Microstructure of Materials
Microstructure and Property of ZrO2 Joint Direct Brazed with Aluminum
2020 年 61 巻 7 号 p. 1252-1257
詳細
2020 年 61 巻 7 号 p. 1252-1257
In order to realize direct brazing ZrO2 with Al, a kind of sputtering Ni/Al double layer on Al solder surface was used. Results showed that ZrO2 could directly brazed to Al solder with sputtering Ni/Al double layer without interface reaction. And then direct brazing was investigated with the change of brazing temperature. Microstructure and joint strength of brazing seam were analyzed. Results showed that joint strength of brazing seam increased from 68.6 MPa to 121.2 MPa with the increasement of brazing temperature from 680°C to 840°C, when the brazing temperature increased up to 920°C, the joint strength gradually increased to 122.6 MPa. With the joint strength enhancement of brazing seam, the fracture of the joint is gradually transferred from interface to the brazing seam.
Fig. 7 Schematics of removing Al2O3 film from Al foil (a) uncoated Al foil before Al melting (b) uncoated Al foil after Al melting (c) coated Ni/Al double layer on Al foil before Al melting (d) coated Ni/Al double layer on Al foil after Al melting.
As an important structural and functional ceramic, ZrO2 not only possesses high strength, high fracture toughness, good biocompatibility, but also has excellent ionic conductivity and chemical stability at high temperature, which has been widely used in turbine engines, biological medicine equipment and solid oxide fuel cells.1)
Brazing is the main technology to connect ceramic with ceramic or metal, and the wettability between them is one of the most important key point. Owing to the strong ion binding property of ZrO2, it cannot wet with most of metals (Cu, Co, Sn and Ni) which the wetting angle is above 100°.2–4) The typical brazing techniques to resolve the problem include adding active metals (such as Ti, Zr, V) into the brazing seam5–7) and depositing the active metal layer on ceramic surface used by sintering8–10) or coating.11–13) However, the interface reaction transition layer maybe harmful to the bonding strength, thermal fatigue and other joint properties. Song14) studied brazing of ZrO2 used Sn–Ti alloys with the brazing temperature of 1000°C and holding time of 10 min, and the shear strength located in the range of 22–28 MPa, which the joint fractured through β-Sn matrix owing to its weakness.
As a kind of brazing filler, Al and its alloy have excellent properties as well as Cu, Ti and Ag, and also the melting temperature of 660°C helps to reduce the brazing temperature. Although molten aluminum can not directly wet the ceramic, the wetting angle decreased from 145° to 59° as the temperature increased from 900°C to 1200°C.15) Sobczak N et al.16) studied the wettability between Al and ZrO2 and found that an oxide films with nanoscale always covered on liquid aluminum which have a significant impact on the macro properties of Al/ceramic joints. And research showed that the transition temperature from non-wetting to wetting in Al/Ceramic system occurs at 1000°C.17) Hioyuki Takeda18) studied the the joint strengthening of zirconia–zirconia joint with aluminum foil by applying a DC voltage when the brazing temperature was 1000°C. The interfacial structure changed with the addition of DC voltage across the zirconia specimens which greatly influenced the joining strength.
Aluminum is active and easily oxidized into Al2O3, whose existence is an important factor to affect the wettability of ZrO2 with Al. A dense Al2O3 film with the thickness of 10 nm19) and the melting point of 2050°C existed on Al surface, which cannot break down unless the temperature reaches 1000°C and oxygen partial pressure drops to 10−30 Pa.20) Although in this situation, dense Al2O3 film would cracked, but still attached on the surface of liquid aluminum, which impede the direct brazing of ZrO2 with Al fillers. In a non-vacuum environment, aluminum-based brazing fillers adding chloride and fluoride (such as activated CsF–AlF3 flux21)) could dislodge Al2O3 film covered on molten aluminum. However, the existence of chloride and fluoride retained in brazing seam had a bad effect on joint performance.22)
In order to realize direct brazing of ZrO2 with Al solder, a method of removing Al2O3 film covered on Al solder surface was proposed in this paper, which more specifically a kind of Ni/Al double layer was deposited on Al solder surface. The direct brazing of ZrO2 with Al was successfully achieved with the increasement of brazing temperature. The removal mechanism of oxidation film on Al solders was further discussed. And then, the relationship between joint strength of brazing seam with brazing temperature was investigated.
