Purpose of Coatings — Хонинговальные головки

Grit coatings are applied for various reasons, such as grit retention in the bonding, grit protection, grit alignment, or heat transfer during the tool manufacturing process or tool use. One coating can have several useful purposes, which are described in the following.

2.7.1.2 Grit Retention in the Bond

Grit morphology and grit surface define grit retention in the bonding matrix [BAIL02]. The metal nodules of coatings enlarge the effective grit surface and increase the roughness of the grit leading to a better mechanical grip (Fig. 2.23) [METZ86, p. 42, YEGE86]. In addition, the coating changes the wettability of the coated grit by the bond ingredients [METZ86, p. 42]. Especially for diamonds in resin bonds, metal coatings form a more cohesive connection with the bond than uncoated diamond surfaces [DYER79].

Table 2.3 Metal grit coating methods

Coating method	Processing temperature (°C)	Coating types	Bonding and example applications	Reference
Electroless coating — the abrasive grits are immersed in a metal alloy solution with reducing agents	<100	Ni, Cu, Ag alloys, thick layer	Phenolic and polyimide resin bonds for carbide and cermet grinding	[KOMP05]
Electrolytic coating, electroplating—the abrasive grits are wired as cathode; the metal to be deposited is the anode.	<100	Thick layer	Phenolic and polyimide resin bonds for carbide and cermet grinding, bronze saw tools for tile trimming and stone polishing	[KOMP05]
Physical vapor deposition (PVD)	*500	Elemental metals, such as Ti, W, thin, 2-10 pm layer	Unproven utility in metal bonds	[KOMP05]
Chemical vapor deposition (CVD)	600-800	On diamond: Carbides of transitional metals, on CBN: Metal nitride or boride, thin, 2-10 pm layer	Metal bond saws for reinforced concrete sawing, stone or rock-drilling; glass grinding wheels; ferrous material honing stones	[KOMP05]
Dry deposit method —grits are mixed with metal powder until they are covered, then the covered grits are heated	350-1000	Transition metals		[FARK72]

Fig. 2.23 Schematic appearance of two common grit coatings

Copper Ti

Purpose of Coatings

In the manufacturing of vitrified bonded tools, chemical reactions between superabrasives and the bond likely occur leading to sufficient grit retention. In resin or metal bonds chemical reactions between grit and bond are less likely. Therefore, grit coatings are applied on superabrasives to allow chemical alloying between coating and bonding and to enhance grit retention [KOMP05]. For metal bonds, common coatings for Ni-, Co — or Fe-based bonds consist of Ti; for bronze-based bondings Cr is used as grit coating material [MARI04].