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Joining of alumina using a V-Active filler metal
Scripta Materialia, Vol. 41, No. 10, pp. 1137–1146, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter
Pergamon
PII S1359-6462(99)00273-0
JOINING OF ALUMINA USING A V-ACTIVE FILLER METAL M.R. Rijnders and S.D. Peteves Institute for Advanced Materials, JRC, European Commission, 1755 ZG Petten, The Netherlands (Received March 11, 1999) (Accepted in revised form August 12, 1999)
Keywords: Bonding; Functional ceramics; Iron alloys; Fracture
Introduction Joining of ceramics is a viable alternative manufacturing route to these of processing and shaping of large ceramic components with often complex geometries. Further, joining can offer cost and reliability advantages. Similarly ceramic-to-metal joining is usually essential for the fabrication of components that have ceramic inserts in otherwise metallic structures. Since ceramics have some highly desirable properties, techniques for their joining have progressed greatly, despite difficulties due to some inherent ceramic characteristics. Numerous joints are presently in use for structural, electronic, functional as well as biomedical applications. However, one of the most persistent difficulties that still remains is that of fabricating easily and reliably strong and refractory joints for, notably, structural applications. Most effort has been directed on the brazing of ceramics with active filler metals based primarily on the Ag-Cu eutectic, which melts at 780°C, with Ti as the active ingredient (1–14). These filler metals readily form strong ceramic joints, but the service temperatures achievable by these brazes are too low, ⫾400°C, for many present day applications of ceramics. The obvious way to increase the refractoriness of the joints is by using braze alloys with higher melting temperatures. Recently, joining of Si3N4 ceramics via brazing with a V-active commercial filler metal, Nioro® ABATM (melting range 940 –960°C), has been investigated (15,16). This study revealed that joints with far superior high temperature strengths than those achieved by using Ag-Cu-Ti based filler metals could be fabricated when using the Nioro ABA braze. However, the use of V as an active element to induce wetting and reactions with alumina ceramics is not well established (17–19). V as constituent in filler metals based on Ga does not induce wetting of sapphire (17), but diffusion bonding of V to Al2O3 at 1600°C yielded strong joints (18). However, neither study provided information on the interfacial microstructures. In the present study the ability of Nioro ABA to form strong and refractory Al2O3 joints is investigated. Flexural strengths of joints were determined at room temperature and at elevated temperatures up to 900°C, and the effects of air annealing at high temperatures on the joint integrity was also studied. The joint microstructures and fracture surfaces were examined optically and with SEM/EDX/EPMA techniques. 1137
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TABLE 1 Some Properties of the Nioro® ABA TM (Wesgo, CA, USA) Filler Metal Composition
Melting Range
Mechanical Properties
Au-36.5Ni-4.76V-1Mo (at%)
940-960°C
Young’s Modulus: 112GPa Yield stress: 483MPa
Experimental Commercial 99.5% pure (AL995, Wesgo Ceramics, Germany) and 99.8% pure (Al998, SCT, France) Al2O3 were used in this study. For wetting and subsequent microstructural studies samples 15 ⫻ 15 mm2 in area and 10mm thick were prepared with the surfaces to be contacted by the braze diamondpolished down to 1m finish. For mechanical tests pairs of Al2O3 cubes (25 ⫻ 25 ⫻ 25 mm3) were joined with the filler metal in between. The faces of the ceramics to be bonded were either finely machined or diamond-polished to a 1 m finish. The machined and polished surfaces had an Ra value of 0.45m and 0.25 m, respectively. Some of the ceramic cubes were annealed at 1100°C in air for 1hr before brazing. Besides to itself, Al2O3 was also brazed to FeNi42 alloy (N42, Imphy, France; 42%Ni, 0.02%C, 0.15%Si, 0.8%Mn, Fe balance in wt%). For these joints two configurations were used: (I) the FeNi42 in a 2mm thick foil form was sandwiched between the Al2O3 workpieces and (II) as a 20 ⫻ 15 ⫻ 25 mm3 block was brazed directly to Al2O3, the 20 ⫻ 15 side being the bonding face. The filler metal, some properties of which are given in Table 1, was in the form of 50m thick foils. The braze foil was abraded slightly and then all the materials were degreased and cleaned with acetone in an ultrasonic bath. Then they were placed in an alumina jig and loaded into a vacuum furnace. Brazing was performed at 1000°C or 1050°C for times from 5min to 60 min under vacuum (p⬍10⫺5 mbar). Heating rate was 10°/min up to 910°C (just below the solidus), then 5°/min up to the brazing temperature. Cooling rate was 5°/min until 650°C, after which furnace power was shut off. Flexural test bars were machined from the bonded blocks to final dimensions of 50mm ⫻ 3mm ⫻ 3mm, using standard procedures. All mechanical tests were carried out under 4-point-bend loading conditions using a displacement rate of 0.1mm/min. Some bars were tested at room temperature, whereas others were first annealed in air for 100 hrs at 800°C or 900°C prior to room temperature
Figure 1. Cross-section of a solidified Nioro ABA sessile drop on Al2O3 (Al998), 30m/1000°C.
