Mechanical properties of a-C:H/Si-containing a-C:H multilayered coatings grown by LF-PECVD

Amorphous hydrogenated carbon (a-C:H), Si-containing a-C:H and a-C:H/Si-containing a-C:H multilayered films have been deposited by low frequency plasma enhanced chemical vapour deposition (LF-PECVD) from cyclohexane and/or tetramethylsilane gas mixtures. Structural and mechanical properties of single layers have been first studied in relation with the working pressure, the gas mixture and especially the plasma power. By optimising these parameters, a-C:H films reveal very interesting mechanical and tribological properties (hardness of 23 GPa, friction coefficient of about 0.04 and wear rate of 1.50 10(-7) mm(3) N-1 m(-1) (under air, against Al2O3, with Hertz pressure of 0.65 GPa)) with compressive residual stress values about -1 GPa. Inversely, Si-containing a-C:H films offer a wider range of mechanical behaviour (7-20 GPa for the hardness and 50-170 GPa for the Young modulus) while preserving lower residual stress values (-0.5 GPa). Then, previous results have been exploited to study a-C:H/Si-containing a-C:H multilayered coatings. Transmission Electron Microscopy, nanoindentation, stress measurements and ball-on-disk tribometer were used to characterize these multilayered films. First results show the possibility to obtain from precursor gas inlet modulation thick multilayered coatings (similar to 10 mu m) with period thicknesses down to 15 nm. Period thickness has no influence on hardness, Young modulus and residual stress within the investigated domain. However, friction coefficient and wear resistance are improved by decreasing the period or by optimising the properties of the Si-containing layer. (C) 2008 Elsevier B.V. All rights reserved.