The pure Al foil with the length and width both of 50 mm and the thickness of 50 µm was ultrasonically cleaned in alcohol and then dried before mounted on the substrate holder in the vacuum chamber of the ANELVASPC-350 multi-target magnetron sputtering system. When the background pressure reached below 4.0 × 10−4 Pa prior to the deposition, high purity Ar gas (99.999%) with a pressure of 0.6 Pa was aerated into the vacuum chamber. The target of Ni (99.99%) and the target of Al (purity) were controlled by the RF cathode and the DC cathode respectively. The thickness of double-layer film (in other word, deposition time) was estimated according to the average deposition rate by observing the film thickness deposited on silicon slice at a certain time. A thick film of aluminum or nickel was deposited on silicon slice for 120 minutes, and the deposition thickness (about 3 µm) was measured using SEM observation. The average deposition rate was equal to the deposition thickness divided by the deposition time, which it is about 25 nm/min. The first layer of aluminum was deposited on Al foil for 80 minutes and then the second layer of nickel was deposited on aluminium film for 4 minutes. And then, the double-layer film of 2 µm Al and 0.1 µm Ni were successively deposited on both sides of Al foil. After film depositing, the components variation with the film depth were analyzed using energy scattering spectrometer.
2.2 Direct brazingThe brazed ZrO2 ceramic with the thickness of 1 mm was polished by 1 µm diamond grinding paste and washed in alcohol ultrasonically.
Direct brazing was operated in vacuum chamber with a pressure of 10−3 Pa. The coated Al solder was placed in two ZrO2 ceramics, and a stainless-steel mass was placed over the ceramic, providing pressure at approximately 1 kPa. And then the brazing was carried out at various temperatures from 680°C to 920°C for 30 minutes. The seam morphology were observed using SEM.
2.3 Joint strength testThe joint strength of brazing seam was measured by an electronic tensile test machine. The brazing samples were cut into pieces with the size of 2 × 2 mm, where were fixed in the special mold, as shown in Fig. 1.
Fixture schematic diagram for measuring shear strength of ceramic brazed joints.
The result of joint strength for each parameter is the average of ten effective measured values. And the fracture morphology were observed using optical microscope.
The energy scattering spectrometer was used to analyze the element distribution of Al solder with Ni/Al double layer. To save equipment analysis time, the nickel and aluminum layers were reduced to 60 nm, which were presented respectively in Fig. 2. As shown in this figure, the film surface is nickel layer which also showed the existence of NiO. With the depth of film, sharp interface between nickel layer and aluminum layer appeared. After Ni/Al interface, sputtering Al layer arised, which accorded with the characteristics of double-layer film. After that, there existed O-riched area with the thickness of 20 nm between sputtering Al layer and Al solder, which showed that Al2O3 film on Al solder was buried by Ni/Al double-layer film. The effect of Ni/Al double-layer film on removing Al2O3 film would be discussed in section 4.1.
Depth profile of Ni, Al and O in coated Al foil filler.
After brazing experiments, the brazed joint were observed using SEM. According to the observation, brazing joints after different brazing temperatures had similar structures, which were all shown as Fig. 3 of brazed seam at 840°C. The brazed seam was tight and full without incomplete penetrations and pores. In particular, there did not exist reaction interface transition layer between Al and ZrO2. As given in Fig. 4, the element map scanning of Al, Zr and Ni were detected. According to the result, element of Al was detected in the whole brazing seam where there was no nickel. It is likely that the low content of nickel was not detected.
