J.T.M. de Hosson

Metal–ceramic interfaces studied with high-resolution transmission electron microscopy

Tailoring materials with a desirable set of physical and chemical properties has always been a dream of materials scientists and engineers. It is accurate to say that important properties of materials in high-technology applications are strongly affected or controlled by the presence of solid interfaces. For example, a great deal of the electronic industry is based on the interesting electrical properties of semiconductor interfaces, with ceramic-semiconductor, metal-semiconductor and also metal-ceramic interfaces playing a crucial role. Interfaces are also important in the field of surface engineering. For techniques designed to enhance corrosion resistance of surfaces or to optimise their performance in catalytical or tribological applications, interfaces play a determining role. In the field of semiconductor technology as well as in the area of surface engineering, metal-oxide interfaces are frequently encountered. Despite their obvious technological importance, our basic understanding of interfaces, even relatively simple interfaces like grain-boundaries, is still rudimentary in relation to materials properties. The importance of interfaces is determined primarily by their inherent inhomogeneity, i.e. the fact that physical and chemical properties may change dramatically at or near the interface itself. It should be realised that physical properties, like elastic moduli, thermal expansion or electrical resistivity may differ near interfaces by orders of magnitude from those in bulk regions. As a result of these sharp gradients an isotropic bulk solid may change locally into a highly anisotropic medium. Consequently all processes that are controlled by interface phenomena, such as de-cohesion, segregation, cavitation and diffusion, occur in a very narrow region, of the order of a few lattice spacings, where the two materials join. Thus, the atomic structure of interfaces needs to be understood in order to establish the physical mechanisms of various boundary phenomena. Experimental techniques capable of revealing the structure with atomic resolution are necessary for their investigation. In this work the emphasis is on the understanding of interfaces between metals and oxides at the atomic structure level, using High Resolution (Transmission) Electron Microscopy (HRTEM) as the experimental method.

Reinforced SiC/Al composite layer produced by laser particle injection

SiC particles with a mean size of 80 mu m were injected into Al substrate:by the laser particle injection process with the aim to improve the surface properties of aluminium. Experimental difficulties induced by the big difference in absorptivity of laser energy between Al and SiC lead to an extremely small operational window of processing parameters. A combination of parameters of Nd:YAG laser beam, SiC powder stream and Al substrate pre-heating temperature which leads to the formation of single laser tracks was found experimentally. The injection depth is discussed from the point of view of a simple injection model. Optical, scanning and (high-resolution) transmission electron microscopy were used to study the microstructure of the produced particle reinforced metal matrix composite. Partial thermal decomposition of SiC and reaction with liquid Al lead to the formation of new phases presented in the laser tracks, detected as Al4C3 and elemental Si.

Structural analyses of reaction layers between SiC and Ti-6Al-4V after laser embedding

Metal-matrix-composites are produced by the laser particle injection processing route. SiC particles are entrapped during solidification of liquid Ti. Since the injected particles partially dissolve, depending on the process conditions, new phases are formed in the matrix. The degree of dissolution will significantly influence the mechanical performance of the MMC. In particular, the interfacial region between Ti and SiC particles is closely investigated with (high resolution) transmission electron microscopy. Furthermore, hardness indents are performed to examine the crack initiation and the crack propagation behaviour at the interface.

Residual stress in magnetron sputtered TiN

In this study magnetron sputtered TiN layers are investigated with X-ray diffraction. The measurements show that there is a texture present and in these layers a non-linear d-sin(2)psi behavior for the (200) planes was found. The latter cannot be explained by the well-known causes that may generate an oscillating d-sin(2)psi relation like shear stresses or a stress gradient. Assuming that the driving force behind the texture evolution is the ion bombardment, than the preferential orientation is a consequence from the dependence of the sputtering yield on grain orientation. This process may also have an influence on the atomic peening process, which is the main cause of stress generation. The atomic peening generates a hydrostatic pressure in the crystals and a biaxial stress state results because the layer is attached to the substrate. The hydrostatic stress has an influence on the lattice spacing; and becomes d(0)(hydro). Assuming that the peening process is influenced by the grain orientation than d(0)(hydro) becomes also orientation dependent and the plane spacing as a function of the sin(2)psi may exhibit a non-linear behavior.

Temperature effects and fast-moving screw dislocations at high strain rate deformations

In this paper, shear deformation at high strain rates is modeled within the framework of discrete dislocation plasticity. The method of discrete dislocation plasticity is extended to incorporate the temperature rise induced by moving dislocations. Also, the stress and displacement fields of a screw dislocation on inclined planes in a periodic structure are developed. The influence on the temperature rise on various micro-mechanical processes is discussed.