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Figure 2. Interface microstructure of a Nioro ABA brazed Al2O3 (AL995), 1h/1050°C.
testing. Finally some bars were tested at high temperatures in air; in these tests the heating rate was 15°C/min with the samples being kept at the test temperature for about 10 min before being loaded. At least 4 bars were fractured for each testing condition. It should be noted for these tests that more than one joint block was fabricated and randomly selected bars were used at each test condition. The fracture surfaces and the microstructures of cross-sectioned joints were examined using SEM, with the composition of the joint seam being analysed by EDX and EPMA. Results and Discussion Al2O3/Al2O3 Joints Preliminary investigations had shown that the filler metal wets the ceramic. Post-mortem contact angles of about 50° were measured on cross-sections of solidified sessile drops of the braze alloy on top of the Al2O3, Fig. 1. Although good interfacial contact (Figs. 1, 2, the gap in Fig. 1 was caused by microstructural preparation) was observed between the ceramic and the braze alloy, no reaction product
Figure 3. Weibull strength distribution of Nioro ABA brazed Al2O3 (AL995) at 1050°C for 30 min, as compared with these of as-received and annealed Al2O3.
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TABLE 2 Strength Test Data of Nioro-ABA Brazed Al2O3 (AL995) Joints Material
Average Strength (⫾std)
Weibull Modulus
As-received Al2O3 Annealed Al2O3 Nioro ABA brazed Al2O3, Batch 1 Nioro ABA brazed Al2O3, Batch 2
274.5 (⫾14.7)MPa 283 (⫾20.7)MPa 282.6 (⫾22.4)MPa 281 (⫾29.9)MPa
21.9 15.8 15.2 10.3
layers or even segregation of V at the interface could be found with the analytical techniques employed in this study. The room temperature strength results of the brazed Al2O3 joints are shown in Fig. 3, as compared with the flexural strength measurements of the as-received and annealed Al2O3 and summarized in Table 2. As can be seen brazing under optimal conditions and with a proper surface preparation of the workpieces can yield joints with similar strength distributions to the monolithic ceramic. For example the average strength and Weibull modulus for the brazed joints, 282.5 MPa and 15.2, respectively compares very well with the respective values of 283.6 MPa and 15.8 for the annealed Al2O3. 9 out of the 13 samples tested broke in the ceramic away from the joint seam, while the remainder fractured at the interface. SEM observations of the corresponding fracture surfaces of the ceramic and metal sides (i.e. those with the majority of the braze foil) of bars broken macroscopically at the interface revealed that the fracture path alternated between the ceramic and the braze foil, Fig. 4, but showed no signs of specific defects, e.g. non-wetted or dewetted areas. The high temperature strength results are shown in Fig. 5. There is little loss of strength up to 600°C, and then a gradual decrease down to about 100MPa at 900°C. It should be noted that Al2O3 joints brazed with filler metals from the Ag-Cu-Ti family completely loose their room temperature strengths at temperatures of about 500°C.(20) There were no obvious microstructural differences between the as-bonded joints and these that were tested at high temperatures. At temperatures higher than 600°C all the joints fractured at the interface. Similar effects of the test temperature on the joint strength were also found for the Nioro ABA brazed Si3N4 as shown in Fig. 6. In this Figure the yield stress of the braze
Figure 4. SEM micrographs of corresponding regions of the metal-side (left) and ceramic-side (right) of fracture surfaces of a brazed joint; fracture strength ⫽ 252 MPa. (bright areas belong to the filler metal); tensile sides of the fractured bars are facing each other.
