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We observe that an Au interlayer markedly enhances the electrical field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films on Si substrates. The EFE properties of UNCD/Au/Si films can be turned on at a lower field and attain a higher current density than in UNCD films grown on Si substrates without an Au interlayer. Transmission electron microscopy reveals that the Au interlayer induces the formation of SiC clusters, preventing the formation of a resistive amorphous carbon layer that nucleates the diamond clusters. This improves the diamond-to-substrate interfacial conductivity. Moreover, there is an abundant nano-graphite phase, which is presumably induced by the coalescence of nano-sized diamond clusters. The percolation of the nano-graphite clusters helps transport electrons, improving the conductivity of the UNCD films. We believe that the simultaneous increase in the conductivity of the UNCD-to-Si interface and the bulk of the UNCD films is the main factor enhancing electrical conductivity and EFE properties of the films.