Metal coatings (also called claddings) came up in the mid 1960s for diamond grits in resin bonded wheels [DYER79]. The metallic coatings were in weight equal or exceeded the weight of the diamond grit [DYER79]. Earlier attempts with very thin metallic coatings, primarily in metal bond wheels, had not been successful [DYER79].
Diamond in resin bonded tools, in particular for grinding cemented carbides, are usually coated with nickel comprising about 55 % by weight of the grit and coating [MALK08, p. 25, DYER79]. The optimum percentage of nickel coating does not change significantly with the grit size [DYER79].
2.7.1.1 Manufacture of Metallic Coatings
Grit coatings are applied by various technologies, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless coating, electrolytic wet chemical methods, or dry deposition (Table 2.3) [KOMP05, [FARK72]. The different coating processes lead to certain chemical signatures, coating morphologies, and effects on the grit material [KOMP05].
Several layers can be applied sequentially; for example, the diamond is coated with a Ti layer by electroplating, electroless plating or PVD processes first, then a Ni-coating is applied by electroplating [CAVE75]. An additional heating process induces the formation of alloys between the layers and the substrate [CAVE75]. The coated grains can additionally be etched to enhance grinding performance in resin bonded wheels [SEAL70].
The tool manufacturer has to control the tool production process carefully, because grit coatings change the grit weight and grit size. Therefore, the theoretical and real grit concentrations differ. Measuring of coating weight for diamond and CBN grains is also introduced in the FEPA 62/93 standard [MENA00].