Extremely smooth (6 nm roughness) and continuous ultrananocrystalline diamond (UNCD) thin films were achieved by microwave plasma chemical vapor deposition using a thin 10 nm tungsten (W) interlayer between the silicon (Si) substrate and the diamond film. The W interlayer significantly increased the initial UNCD nucleation density to >10¹² sites/cm², thereby lowering the surface roughness and eliminating interfacial voids. A method is also disclosed to make various articles.