Metal bond, according to its main purpose, should hold the diamond grains on the working surface of the tool and at the same time withstand mechanical and thermal loads. Naturally, the main criterion of its quality is the intensity of its wear, it should be minimal. This is achieved by simply increasing the hardness of the bond to the maximum possible. Baking of such a binder usually takes place at temperatures of 900-1300 degrees Celsius. Plasticity also plays a major role in the performance of the binder, as the binder must provide complete coverage of the abrasive grains, without gaps. In addition, the effectiveness of the binder depends on the thermal conductivity, which determines the intensity of heat flux removal and ensuring minimal heating of the working area of the tool. The number of materials that can give such characteristics is not great. The most common are copper-aluminum, copper-tin and aluminum-zinc bonds.
For example, in the industry, special metal binders M1-01 and M1-06 are used for grinding carbide at elevated regimes. These are aluminum-zinc binders. In addition to the two metals, the binder also contains copper. This alloy has a very complex composition and technology of its melting. In the material structure of the binder there are three phases: a solid solution based on the intermetallide CuAl2, an alloy of aluminum with zinc and silicon crystals. Due to the complexity of production and price, this binder is not relevant for the purposes of knife sharpening and is practically not used in the manufacture of sharpening stones.
The most suitable composition and qualities for sharpening knives and tools, has become a copper-based binder. Copper has above all high ductility with sufficiently high hardness and wide availability, and acts as a matrix material for the manufacture of diamond tool bonds. However, it is not used in pure form in bonding, as it leads to very intensive “salting” and thus to loss of cutting ability of the diamond layer, especially when sharpening and grinding hard alloys and composites. During processing, the hard alloy forms heaps on the side surface of diamond grains and buildup (growth) on the tops, which, interacting with the bond, contribute to the development of “salting”. Therefore, it is necessary to add to copper other materials that reduce its negative qualities. Among such materials are usually lead, nickel, boron, but the most popular is tin. It is these two metals: copper as a base and 20% tin as an additive, and created the most popular metal bond M2-01.
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M2-01 (previous designation – M1), is used for manufacturing diamond tools recommended for grinding carbide products, carbide multi-blade tools and their sharpening. The structure of M2-01 bond of copper-tin system consists of grains of α-(solid solution of tin in copper) phase, between which there are columns of eutectoid (alloy in which recrystallization (polymorphic transformation) occurs with simultaneous formation of two new phases), consisting of a mixture of δ- and α-phases. The α-phase is characterized by relatively high strength and ductility. The strength and ductility of the δ phase are lower, but this phase has a higher hardness than the α phase. The ratio between the volumes of α-phase and eutectoid is 40:60.
Due to this structure, M2-01 bond has high strength properties, in particular, its compressive strength is 830 MPa and hardness is 89 HRB (Brinell scale). At the same time it has a good enough plasticity and thermal conductivity, almost 2 times higher than the thermal conductivity of the binder based on the copper-aluminum-zinc system. This bond also has a higher strength of diamond-holding – the tear-out force of a diamond grain from this bond has a value of 8.2 N (Newton), approximately 0.836 kilogram-force. The main disadvantage of this bond is the previously mentioned rapid “salting” of the bar. But for the purposes of manual knife sharpening at low speed, this aspect does not play a major role. That is why manufacturers actively use such bonds in the production of powder-filled stones based on natural or synthetic diamonds, as well as elboros.
An example would be a premium grade elboror bar. The bar is made from cubic boron nitride (elboron). The volume content of elboron in the bar is 100% with a uniform distribution throughout the entire volume. This allows working on both sides of the bar (monolayer bars). The geometry of the working plane on both sides is even, the bar does not need leveling, the working layer is opened during work. The bar works effectively on almost all types of steel, including super-hard powdered high-speed cutters such as Rex 45 or Maxamet.