Tailoring the mechanical and tribological properties of sputtered boron carbide films via the B1-xCx composition

Boron carbide films are very attractive due to their high hardness and interesting tribological properties. In the present work, the compositional effect of boron carbide B_1-xC_x (01-xC_x layers were deposited on Si (100) and M2 high speed steel substrates using a pilot-scale closed-field unbalanced magnetron sputtering system equipped with one graphite and two boron targets. Different compositions were obtained by tuning the electrical power applied to the graphite target. The hardness of the B_1-xC_x films decreases from 28 to 20GPa as the carbon content [C] rises from 19 to 56at.%, but thereafter it increases again up to ~25GPa when [C] reaches 76at.%. The hardness variation is explained by changes in the film microstructure, namely formation of either a nanocrystalline boron carbide in the case of boron-rich films, or boron carbide nanocrystals dispersed in an amorphous boron carbide/a-C matrix in the case of carbon-rich films as deduced from the combined XPS, micro-Raman and XRD measurements. The friction coefficient of the B_1-xC_x films reduces from 0.66 to 0.13, and the wear rate against alumina ball drops from 6.4×10^-5mm³/Nm to 1.3×10^-7mm³/Nm with increasing the [C]. Raman analyses indicate that improvement of the tribological properties of the B_1-xC_x films is primarily caused by the presence of the amorphous carbon phase that leads to the formation of a graphitic tribolayer acting as a solid lubricant during the wear process. Deposition of the B_0.81C_0.19 film with high hardness (28GPa) improves the corrosion resistance of the M2 steel substrate by four orders of magnitude, documented by the decrease of the corrosion current from 3×10^-6A/cm² to 8×10^-10A/cm². We demonstrate that adjustment of the x-portion in the B_1-xC_x system allows one to tailor the tribo-mechanical properties over a large range from high hardness/high friction to high hardness/low friction structures.