Interface structure and formation mechanism of diffusion-bonded SiC/Ni-Cr joint

Abstract

It is necessary to bond SiC ceramics to metals in order to expand the engineering applications of SiC ceramics. There have been some reports on the interface structures and reaction mechanisms of SiC ceramics to pure metals and intermetallics such as Ni [1–3], Cr [4, 5] and TiAl [6]. For SiC/Ni-Cr joints, however, only partial information is available and the understanding of the interface reaction is still missing. This letter aims to study the interface structure and to clarify the reaction process of diffusion-bonded SiC/Ni-Cr joints to lay a foundation for their practical use. The materials used in experiments were cylindrical SiC rods (diameter 6 mm, height 4 mm), and Ni-Cr alloy foils (thickness 0.2 mm) with a normal composition of Ni-25at.%Cr. The SiC/Ni-Cr/SiC couples were diffusion-bonded at 1273 K for 1.8–7.2 ks under a pressure of 7.2 MPa in a vacuum furnace equipped with a graphite heating tube. The cross-sections of diffusionbonded SiC/Ni-Cr joints were prepared for metallographic analysis by standard polishing techniques. The morphologies, crystal structures and chemical compositions of the formed phases were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe X-ray microanalysis (EPMA). Fig. 1 shows the back-scattered electron image of the cross-section of the SiC/Ni-Cr joint bonded at 1273 K for 3.6 ks. It can be seen from the figure that three kinds of reaction layers have occurred between SiC ceramic and Ni-Cr alloy. For the sake of convenience, the layer adjacent to SiC ceramic and the layer adjacent to Ni-Cr alloy are called A layer and C layer, respectively, and the layer between A and C is called B layer. Obviously, A layer and B layer are composed of duplex phases, and C layer is a single-phase one. Fig. 2 shows the XRD patterns from the revealed surfaces, parallel to the original bonding interface, of the reaction layers of the SiC/Ni-Cr joint bonded at 1273 K for 3.6 ks. Obviously, the phases identified from the revealed surface on the SiC side are SiC, Ni2Si and graphite where SiC is an original phase existing in the SiC base material. The phases identified from the revealed surface on the Ni-Cr side are Ni(ss.Cr), Ni5Cr3Si2 and Cr3Ni2SiC where Ni(ss.Cr) is the solid solution of Cr in Ni and is the original phase in the Ni-Cr base material. Therefore, four kinds of new reaction phases have formed during the diffusion bonding of SiC ceramic to Ni-Cr alloy. They are orthorhombic Ni2Si, hexagonal graphite, cubic Ni5Cr3Si2 and cubic Cr3Ni2SiC. Table I shows the chemical compositions of each phase in the reaction layers. Based on the XRD results mentioned above, the synthetical analysis to stoichiometric compositions of each phase indicates that the white and black phases in A layer are Ni2Si and graphite, respectively; the grey and blackish phases in B layer are Ni5Cr3Si2 and Cr3Ni2SiC, respectively; and the grey phase in C layer is also Ni5Cr3Si2. In other words, A layer, B layer and C layer are composed of