Specimen observation

The graphite-Ticusil-AlN specimen prepared at 1050 ^oC in flowing nitrogen could achieve fully wetted interfaces. As the image of the cross section shown in Fig. 4-6, The distribution of reaction phases was significantly changed. At the interface between AlN and Ticusil, the containing range of dispersed particles was much larger compared with that in AlN-Ticusil-AlN system. Also, the size of the dispersed particles was also increased. Besides, there were some small crystals between the TiN layer and the dispersed particles. When we focused on the AlN-Ticusil interface, as the image shown in Fig. 4-7, the small column-like crystals were found formed beside the TiN layer. The growth direction was vertical to the interface. Nonetheless, the thickness of the TiN layer was around 2.4 um, which was similar to the TiN layer in AlN-Ticusil-AlN system. As the image Fig.4-8 focused on those column-like crystal. The behavior of the growth of these crystals was similar to the distribution of titanium in the AlN-Ticusil-AlN system.

On the other hand, the containing range of the particles at the AlN-Ticusil interface was much wider than those in AlN-Ticusil-AlN system. The alignment of these particles was also more loose in graphite-Ticusil-AlN system.

For the other side, the interface was between the graphite paper and Ticusil braze foil. At first, some cracks were found near the interface. However, when focused on the interface, as the image shown in Fig. 4-9, the cracks were found only within the graphite

temperature, the thermal induced stress would cause the crack to form within the graphite paper and elongated between the layer of graphite paper.

The observation was also conducted on the interface products at the graphite-Ticusil interface. The interface products were composed of a continuous layer, which connected the graphite layer and Ticusil, and some small reaction particles, as shown in Fig. 4-10.

The thickness of the layer product was thinner than that of TiN layer at the other interface.

By the analysis of EDS and WDS, the layer products were found to be TiC, which was stable at room temperature. Though the thickness of TiC layer was small, around 0.6 um, this layer still connected the graphite layers and Ticusil. No obvious cracks were found between TiC layer and graphite layer. For those reaction particles, it was found that the composition was rich of titanium, copper and carbon. However, the reaction products at the graphite-Ticusil side was much fewer than that at the AlN-Ticusil side according to the thickness of reaction products layers.

Figure 4-6 Cross section of graphite-Ticusil-AlN specimen joined at 1050 ^oC in flowing N2 for 15 min.

Figure 4-7 The AlN-Ticusil interface with graphite-Ticusil-AlN system joined at 1050

oC in flowing N2 for 15 min

Figure 4-8 The magnified image on AlN-Ticusil interface within graphite-Ticusil-AlN

Figure 4-9 The graphite-Ticusil interface within graphite-Ticusil-AlN system

4.2.3 Composition analysis

The composition mapping for the graphite-Ticusil-AlN system was also conducted, as shown in Fig. 4-11. The distribution of copper indicated the remained region of Ticusil matrix. The region of ceramic substrate AlN and the reaction products TiN layer at the AlN-Ticusil interface could be indicated by the distribution of nitrogen. However, within the distribution of titanium, there were some difference from the previous EPMA mapping on the AlN-Ticusil-AlN system. First, though the titanium would still move to both interfaces to form the reaction products, the amount of titanium on the both side of interfaces were different. There was much more titanium on the AlN-Ticusil side, while there was only few titanium on the graphite-Ticusil side. This phenomenon indicated that the titanium would tend to move to the AlN side, which means that the affinity of AlN to Ti was higher than the affinity of graphite to Ti. The other difference from AlN-Ticusil-AlN system was that the distribution of titanium was not only concentrated at the interfaces. Some of the titanium dispersed within the matrix of Ticusil, and the distribution of those in Ticusil was matched to the distribution of nitrogen within the Ticusil, though the intensity was small. This indicated that some TiN particles would form within the remained filler.

When the mapping focused on the AlN-Ticusil interface, the result was shown in Fig.

4-12. There was little silver content found at the TiN layer, while there was no copper content within this layer. The copper content matched the position of those dispersed reaction particles. This indicated that the copper only formed those reaction particles with titanium but not formed the layer-structure products. Besides, the distribution of nitrogen was also corresponded to the position of dispersed particles. The sites of those dispersed particles were composed of titanium, copper and nitrogen mixed with each other.

For the graphite-Ticusil interface, the composition mapping was also conducted as

shown in Fig. 4-13. By the comparison of those elements, it could be found that there was no obvious composition change at the cracks within the graphite layers. This result indicated that these cracks were not resulted from the chemical reaction. The layer structure at the graphite-Ticusil interface were corresponded to the distributions of the carbon and titanium. It indicated the layer was composed of titanium carbide. However, those particles near the graphite-Ticusil interface were not only composed of carbides. By the mapping of those elements, some of the particles were composed of titanium, copper and nitrogen while some of the particles were titanium carbides.

Figure 4-12 Composition mapping of the interface reaction product at the cross section at AlN-Ticusil interface for graphite-Ticusil-AlN. joined at 1050 ^oC in flowing N2 for 15

min

Figure 4-13 Composition mapping of the interface reaction product at the cross section at graphite-Ticusil interface for graphite-Ticusil-AlN joined at 1050 ^oC in flowing N2

for 15 min