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-S divide H dominant to a crystalline beta-WC dominant WC-a-C divide H mixture. The determined elastic constants meet well the results obtained from depth-sensing indentation measurements. The theoretical calculation of the elastic constant of the composite was also performed as a function of WC volume fraction. The result 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.