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SiC joint brazed with CuTi alloys
Engineering Frachue Mechanics Vol. 40, No. 415, pp.
829-836, 1991
Printed in Great Britain.
0013-7944/91 $3.00+ 0.00 Pergamon Bress pk.
INTERFACE MICROSTRUCTURE AND STRENGTH SiC/SiC JOINT BRAZED WITH Cu-Ti ALLOYS TOSHITSUGU
OF
NISHINO and SHIGEO URAI
Toyama National College of Technology, 13 Hongomachi, Toyama 939, Japan MASAAKI NAKA Welding Research Institute, Osaka University, 11-l Mihogaoka, Ibaraki, Osaka 567, Japan Abstract-The joining of Sic to Sic was performed using amorphous Cu-Ti tiller metals with the Ti content ranging from 34 to 57 at%. The strength of joint was measured by fracture shear loading, and the joining mechanism was clarified by observing the microstructure and analyzing reaction phases in the joining layer. The fracture shear testing of Sic joints was carried out at elevated temperatures up to 973 K. The joining strength of Sic using C%Ti, filler is higher than that of Sic using other Cu-Ti tillers at any brazing temperature at constant brazing times of 1.8 and 3.6 ks. The joining strength of the Sic joint decreases with increasing Ti content of the tiller at constant brazing conditions. The Ti in Cu-Ti alloys reacts with Sic, and forms Tic, T&Sic, carbides and TiSi, silicides in the joining layer. For the Sic joint brazed with Ti-rich Iillers, the excess amounts of reaction phases in the joining layer degrade the strength of the joints.
1. INTRODUCTION THE USEOF ceramics in structural components has received increasing attention in recent years. In particular, silicon-based ceramics such as S&N, and Sic possess excellent mechanical properties. However, the inherent brittleness that arises from the covalent or ionic bonding in the ceramics requires the joining of ceramics[ 11.Since copper does not wet ceramics such as S&N,, and SiC[2,3], brazing filler metals such as Ag-Cu-Ti containing active elements of Ti are used for joining the ceramics to metals[4]. This work tries to improve the wettability of copper against Sic by addition of Ti, and further, to join Sic to Sic using the amorphous Cu-Ti filler metal. Further, the joining mechanism is clarified by observing and analyzing the microstructures of Sic joints. 2. EXPERIMENTAL PROCEDURE The ceramics used were pressureless sintered Sic with a few percent of alumina. The amorphous Cu-Ti alloy fillers Cuti TiY , Cu,Ti, and Cu,,Ti,, of width 10 mm and thickness 45 pm (Table 1) were produced by a melt spinning method, where the numbers attached are denoted as atomic percent of elements. Sic 15 mm in diameter and 3 mm in thickness was polished mechanically with diamond paste 30 pm in diameter, and then a lap joint was made with the amorphous filler metal in a vacuum. The brazing of the SiC/SiC joint was conducted at the desired temperature in 1.33 MPa or below with a heating and cooling rate of 0.33 K/s. The joining strength of the lap joint was evaluated by fracture shear loading using a cross-head speed of 1.67 x 10e2 mm/s. The microstructure and element distribution of specimens joined were determined by means of an EPM analyzer. The phases in the microstructure were also determined by X-ray diffractometry with Cu Ka. Table 1. Chemical composition of the reaction phases in the joint with Cu,Ti, alloy filler
829
830
T. NISHINO et al. SiC/Cu,,Ti,/SiC
SiC/Cu,Ti,/SiC
1373 K
I.8 ks
e
0
Bmring
temperature
I
2
3
4
5
6
7
Brazing time (ks)
(K)
Fig. 1. Brazing temperature dependence of strength of Sic/Sic joints brazed for 1.8 ks using Cu,Ti, filler metal.
3. RESULTS
Fig. 2. Brazing time dependence of strength of Sic/Sic joints brazed using C&,Ti% filler metal.
