New joining technologies are increasingly needed to support the more demanding use conditions targeted by Ultra-High-Temperature Ceramics. Joints must be reliable and strong, and the methods of joining must be improved to successfully accommodate new materials combinations and to fabricate structures of increasing geometric complexity and size.
The reliability, strength, and other performance characteristics of joints are strongly affected by the chemical and physical properties of the interface generated during the bonding/joining process. The degree to which the brazing alloy flows to fill gaps over the adjoining surfaces is affected by the energetics of the interfaces being created and removed, and these in turn are modified by interfacial reactions that can intersperse a new material between the liquid and the solid being joined.
Thus, the study of the solid-surfaces wettability, the interplay between the interfacial energetics and the control of interfacial reactions are essential to develop new joining approaches.
ZrB2 plays a key role for many applications in which stability to extremely high temperatures and damage tolerance are required, but still poses problems of production and joining to itself or to the supporting metal structure.
In this work, the reactivity issues at the solid-liquid interface between the ceramic body and the brazing alloy are addressed from both the theoretical and the experimental points of view.
In particular, the case of Ni-base brazing alloys is discussed. It is shown how the reactivity and the dissolution of the solid phase can be controlled and even suppressed by adjusting the brazing alloy composition on the basis of thermodynamic calculations and by making reference to recently computed multi-component phase diagrams. The role of sintering aids on the interfacial reactivity and on the brazing alloy performances is discussed as well.
Real interfaces obtained by sessile-drop and joining tests are described and analyzed by SEM, EDS and WDS techniques in order to identify their compositions and to interpret the interfacial structures.