ceramic joints

Materials Letters 12 ( 199 1) 84-88 North-Holland

An improvement of the oxidation resistance of Ag-Cu eutectic-5 at% Ti brazing alloy for metal/ceramic

joints

A.P. Xian a, Z.Y. Si ‘, L.J. Zhou b, J.N. Shen b and T.F. Lib a Institute ofMetal Research, Academia Sinica. 72 Wen-Hua Road, Shenyang IlOOlS. China b Corrosion Science Laboratory, Academia Sinica, 62 Wen-Cui Road, Shenyang 110015, China Received 22 March 199 1; in final form 30 May 199 1

The oxidation kinetics ofthe Ag-Cu eutectic-5 at% Ti brazing alloy for metal/ceramic joints was studied by the TGA technique and X-ray diffraction. The results revealed that the oxidation resistance of the alloy was very poor, but can be improved by adding 5 at% Al to form an adherent protective CuA1204film without sacrificing its excellent wettability, whereas small additions of Cr, Ni or RE were ineffective.

2. Experimental

1. Introduction

Earlier work has sought to improve the wettability of brazing alloy on ceramic by adding reactive metals, such as Ti and Zr, into conventional brazing tillers [ l-101. A promising brazing alloy, Ag-Cu eutectic-5 at% Ti ternary alloy, has been developed in recent years [ 7-101. In particular, the brazing tiller developed by Mizuhara et al. [ 7,8] has led to production of a commercially available active brazing alloy for ceramic/metal joints. Since the joints are mainly applied at high temperature, it is critical to have adequate oxidation resistance and corrosion resistance in the ceramic/ metal joints. For example, the interfacial temperature of the ceramic/metal joint in a heat engine would be up to 873 K [ 111. Earlier work by Kapoor and Eagar [ 9 ] has shown that the oxidation resistance of the basic Ag-Cu eutectic-5 at% Ti brazing alloy at this temperature was poor, which can be improved by adding small amounts of Al. The present study was undertaken to understand the protective mechanism of the additions. Al, Cr, Ni and RE were chosen as it was expected that they would promote formation of an adherent protective scale.

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The alloys were prepared by arc melting the constituent metals on a copper hearth under a protective argon atmosphere with purity of 99.999%. The remelt technique was used to ensure uniformity; the weight loss being less than 5% during arc melting. Cr, Ni, Al and RE( Y, Ce) were added in the form of a eutectic compound with a low melting point such as Al,Ti, NisOTiSO, CrS,,TiSo, etc. In particular, for the brazing alloys with RE, Cu-Y and Al-Ce alloy were used, in order to avoid rapid oxidation of RE during arc melting. The as-cast ingots were cold forged, vacuum annealed, and cold rolled to about 0.2 mm thickness, then cut into square sheets of 1 cm x 1 cm. The sheets were vacuum annealed, and polished with emery paper No. 1000. Finally, they were carefully cleaned in acetone before the oxidation test. The oxidation resistance were measured using a precision microbalance in conjunction with a thermogravimetric analyzer (TGA), sensitive to 0.01 mg. The specimens were exposed in flowing air at 873 K, this temperature representing the maximum operating temperature of Ag-Cu eutectic based brazing alloys. The specimens were heated at a rate of 10 K/ min to 873 K, then held for 48 h. The melting points for all tested alloys were measured by DTA in air with a 10 K/min heating rate. The mechanical properties

0167-577x/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

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of the alloy were evaluated by the HV hardness, the hardness values of the as-cast alloys were determined under a load of 4.9 N for 15 s. The contact angles of the candidate brazing alloys on a sialon ceramic [ 12 ] were measured by the sessile drop method in vacuum (less than 16 mPa), the details of the wetting test are reported elsewhere [ 13 1.

3. Results and discussion The oxidation kinetics of Ag-Cu eutectic and AgCu eutectic-5 at% Ti brazing alloy are shown in fig. 1. The weight gain-time relationship of both alloys conforms to classical parabolic behaviour, which implies that the diffusion of the oxidants through the oxide was rate-determined, as in the equation W==kt ,

(1)

where W is the weight gain, t the oxidation time and k is a parabolic rate constant (see table 1) which depends on temperature, materials, atmosphere, etc. A semi-quantitative analysis for the composition of the oxide film of both alloys by EDAX (energy dispersive analysis of X-rays) revealed that, apart from some Ag, the oxide film was comprised of almost entirely copper oxide (table 2), and there was virtually no Ti in the oxide of Ag-Cu eutectic-5 at% Ti alloy. X-ray diffraction patterns (fig. 2 ) revealed that there were two copper oxide phases , CuO and

“li!LA 2

fi

4

6

8

Ihour@

Fig. I. Oxidation kinetics of Ag-Cu eutectic and Ag-Cu tic-5 at% Ti brazing alloy in flowing air at 873 K.

