Elastic constants of WC–a-C:H composite films studied by Brillouin spectroscopy

Brillouin light-scattering experiments have been performed on tungsten-carbide containing amorphous-hydrogenated-carbon (WC--a-C:H) films which were prepared on (111) silicon wafers by a reactive rf sputtering process. From the measured velocity of the Rayleigh surface modes the shear modulus G is calculated as a function of the tungsten-carbide volume fraction, reflecting a phase transition of the films from an a-C:H-dominated to a crystalline {β}-WC-dominated WC--a-C:H mixture. The elastic constants determined agree well with results obtained from depth-sensing indentation measurements. Theoretical calculation of the elastic constant of the composite was also performed as a function of WC volume fraction. The results show a small deviation from the experimental data in the range of low WC fraction, which is probably caused by the phase change of the a-C:H matrix. The relatively large discrepancy of the theoretical calculation in the high-WC-fraction region may be a result of the invalidity of the theory.