STUDY OF SOME MECHANICAL PROPERTIES OF THIN HARD COATING CHROMIUM CARBIDES

In the present work, some mechanical properties of chromium carbides coating on steel substrates were studied. The samples were prepared by d.c. magnetron sputtering of a pure Chromium (Cr) layer on steel substrates containing 1%wt of Carbon. The system (substrate/coating) was then annealed in vacuum at a temperature at which the carbon of the substrate may diffuse into the Cr metallic film. This fact leads to the formation of chromium carbides, which are identified by X-ray diffraction as Cr7C3, Cr23C6 and (Cr,Fe)7C3. The study by nanoindentation shows that the microhardness decreases with increasing of annealing temperature until 800°C, then increases rapidly to reach a maximum (≈2800 Kg.mm-2) at 1000°C. The internal stresses, measured by X-ray diffraction, are compressive at 900°C and tensile at 1000°C in the chromium coating. Nevertheless, they are tensile in the substrates regardless the temperature. The sign change of the stresses in the coating is probably due to the phase transformation (Cr7C3→(Cr,Fe)7C3))