Amorphous diamond (a:D) is a hard, electrically insulating, inert and transparent form of carbon that has the sp(3) bond character of crystalline diamond, but lacks a long-range ordered structure. Using our filtered cathodic are system, we have produced a:D films at room temperature that demonstrate Vickers hardness above 8000 H-v, hydrogen content below 0.1%, density of 2.7 +/- 0.3 g cm(-3), and adhesion on tungsten carbide and silicon substrates above 70 MPa. The fine structure of a:D was characterized by transmission electron microscopy (TEM) and electron diffraction. TEM showed no evidence of any ordered structure down to 1 nm. Unlike natural diamond or diamond-like carbon, a:D has a flat transmission spectrum from 0.8 to over 50 mu m, which is due to its amorphous nature and the lack of hydrogen. Also, we determined the index of refraction of our a:D to be 2.47-2.57. The thermal diffusivity of a:D has been measured and reported for the first time. We have lowered the intrinsic stress in the films by the use of bias and the inclusion of impurity atoms, and produced films up to 8 mu m thick on carbide tool bits. In addition, a molecular dynamics code has been used to model this material.