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Figure 5. Average strengths versus test temperature in air for Nioro ABA brazed joints with error bars noting 2std from the mean value; 䡺:Batch 1, E:Batch 2. (different joints brazed under nominally identical conditions) The strength of the Al2O3 within the temperature range is also indicated.
alloy and of the Au-18Ni parent alloy of the braze is also given as a function of temperature.(19). The joints strength decreases with temperature as do the alloy yield strengths. The effect of oxidation on joint strength was investigated by annealing flexural bars in air for 100 hr. The average room temperature joint strength after annealing at 700, 800 and 900°C is shown in Fig. 7. It is seen that the joints had a remnant strength of 70 MPa even after an anneal at 900°C for 100h. The joints fractured at the interface, with the fractographs of those annealed at 900°C revealing the formation of Ni-oxide, Figs.8 –10. Fig. 10, in particular is showing the oxidation of the flexural bar along its periphery. Bonding of Al2O3 to FN42 was more difficult, especially when joining directly the large blocks (cubes of 25mm in side face) of materials. Alumina was used in the as-received/annealed condition in the case of large blocks of FN42. In this case less than 50% of the maximum possible number of bars survived the machining process of the bonded block. “Perimeter” cracks could often be observed in the as bonded blocks; these cracks are common in ceramic/metal joints and start from the joint corner edges in the ceramic near the interface. (21). The survival rate of test bars upon machining was much better when the FN42 was in the form of a 2mm thick insert between two Al2O3 blocks (12 out of 12 for the foil insert against 6 out of 12 for the bulk blocks). Here, alumina was used in the polished/ annealed condition. The strength data results for these samples are shown in Fig. 11. The results indicate that strong bonds can be formed and indirectly manifest the degrading effects of residual stresses on joint strength caused by mismatched thermal expansions and elasticities. When the FN42 is bonded to Al2O3 as an insert the maximum critical residual stress in the ceramic is estimated (22) to be
Figure 6. The HT strength of Nioro ABA brazed Si3N4 and Al2O3 joints. The yield stress of the Au-18Ni and the Nioro ABA filler metal is included herewith. (20).
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Figure 7. The RT strength of joints that had been annealed at high temperatures in air for 100hrs.
Figure 8. Fracture surface of a Nioro ABA brazed AL995 after 100h anneal at 900°C and tested at RT.
Figure 9. EDX spectra from regions in Figs 8 and 10 (marked by A) showing the formation of NiO.
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Figure 10. Fracture surface of a Nioro ABA brazed AL995 after 100h anneal at 900°C and tested at RT.
Figure 11. Weibull strength distribution of Al2O3/FN42 and Al2O3/FN42/Al2O3 joints as compared with that of Al2O3.
TABLE 3 Room Temperature Mechanical Properties Used for Residual Stress Calculation
Al998 Nioro ABA FN42
␣ (*10⫺6/°C)
E (/GPa)
7.5 14 9
320 112 147
0.2–0.25 0.38 0.3
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Figure 12. Fracture surface of an AL998/FN42/AL998 joint; arrow indicates tensile face and likely fracture origin.
Figure 13. Fracture surface of an AL998/FN42 joint; arrow indicates tensile face and likely fracture origin.
Figure 14. Interface microstructure of an Al998/FN42/Al998 joint; A marks the Au-Ni-Fe solid solution.
