Details are given of experiments carried out to compare the field electron emission characteristics of single crystal silicon lip arrays with and without amorphous diamond (a-D) coating. The coatings on the tip arrays were prepared using the filtered vacuum ire plasma deposition technique. In addition, the optical and electrical properties of the a-D coatings were measured. An: anode probe technique was employed to measure the current-voltage characteristics and turn-on fields of the tip-array emitters before and after coating. It is found that the coating enhances the emission. The physical reasons for these effects are discussed.