Solid state diffusion bonding of silicon nitride using vanadium foils : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

This paper describes the relation between the interfacial microstructure and the fracture strength of the joints of silicon nitride (Si 3 N 4 ) and vanadium (V) formed by solid state diffusion bonding. At first, the interfacial microstructure and its evolution process were analyzed in detail. The phase sequence at the interface changes with the bonding time showing five typical stages. In the first stage, a V 3 Si layer and V 2 N grains are formed. The V 2 N grains contact with the V 3 Si layer at 1473 K and below, while the contact is prohibited at 1498 K and above. The Si 3 N 4 /V 3 Si interface is metastable. In the second stage, a V 5 Si 3 N 1-X layer appears. In the third stage, V is annihilated. In the fourth stage, the V 3 Si layer is annihilated and VN grains are formed. In the fifth stage, V 2 N is annihilated. This evolution process of the interfacial microstructure agrees well with the proposed chemical potential diagram, except the metastable state of the interface. The increase and decrease behavior in the thickness of each reaction product interact with each other. The behavior of the V 3 Si layer is affected not only by the formation and growth of the V 5 Si 3 N 1-X layer but also by the formation of a V(Si) zone at the V 3 Si/V interface. The formation of the V 5 Si 3 N 1-X layer starts when the spatial gradient of the chemical potential of vanadium in the V 3 Si layer decreases to a certain value. The fracture strength of the joints changes depending on the bonding temperature and time. The higher bonding temperature leads to the higher maximum fracture strength. The maximum strength at each bonding temperature is achieved when the thickness of the V 3 Si layer is 2.0μm. The prolonged bonding time gradually reduces the fracture strength down to 42 MPa.