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Figure 15. EPMA elemental line profile across the Nioro ABA/FN42 interface (Fig. 14).
in the order of 300 MPa for perfect bonding and the assumed material properties (23) (See Table 3 for key mechanical properties). The AL998/FN42/AL998 samples broke primarily in the ceramic (Figure 12). The one sample that fractured at the joint interface was the weakest of the tested batch. The fracture strength distribution of the joints showed an average strength 70 MPa less than that of the monolithic alumina, but a similar Weibull modulus, Fig. 11. This suggests the same size and distribution of defects in the joints and bulk Al2O3, with the strength of the joints undermined by a fairly constant (tensile) residual stress. Obviously this stress is much less than that previously inferred of 300 MPa and can be accounted for by a presumed redistribution of stresses upon machining and the plasticity of the FN42 alloy. Fractographic examinations on a few tested AL998/FN42 samples showed that all but one of the samples fractured at the interface. (Figure 13) The exception fractured in the ceramic and was the strongest of the tested batch. Yet the latter sample was only as strong as the weakest of the AL998/FN42/AL998 joints, demonstrating the influence of higher residual stresses in the AL/FN42 joints. A Fe-Ni-Au solid solution band is formed at the interface between the FN42 and Nioro ABA alloy, Fig. 14, and Fe and Ni diffuse throughout the braze foil under the given brazing conditions, Fig. 15. This does not seem to affect the joinability of Al2O3 as corroborated by the microstructural studies and the joint strength results. However, assuming a composition for the solid solution reaction product as measured by EPMA a solidus of about 1000°C is estimated (24), that may pose some problems when brazing at about this temperature range.
Conclusions Alumina joints can be produced by brazing with the commercial Au-Ni-V-Mo filler metal, Nioro ABA. Optimum brazing conditions and surface preparation of the workpieces lead to Al2O3/Al2O3 joints with strength distributions similar to that of the monolithic ceramic. Of more significance is the fact that these joints retain measurable strengths up to 900°C and after 100h oxidation. A reaction layer between the ceramic and braze alloy could not be found with the analytical techniques employed. The filler metal was used also to join Al2O3 to Fe-42%Ni alloy. Fe and Ni dissolve in the filler metal extensively, with no apparent effect on the brazing process. Despite the relatively small thermal expansion mismatch between Al2O3 and Fe-42%Ni, residual stresses play a role in the quality of these joints.
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Acknowledgments Part of this work has been performed within the Commission research and development programme. Part of the work has been performed within the Brite-Euram Project BE-3213 funded by the CEC under the contract BRPR-414, and the authors thank all partners of this project for helpful discussions. M. Nicholas is gratefully acknowledged for revision of the original manuscript. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
H. Mizuhara and K. Mally, Weld. J. 64, 27 (1985). J. K. Boadi, T. Yano, and T. Iseki, J. Mater. Sci. 22, 2431 (1987). A. J. Moorhead, Adv. Ceram. Mater. 2, 159 (1987). H. Mizuhara, E. Huebel, and T. Oyama, Bull. Am. Ceram. Soc. 68, 1591 (1989). M. G. Nicholas, D. A. Mortimer, L. M. Jones, and R. M. Crispin, J. Mater. Sci. 25, 2679 (1990). J. J. Pak, M. L. Santella, and R. J. Fruehan, Metall. Trans. B. 21B, 349 (1990). A. H. Carim, Scripta Metall. Mater. 25, 51 (1991). H. Kobayashi, Y. Arai, H. Nakamura, and T. Sato, Mater. Sci. Eng. A. A143, 91 (1991). M. Paulasto and J. Kivilahti, in Structural Ceramics Joining II, ed. A. J. Moorhead, R. E. Loehman, and S. M. Johnson, p. 165, American Ceramic Society, Westerville OH (1992). 