The elastic constants of tetrahedral amorphous carbon (ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H) thin films were determined nondestructively by surface Brillouin scattering. Besides the usual Rayleigh surface mode, we also observe a new pseudosurface acoustic mode of longitudinal polarization, which is a modified version of the longitudinal guided mode usually found in slow-on-fast supported films. The Young’s modulus E of a ta-C with 88% ${\mathrm{sp}}^3$ is 757 GPa, and the shear modulus G is 337 GPa. The moduli of ta-C:H with 70% ${\mathrm{sp}}^3$ and 30 at. % H are lower, $\text{E=300\hspace{0.05em}}\mathrm{GPa}$ and $\text{G=115\hspace{0.05em}}\mathrm{GPa}.$

Ultrathin 2-nm-thick carbon coatings are needed to increase the storage density in magnetic hard disks. We show how x-ray reflectivity, surface Brillouin scattering, resonant Raman scattering, and electron energy loss spectroscopy can measure consistently the structural and mechanical properties of these thin films. 2 nm films retain a Young’s modulus of 100 GPa.