SEM image of brazed joint when the brazing temperature was 840°C (a) 2000X; (b) 5000X.
Element map scanning of brazed joint when the brazing temperature was 840°C (a) SEM micrograph of cross section; (b) map scanning of element Ni; (c) map scanning of element Al; (d) map scanning of element Zr.
And then, the joint strength of brazing seam after different brazing temperature was measured by an electronic tensile test machine. Figure 5 showed the relationship between the joint strength of brazing seam and brazing temperature. As shown in Fig. 5, the joint strength of brazing seam at the temperature of 680°C was 68.6 MPa. With the increase of brazing temperature, the joint strength gradually increased up to 122.6 MPa at the brazing temperature of 920°C.
The relationship between joint strength and brazing temperature.
Observation of the fracture morphology of the joints by optical microscope was shown in Fig. 6. The fracture morphology of joints changes significantly as the brazing temperature increases. When the brazing temperature was 680°C, it can be seen dark and light areas as shown in Fig. 6(a). It can be seen that ceramic particles of ZrO2 with small amount of metal Al can be observed in the dark area, which indicated that the fracture occurred at the interface between coated aluminum foil and ZrO2 ceramic. Furrow shaped morphology was observed in light areas, which presented the trace of high plastic Al during shear fracture. It indicated that fracture of light areas occurred in brazing seam. When the brazing temperature increased to 760°C, the proportion of furrow morphology had been increased (Fig. 6(b)). With the brazing temperature was up to 840°C, a screen full of furrows was observed in Fig. 6(c). When the temperature rose further to 920°C, the fracture morphology observation (Fig. 6(d)) was similar to Fig. 6(c) at the brazing temperature of 840°C.
OM images with large depth field of the fracture morphologies of joints brazed at 680°C (a), 760°C (b), 840°C (c) and 920°C (d).
The fracture morphology at 680°C (Fig. 6(a)) exhibits that the fracture occurred at the interface between Al solder and ZrO2, because only a small part of Al foil wetted ZrO2. At 760°C (Fig. 6(b)), the coated Al solder had basically wetted ZrO2, and some furrows were observed near the center because the shear fracture occurred in highly plastic Al solder. But part of Al solder in the center area still could not wet ceramic. With the brazing temperature increased, the fracture area changed from the interface into the brazing seam. Combined with the joint strength variation with brazing temperature, the enhancement of joint strength was driven by the increasement of brazing temperature, it was because of the increase of interface strength that the shear fracture was gradually transferred from the interface to the brazing joint, which made the joint strength increase.
The above experimental results showed that the methods of Al solder with sputtering Ni/Al double-layer film and heating brazing could resolve the two key points effectively on removing Al2O3 film and enhancing joint strength.
4.1 The mechanism of removing Al2O3 filmAlthough Al and its alloy have not been widely used in ceramic bonding at present, which had often been used to braze Al with other metals. And how to effectively remove Al2O3 film on Al solder is an important step for direct brazing. In past studies, adding a chloride flux in non-vacuum brazing or adding Mg to react with Al2O3 film in vacuum brazing were used to remove Al2O3 film. But they are not suitable for Al brazing with ZrO2 because of the difficulty of disposing reaction products. Therefore, joining of Al and ZrO2 is of great importance in both scientific research and industrial application. According to the experiment observation and literature research, it means that the wetting of zirconia ceramics with metals may be controlled by the affinity of the metals with oxygen in ZrO2. In this paper, the Ni/Al double layer was deposited on Al solder, and then this kind of Al solder was used to braze ZrO2 which could reduce the influence of Al2O3 film on wetting and brazing. This method not only avoids the pollution of the reaction products on work pieces and heating chamber, but also makes the operation easier. The mechanism of removing Al2O3 film from Al solder is shown in Fig. 7.