AND DISCUSSION
3.1. Strength of Sic/Sic joints Since the addition of titanium considerably improves the wettability of molten copper on SiC[S, 61, copper alloys with high Ti content could be applicable to filler metals for joining Sic. The joining of Sic to Sic was done using amorphous Cu-Ti alloy fillers with Ti contents of 34-57 at%. Figure 1 shows the brazing temperature dependence of SIC joints brazed during a brazing time of 1.8 ks using amorphous 50 at% Ti fillers. The joints exhibit maximum strength at 190 MPa at the brazing temperature of 1323 K. The change in strength of the joints using Cu-50 at% Ti filler metal with brazing time at 1373 K is shown in Fig. 2. The joint shows a maximum strength at 3.6 ks, and definitely decreases the strength with further increasing brazing time, and in particular, drastically lowers the value at brazing times of 4 ks or longer. Figures 1 and 2 imply that the optimum amounts of reaction between the filler metal and Sic ceramics provide the highest strength of Sic joint. In other words, the excess reaction of the filler metal with Sic degrades the strength of the joint. These effects are discussed later. Figure 3 shows the change in strength of Sic/Sic joints with Ti content in Cu-Ti filler metals at brazing conditions of 1373 K and 1323 K for 1.8 ks. The strength of Sic joint using 34 at% Ti
1373K,I.Bks
SC/Cu-Ti/SiC
I
0
0 :
f’ we
(2-9(1323K,l.B w
0
30
ksl \
(1373K ,I.8 ks) I
I
I
40
50
60
Titanium
content (at%)
Fig. 3. Titanium content dependence of strength of Sic/Sic joints using Cu-Ti tiller at 1323 K for 1.8 ks and 1373 K for 1.8 ks.
I
I
a %30 3x
I
I
I
400 SJO a0 Testing
temperature
I
I
700
600
I
)
600
looo
(K)
Fig. 4. Testing temperature dependence of joining strength of Sic/Sic joint brazed at 1373 K for 1.8 ks using Cu,Ti, filler.
Interface microstructure and strength of Sic/Sic joint
; .i
i-,”
(5)
i -U;-i*--,---.,“,“,..
______;.. .I.“i,‘&__
JR, [ f,.:,.,: I.”
lhzing:1373 K ~1.8ks Testing : 373K
..“i-:‘,
-
Fig. 5. Fractured surface of Sic/Sic joint tested at 373 K.
Fig. 6. Fractured surface of Sic/Sic joint tested at 973 K.
831
832
T. NISHINO et 01.
Fig. 7. Microstructure of Sic/Sic joint with Cu%Ti, alloy filler.
Fig. 8. Microst~cture and EDX analyses of SiC/SiC joint with Cu,Ti,
alloy fiiier.
Interface microstructure and strength of Sic/Sic joint
IO,,,
(IO) Fig. 10. Microstructure
Fig. 11. Microstructure
of Sic/Sic joint with Cu,,Ti,
alloy filler.
and EDX analyses of Sic/Sic joint with Cu., Ti,, alloy filler.
833
835
Interface microstmctu~ and strength of Sic/Sic joint
28 (degree) Brozing:1373K x I.8 ks Testing : 373 K
Fig. 9. X-ray diffraction pattern of fracture surface of SiC/SiC joint with Cu,Ti,
filler.
alloy filler was higher than that of the Sic joint using other fillers at any brazing temperature. The strength of Sic joints brazed at 1323 K for 1.8 ks gives a higher value than that of joints brazed at 1373 K for 1.8 ks. An increase in Ti content in the filler reduces the strength of joints. The excess reaction of alloys with SiC with fillers with higher Ti content, or at higher brazing temperatures, is attributable to the reduction of the strength. The elevated temperature strength of the SIC joint with 50 at% Ti filler metal brazed at 1373 K for 1.8 ks is shown in Fig. 4. The strength of the joint increases with temperature up to 573 K, and then decreases at higher testing temperatures. The decrease in strength of the joint arises from the decrease in the strength of filler from observing the fracture surface of the joints. The fracture surfaces of joints are shown in Figs 5 and 6. Figure 5 is the fractured surface of a joint tested at 373 K. The crack passes through the joining layer and then penetrates Sic. According to the EDX spectrum, an initial fracture with no slip deformation occurs at the joining layer containing the filler metal. Figure 6 shows the fractured surface of a joint tested at the higher temperature of 973 K. The decrease in strength of the Cu-Si matrix identified by EDX analysis results in the decrease of strength of the joint interface. 3.2. Microstructures of Sic/Sic joints Figure 7 shows the microstructure of the Sic/Sic joint interface with CkTi, filler brazed at 1373 K for 1.8 ks. The layer phases at the interface between Sic and filler metal, and granular phases in the joining layer are observed. The microstructure and X-ray image analyses of the Sic/Sic joint with Cu,Ti, filler brazed at 1373 K for 1.8 ks are shown in Fig. 8. The layer phases grow from the interface containing Ti, C and Si, as indicated in the X-ray image analyses of Fig. 8.
