Formation Of Such Layers Research Articles

Kinetics of nitride formation on titanium targets during reactive sputtering

During reactive sputtering processes formation of surface compounds on the target leads to large reductions in deposition rate. The formation of such layers during the reactive sputtering of titanium with Ar-N 2 ion beams have been studied “in situ” using Auger electron spectroscopy. The kinetics of the process were studied by monitoring the AES nitrogen signal with time. The amount of nitrogen on the target surface was estimated using a simple kinetic model. The time dependence was found to be in agreement with the exponential rate predicted by the model. To explain fully the processes occurring, chemical reactions at the surface, altering, e.g., the sticking probability and penetration of reactive ions into the target lattice have to be taken into account. It is also shown that the stable δ -TiN phase is formed on the target surface. The most important parameter for the formation of the nitride layer on the target was the ratio between the number of Ar and N 2 ions impinging. The total number of N 2 ions at a fixed N 2 -Ar ratio, i.e. the degree of ionization, was also found to be important. The energy, however, was found to have only a minor influence on the formation of the nitride layer. The resulting nitride layers during ion beam sputtering are also compared with layers formed on the target during reactive sputtering of titanium in an ordinary RF sputtering apparatus.

Variations in Cr content of magnetite from the upper zone of the Bushveld Complex — evidence for heterogeneity and convection currents in magma chambers

Mineral separates of pure magnetite from the upper zone of the Bushveld Complex have been analysed for Cr. Detailed sampling within layers of massive magnetite indicates that the Cr content frequently shows an extremely rapid but regular depletion with height (for example, by a factor of seven over 85 cm of massive magnetite), with sudden reversals of variable magnitude. Rayleigh Law fractionation, from a homogeneous liquid of the volume now seen as cumulate rocks overlying the sampled horizons, cannot satisfy these observed concentration gradients. A diffusion-controlled crystallization model is preferred. In another vertical section of massive magnetite the Cr content remains constant with increasing height, and is interpreted as indicating a steady state situation where the rate of depletion of Cr by magnetite fractionation is balanced by the rate of diffusive addition into the crystallization zone. Reversals in Cr content sometimes occur in the middle of pure magnetite layers and are attributed to convective overturn in the magma chamber. These results provide geochemical support for the model of convection cells and bottom crystallization in large magma reservoirs as proposed by Jackson [20]. Depletion of compatible elements in this bottom layer causes chemical inhomogeneity in the magma. Sections, composed of several layers of magnetite sandwiched between magnetite-bearing gabbro, have also been studied. In one case, a steady decrease in Cr content with increasing height in the separated magnetite was observed; in another, several irregular reversals were found. There appears to be no systematic relation between convection cycles, the depletion in Cr and the formation of pure magnetite layers, indicating that the chemical composition of the magma does not control the production of monomineralic layers. The fluctuating pressure model envisaged by Cameron [16] for the formation of such layers is entirely consistent with these data.

Use of x-ray photoelectron spectroscopy for the investigation of the surfaces of hard-alloy parts machined with polycrystalline superhard materials

The thin (<600 A) surface layers forming on VK20 and VK20K hard alloys during machining with polycrystalline superhard tool materials and diamond grinding under recommended conditions consist chiefly of tungsten and cobalt oxides, cobalt containing dissolved tungsten and carbon, and pure tungsten (or the compound W2C). The formation of such layers must be regarded as a beneficial phenomenon, because the presence on the surface of a tool material of a thin layer of oxides of the elements of which the material is composed helps reduce tool wear.

Formation of closed magnetic field line plasma systems using intense relativistic electron beams

Closed magnetic field plasma confinement geometries produced by plasma currents induced by rotating intense relativistic electron beams are discussed conceptually. Experimental evidence for the formation of such layers is presented.

Cutting capabilities of lathe tools

In cemented carbides, the surface-region composition can be modified to provide added toughness for coated cutting-tool substrates by developing a cobalt-enriched, cubic-carbide-free surface layer. Typically, such modifications have been prepared by establishing nitrogen gradients in the cemented carbide through the additions of cubic nitrides or nitrification of the cubic carbides during sintering. Recently, methods have been developed for forming cubic-carbide-free surface layers in compositions without the intentional addition of nitrogen. In this study, the kinetics of formation of such layers are compared for compositions with and without nitrogen. The effects of sintering atmosphere and cubic-carbide composition are also discussed.