engleski

Surface Layer Morphology of Tungsten-Carbon Thin Films

Transition metals carbides, and tungsten carbides in particular, exhibit many properties of interest to technology - high metling point, extreme hardness, low friction coefficient, chemical inertness, oxidation resistance, and good electric conductivity. The surface conditions of these materials are critical for some applications, and preparation processes are optimized to meet such requirements. Surface layer of W-C thin films prepared by reactive magnetron sputtering (argon + benzene) on monosilicon substrates has been examined by GISAXS, AFM and SEM methods. Preparation conditions have been varied in a wide range of deposition parameter values: benzene partial pressure in the 1-10% range, substrate temperature in steps RT, 200°C, and 400°C, and beside deposition onto substrates at floating potential a substrate bias of -70 V has been applied . It was found that WC1-x is a dominant carbide in all prepared films, with (111) preferential orientation in films deposited at room temperature. The surplus/unreacted carbon forms a fine dispersion of graphitic nanoparticles imbedded in the intergranular space. The W-C grainsize is determined at about 3 nm by the GISAXS method, with a very smooth surface - roughness about 0, 5 nm - and a very short height-height correlation length at the surface. The AFM examination show roughness depending upon the preparation conditions - from about 0, 5 nm on films prepared at low benzene partial pressure increasing towards a few nanometers when high benzene partial pressure was used. A corresponding average lateral grainsize has been estimated at 10-20 nm, revealing a scale-like morphology of the film surface. The SEM results corroborate the surface morphology, but reveal the subsurface film structure: at the top of the film there is a 10-20 nm thick layer which clearly differs from the main body of the film beneath. The "bulk" of the W-C films (about 200 nm thick) consist of columns 50-100 nm in diameter, atop of which there is a continuous segregated fine-structured layer. The observed structural discontinuity in the prepared W-C films inevitably influence their mechanical properties.

Thin films; surface morphology; tungsten-carbon

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