-
SiC/Cu-Ti/SiC joining layer
SC
40 ‘=
t
Lo f, ;_
I
IO 0 Fig. 12. Schematic structure diagram of Sic/Sic joints with Cu,Ti, and clLITi,, alloy fillers.
’
40
’ eo
’
80
Titanium content (at%)
Fig. 13. Change in thickness of the carbide and the joining layer with Ti content at the Sic/Sic joint interface.
836
T. NISHINO et al.
The quantitative spot analyses shown in Table 1 indicate that the layer phase is TijSiCz carbide, as reported in ref. [6]. Further, the titanium and silicon enriched phases observed in the central part of the joining layer are identified as TiSi, in Table 1. The X-ray diffraction analyses of the fracture surface of the joint with Cu,Ti, filler reveal that the layer phases at the interface are composed of Tic and Tij Sic, carbides, as shown in Fig. 9. The existence of TiC was also identified in the Cu-Ti/SiC sessile drop[7]. The two carbides of the Tic and Ti,SiC* phases are more clearly seen in the microstructure and the X-ray image analyses of the Sic/Sic joint with Cu,,Ti,, filler metal in Figs 10 and 11. The carbon image analysis shows the existence of a carbon-rich phase of Tic. Based on microstructural observation and chemical analyses of the joining layer in Sic/Sic joints, Fig. 12 illustrates the schematic structures of Sic/Sic joints with Cu,Ti, and Cud3Ti5, fillers. The titanium in Cu-Ti alloys reacts severely with SIC to form the Ti,SiC,(T,) phase at the interface in the following reactions: Sic + Ti (in alloys)-rTiC
+ Si
2SiC + 3Si (in alloys)+Ti,SiC, Ti (in alloys) + 2Si+TiSi,,
(1) (2) (3)
Also, the T, phase, with a regular shape, and TiSi, silicide grow in the central part of the joints. Figure 13 shows the changes in thickness of the carbide and the joining layer with Ti content at the Sic/Sic joint interface. The joining layer increases with increasing Ti content. The amounts of reaction for titanium with Sic increase with the increase in Tic and Ti$iC2(T,) phases and TiSiz silicide. The embrittlement of the joining layer arises from the excess reaction in the Ti-rich fillers, degrading the strength of the Sic joint. 4. CONCLUSIONS The joining of Sic to Sic was done using Cu-Ti amorphous fillers containing 34-57 at% Ti in a vacuum. The joining mechanism was evaluated from the measurements of the strength of joints and the observation of the joining layer. The results obtained are summarized as follows. 1. The joint with 34 at% Ti filler shows the highest strength, and the joining strength decreases with increasing Ti content in the filler at a constant brazing temperature and time. 2. The excess amounts of reaction of Cu-Ti filler with Sic at higher Ti contents or higher temperatures result in the degradation of strength of the Sic joint. 3. The Ti in Cu-Ti alloys reacts with SIC to form Tic, Ti,SiC,(T,) carbides at the alloy/Sic interface, and also TiSi, silicides in the central part of the joining layer. REFERENCES [I] M. Naka and I. Okamoto, Welding Technique 37, 82 (1989). 121 M. Naka. M. Kubo and I. Okamoto. J. Mater. Sci. Letr. 6. 965 (19871. i3j M. Shindo, M. Naka and I. Okamotb, J. Mater. Sci. Left. k, 663‘(1989). [4] T. Yano, H. Suematsu and T. Iseki, J. Mater. Sci. 23, 3362 (1988). [S] T. Nishino, S. Urai, I. Okamoto and M. Naka, Pm. 5th Inr. Symp. Japan Weld. Sm., p. 721 (1990). [6] S. Morozumi, M. Endo, M. Kikuchi and K. Hamajima, J. Mater. Sci. 20, 3976 (1985). [A M. Naka, 1. Okamoto, T. Nishino and S. Urai, Trans. JWRI 18,27 (1989).