Schematics of removing Al2O3 film from Al foil (a) uncoated Al foil before Al melting (b) uncoated Al foil after Al melting (c) coated Ni/Al double layer on Al foil before Al melting (d) coated Ni/Al double layer on Al foil after Al melting.
As given in Fig. 7(a) of uncoated Al solder, it can be seen that Al2O3 film on Al solder was directly in contact with ZrO2. The coefficient of thermal expansion of Al2O3 is only 1/3 of that of Al, brittle Al2O3 cracked during the heating process. Even after Al melted, the flow of molten aluminum could not push the broken oxide film away from the interface, most of them still covered the interface between Al solder and ZrO2, which was shown in Fig. 7(b).
As shown in Fig. 7(c) of Al solder with Ni/Al double-layer film, Al2O3 film on Al solder surface was covered by Ni/Al double-layer film, which also cracked during heating process. After Al solder with the double-layer film melted. Cracked Al2O3 would be sucked into the brazing seam by melting aluminum (Fig. 7(d)). This kind of physical self-removing Al2O3 method not only avoided the pollution of reaction products to workpieces and heating chamber, but also was more convenient to use.
In addition, there existed NiO film according to the results of element distribution. Although there formed NiO film in this method, which would reacted with Al on the brazing process by the following reaction:23)
| \begin{equation} \text{Al} + \text{NiO} \to \text{Al$_{2}$O$_{3}$} + \text{Ni} \end{equation} | (1) |
After reaction, Ni–Al alloy solution directly contact with ceramic surface. The products were buried into the brazing seam, which could play a role in joint strengthening.
4.2 The influence of brazing temperature on joint strength of brazing seamThe conventional brazing temperature was set at the solder melting point above 20–50°C. In this paper, the method of heating brazing is adopted, and the results showed that the joint strength enhances significantly with the increasement of brazing temperature. Both the improvement of joint strength and fracture mode variation can prove that the bonding strength of brazing seam has been greatly improved. The wetting of ceramics by liquid metals and alloys is a major factor controlling the effectiveness of brazing which is used extensively in the joining of ceramics.24) According to the results of joint strength, it demonstrated that the wetting of Al liquid with ZrO2 has been gradually developed with the increase of brazing temperature.
In fact, the increase of wetting state of the system mainly comes from the reduction of interfacial tension between solid and liquid. Aluminium has a higher ability to join ceramics, with the property of relaxing the residual stress generated in the ceramics after the joining of ceramics.25) In essence, Al atoms in the molten solution and non-metal atoms on the ceramic surface form a common bond of electrons,26) the rising of system temperature provides more kinetic energy for the Al atom, so that it can overcome the energy barrier and forms a chemical bond. And the strength of Al/ZrO2 interface was significantly advanced after the formation of chemical bonds.
The change of Al/ZrO2 brazed joint strength and fracture mode also provide two important information about interface strengthening:
The experimental results showed that there contained two key problems: removing Al2O3 film from Al solder surface and enhancing the joint strength. In this paper, Al solder with sputtering Ni/Al double-layer film and heating brazing were used to resolve the problems mentioned above.
In this paper, a kind of Al solder with Ni/Al double-layer film were proposed to braze ZrO2 directly. On heating brazing, Al2O3 film cracked and was drawn into the brazing seam, which led to the direct contact between melting Al solder and ZrO2.
The method of heating brazing was used to improve the joint strength of Al/ZrO2. The joint strength of the brazing seam is 68.6 Mpa at 680°C, which the fracture occurred in the interface between Al solder and ZrO2. With the increase of brazing temperature, the joint strength gradually ascended to 121.2 Mpa at 840°C. When the temperature further raised to 920°C, the joint strength improved up to 122.6 Mpa. The fracture of the joint is gradually transferred from the interface to the brazing seam.
The authors would like to acknowledge the financial support provided by University Teachers Training Foundation (No. ZZSDJ15048) and Undergraduate Innovation and Entrepreneurship Project Funding provided by Shanghai Municipal Education Commission (No. 201911458004).
