Deposition process and characterization of chromium-carbon coatings produced by direct sputtering of a magnetron chromiu
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Chromium-carbon coatings have been deposited on various substrates by direct sputtering of a chromium carbide, Cr 3 C 2 , target in pure argon atmosphere. The composition of coatings determined by Rutherford backscattering spectroscopy and the deposition rate were investigated as functions of the sputtering gas pressure and self-bias voltage applied to substrates. The atom number ratio C/Cr in the coatings was equal to 0.7 regardless of the deposition conditions investigated. Oxygen and argon atoms were the major impurities incorporated in the amorphous coatings. Oxygen-free C r - C coatings were prepared at low argon pressures or on substrates biased to a voltage in the range —100 to —320 V. The C r - C coatings deposited on biased substrates contained less than 2 at. % of argon. The morphological features of C r - C coatings examined by scanning electron microscopy were also dependent on the sputtering gas pressure and bias voltage of substrates. Fully dense structures were observed for coatings deposited at low argon pressures or on biased substrates. The electrical resistivity of C r - C coatings was extremely dependent on the concentration of oxygen atoms incorporated in the coatings. Oxygen-free C r - C coatings exhibited electrical resistivity values as low as 120 /nCl cm, i.e., less than twice the bulk resistivity of Cr 3 C 2 . The residual stresses in the coatings and microhardness of the deposited material were also investigated as functions of the deposition parameters. Tensile residual stresses were lower than 0.8 GPa, and the maximum microhardness of coatings was about 13 000 MPa, i.e., similar to that of the bulk material.
I. INTRODUCTION Metal-carbon coatings produced by physical or chemical vapor deposition techniques are of interest for the wear protection of mechanical components; in particular, carbon-rich compounds of tungsten, chromium, rhenium, iron, and titanium of various compositions exhibit promising tribological properties for terrestrial applications.1"3 The microhardness of this type of coatings is firmly connected with the crystallographic structure of the carbonaceous material which is strongly dependent on both the carbon concentration in the coatings and deposition temperature. For instance, the carbon incorporation in reactively sputter-deposited W - C coatings led to an increase in Vickers microhardness from 13 000 to a maximum value of 26 000 MPa for a carbon content of 15 at. %. Then, the microhardness of W - C coatings decreased progressively as the carbon concentration increased up to 25-30 at. %. An additional carbon incorporation resulted in a new increase of the microhardness up to a maximum value of 26 000 MPa for a carbon con-
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address: National Polytechnic Institute of Grenoble, CENG-CEREM DEM-SGSA-LTS, 17 Rue Des Martyrs, F-38054 Grenoble Cedex 9, France.
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J. Mater. Res., Vol. 9, No. 7, Jul 1994
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tent of 40-45 at. %.4"6 The structure of W - C layers containing 15 at. % of carbon was identified wit
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