10. H. Hao, Y. Wang, Z. Jin, and X. Wang, J. Am. Ceram. Soc. 78, 2157 (1995). 11. M. Paulasto, F. J. J. van Loo, and J. K. Kivilahti, J. Alloys Compounds. 220, 136 (1995). 12. H. Hao, Y. Wang, Z. Jin, and X. Wang, J. Mater. Sci. 32, 5011 (1997). 13. M. Paulasto and J. Kivilahti, J. Mater. Res. 13, 343 (1998). 14. S. D. Peteves, G. Ceccone, M. Paulasto, V. Stamos, and P. Yvon, JOM. 48, 48 (1996). 15. M. Paulasto, G. Ceccone, S. D. Peteves, R. Voitovich, and N. Eustathopoulos, Ceram. Trans. 77, 91 (1997). 16. M. Paulasto, G. Ceccone, and S. D. Peteves, High Temperature Capillarity HTC-97, p. 290, Foundry Research Institute, Cracow, Poland (1998). 17. Ju. V. Naidich and Ju. N. Chuvashov, J. Mater. Sci. 18, 2071 (1983). 18. A. Hasegawa, Y. Kawamura, M. Satou, and K. Abe, J. Nucl. Mater. 233–237, 1279 (1996). 19. J. J. Stephens, P. T. Vianco, and F. M. Hosking, JOM. 48, 54 (1996). 20. S. D. Peteves and M. G. Nicholas, J. Am. Ceram. Soc. 79, 1553 (1996). 21. A. Bartlett and A. G. Evans, Acta Metall. Mater. 39, 1579 (1991). 22. M. Y. He and A. G. Evans, Acta Metall. Mater. 39, 1587 (1991). 23. A) For FN42 alloy: N42 alloy product sheet, Me´talImphy S.A., Paris, France, and B) For AL997, product sheet Al99.7 SCT, France. 24. A. Prince, G. V. Raynor, and D. S. Evans, ed., Phase Diagrams of Ternary Au-Alloys, Institute of Metals, London, UK (1990).
Pergamon
PII S1359-6462(99)00273-0
JOINING OF ALUMINA USING A V-ACTIVE FILLER METAL M.R. Rijnders and S.D. Peteves Institute for Advanced Materials, JRC, European Commission, 1755 ZG Petten, The Netherlands (Received March 11, 1999) (Accepted in revised form August 12, 1999)
Keywords: Bonding; Functional ceramics; Iron alloys; Fracture
Introduction Joining of ceramics is a viable alternative manufacturing route to these of processing and shaping of large ceramic components with often complex geometries. Further, joining can offer cost and reliability advantages. Similarly ceramic-to-metal joining is usually essential for the fabrication of components that have ceramic inserts in otherwise metallic structures. Since ceramics have some highly desirable properties, techniques for their joining have progressed greatly, despite difficulties due to some inherent ceramic characteristics. Numerous joints are presently in use for structural, electronic, functional as well as biomedical applications. However, one of the most persistent difficulties that still remains is that of fabricating easily and reliably strong and refractory joints for, notably, structural applications. Most effort has been directed on the brazing of ceramics with active filler metals based primarily on the Ag-Cu eutectic, which melts at 780°C, with Ti as the active ingredient (1–14). These filler metals readily form strong ceramic joints, but the service temperatures achievable by these brazes are too low, ⫾400°C, for many present day applications of ceramics. The obvious way to increase the refractoriness of the joints is by using braze alloys with higher melting temperatures. Recently, joining of Si3N4 ceramics via brazing with a V-active commercial filler metal, Nioro® ABATM (melting range 940 –960°C), has been investigated (15,16). This study revealed that joints with far superior high temperature strengths than those achieved by using Ag-Cu-Ti based filler metals could be fabricated when using the Nioro ABA braze. However, the use of V as an active element to induce wetting and reactions with alumina ceramics is not well established (17–19). V as constituent in filler metals based on Ga does not induce wetting of sapphire (17), but diffusion bonding of V to Al2O3 at 1600°C yielded strong joints (18). However, neither study provided information on the interfacial microstructures. In the present study the ability of Nioro ABA to form strong and refractory Al2O3 joints is investigated. Flexural strengths of joints were determined at room temperature and at elevated temperatures up to 900°C, and the effects of air annealing at high temperatures on the joint integrity was also studied. The joint microstructures and fracture surfaces were examined optically and with SEM/EDX/EPMA techniques. 1137
