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Nanodiamond ͑ND͒ powder coatings are very promising for cold cathode applications. The suitability of a ͑di͒electrophoretically deposited ND coating ͑crystallite size: 1-10 nm͒ for high current applications was analyzed depending on three types of Si substrates ͑flat, rough, Si tip array͒. Nondestructive direct current field emission ͑FE͒ investigations were performed by means of a field emission scanning microscope. FE properties like the electrical onset field strength E on ͑at 0.5 nA͒, current I vs E behavior, maximum current carrying ability as well as the local distribution and uniformity of emitters were analyzed by using anodes of different apex diameters D. A decrease of E on for Dϭ10/100/1000 m from 203/190/109 V/m of the flat to 67/44/20 V/m of the rough, and to Ϸ7/6.7/4.5 V/m of the tip array sample is achieved by the geometrical field enhancement of the structures. The reproducible high-current density J REP Ͼ15 A/cm 2 for Dϭ10 m of the flat and rough sample exhibits the large potential of the ND coating for power applications. Their nonuniformity due to the m-roughness results, however, in a low J REP for large areas. In contrast, ND coated tips showed a good uniformity over mm 2 size and a moderate J REP Ϸ0.2 A/cm 2 , basing on about 40% of significantly emitting tips.