Present knowledge on metal-oxide interfaces is reviewed, emphasizing fundamental properties and behaviour. Methods have become available to fabricate ‘ideal’ metal-oxide interfaces with precisely controlled chemistry, special crystallography and high structural perfection. Recent experimental investigations of such model systems have advanced the understanding of adhesion, structure, mechanical behaviour and diffusion reactions at metal-oxide interfaces. In parallel, physical theories and powerful methods of computer modelling have been pushed forward to predict and to interpret the experimental observations. As an important development, recent work increasingly explores the correlation between microscopic properties of metal-oxide interfaces and their technically important macroscopic behaviour.

Metal-oxide interfaces

A one-dimensional calculation of thermal residual stresses, arising from the fabrication of a Functionally Graded Material (FGM) system, is presented. As a first step, calculations have been limited to the linear elastic case. The FGM system consists of ceramic (Al2O3) and metal (Ni) phases varying with distance in one direction. Several functional forms of gradation of constituents were examined to arrive at the optimum profile giving the minimum residual stress level. A linear variation in composition from fully ceramic to fully metal showed the least residual stress. Residual stresses were found to increase when fully ceramic and/or fully metal regions are included in the structure, adjoining the graded zone. The character (tension or compression) of stresses is dependent on the functional form of gradation. A low residual stresses state close to that of the continuously graded structure can be achieved also in a multilayer FGM system having several layers (typically > 11) of constant composition. The effects of temperature dependent elastic and thermal expansion characteristics of constituents on residual stress were found to be small.

Thermal residual stresses in a functionally graded material system

Metal-ceramic joining has slowly but steadily become an important manufacturing step. The evolution of joining processes has allowed ceramics to be used in combination with metals in a number of hybrid devices from traditional light bulbs and seals to improved cutting tools and modern monitoring and measuring electronic devices. New joining methods and newer approaches to conventional methods have been developed aiming at joints characterized by improved reliability, and interfaces capable of withstanding high-temperature resistance with minimum residual stresses. A summary of recent improvements on alternative approaches to ceramic-metal joining as well as new developments on brazing are presented herein. The present review also focuses on recent advances towards brazing metallized ceramics and the selection of filler alloys, since in a scenario that includes joining by laser and direct bonding with liquid transient phases, brazing continues to be by far the most widely used approach to joining as a result of its low-cost and possibility to join intricate geometries for large-scale production. Finally, methods to evaluate the mechanical strength and residual thermal stresses are presented in addition to alternative approaches to minimize residual stresses and, consequently, improve joint reliability.

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Review Article: recent advances in metal-ceramic brazing

Current manufacturing routes for advanced continuous fibre composites are compared. The different routes lead to significant differences in the deformation required to consolidate the composite and in the fibre distribution in the composite. Excessive deformation increases the chances of fibre breakage or fibre surface damage and with non-uniform fibre distribution adversely affects composite mechanical properties. A new route for making composites is described which involves coating the fibre with the matrix by physical vapour deposition (PVD) before consolidation. Examples of Ti alloy, Al alloy, and Ti aluminides made by this route are given. It is shown that the PVD route may have many important advantages over alternative routes. It would enable a wide range of composites to be produced with precise control over matrix composition, matrix microstructure, fibre distribution, and fibre volume fraction. This could lead to increased toughness in intermetallic and ceramic composites. The PVD route ...

Processing of Advanced Continuous Fibre Composites: Current Practice and Potential Developments

Self-limiting growth of copper islands on TiO2(110)-(1×1)

This review article on metal/ceramic joining is subdivided into the description of research activities in the fields of active metal brazing and diffusion bonding published in the last decade. Informations are given on active metal brazing of oxide, nitride, and carbide ceramics and on diffusion bonding of alumina, zirconia, magnesia, silicon nitride, aluminum nitride and silicon carbide ceramics to metals. Ultra high vacuum diffusion bonding and experiments using the model combination Nb/alumina are also regarded. Emphasis is laid on a concise reproduction of experimental data concerning the bonding conditions and the determintion of bond strength. The review demonstrates that much effort was devoted to studies on the formatin of interfacial reaction layers and on the efficiency of interlayers additonally introduced between the ceramic and the metal part to reduce internal stresses caused by thermal expansion misfit of the materials to be bonded.

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G. Elssner

Papers

Metal/ceramic joining.