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TABLE 1 Some Properties of the Nioro® ABA TM (Wesgo, CA, USA) Filler Metal Composition
Melting Range
Mechanical Properties
Au-36.5Ni-4.76V-1Mo (at%)
940-960°C
Young’s Modulus: 112GPa Yield stress: 483MPa
Experimental Commercial 99.5% pure (AL995, Wesgo Ceramics, Germany) and 99.8% pure (Al998, SCT, France) Al2O3 were used in this study. For wetting and subsequent microstructural studies samples 15 ⫻ 15 mm2 in area and 10mm thick were prepared with the surfaces to be contacted by the braze diamondpolished down to 1m finish. For mechanical tests pairs of Al2O3 cubes (25 ⫻ 25 ⫻ 25 mm3) were joined with the filler metal in between. The faces of the ceramics to be bonded were either finely machined or diamond-polished to a 1 m finish. The machined and polished surfaces had an Ra value of 0.45m and 0.25 m, respectively. Some of the ceramic cubes were annealed at 1100°C in air for 1hr before brazing. Besides to itself, Al2O3 was also brazed to FeNi42 alloy (N42, Imphy, France; 42%Ni, 0.02%C, 0.15%Si, 0.8%Mn, Fe balance in wt%). For these joints two configurations were used: (I) the FeNi42 in a 2mm thick foil form was sandwiched between the Al2O3 workpieces and (II) as a 20 ⫻ 15 ⫻ 25 mm3 block was brazed directly to Al2O3, the 20 ⫻ 15 side being the bonding face. The filler metal, some properties of which are given in Table 1, was in the form of 50m thick foils. The braze foil was abraded slightly and then all the materials were degreased and cleaned with acetone in an ultrasonic bath. Then they were placed in an alumina jig and loaded into a vacuum furnace. Brazing was performed at 1000°C or 1050°C for times from 5min to 60 min under vacuum (p⬍10⫺5 mbar). Heating rate was 10°/min up to 910°C (just below the solidus), then 5°/min up to the brazing temperature. Cooling rate was 5°/min until 650°C, after which furnace power was shut off. Flexural test bars were machined from the bonded blocks to final dimensions of 50mm ⫻ 3mm ⫻ 3mm, using standard procedures. All mechanical tests were carried out under 4-point-bend loading conditions using a displacement rate of 0.1mm/min. Some bars were tested at room temperature, whereas others were first annealed in air for 100 hrs at 800°C or 900°C prior to room temperature
Figure 1. Cross-section of a solidified Nioro ABA sessile drop on Al2O3 (Al998), 30m/1000°C.
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Figure 2. Interface microstructure of a Nioro ABA brazed Al2O3 (AL995), 1h/1050°C.
testing. Finally some bars were tested at high temperatures in air; in these tests the heating rate was 15°C/min with the samples being kept at the test temperature for about 10 min before being loaded. At least 4 bars were fractured for each testing condition. It should be noted for these tests that more than one joint block was fabricated and randomly selected bars were used at each test condition. The fracture surfaces and the microstructures of cross-sectioned joints were examined using SEM, with the composition of the joint seam being analysed by EDX and EPMA. Results and Discussion Al2O3/Al2O3 Joints Preliminary investigations had shown that the filler metal wets the ceramic. Post-mortem contact angles of about 50° were measured on cross-sections of solidified sessile drops of the braze alloy on top of the Al2O3, Fig. 1. Although good interfacial contact (Figs. 1, 2, the gap in Fig. 1 was caused by microstructural preparation) was observed between the ceramic and the braze alloy, no reaction product
Figure 3. Weibull strength distribution of Nioro ABA brazed Al2O3 (AL995) at 1050°C for 30 min, as compared with these of as-received and annealed Al2O3.
