Nanostructured diamond (NSD) films were deposited by chemical vapor deposition (CVD) on both 440C stainless steel ball bearings and 316 stainless steel discs through the use of a metal-boride interlayer. The interlayer is deposited by means of a novel method using microwave plasma CVD with a feedgas mixture composed of hydrogen (H2) and diborane (B2H6). Substrate temperature was found to have a profound effect on the boride interlayer composition and stoichiometry, forming either a predominantly CrB or Fe2B interlayer. Fe2B-based interlayers were found to be effective in blocking the mutual diffusion of carbon and iron that would otherwise produce graphitic carbon and lead to poor NSD adhesion. Fully continuous and adhered nanostructured NSD films are grown using this interlayer, particularly on 316 steel discs.