829-836, 1991
Printed in Great Britain.
0013-7944/91 $3.00+ 0.00 Pergamon Bress pk.
INTERFACE MICROSTRUCTURE AND STRENGTH SiC/SiC JOINT BRAZED WITH Cu-Ti ALLOYS TOSHITSUGU
OF
NISHINO and SHIGEO URAI
Toyama National College of Technology, 13 Hongomachi, Toyama 939, Japan MASAAKI NAKA Welding Research Institute, Osaka University, 11-l Mihogaoka, Ibaraki, Osaka 567, Japan Abstract-The joining of Sic to Sic was performed using amorphous Cu-Ti tiller metals with the Ti content ranging from 34 to 57 at%. The strength of joint was measured by fracture shear loading, and the joining mechanism was clarified by observing the microstructure and analyzing reaction phases in the joining layer. The fracture shear testing of Sic joints was carried out at elevated temperatures up to 973 K. The joining strength of Sic using C%Ti, filler is higher than that of Sic using other Cu-Ti tillers at any brazing temperature at constant brazing times of 1.8 and 3.6 ks. The joining strength of the Sic joint decreases with increasing Ti content of the tiller at constant brazing conditions. The Ti in Cu-Ti alloys reacts with Sic, and forms Tic, T&Sic, carbides and TiSi, silicides in the joining layer. For the Sic joint brazed with Ti-rich Iillers, the excess amounts of reaction phases in the joining layer degrade the strength of the joints.
1. INTRODUCTION THE USEOF ceramics in structural components has received increasing attention in recent years. In particular, silicon-based ceramics such as S&N, and Sic possess excellent mechanical properties. However, the inherent brittleness that arises from the covalent or ionic bonding in the ceramics requires the joining of ceramics[ 11.Since copper does not wet ceramics such as S&N,, and SiC[2,3], brazing filler metals such as Ag-Cu-Ti containing active elements of Ti are used for joining the ceramics to metals[4]. This work tries to improve the wettability of copper against Sic by addition of Ti, and further, to join Sic to Sic using the amorphous Cu-Ti filler metal. Further, the joining mechanism is clarified by observing and analyzing the microstructures of Sic joints. 2. EXPERIMENTAL PROCEDURE The ceramics used were pressureless sintered Sic with a few percent of alumina. The amorphous Cu-Ti alloy fillers Cuti TiY , Cu,Ti, and Cu,,Ti,, of width 10 mm and thickness 45 pm (Table 1) were produced by a melt spinning method, where the numbers attached are denoted as atomic percent of elements. Sic 15 mm in diameter and 3 mm in thickness was polished mechanically with diamond paste 30 pm in diameter, and then a lap joint was made with the amorphous filler metal in a vacuum. The brazing of the SiC/SiC joint was conducted at the desired temperature in 1.33 MPa or below with a heating and cooling rate of 0.33 K/s. The joining strength of the lap joint was evaluated by fracture shear loading using a cross-head speed of 1.67 x 10e2 mm/s. The microstructure and element distribution of specimens joined were determined by means of an EPM analyzer. The phases in the microstructure were also determined by X-ray diffractometry with Cu Ka. Table 1. Chemical composition of the reaction phases in the joint with Cu,Ti, alloy filler
829
830
T. NISHINO et al. SiC/Cu,,Ti,/SiC
SiC/Cu,Ti,/SiC
1373 K
I.8 ks
e
0
Bmring
temperature
I
2
3
4
5
6
7
Brazing time (ks)
(K)
Fig. 1. Brazing temperature dependence of strength of Sic/Sic joints brazed for 1.8 ks using Cu,Ti, filler metal.
3. RESULTS
Fig. 2. Brazing time dependence of strength of Sic/Sic joints brazed using C&,Ti% filler metal.
