Focused Session CA-12
Ceramic Joining: From Macro- to Nano-length Scales
ABSTRACTS
CA-12.1:IL01 Agglomeration of Thin Metal Bilayer Films
K. VAN BENTHEM, Department of Materials Science and Engineering, University of California, Davis, Davis, CA, USA
Solid state dewetting describes the break-up of kinetically constrained thin solid films at elevated temperatures. The driving force for this process, which is often referred to as thin film agglomeration, is the minimization of the total free energy of film and substrate free surface and film-substrate interface, including consideration of potential inter-diffusion and solid-state reactions. Dewetting transitions of two separate metal bilayer films are expected to be strongly affected by the metal/metal interface and potential alloying. In this study, the agglomeration of Au/Ni bilayer films supported by SiO2/Si substrates was studied by plan-view in-situ Scanning Electron Microscopy and cross-sectional in situ Transmission Electron Microscopy. The observations reveal short-range and long-range agglomeration behavior depending on deposition sequence and quality of the metal/metal interface. Inter-diffusion and alloying across the Au/Ni interface is observed due to relatively high interfacial stress resulting from the local lattice mismatch.
CA-12.2:IL01 Advanced Routes for Brazing SiC: Wetting and Interfacial Phenomena
F. VALENZA, S. GAMBARO, M.L. Muolo, A. Passerone, CNR-ICMATE, Genova, Italy
The full exploitation of SiC and SiC-based composites needs effective joining technologies for the integration of ceramic components into existing structures or the assembling in complex shapes. When developing liquid-assisted joining methods, such as brazing, understanding how well the filler alloy wets the adjoining surfaces and the interfacial phenomena is essential. Then, microchemical and microstructural characteristics of the interfaces must be related to the processing conditions and to the ultimate mechanical properties so as to provide the processing-microstructure-property relationships needed to optimise joining processes. Recent systematic studies, referred to the wetting and interactions of various liquid alloys with SiC and SiC-based composites are presented. These studies address both basic (wettability, phase equilibria determination) and application aspects. In particular, a special attention is paid to elucidate the role that dissolution, interfacial reactivity, additions of active metals to the molten matrix have in the wetting process and in the solid-liquid adhesion. The utilization of phase diagrams for the optimization of brazing processes along with real interfaces and mechanical properties of joined samples will be also shown.
CA-12.2:IL02 Active Metal Brazing of Alumina
K.M. KNOWLES, M. ALI, University of Cambridge, Department of Materials Science and Metallurgy, Cambridge, UK; P.M. Mallinson, AWE plc, Aldermaston, Reading, Berkshire, UK; J.A. Fernie, School of Science, Engineering & Design, Stephenson Building, Teesside University, Tees Valley, UK
There are a number of possible technologies available for joining high temperature engineering ceramics to another material, such as a ceramic or a metal. Joining with active braze alloys (ABAs) is particularly attractive because, under favourable circumstances, bonds with good levels of strength can be produced. As Fernie et al. noted in their 2009 review on the joining of engineering ceramics (International Materials Reviews 54, 283-331), microstructural investigations of ceramic bonds in both industry and academia tend to be limited to relatively routine optical and scanning electron microscopy, perhaps with X-ray diffraction, to characterise interfacial reaction products. Until relatively recently this has been true for the characterisation of alumina-alumina and alumina-metal joints using Ag-Cu-Ti ABAs. In this presentation we will show how recent work using transmission electron microscopy has enabled the fundamental chemical processes occurring in alumina-alumina and alumina-Kovar braze joints using Ag-Cu-Ti and Copper ABAs to be established unequivocally, so that recommended values for brazing temperature and brazing time for particular combinations of ABA and components to be joined have the necessary validated underpinning science.
CA-12.2:L03 Interfacial Energy as the Driving Force for Diffusion Bonding of Ceramics
S. Kovacevic, S.Dj. MESAROVIC, School of Mechanical and Materials Engineering, Washington State University, Pullman, USA; R. Pan, Laser Materials Processing Research Center, School of Materials Science and Engineering, Tsinghua University, Beijing, China; D.P. Sekulic, Department of Mechanical Engineering, College of Engineering, University of Kentucky, Lexington, KY, USA; State Key Laboratory of Advanced Welding and Joining, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, China
Diffusion bonding of ceramics with a metallic interlayer can deliver a variety of joint microstructures. When applied to ZrC with Ti interlayer such procedure can deliver the seamless joint, depending on the thickness of the interlayer. Experiments indicate existence of the critical interlayer thickness, below which the seamless homogeneous joint is obtained, and above which the joint does not homogenize. We analyze the thermodynamics and kinetics of the diffusion bonding process with metallic interlayer, uncover the driving forces for the diffusion and phase transformations, and, explain the critical thickness of the interlayer. Analysis of phase diagrams and elastic strain energies indicate that the changes in bulk free energies resulting from small changes in C-concentrations oppose the C-diffusion. We show that the only component of the total system energy that decreases with carbon transfer from ZrC to Ti is the interface energy. Specifically, the interface energy must depend on the jump in the C-concentration across the interface. The sharp interface model yields an estimate for the critical thickness of 10 microns, while the phase field simulations predicts the value of 32 microns, both in good agreement with experimental findings (10-50 microns).
CA-12.2:L06 Atmospheric Plasmas for Improving Mechanical Performances of Joined SiC Components
A. DE ZANET, M. Salvo, V. Casalegno, Department of Applied Science and Technology (DISAT), Politecnico di Torino, Italy
An effective approach to improve the joint strength of components is the modification of the surface texture. More in-depth, increasing the surface specific area and promoting the formation of anchor points is expected to provide higher joint strength because of mechanical interlocking. Several strategies can be adopted. However, when the material to be prepared is SiC few techniques are available. Usually the existing solutions are complex, time-consuming and they impose strict limitations on the maximum size of components. So, when the focus is on treating large components or multiple batches it is important to identify a process that is cost-effective, time-saving and viable also for complex geometries. This activity aimed to assess the feasibility of a commercial plasma generator as a surface modification treatment to improve the mechanical properties of joined SiC. Samples were characterized by electron microscope before and after the plasma exposure. Afterward, each sample was treated using a corona plasma and then joints were manufactured using an epoxy adhesive as glue. Then, treated and untreated specimens underwent shear strength tests. The results obtained by the surface characterization together with those derived by mechanical testing are discussed.