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TABLE 2 Strength Test Data of Nioro-ABA Brazed Al2O3 (AL995) Joints Material
Average Strength (⫾std)
Weibull Modulus
As-received Al2O3 Annealed Al2O3 Nioro ABA brazed Al2O3, Batch 1 Nioro ABA brazed Al2O3, Batch 2
274.5 (⫾14.7)MPa 283 (⫾20.7)MPa 282.6 (⫾22.4)MPa 281 (⫾29.9)MPa
21.9 15.8 15.2 10.3
layers or even segregation of V at the interface could be found with the analytical techniques employed in this study. The room temperature strength results of the brazed Al2O3 joints are shown in Fig. 3, as compared with the flexural strength measurements of the as-received and annealed Al2O3 and summarized in Table 2. As can be seen brazing under optimal conditions and with a proper surface preparation of the workpieces can yield joints with similar strength distributions to the monolithic ceramic. For example the average strength and Weibull modulus for the brazed joints, 282.5 MPa and 15.2, respectively compares very well with the respective values of 283.6 MPa and 15.8 for the annealed Al2O3. 9 out of the 13 samples tested broke in the ceramic away from the joint seam, while the remainder fractured at the interface. SEM observations of the corresponding fracture surfaces of the ceramic and metal sides (i.e. those with the majority of the braze foil) of bars broken macroscopically at the interface revealed that the fracture path alternated between the ceramic and the braze foil, Fig. 4, but showed no signs of specific defects, e.g. non-wetted or dewetted areas. The high temperature strength results are shown in Fig. 5. There is little loss of strength up to 600°C, and then a gradual decrease down to about 100MPa at 900°C. It should be noted that Al2O3 joints brazed with filler metals from the Ag-Cu-Ti family completely loose their room temperature strengths at temperatures of about 500°C.(20) There were no obvious microstructural differences between the as-bonded joints and these that were tested at high temperatures. At temperatures higher than 600°C all the joints fractured at the interface. Similar effects of the test temperature on the joint strength were also found for the Nioro ABA brazed Si3N4 as shown in Fig. 6. In this Figure the yield stress of the braze
Figure 4. SEM micrographs of corresponding regions of the metal-side (left) and ceramic-side (right) of fracture surfaces of a brazed joint; fracture strength ⫽ 252 MPa. (bright areas belong to the filler metal); tensile sides of the fractured bars are facing each other.
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Figure 5. Average strengths versus test temperature in air for Nioro ABA brazed joints with error bars noting 2std from the mean value; 䡺:Batch 1, E:Batch 2. (different joints brazed under nominally identical conditions) The strength of the Al2O3 within the temperature range is also indicated.
alloy and of the Au-18Ni parent alloy of the braze is also given as a function of temperature.(19). The joints strength decreases with temperature as do the alloy yield strengths. The effect of oxidation on joint strength was investigated by annealing flexural bars in air for 100 hr. The average room temperature joint strength after annealing at 700, 800 and 900°C is shown in Fig. 7. It is seen that the joints had a remnant strength of 70 MPa even after an anneal at 900°C for 100h. The joints fractured at the interface, with the fractographs of those annealed at 900°C revealing the formation of Ni-oxide, Figs.8 –10. Fig. 10, in particular is showing the oxidation of the flexural bar along its periphery. Bonding of Al2O3 to FN42 was more difficult, especially when joining directly the large blocks (cubes of 25mm in side face) of materials. Alumina was used in the as-received/annealed condition in the case of large blocks of FN42. In this case less than 50% of the maximum possible number of bars survived the machining process of the bonded block. “Perimeter” cracks could often be observed in the as bonded blocks; these cracks are common in ceramic/metal joints and start from the joint corner edges in the ceramic near the interface. (21). The survival rate of test bars upon machining was much better when the FN42 was in the form of a 2mm thick insert between two Al2O3 blocks (12 out of 12 for the foil insert against 6 out of 12 for the bulk blocks). Here, alumina was used in the polished/ annealed condition. The strength data results for these samples are shown in Fig. 11. The results indicate that strong bonds can be formed and indirectly manifest the degrading effects of residual stresses on joint strength caused by mismatched thermal expansions and elasticities. When the FN42 is bonded to Al2O3 as an insert the maximum critical residual stress in the ceramic is estimated (22) to be
Figure 6. The HT strength of Nioro ABA brazed Si3N4 and Al2O3 joints. The yield stress of the Au-18Ni and the Nioro ABA filler metal is included herewith. (20).
