REACTION LAYERS IN DIFFUSION BONDED OF SIALON TO FERRITIC STEEL

Besides dissimilar bonding nature that makes difficulties in joining ceramics with metals, different thermal expansion coefficient of the materials is another important issue. This can cause residual stress which may crack the ceramics. In diffusion bonding, ceramic-metal joint is attained by formation of reaction layers in the interface. Mechanical properties, thickness and morphology of the layers highly affect the joint strength. Thus, understanding the reaction layer is important. This work investigated reaction layers in sialon – AISI 430 FSS and sialon – 7.5%-Cr FS joints. The materials were diffusion bonded to produce the joint and grow the reaction layers. The sialon was also joined with nitrided steels. The reaction layers were studied using OM, FESEM, EDX, XRD, and XPS. Microstructure of the interface illustrated the interfacial joint. Sialon decomposed and liberated Si and N. EDX analyses found Si in the steel adjacent to the interface. Thus, Si diffused into the steel which had lower concentration of Si and created silicon-diffusion layer. Since Cr and Si are ferrite formers, Si dissolved in α-Fe(Cr, Si) in the layer. Similarly, the EDX analyses found more-dominant Fe and Cr and less Si in the interface layer in sialon side. This revealed the transfer of Fe and Cr from the steel into the sialon-decomposed part. XRD and XPS confirmed silicide and oxides precipitation in the interface layer. They also revealed α-Fe(Cr, Si) in the matrix which was same with the phase in diffusion layer. It depicted that the interface-layer matrix and the diffusion layer was one species that composed the joint. Hence, cohesive joint was formed through ferrite-solid-solution bridge. Pre-dissolved nitrogen in nitrided steel might have occupied the space in steel lattice; therefore it reduced the nitrogen solubility in the steel during the joining. This impeded sialon – nitrided steels reactivity and suppressed FeSi2 formation as verified by XPS and XRD on the interface-layer surface. The sialon – steel joint could be potentially useful to serve high-temperature and abrasive environment such as agitator for abrasive slurry, turbine rotor, rocker arm or other automotive components.