UC Berkeley

Partial-transient-liquid-phase (PTLP) bonding of advanced ceramics employs an A/B/A sandwich-style interlayer that is designed such that the outer cladding, A, forms a transient-liquid phase that disappears at the bonding temperature due to diffusion of A into the core layer, B. The resultant bonds can have re-melt temperatures that are significantly higher than the bonding temperature. The success of PTLP bonding relies on the proper selection of the interlayer components: the transient liquid must be able to flow into and fill strength-limiting interfacial flaws, the adhesion between the interlayer and the bulk ceramic must be sufficiently high to prevent interfacial failure, the formation of strength-reducing brittle phases at the interface should be minimized, and the residual stresses due to coefficient of thermal expansion (CTE) mismatch should be minimized. The composition of the transient liquid predominately determines the interfacial characteristics of the bond, while the core composition determines the residual stresses in the assembly. In recent work, Al2O3 bonded using Ni/Nb/Ni interlayers produced joints that were of such high strength that all bonded samples failed exclusively in the ceramic and not at the joint during 4-point bend testing.

The wetting characteristics of the Ni-Nb transient-liquid and the CTE of Nb are favorable for the fabrication of strong PTLP-bonded Al2O3. However, for other ceramic systems, using a binary interlayer system such as Ni-Nb may not be desirable. When using binary interlayers it is not possible to control the composition of the transient liquid and the core independently. In order to expand PTLP bonding to other advanced ceramics, this study examined a new interlayer design that employs a surface-modified core, such as Mo-surface-modified Nb, instead of a homogeneous core, such as pure Nb. A surface-modified core is a core layer with an intentionally inhomogeneous composition in order to better control the composition of the transient-liquid and the core layer independently. It was found that Al2O3 PTLP bonded using a Mo-surface-modified V core and a Ni cladding had fracture strengths of 302±29 MPa. This is comparable to those using Ni/Nb/Ni interlayers, 341±28 MPa. In both assemblies, all of the samples failed in the ceramic bulk. The insights gained from these experiments were used to develop interlayer design guidelines for the PTLP bonding of other advanced ceramics.