Fracture behavior

After the analysis on each interface within the specimen, the measurement of the properties of the whole specimen was then conducted. For the studies of joining, the bonding strength was an important factor to evaluate the joining properties of the joining system. When two components were bonded, an interface would form between the contacted surfaces. The interface could be characterized by either mechanical or reaction bonding, depending on the activity of the components and the roughness on the substrates.

The strength of an interface in this study was defined as how difficult was the interface separated or the assembly of those components broken by the application of the external force. The interface strength was then measured when the interface was separated under an external load. The strength of an interface compared with the substrates would affect the fracture behavior when the applied load reached the strength of the weakest part within the specimen. The cracks would propagate through the weakest part of the specimen. As the result, if the weakest part were on the interface, the fracture would occur between the two components, the specimen would then be separated into two parts with the both surfaces not containing the component of the other side. On the other hand, if the weakest part were within the component substrate, the cracks would then propagate through the whole component. The fracture surfaces would then be some of the component remained on one surface while some could be found on the other surface. As

gradually occurred and be regarded as a graceful fracture.

In the case of the bending test on AlN substrate only, the fracture of AlN substrates was similar to brittle materials. Once the maximum strength was reached, the cracks propagate swiftly and the material broke suddenly. This was the same as typical ceramic materials as the structure of ceramics was dominant by covalent bonding. The bonding was difficult to move to release the stress and the defect. The material would then fracture suddenly as the strength was reached. When the joining of AlN was conducted, the fracture behavior would be influenced since the interfaces between different materials was introduced to the system. In the case of AlN-Ticusil-AlN system, the both side of the specimen were the AlN plates. The specimen could then be regarded as AlN / AlN-Ticusil interface / Ticusil / AlN-Ticusil interface / AlN system in the sequence. The fracture behavior of the AlN-Ticusil-AlN system was similar to the AlN substrate. Besides, by the observation on the fracture origin of the fractured specimen, the fracture started from the substrates and no cracks were found on the interface between AlN and Ticusil. As the result, the interfacial strength between AlN and Ticusil was higher than the strength of the AlN substrate. However, the flexural strength was slightly lower in the AlN-Ticusil-AlN system than AlN-Ticusil-AlN substrates only. The might be resulted from the reason that once the joining process was applied, the difference of the coefficient of thermal expansion and the high temperature difference from room temperature to the joining temperature would lead to a large thermal induced stress [108-111]. Some micro cracks would then form by these thermal induced stress occurred within the joining process. These micro cracks propagated and caused the AlN substrate broken when the external load was applied. As the result, the flexural strength of the joined AlN-Ticusil-AlN system was then decreased.

In the case of the bending test on graphite-Ticusil-AlN system, the substrates on both sides were different. When the load was applied from the top, the substrate on the top side

would afford the direct compressive stress. On the other hand, the substrate bottom side would afford the tensile stress. Thus, a directional-dependence on the stress was then occurred. When the AlN side was put on the top side, as shown in Fig. 4-32, the stress-strain curve gradually raised up and a drop would occur as the peak strength was reached.

However, the drop was relatively small and the specimen was still connected together.

When the following stress still applied to the specimen, the stress-strain curve showed a gradually fall and a large deflection could be achieved as the loading continuously applied.

This phenomenon was resulted from that though the AlN substrate was ruptured as the first drop on the curve occurred, the Ticusil layer still connected both AlN substrate and graphite paper to remain assembly. Since Ticusil was mainly composed of copper-silver alloy, the ductility of Ticusil was very good. When the further loading was applied, the specimen was bended to a curve shape as the Ticusil layer was elongated. Thus, the gradually drop of the stress-strain curve was corresponded to the elongation of the Ticusil layer. This phenomenon also indicated that the bonding between AlN-Ticusil and between graphite-Ticusil were strong since the fracture of AlN substrate and graphite paper occurred before the interfaces ruptured.

On the other hand, when the graphite paper side was put to the top side, the fracture behavior of the specimen was similar to brittle material. When the external load exceeded the strength, the specimen was then fractured with cracks propagated through the specimen. This was resulted from the low tensile strength of the ceramic substrate AlN.

bonds of the structure, the flexural strength was then much lower when the graphite side was on the top. Moreover, when the specimen was ruptured, it was broken into two pieces with the cracks in the middle. This also indicated that the bonding between AlN-Ticusil and between graphite-Ticusil were strong. If the interface were not strong enough, the crack should propagate on the weak interface so that the fracture should occur on the weak interface. Nonetheless, the peak strength of graphite-Ticusil-AlN joined system was much lower than that of AlN or AlN-Ticusil-AlN joined system. This was resulted from the larger difference of the coefficient of thermal expansion between each component.

Larger difference of the coefficient of thermal expansion was then resulted in a large thermal induced stress and made the cracks easier to propagate.