Silver-copper oxide based reactive air braze for joining yttria-stabilized zirconia
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We investigated a new method of ceramic-to-metal joining, referred to as reactive air brazing, as a potential method of sealing ceramic components in high-temperature electrochemical devices. Sessile drop wetting experiments and joint strength testing were conducted using yttria stabilized zirconia (YSZ) substrates and CuO–Ag-based air brazes. Results from our studies indicate that the wettability of the braze improves substantially with increasing CuO content, over a compositional range of 1–8 mol% CuO, which is accompanied by an increase in the bend strength of the corresponding brazed YSZ joint. The addition of a small amount of TiO2 (0.5 mol%) to the CuO–Ag braze further improves wettability due to the formation of a titanium zirconate reaction product along the braze/substrate interface. However, with one notable exception, the bend strength of these ternary braze joints remained nearly identical to those measured in comparable binary braze joints. Scanning electron microscopy analysis conducted on the corresponding fracture surfaces indicated that in the binary braze joints, failure occurs primarily at the braze/YSZ interface. Similarly in the case of the ternary, TiO2-doped brazes joint failure occurs predominantly along the interface between the braze filler metal and the underlying titanium zirconate reaction layer.
I. INTRODUCTION
Ceramics are often used in high-temperature applications because of their excellent high-temperature mechanical properties and high levels of wear and corrosion resistance. However, one limitation with these materials is that it is generally difficult to economically manufacture large or complex-shaped ceramic components. An alternative to monolithic part manufacture is to join small simple-shaped pieces into the more complex structure. Although considerable effort has been directed toward developing various methods of ceramic joining, each technique incurs some form of trade-off, typically in terms of cost, ease of processing, and the final properties of the joint.1–4 For example, glass joining is a cost-effective and relatively simple method of bonding ceramics. However, there are several operational limitations inherent to this technique, including low practical exposure temperature, which is defined by the softening point of the glass, and variations in thermal expansion and other thermomechanical properties upon long-term thermal exposure due to the devitrification.1 Reaction bonding, which is an alternate joining technique, often yields joints that contain residual porosity, unconverted reactants, and DOI: 10.1557/JMR.2005.0088 636
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 3, Mar 2005 Downloaded: 11 Mar 2015
undesired secondary product phases, any of which can reduce joint strength by acting as sites for crack initiation.2 On the other hand, joints formed by converting a polymeric precursor to the final ceramic bonding phase often experience cracking during processing because of the large volumetric shrinkage that accompanies pyrolysis.3 A common alte
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