Chapter 2 Literature survey
2.5 Brazing of AlN with Ag-Cu-Ti braze foil
Since the active brazing process was very useful in joining ceramics to another ceramics or to metals, many studies were reported using this technique [50-56].
However, selecting a proper braze filler to join nitride is a critical issue. The addition of titanium within the braze foil plays an important role in the brazing process. Rhee studied on the wetting angle of Cu and Ag on AlN [57]. The result of the contact angle showed that the wettability of either Cu or Ag was very poor, as shown in Fig. 2-6.
Sugihara also conducted the wetting of pure Ag and Cu on AlN, as shown in Fig. 2-7 [58]. The image clearly showed that the wetting angles of Cu and Ag to nitride were larger than 90o even at very high temperature so that it was impossible to directly join these metals to AlN. As the result, the addition of active brazing element into the braze filler was needed.
One of the common used braze fillers is Ag-Cu-Ti alloys, this kind of braze foil exhibits very high wettability and can be used to join many ceramic substrates [59]. By the addition of titanium into the Ag-Cu based matrix, the wettability to the aluminum nitride could be significantly increased [60-62]. The contact angle could be decreased to around 15o at 900 oC with 3 wt% of the Ti into silver-copper based brazing alloy.
Moreover, the contact angle could still be decreased as the joining temperature increased. The dependence of the joining time and temperature to the contact angle is shown in Fig. 2-8. As the result, the content of the titanium in braze foil played a critical role in joining system.
For the joining of AlN using Ag-Cu-Ti braze alloys, Carim studied on the interface between AlN and Ag-Cu-Ti [1]. An intermetallic compound ŋ-phase (Ti, Cu, Al)6N was reported to be found beside the TiN interface products. A sequence of AlN / TiN /
ŋ-phase was then proposed. The ŋ-ŋ-phase possessed a near cubic structure with M6X structure in which M denotes metal elements and X denotes anions. The composition difference of the braze foil would affect the formation of the reaction compounds. When Cusil-ABA (63.06 wt. % Ag, 35.26 wt. % Cu, and 1.68 wt. % Ti) was used, a layer of 0.5 um to 1 um ŋ-phase formed beside the TiN layer. However, when a braze foil with higher titanium content, which was Ticusil (68.8 wt.% Ag, 26.7 wt. % Cu, 4.5 wt. % Ti), a series of Ti-Cu compounds were formed due to the excess of the titanium active element. As the result, a rugged and irregular interface with complex reaction
compounds was then formed.
However, the present of carbon would significantly affect the behavior of the joining system. The study of active brazing using Ag-Cu-Ti braze foil on carbon/carbon composite was proposed by Singh [63]. Though the reactivity on the surface of the carbon composites was low, the interdiffusion of solutes and the substrate occurred so that the interface was microstructurally sound. The formation of the secondary phases also help the interface well-bounded. However, the porous structure within the carbon composites would lead to the impregnation of the braze foil. During the joining process, the Ag-Cu-Ti filler not only wetted the surface of the carbon composites but infiltrated through the porous structure. The formation of the reaction products TiC was discontinuous and inhomogeneous so that the braze filler could penetrate through the substrate. The wettability of Ag-Cu-Ti braze filler Cusil-ABA and Ticusil to carbon/carbon composite
The formation of other reaction compounds would be possible during the brazing process using Ag-Cu-Ti braze alloy, such as nitrides and oxides. For example, titanium-oxides were also a stable phase under ultra-low oxygen pressure around 10-28 atm since titanium exhibits high affinity to oxygen [55]. As the result, a series of TiO reaction phases might be found around the interface between the Ag-Cu-Ti filler and the substrates.
The large difference of the coefficient of thermal expansion (CTE) would also lead to serious problems during brazing. The CTE of Ag-Cu-Ti braze foils is relatively large (19.5 x 10-6 for Cusil-ABA and 18.5 x 10-6 for Ticusil) compared to the ceramic substrate and carbon composites (4.5 x 10-6 for AlN and 0 – 4.0 x 10-6 for carbon composites). Since the brazing temperature was high, the resulted thermal strain (ΔαΔT) then became a significant number. The thermal induced stress and cracks would then be easily formed.
However, the large ductility of Ag-Cu-Ti braze foil compared to ceramics or carbon composites would help to prevent the crack formation [64]. The resulting stress would be accommodated by the braze foil and thus the formation of thermal induced cracks was significantly lower than that obtained by calculation. The failure of brazing assembly was also prevented.
Table 2-4 The contact angle of the Ag-Cu-Ti braze foil on carbon/carbon composites
Figure 2-6 The image of the wetting behavior for liquid Ag and liquid Cu to AlN [58]
Figure 2-7 The dependence of joining temperature and time to contact angle for (a) braze filler ( Ag 70.5 wt%, Cu 26.5 wt%, Ti 3 wt%) (b) braze filler ( Ag 64 wt%, Cu 34.2 wt%, Ti 1.8 wt%) to AlN. The close points denotes for dense ceramic substrate and
open points for porous ceramic substrate [60]