Iron-Alumina Joining with Fe-Y₂O₃ Composite Interlayer and Control of Its Reaction Layer

Abstract

In order to join Al₂O₃ with iron, an Fe-M₂O₃ composite (I) as an interlayer was applied. Here, M₂O₃ represents for Y₂O₃, Al₂O₃ and Y₂O₃·Al₂O₃. The Al₂O₃/I/Fe joint was prepared by hot-pressing. In the case that M₂O₃ is Y₂O₃, Al₂O₃/I/Fe joining can be completed through formation of a series of reaction sublayers: at the interface of the Al₂O₃/I, a chemical reaction sublayer, next to Al₂O₃, including iron, aluminium and oxygen is formed; and then next to it has a second sublayer of yttrium, aluminium and oxygen matrix mixed with dispersed iron as a different phase, the Fe-M₂O₃ interlayer and subsequently the I/Fe interface, where the inter-diffusion of iron has occurred. The thickness of second sublayer changes with the Y₂O₃ content. This layer plays a role in relieving thermal stress caused by a different thermal strain of Al₂O₃ and iron bulks, but the increase of its thickness makes the joint easier to break. With increase in Y₂O₃ content in Fe-Y₂O₃ interlayer, the layer tends to agglomerate, which results in the weakening of the joint. To bring the thickening of the reaction layer and the agglomeration under control, Fe-Y₂O₃·Al₂O₃ interlayer was found to be effective. For analysis of the boundary region, electron probe microanalysis (EPMA) was used. Tensile strength and thermal expansion for Fe/I/Fe, Al₂O₃/I/Al₂O₃ joints and Fe-Y₂O₃ composite sintered materials were measured.