AND DISCUSSION
3.1. Strength of Sic/Sic joints Since the addition of titanium considerably improves the wettability of molten copper on SiC[S, 61, copper alloys with high Ti content could be applicable to filler metals for joining Sic. The joining of Sic to Sic was done using amorphous Cu-Ti alloy fillers with Ti contents of 34-57 at%. Figure 1 shows the brazing temperature dependence of SIC joints brazed during a brazing time of 1.8 ks using amorphous 50 at% Ti fillers. The joints exhibit maximum strength at 190 MPa at the brazing temperature of 1323 K. The change in strength of the joints using Cu-50 at% Ti filler metal with brazing time at 1373 K is shown in Fig. 2. The joint shows a maximum strength at 3.6 ks, and definitely decreases the strength with further increasing brazing time, and in particular, drastically lowers the value at brazing times of 4 ks or longer. Figures 1 and 2 imply that the optimum amounts of reaction between the filler metal and Sic ceramics provide the highest strength of Sic joint. In other words, the excess reaction of the filler metal with Sic degrades the strength of the joint. These effects are discussed later. Figure 3 shows the change in strength of Sic/Sic joints with Ti content in Cu-Ti filler metals at brazing conditions of 1373 K and 1323 K for 1.8 ks. The strength of Sic joint using 34 at% Ti
1373K,I.Bks
SC/Cu-Ti/SiC
I
0
0 :
f’ we
(2-9(1323K,l.B w
0
30
ksl \
(1373K ,I.8 ks) I
I
I
40
50
60
Titanium
content (at%)
Fig. 3. Titanium content dependence of strength of Sic/Sic joints using Cu-Ti tiller at 1323 K for 1.8 ks and 1373 K for 1.8 ks.
I
I
a %30 3x
I
I
I
400 SJO a0 Testing
temperature
I
I
700
600
I
)
600
looo
(K)
Fig. 4. Testing temperature dependence of joining strength of Sic/Sic joint brazed at 1373 K for 1.8 ks using Cu,Ti, filler.
Interface microstructure and strength of Sic/Sic joint
; .i
i-,”
(5)
i -U;-i*--,---.,“,“,..
______;.. .I.“i,‘&__
JR, [ f,.:,.,: I.”
lhzing:1373 K ~1.8ks Testing : 373K
..“i-:‘,
-
Fig. 5. Fractured surface of Sic/Sic joint tested at 373 K.
Fig. 6. Fractured surface of Sic/Sic joint tested at 973 K.
831
832
T. NISHINO et 01.
Fig. 7. Microstructure of Sic/Sic joint with Cu%Ti, alloy filler.
Fig. 8. Microst~cture and EDX analyses of SiC/SiC joint with Cu,Ti,
alloy fiiier.
Interface microstructure and strength of Sic/Sic joint
IO,,,
(IO) Fig. 10. Microstructure
Fig. 11. Microstructure
of Sic/Sic joint with Cu,,Ti,
alloy filler.
and EDX analyses of Sic/Sic joint with Cu., Ti,, alloy filler.
833
835
Interface microstmctu~ and strength of Sic/Sic joint
28 (degree) Brozing:1373K x I.8 ks Testing : 373 K
Fig. 9. X-ray diffraction pattern of fracture surface of SiC/SiC joint with Cu,Ti,
filler.
alloy filler was higher than that of the Sic joint using other fillers at any brazing temperature. The strength of Sic joints brazed at 1323 K for 1.8 ks gives a higher value than that of joints brazed at 1373 K for 1.8 ks. An increase in Ti content in the filler reduces the strength of joints. The excess reaction of alloys with SiC with fillers with higher Ti content, or at higher brazing temperatures, is attributable to the reduction of the strength. The elevated temperature strength of the SIC joint with 50 at% Ti filler metal brazed at 1373 K for 1.8 ks is shown in Fig. 4. The strength of the joint increases with temperature up to 573 K, and then decreases at higher testing temperatures. The decrease in strength of the joint arises from the decrease in the strength of filler from observing the fracture surface of the joints. The fracture surfaces of joints are shown in Figs 5 and 6. Figure 5 is the fractured surface of a joint tested at 373 K. The crack passes through the joining layer and then penetrates Sic. According to the EDX spectrum, an initial fracture with no slip deformation occurs at the joining layer containing the filler metal. Figure 6 shows the fractured surface of a joint tested at the higher temperature of 973 K. The decrease in strength of the Cu-Si matrix identified by EDX analysis results in the decrease of strength of the joint interface. 3.2. Microstructures of Sic/Sic joints Figure 7 shows the microstructure of the Sic/Sic joint interface with CkTi, filler brazed at 1373 K for 1.8 ks. The layer phases at the interface between Sic and filler metal, and granular phases in the joining layer are observed. The microstructure and X-ray image analyses of the Sic/Sic joint with Cu,Ti, filler brazed at 1373 K for 1.8 ks are shown in Fig. 8. The layer phases grow from the interface containing Ti, C and Si, as indicated in the X-ray image analyses of Fig. 8.
