Abstract: The present paper is focused on ceramic–metal composites obtained via different
technologies which leads to different microstructures in terms of size and distribution of metal phase. Composites analysed in paper were produced by the following methods:(a) infiltration of porous ceramics by metal, (b) consolidation under high pressure and (c) sintering of ceramic powder coated by metal. Their microstructures were investigated by scanning and transmission electron microscopy methods. The three methods of composite fabrication employed in the present study result in specific spatial distribution and dispersion of metal phase. Presureless infiltration of porous ceramics by liquid metal is driven by capillary force and make it possible to produce microstructure with percolation of metal phase in ceramic matrix. The volume fraction of metal phase in this case depends on the size
of pores. The size of pores influence also the kinetics and extent of infiltration. Ceramic preforms with small size of pore are not fully infiltrated. This method is useful for composite with size of metal phase in the range of micrometers. Hot pressing under high pressure produces microstructures of composites with metal phase grain size in the range from nano to micrometers. Moreover, it allows to achieve the nanometric size of ceramic grains. In the case of ceramic powders covered by metal, compression and hot pressing preserves nanometric size of metal. The grain growth of ceramic grains is suppressed.
Abstract: Practical applications of metal/ceramic joints can be found in the biomedical field
regarding the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or in the production of drug delivery systems, biomedical sensors and electrodes. Most of metal/ceramic joints are produced by the active metal brazing technique, which originates a multi-layered interface which should be able of accommodating the abrupt electronic,
crystallographic, chemical, mechanical and thermo-mechanical discontinuity that characterize these systems. Additionally, when considering biomedical applications, corrosion resistance becomes of prime importance. In this work, the corrosion resistance of Ti/glass-ceramic interfaces obtained by active metal brazing was evaluated by electrochemical impedance spectroscopy (EIS) tests. The electrochemical behaviour of the interface was monitored, as a function of time, in a simulated physiological solution at room temperature. In order to evaluate the contribution of each layer and galvanic interactions between them, to the degradation mechanism of the interface, individual samples, representative of reaction layers present at the interface, were fabricated and electrochemically
tested. Results show that the corrosion behaviour, of the whole interface was strongly influenced by the chemical composition of its constitutive layers. Thus, layers containing high contents of both titanium and silver showed a polarisation resistance increase with the immersion time, as a result of the formation of a thermodynamically stable passive film. On the other hand, the copper rich layer,
appears to be the main responsible for the interface degradation. In fact, for high immersion times, an instable passive film is formed and, as a consequence, large amounts of copper are released. Galvanic interactions between the copper and the silver rich layers where also identified.
Abstract: A model for designing sandwich nanocomposite ceramic tool materials with symmetrical distribution was presented. By adding nano-sized Al2O3 particles into the submicro-sized Al2O3 and TiCN, Al2O3/TiCN sandwich nanocomposite ceramic tool materials were fabricated by means of powder-laminating and hot-pressing technique. The experimental results showed that optimal mechanical properties were achieved for the composite with the addition of 35 vol.% TiCN particles in the middle layer and 45 vol.% TiCN particles in the outer layers, layer thickness ratio is 0.3, with the flexural strength reaching respectively 900MPa，fracture toughness and Vicker's hardness in the surface layers being 6.5MPa•m1/2 and 19.2GPa.
Abstract: In this work damage micro-mechanisms of two different types of fibre reinforced composites are investigated by acoustic emission, AE. Ceramic based oxide fibre reinforced mullite matrix composite and metallic based SiC fibre reinforced titanium matrix composites exhibit different fracture mechanisms during loading and AE technique could pinpoint these damage mechanisms based on the AE responses detected simultaneously. The results show that in a ceramic matrix composite, the identification of fibre fracture and matrix cracking requires careful analysis of the AE data as both fibres and matrix break in brittle manner. Whereas the separation of fibre fracture from the ductile tearing of matrix ligaments could be easier in metallic based composites, such as titanium matrix composites.
Authors: Siegfried Schmauder, Ulrich Weber, Andreas Reuschel, Markus Willert
Abstract: A model based on the geometry of the phases is introduced in order to investigate the mechanical properties of interpenetrating microstructures. In order to characterize the elastic and elastic-plastic properties of the composite a self consistent unit cell model is applied on a wide range of volume fractions for an Al/TiO2 composite. Besides the volume fraction a microstructural based parameter is used, the matricity, to describe the mutual circumvention of both phases. Computations are carried out for different temperatures and void volume fractions. In addition a conservative fracture criterion based on critical normal stresses is applied to derive realistic stress strain curves.