Repetition Rates, 2014. [13] S. Gambaro, F. Valenza, A. Passerone, G. Cacciamani, M. Muolo, Brazing Ti Cu reaction layer drops as the brazing temperature/holding time 4 transparent YAG to Ti6Al4V: reactivity and characterization, J. Eur. Ceram. Soc. 36 increases. The shear strength of the joint first increases and then de­ (16) (2016) 4185–4196. creases with the increase of the brazing temperature/holding time and [14] C. Zhu, C. Shang, Vacuum brazing of MgAl2O4 transparent ceramics with AgCuTi � – 78.3 MPa is the highest strength that is achieved at 840 C for 10 min. active filler metal, Trans. China Weld. Inst. 36 (4) (2015) 101 105. [15] X. Liu, L. Zhang, Z. Sun, J. Feng, Microstructure and mechanical properties of Therefore, based on the results of this study, it can be concluded that the transparent alumina and TiAl alloy joints brazed using Ag-Cu-Ti filler metal, microstructure has shown a significant influence on the mechanical Vacuum 151 (2018) 80–89. properties of the AlON/Ti6Al4V brazing joint. The brazing parameters [16] J.W. McCauley, P. Patel, M. Chen, G. Gilde, E. Strassburger, B. Paliwal, K. Ramesh, D.P. Dandekar, AlON: a brief history of its emergence and evolution, J. Eur. Ceram. need to be carefully selected to optimise the thickness of the reaction Soc. 29 (2) (2009) 223–236. layer between the AlON ceramic and the brazing filler,which is essential [17] M. Liu, C. Liu, J. Zhang, R. Tao, Q. Zhang, Q. Qi, Microstructure and mechanical for achieving a reliable AlON/Ti6Al4V brazing joint. properties of BN-Si3N4 and AlON joints brazed with Ag-Cu-Ti filler alloy, J. Eur. Ceram. Soc. 38 (4) (2018) 1265–1270. [18] M. Ali, K.M. Knowles, P.M. Mallinson, J.A. Fernie, Interfacial reactions between Funding sapphire and Ag–Cu–Ti-based active braze alloys, Acta Mater. 103 (2016) 859–869. [19] K. Nagatsuka, S. Yoshida, Y. Sechi, K. Nakata, Effect of Ti content in Ag–Cu–Ti This work was supported by the National Natural Science Foundation activated filler metal on dissimilar joint formation of sialon and WC–Co alloy by of China [51805114 and U1737205] and China Postdoctoral Science laser brazing, Sci. Technol. Weld. Join. 19 (6) (2014) 521–526. Foundation [2018M631921]. [20] J.-H. Kim, Y.-C. Yoo, Bonding of alumina to metals with Ag-Cu-Zr brazing alloy, J. Mater. Sci. Lett. 16 (14) (1997) 1212–1215. [21] R.E. Loehman, F.M. Hosking, B. Gauntt, P.G. Kotula, P. Lu, Reactions of Hf-Ag and Data availability Zr-Ag alloys with Al2O3 at elevated temperatures, J. Mater. Sci. 40 (9) (2005) 2319–2324. The raw/processed data required to reproduce these findingscannot [22] O.M. Akselsen, Advances in brazing of ceramics, J. Mater. Sci. 27 (8) (1992) 1989–2000. be shared at this time as the data also forms part of an ongoing study. [23] U. Klotz, C. Leinenbach, C. Liu, J. Loffler, P. Uggowitzer, J. Wang, Cu-Sn-Ti Experimental Investigation and Thermodynamic Assessment, 2013. Declaration of competing interest [24] Y. Wang, Z.Z. Duan, G. Chen, Q.Y. Jiang, W. Dong, K. Lei, Effects of brazing temperature on microstructure and properties of interface between cBN and Co- based active filler metals, Vacuum 145 (2017) 30–38. The authors declare that they have no known competing financial [25] C. Jiang, H. Chen, Q. Wang, Y. Li, Effect of brazing temperature and holding time interests or personal relationships that could have appeared to influence on joint properties of induction brazed WC-Co/carbon steel using Ag-based alloy, J. Mater. Process. Technol. 229 (2016) 562–569. the work reported in this paper. [26] H. Guo, J. Zhang, X. Mao, F. Zhang, F. Sun, F. Chen, J. Wang, R. Tian, J. Liu, S. Wang, Strengthening mechanism of twin lamellas in transparent AlON ceramics, CRediT authorship contribution statement J. Eur. Ceram. Soc. 38 (9) (2018) 3235–3239. [27] J. Lin, M. Huang, W. Yang, L. Xing, Degradation kinetics of Ti-Cu compound layer in transient liquid phase bonded graphite/copper joints, Sci. Rep. 8 (1) (2018) Chun Li: Conceptualization, Methodology, Investigation, Data 15190. curation. Kaiping Zhang: Investigation, Data curation. Xiaojian Mao: [28] W. Kollenberg, J. Margalit, Thermal expansion of AlON and γ-Al 2 O 3, J. Mater. – Conceptualization, Methodology. Xiaoqing Si: Visualization, Writing - Sci. Lett. 11 (14) (1992) 991 993. [29] J. Cao, Z. Zheng, L. Wu, J. Qi, Z. Wang, J. Feng, Processing, Microstructure and review & editing. Bo Lan: Investigation, Data curation. Zhan-Guo Liu: mechanical properties of vacuum-brazed Al2O3/Ti6Al4V joints, Mater. Sci. Eng. A Conceptualization, Writing - review & editing. Yongxian Huang: 535 (2012) 62–67. Writing - review & editing. Junlei Qi: Writing - review & editing. Jicai [30] X. Wang, C. Li, X. Si, J. Qi, J. Feng, J. Cao, Brazing ZTA ceramic to TC4 alloy using the Cu foam as interlayer, Vacuum 155 (2018) 7–15. Feng: Project administration, Writing - review & editing. Jian Cao: Project administration, Funding acquisition, Supervision, Writing -

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