Effect of alumina impurity on microstructure and properties of alumina based conventionally brazed joints

Mo-Mn metallization of alumina ceramics of different purity has been performed at 1400 °C for 10 min in moist hydrogen and nitrogen atmosphere. Nickel coating has been applied onto the metallized alumina ceramics at 1000 °C for 1 h in a reducing hydrogen atmosphere. Finally, metallized and nickel coated alumina ceramics has been brazed with another metallized and nickel coated alumina ceramics using CuAg filler alloy at 900 °C for 10 min in a vacuum furnace at 1×10-6 Torr pressure. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis has been carried out for phase analysis, microstructural investigation and elemental composition analysis. The adhesive strength of the metallizing layer and brazing strength of the joint have been measured by pull down breaking strength method. SEM study has shown that the width of the interfacial reaction region between the metallizing layer and substrate enhances with increasing the impurity content in the alumina ceramics. It has been observed that the adhesive strength of the metallizing layer depends on the interfacial reaction layer thickness. The adhesive strength of the metallizing layer has been increased with increasing the thickness of interfacial reaction layer. High adhesive strength of the metallizing layer as well as brazing strength has beenachieved for alumina ceramics with high impurity content.