The field emission of diamond is promising for high-power applications. According to theoretical analyses, the field emission (FE) of insulating diamond should be governed by the diamond-substrate interface. Therefore, we have investigated the influence of differently p- and n-doped Si(100)-substrate types on the FE properties of thick, oriented, and locally insulating diamond films, grown in a microwave plasma-assisted chemical vapor deposition setup. Local FE measurements were performed by means of a field emission scanning microscope with variable lateral resolution R greater than or equal to 100 nm. By using anodes of 1 mu m tip diameter, very high maximum reproducible local current densities J(REP) (from mu m(2)-sized areas) up to 8860 A/cm(2) could be achieved at vacuum field strengths E of typical 1200-3000 V/mu m, corresponding to 20-50 V/mu m in the diamond near the substrate. The current I vs E behavior, the reproducibility, and JREP depended strongly on the substrate type. FE mapping over a 10 x 10 mu m(2) sized area revealed a uniform emission on a 100 nm scale. In order to realize high currents for power applications, the uniformity of the substrate-diamond interface, as well as of the film smoothness, should be improved.