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CuZO in the oxide film. The oxide film of Ag-Cu eutectic alloy possessed very poor adhesion to the underlying alloy, spalling severely during cooling after the oxidation test. In contrast, the oxide film of AgCu eutectic-5 at% Ti brazing alloy had a good adhesion to the underlying alloy; and no spalling was found after cooling. From fig. 1, it was confirmed that the oxidation resistance of the Ag-Cu eutectic5 at% Ti alloy was very poor. Additions Cr, Ni, Al and RE, were made, but in order to maintain the excellent wettability and others properties of the Ag-Cu eutectic-5 at% Ti alloy, the additional elements were limited to 5 at%. Table 1 summarizes the data of the melting points, hardness and contact angles on the sialon ceramic of the complex alloys. These experiments revealed that the melting points of the alloys vary between 1057 and 1076 K; the HV hardness of the alloys varies between 80.4 and 115.3 kg/mm’, slightly increasing with adding Cr or Al; wettability of the alloy with small additions of Al, Cr, Ni were excellent, but for the alloys with small RE additive it was relatively poor. The oxidation behaviour of the alloys with additions of Cr, Ni, Al and RE are shown in fig. 3. The oxidation kinetics for all alloys, except the Al-containing alloy, conformed to a parabolic law. The parabolic rate constants for various alloys are listed in table 1. The small differences in the constants, apart from the Al-containing alloy, confirmed that no continuous protective oxide film was formed by small additions of Cr, Ni or Y. The oxide film of the alloys consisted of two layers, the outer black layer taking about 90% of the oxide thickness was CuO, they spalled easily. An inner dark red color layer about 10% of the oxide thickness was CuZO, which adhered tightly to the underlying alloy. EDAX analysis of the oxide films are listed in table 2. Again it should be noteworthy that no Ti was found in the oxide films. The oxidation resistance of the Al-containing alloy without RE (curve 6) was greatly enhanced, as shown in fig. 3. The oxidation kinetics did not conform to a parabolic law. In fact, the growth of the oxide film stopped after 20 h exposure at 873 K. EDAX analysis revealed that the outer oxide layer was copper oxide with more Ag than in other alloys, and the inner oxide which contained Al. The oxide has an excellent adhesion to the metal substrate, and it was 85

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Table 1 Melting point T (with 10 K/min heating rate), Vicker hardness HV (under 4.9 N load), contact angels @(on sialon ceramic, at 1150 K for 5 min) and parabolic rate constant k of oxidation (oxidizing at 873 K in flowing air) of the Ag-Cu-Ti-X brazing alloys T

Alloy

(K) Ag-Cu Ag-Cu Ag-Cu Ag-Cu Ag-Cu Ag-Cu Ag-Cu Ag-Cu

eutectic eutectic-5 eutectic-5 eutectic-5 eutectic-5 eutectic-5 eutectic-5 eutectic-5

at% Ti at% Ti-5 at% Ti-5 at% Ti-2 at% Ti-5 at% Ti-1 at% Ti-5

at% Al at% Cr at% Ni at% Ni at% Y at% Al-Ce

1063 a’ 1057 1076 1071 1057 1057 1057 1075

HV (kg/mm’)

4 (deg)

80.4 113.7 115.3 81.2

10 15 12 15

92.1

48 50

10Sk (mg2 crne4 s-l) 4.38 6.29 3.56 7.30 3.60 4.39 3.18

‘) Data from Ag-Cu phase diagram. Table 2 EDAX analysis of the Ag-Cu eutectic-5 atl Ti-X alloys and their oxide scale, X is Cr, Ni, Al or RE (scanning analysis area of 0.5 mm by 0.5 mm, the mean value of two different positions in the alloy, at%) Alloy

Position

Ag

cu

Ag-Cu eutectic

metal oxide metal oxide

77.45 6.44 61.46 6.90 5.54 43.18 14.74 10.92 58.50 6.33 4.48 65.12 5.86 2.86 61.07 6.33 3.42 52.62 5.74 3.01 46.72 4.02 4.19

22.55 93.56 35.54 93.10 94.40 35.87 85.26 78.80 29.31 93.67 95.45 31.24 94.14 96.67 34.52 93.67 87.77 40.2 1 94.36 96.63 31.84 95.98 89.44

Ag-Cu eutectic-5 at% Ti

Ag-Cu eutectic-5 at% Ti-5 at% Al

Ag-Cu eutectic-5 atl Ti-5 at96 Cr

Ag-Cu eutectic-5 at% Ti-2 at% Ni

Ag-Cu eutectic-5 at% Ti-5 at% Ni

Ag-Cu eutectic-5 atoh Ti-I at% Y

Ag-Cu eutectic-5 at% Ti-5 at% Al-O.5 at% Ce

metal oxide metal oxide metal oxide metal oxide metal oxide metal oxide

outer inner outer inner outer inner outer inner outer inner outer inner outer inner

Ti

X

Ce

3.00 0.06 9.31

11.65 a’

0.07 7.43

10.21 4.76

2.52

0.08 1.12

0.12 1.32

0.35 3.11

5.50 5.58

3.32 1.60

0.07 6.09

0.29 15.35 a’

