The mechanism of diamond surface coating in abrasive tools

Development of surface plating on diamond and CBN abrasives

The use of metal-coated diamond particles has been known for more than 90 years, but it was not until 1965 that interest in diamond particles was revived when nicodur suggested, based on the grinding mechanism, that diamond particles coated on the surface could increase the service life of the grinding wheel by more than 50%. After 1966, diamond abrasives with copper and nickel plating appeared in De Beers, the United States and the Soviet union. This abrasive was first used in resin-bonded grinding wheels with great success. Later development of plating titanium, molybdenum, tungsten, niobium, tantalum metal and non-metal materials such as ceramic refractory compounds such as coating, and expand the application in sintering metal bond, electroplated bond, ceramic bond grinding tools and diamond saw blades, drill bits and other products, are also single layer coating to multilayer, such as copper, nickel, titanium, nickel and metal and nonmetal composite coating. Surface coating is not only used for grinding, but also for processing of micro powder and polycrystalline raw materials. 

Metal plating method, initially using wet plating -- electroless plating and electroplating, later, vacuum evaporation plating, sputtering plating, ion plating and other dry plating methods, also have been applied to superhard materials surface plating up.

At present, surface plating has become one of the main approaches for the development of diamond varieties in the world together with single crystal synthesis, separation, polycrystalline sintering and gas deposition.

Surface plating is not only suitable for diamond, but also for cubic boron nitride (CBN). A similar surface coating method is used for some special purpose corundum and silicon carbide abrasives.

The coating can improve the particle strength of the abrasive

For low strength abrasives used in resin-bonded abrasives, the compressive strength of both diamond and cubic boron nitride is significantly improved after plating. When the weight increased by 30%, the strength increased by 30% ~ 50%. When the weight increase is 50%, the strength increases by more than 50%. Copper plating is more significant than nickel plating in strengthening abrasive particles.

After coating the surface of abrasive particles, the defects of particles can be made up, pits, holes, cracks are filled; Moreover, the outer surface of the brittle abrasive particles is covered with a layer of metal with high toughness. At this time, the abrasive particles are no longer just brittle particles, but composite particles with a ductile surface. These are the reasons for the increase in particle strength.

The coating improves the bonding strength of the abrasive and the binder

In resin-bonded grinding wheels, the utilization rate of diamond or CBN abrasives without surface coating is very low, and 60% ~ 70% abrasive particles fall off prematurely when they are not fully utilized. The reason is that the smooth surface of the abrasive particles and resin combination is not firm.When a layer of metal coating on the surface, the coating is cloud, wave or burr, rough surface, uneven, and resin can be very good combination; Coating, on the other hand, tightly wrapped in the inner surface of the abrasive, the coating and the abrasive made mechanical bond strength is much higher than the combination of metal powder and abrasive pressure molding intensity: not only the mechanical bonding strength increased, at the same time, due to the coating metal in the atomic state deposit on the particle surface, combined with surface is very close, the distance is very small, in the range of van der Waals stress field, and therefore the role of van der Waals force; Some metal coatings (such as nickel coatings) and diamond chemical adsorption.

As mentioned above, the bonding strength of the abrasive and the binder is greatly increased by this bridging of the coating between the abrasive and the binder.

In metal binder products, the coating also has a similar effect of improving the bonding strength. Some coated metals, such as titanium and tungsten, even react with diamond to form metallic carbides.

The coating protects the abrasive and prevents graphitization

During high temperature sintering and high temperature grinding, the coating can protect the diamond from oxidation, graphitization, solvation or carbides. The reason is that the coating around the diamond keeps out the oxygen and other harmful impurities.

Some metallic coatings, such as nickel, prevent the graphitization of diamond crystal structures. The metal atoms are attracted to their carbon counterparts to form bonds, just as a tent is secured by a rope. When one side of the diamond is exposed in the grinding process to produce the graphitization tendency, the other side of the metal is around to protect the regular tetrahedron structure of the diamond, thus inhibiting the graphitization process.

Nickel and diamond can be attracted to each other, because of the nickel in the unit cell (111) face is an equilateral triangle, the side with the carbon graphite (0001) of six ring inscribed is the side of the triangle is very close, and nickel - outer shell with d with holes, so prone to three adsorption, make flat carbon crease into six ring as shown in figure 8 and figure 9 diamond structure.

Coatings give abrasives beneficial physical properties

Coating the surface of the abrasive particles with different coatings can correspondingly give the abrasive particles beneficial physical properties (thermal conductivity, electrical conductivity, magnetism), so as to improve their performance.

In the absence of gold plating, the diamond passes the heat generated by grinding directly and rapidly to the resin, which is slow to transfer heat outward and thus vulnerable to thermal damage.

After metallized, metal coating of thermal conductivity is slower than the diamond, in the process of grinding wheel high-speed rotation, the heat generated by the grinding parts of the diamond grains, must pass a metal coating, and then passed to the binder, have played an important role a heat buffer, made contact with grits resin can avoid impulse type of instantaneous high temperature shock and not burnt metamorphic, to maintain its control ability of abrasive.

The surface of resin grinding wheel is not metallized abrasive, the premature peeling rate during grinding is 60% ~ 70%; Metallized abrasives have an early shedding rate that drops to 30 to 40 percent or less, resulting in significant increases in durability, typically 50 to 100 percent, or even 200 percent.The fundamental reason for this effect is that the metal coating protects the resin by slowing down the heat pulse. This is one of the most important functions of metal coatings.