First, we studied the relation between the sp3 bond ratio and the hardness of 100-nm-thick tetrahedral amorphous carbon (ta-C) films deposited by a Filtered Cathodic Arc (FCA) system at different substrate bias voltages. For comparison, sputtered amorphous carbon (a-C) films and Highly Oriented Pyrolytic Graphite (HOPG) were also analyzed. According to the results, ta-C film deposited at a -70 volt substrate bias voltage had high hardness and high sp3 bond ratio of up to 88 GPa and 85%, respectively, whereas those of sputtered a-C were 29 GPa and 28%. Furthermore, we found that the hardness of carbon films, including sputtered a-C and HOPG, increased with increasing sp3 bond ratio. Based on this relation, the chemical bond structures of carbon films are considered to be closely related to their mechanical properties. Secondly, we investigated the relation between sp3 bond ratio and ta-C film thickness, over the range from 1 to 10 nm. The measurements showed that a 2-nm-thick initial layer grown on the surface of CoCrPt magnetic film had low sp3 bond ratios. It is suggested that this reduction in sp3 bond ratio in the initially grown layer seriously degrades the film's performance as a hard disk media overcoat. Further efforts to improve ta-C film processing will be required to improve its mechanical properties.