0.61

5.56

0.3 0.2

‘) The data tend higher for light elements measured by EDAX.

very difficult to spa11 it by deformation. The grey inner oxide layer was shown by X-ray diffraction to be a protective oxide, CuA1204. The enhancement of the oxidation resistance of an alloy by small additions depends on the selective oxidation of the addition to form a continuous, protective oxide film on the surface. The present study demonstrates that Al is more 86

effective to do this than Cr, Ni and RE. An additional test was made to measure the coefficient of linear expansion of the Ag-Cu-Ti-Al alloy; a detail of the test was reported earlier [ 141. The coefficient of linear expansion of the alloy is 17.1 x 10v6 K-' between 293 and 393 K, which is similar to that of the commercial brazing alloy Ag-

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In contrast with many observations [ 17, I8 1, the oxidation resistance of Al-containing alloy with trace RE is not improved. The oxidation kinetics of the alloy conformed to a parabolic law (shown in fig. 3), and there was a smaller amount of Al content in the inner oxide compared with that of the simpler AgCu-Ti-Al alloy. This implies that the protective CuA1204 was damaged by adding trace amounts of Ce. However, it is not possible to explain this effect at this stage.

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4. Conclusions

28 Fig. 2. X-ray diffraction patterns of the oxide film of (A) Ag-Cu eutectic, (B) Ag-Cu eutectic-5 at% Ti and (C) Ag-Cu eutectic5 at% Ti-5 at% Al brazing alloys in flowing air at 873 K for 48 h.

The Ag-Cu eutectic-5 at% Ti brazing alloy for ceramic/metal joints have inherently poor oxidation resistance. It is therefore necessary to improve this alloy by adding other elements. The current work has demonstrated that Al is a very effective element in enhancing the oxidation resistance of Ag-Cu-Ti based alloys by forming a layer of protective CuA1204 film.

Acknowledgement This research was supported by the National Science Foundation of China. The authors would also like to thank Professor J.Z. Zhao for valuable discussions.

References Fig. 3. Oxidation kinetics of the Ag-Cu-Ti based brazing allays in flowing air at 873 K. ( 1) Ag-Cu eutectic-5 at% Ti-2 at% Ni, (2) Ag-Cu eutectic-5 at% Ti-1 at% Y, (3) Ag-Cu eutectic-5 at% Ti-5 at% Ni, (4) Ag-Cu eutectic-5 at% Ti-5 at% Cr, (5) Ag-Cu eutectic-5 at% Ti-5 at% Al-Ce, (6) Ag-Cu eutectic-5 at% Ti-5at% Al.

Cu-In-Ti [ 1.51. Moreover, the joining strength of the brazing alloy for sialon/sialon joint was satisfactory, the bending strength of the joints was more than 300 MPa and the fracture was in the ceramic, independent of the joining interface; some details of the joining test are reported elsewhere [ 161.

[ 11M. Naka, K. Asami, I. Okamoto and Y. Arata, Trans. JWRI 12 (1983) 145.

[ 21 M. Naka, T. Tanaka and I. Okamoto, Trans. JWRI 14 (1985) 29.

[ 31 A.J. Moorhead, Weld. J. 62 ( 1983) 17. [4] A.J. Moorhead and H. Keating, Weld. J. 65 (1986) 17. [ 51M.G. Nicholas, T.M. Valentine and M.J. Waite, J. Mater. Sci. 15 (1980) 2197. [6] R.R. Kapoor and T.W. Eagar, Metall. Trans. 20 B (1989) 919. [ 71 H. Mizuhara and K. Mally, Weld. J. 64 ( 1985 ) 27, [8] H. Mizuhara, US Patent 4,591,535 (1986). [9] R.R.~~r~dT.W.~~, J.Am.Ceram.Soc. 72 (1989) 448. [IO] A.P. Xian and Z.Y. Si, J. Mater. Sci. 25 (1990) 4483.

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[ 111 R.R. Kapoor and T.W. Eagar, Presented at the 89th Annual Meeting of The American Ceramic Society, Pittsburgh, PA, April 28, 1987 (Forum, Paper No. 12-F2-87). [ 12 ] L.P. Huang, Z.K. Huang, Y.R. Qiu and X.R. Fu, Acta Inorg. Mater. Sinica 1 (1986) 123. [ 13 ] A.P. Xian and Z.Y. Si, J. Mater. Sci. Letters ( 199I), in press.

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[ 141 Z.G. Zheng, L.Y. Bai and L.X. Ba, Proceedings of the 1st Asian Thermophysical Properties Conference, April 2 I-24, 1986, Beijing, eds. B.X. Wang and N. Seki, p. 559. [ 151 H. Mizuhara and E. Huebel, Weld. J. 65 ( 1986) 43. [ 161 A.P. Xian, Thesis, Institute of Metal Research, Academia Sinica, Shenyang, China ( 199 1). [ 171 C.M. Cotell, J. Electrochem. Sot. 134 (1987) 1871. [ 181 W.J. Quadakkers, Oxid. Met. 32 (1989) 67.