The term ceramics describes a wide range of materials spanning an equally broad range of properties and applications. The so-called Ultra-High Temperature Ceramics (UHTCs) are the subject of considerable and growing international research interest. These ceramics melt at temperatures >3000 °C. The present work focused on HfB2, which melts at 3380 °C. As is the case for some other ceramics, HfB2 exhibits good thermo-mechanical and thermo-chemical properties, and good resistance to erosion and corrosion.
A major obstacle to more widespread use of HfB2 and other closely related diborides, has been the difficulty of processing. The fabrication of large complex-shape parts will be very challenging, and for many applications, the practical use of these materials in complex structures will require effective methods of joining. Without joining methods, the full promise of these materials will not be realized.
Joining strategies that exploit a transient liquid phase (TLP) seem ideally suited for such challenging situations and are the subject of the present research. In TLP joining UHTCs the interactions between the liquid metal, the UHTC and sintering aids can affect θ and the amount and nature of the liquid present. Wetting experiments will help establish a fundamental understanding of the reactivity/compatibility among the materials being joined, and in optimizing interlayer constituents and designs.
In the present work, the wettability of polycrystalline HfB2 ceramics by molten Ni-Nb alloy that is a candidate of the interlayer for TLP joining of UHTCs have been systematically investigated by the sessile drop method up to 1500 °C under vacuum, then time changes of the contact angle and interfacial reactions are discussed. Substrates of HfB2 ceramics employed were fabricated by pressure less sintering with MoSi2 sintering aid. Ni-Xat%Nb (X=0, 20, or 40) alloys were found to melt at the temperature much below their melting point during the heating period, which is mainly attributed to the interfacial reaction of HfB2 and/or MoSi2 with the alloys.
The contact angles of all Ni-Nb alloys were rapidly decreased with time and showed good wetting against HfB2 ceramics. Among them, pure Ni (-0at%Nb) melt totally disappeared around 1400 °C during heating by the diffusion of Ni into HfB2 ceramics. EMPA microstructural analysis on the cross section of the wetting sample revealed that the reactant was an intermetallic compound consisted of Ni-Mo-Si, which is suggested that Ni preferentially reacted with sintering aid MoSi2. With Nb addition, the interfacial reaction of Ni-Nb alloys with HfB2 ceramics was suppressed, which would be attributed less solubility limit of MoSi2 into Ni-Nb alloy. More microstructral analyses in detail, and TLP joining results of HfB2 with Ni-Nb interlayer will be presented at the conference.