-
SiC/Cu-Ti/SiC joining layer
SC
40 ‘=
t
Lo f, ;_
I
IO 0 Fig. 12. Schematic structure diagram of Sic/Sic joints with Cu,Ti, and clLITi,, alloy fillers.
’
40
’ eo
’
80
Titanium content (at%)
Fig. 13. Change in thickness of the carbide and the joining layer with Ti content at the Sic/Sic joint interface.
836
T. NISHINO et al.
The quantitative spot analyses shown in Table 1 indicate that the layer phase is TijSiCz carbide, as reported in ref. [6]. Further, the titanium and silicon enriched phases observed in the central part of the joining layer are identified as TiSi, in Table 1. The X-ray diffraction analyses of the fracture surface of the joint with Cu,Ti, filler reveal that the layer phases at the interface are composed of Tic and Tij Sic, carbides, as shown in Fig. 9. The existence of TiC was also identified in the Cu-Ti/SiC sessile drop[7]. The two carbides of the Tic and Ti,SiC* phases are more clearly seen in the microstructure and the X-ray image analyses of the Sic/Sic joint with Cu,,Ti,, filler metal in Figs 10 and 11. The carbon image analysis shows the existence of a carbon-rich phase of Tic. Based on microstructural observation and chemical analyses of the joining layer in Sic/Sic joints, Fig. 12 illustrates the schematic structures of Sic/Sic joints with Cu,Ti, and Cud3Ti5, fillers. The titanium in Cu-Ti alloys reacts severely with SIC to form the Ti,SiC,(T,) phase at the interface in the following reactions: Sic + Ti (in alloys)-rTiC
+ Si
2SiC + 3Si (in alloys)+Ti,SiC, Ti (in alloys) + 2Si+TiSi,,
(1) (2) (3)
Also, the T, phase, with a regular shape, and TiSi, silicide grow in the central part of the joints. Figure 13 shows the changes in thickness of the carbide and the joining layer with Ti content at the Sic/Sic joint interface. The joining layer increases with increasing Ti content. The amounts of reaction for titanium with Sic increase with the increase in Tic and Ti$iC2(T,) phases and TiSiz silicide. The embrittlement of the joining layer arises from the excess reaction in the Ti-rich fillers, degrading the strength of the Sic joint. 4. CONCLUSIONS The joining of Sic to Sic was done using Cu-Ti amorphous fillers containing 34-57 at% Ti in a vacuum. The joining mechanism was evaluated from the measurements of the strength of joints and the observation of the joining layer. The results obtained are summarized as follows. 1. The joint with 34 at% Ti filler shows the highest strength, and the joining strength decreases with increasing Ti content in the filler at a constant brazing temperature and time. 2. The excess amounts of reaction of Cu-Ti filler with Sic at higher Ti contents or higher temperatures result in the degradation of strength of the Sic joint. 3. The Ti in Cu-Ti alloys reacts with SIC to form Tic, Ti,SiC,(T,) carbides at the alloy/Sic interface, and also TiSi, silicides in the central part of the joining layer. REFERENCES [I] M. Naka and I. Okamoto, Welding Technique 37, 82 (1989). 121 M. Naka. M. Kubo and I. Okamoto. J. Mater. Sci. Letr. 6. 965 (19871. i3j M. Shindo, M. Naka and I. Okamotb, J. Mater. Sci. Left. k, 663‘(1989). [4] T. Yano, H. Suematsu and T. Iseki, J. Mater. Sci. 23, 3362 (1988). [S] T. Nishino, S. Urai, I. Okamoto and M. Naka, Pm. 5th Inr. Symp. Japan Weld. Sm., p. 721 (1990). [6] S. Morozumi, M. Endo, M. Kikuchi and K. Hamajima, J. Mater. Sci. 20, 3976 (1985). [A M. Naka, 1. Okamoto, T. Nishino and S. Urai, Trans. JWRI 18,27 (1989).