metal layers may be applied by variations of the chemical vapor deposition technique discussed above while the nib layer may be applied by electroless coating some interfacial bonding may occur during coating and subsequent processing and this is not at all undesirable thus in the three layer structure the conditions may be such that the chromium chemically bonds to the diamond material to form a layer of chromium carbide during coating while the tungsten subsequently bonds to the chromium carbide layer after the larger size particles are cleaned and coated as described above they are placed into a suitable container and vibrationally packed the vibrational packing operation involves vibrating the container and its contents while applying a weight to the layer of superhard particles this causes the larger size particles to distribute themselves evenly along the bottom of container in one or more layers thereafter if desired a secondary charge of smaller size particles about 1 6 to 1 9 the size of the larger size particles is introduced into the container the secondary charge comprises preferably superhard particles generally single crystal diamonds or other abrasive particles in the size range of about 25 75 microns preferably about 50 65 microns 230 270 mesh to which a coating of nib about 1 10 microns thick preferably about 5 microns thick has been applied by methods such as those discussed above the secondary charge is added in amounts which comprise about 10 to 30 by volume of the initial charge of larger size particles the purpose of the secondary charge is to increase the concentration of abrasive material by.