We have grown amorphous hydrogenated boron carbide thin films on a cathode substrate by rf plasma decomposition of diborane and methane. The chemical composition, infrared absorption, optical absorption, microhardness, and adhesion of these thin films were measured. As a function of increasing diborane concentration in the feedstock, we observe increasing boron and decreasing hydrogen concentrations, increasing infrared absorption at 1300 cm−1 due to boron icosahedra, increasing optical band gaps, dramatically increased microhardness, and increased adhesion to the underlying substrates of these thin films. These results provide evidence that the presence of boron icosahedra increases microhardness, adhesion, and optical band gaps.