We have investigated the suitability of nanodiamond (ND) and ALN coated Si tip arrays for cold cathodes by means of a field emission scanning microscope combined with. scanning electron microscopy. The ND coated tips required an average extraction voltage U(10 nA) of 360 V for 10 mum electrode distance and yielded stable emission up to currents of 10-50 muA. The rather nonuniform emission distribution of the arrays (80 V < U(10 nA) < 1800 V) was correlated with the varying morphology of the ND tips. Field-enhancing nanoprotrusions were observed at the apex of the strongest emitters. The ALN coated tips showed strong activation effects at emission currents below 10 nA or after annealing at 250 degreesC resulting in an average U(10 nA) of 410 V, improved lateral emission uniformity (150 V < U(10 nA) < 600 V), and stable currents up to 1-10 muA. Direct high current processing of such emitters led to a partial destruction of the apex. Different field emission mechanism will be discussed for ND and AIN coatings.