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Figure 7. The RT strength of joints that had been annealed at high temperatures in air for 100hrs.
Figure 8. Fracture surface of a Nioro ABA brazed AL995 after 100h anneal at 900°C and tested at RT.
Figure 9. EDX spectra from regions in Figs 8 and 10 (marked by A) showing the formation of NiO.
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Figure 10. Fracture surface of a Nioro ABA brazed AL995 after 100h anneal at 900°C and tested at RT.
Figure 11. Weibull strength distribution of Al2O3/FN42 and Al2O3/FN42/Al2O3 joints as compared with that of Al2O3.
TABLE 3 Room Temperature Mechanical Properties Used for Residual Stress Calculation
Al998 Nioro ABA FN42
␣ (*10⫺6/°C)
E (/GPa)
7.5 14 9
320 112 147
0.2–0.25 0.38 0.3
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Figure 12. Fracture surface of an AL998/FN42/AL998 joint; arrow indicates tensile face and likely fracture origin.
Figure 13. Fracture surface of an AL998/FN42 joint; arrow indicates tensile face and likely fracture origin.
Figure 14. Interface microstructure of an Al998/FN42/Al998 joint; A marks the Au-Ni-Fe solid solution.
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Figure 15. EPMA elemental line profile across the Nioro ABA/FN42 interface (Fig. 14).
in the order of 300 MPa for perfect bonding and the assumed material properties (23) (See Table 3 for key mechanical properties). The AL998/FN42/AL998 samples broke primarily in the ceramic (Figure 12). The one sample that fractured at the joint interface was the weakest of the tested batch. The fracture strength distribution of the joints showed an average strength 70 MPa less than that of the monolithic alumina, but a similar Weibull modulus, Fig. 11. This suggests the same size and distribution of defects in the joints and bulk Al2O3, with the strength of the joints undermined by a fairly constant (tensile) residual stress. Obviously this stress is much less than that previously inferred of 300 MPa and can be accounted for by a presumed redistribution of stresses upon machining and the plasticity of the FN42 alloy. Fractographic examinations on a few tested AL998/FN42 samples showed that all but one of the samples fractured at the interface. (Figure 13) The exception fractured in the ceramic and was the strongest of the tested batch. Yet the latter sample was only as strong as the weakest of the AL998/FN42/AL998 joints, demonstrating the influence of higher residual stresses in the AL/FN42 joints. A Fe-Ni-Au solid solution band is formed at the interface between the FN42 and Nioro ABA alloy, Fig. 14, and Fe and Ni diffuse throughout the braze foil under the given brazing conditions, Fig. 15. This does not seem to affect the joinability of Al2O3 as corroborated by the microstructural studies and the joint strength results. However, assuming a composition for the solid solution reaction product as measured by EPMA a solidus of about 1000°C is estimated (24), that may pose some problems when brazing at about this temperature range.
Conclusions Alumina joints can be produced by brazing with the commercial Au-Ni-V-Mo filler metal, Nioro ABA. Optimum brazing conditions and surface preparation of the workpieces lead to Al2O3/Al2O3 joints with strength distributions similar to that of the monolithic ceramic. Of more significance is the fact that these joints retain measurable strengths up to 900°C and after 100h oxidation. A reaction layer between the ceramic and braze alloy could not be found with the analytical techniques employed. The filler metal was used also to join Al2O3 to Fe-42%Ni alloy. Fe and Ni dissolve in the filler metal extensively, with no apparent effect on the brazing process. Despite the relatively small thermal expansion mismatch between Al2O3 and Fe-42%Ni, residual stresses play a role in the quality of these joints.
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Acknowledgments Part of this work has been performed within the Commission research and development programme. Part of the work has been performed within the Brite-Euram Project BE-3213 funded by the CEC under the contract BRPR-414, and the authors thank all partners of this project for helpful discussions. M. Nicholas is gratefully acknowledged for revision of the original manuscript. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
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