Diamonds and CBNs for sharpening knives

Natural diamonds

A diamond is a mineral, the only precious stone consisting of a single element. Also, diamond is one of the allotropic modifications of carbon and the hardest of substances. The density of the diamond is 3.48—3.56 g/cm3, and the microhardness is 8600-10000 kgf/mm2.

Diamond is 96-99% carbon. Also, its composition contains (within 0.2-0.3%) other elements too: boron, silicon, nitrogen, oxygen, aluminum, manganese, copper, as well as impurities of iron, nickel, titanium, zinc, and other elements. There are inclusions of graphite, olivine, pyrope, etc. Diamond crystallizes in the cubic (isometric) classification of crystal symmetry groups and occurs as octahedron or crystals of similar shape. Usually, diamonds have a transparent and yellowish color, but there are also blue, green, cherry, bright yellow, pink-purple stones. Diamonds can be completely transparent or cloudy. 

Top-quality diamonds are produced in South Africa and Russia in kimberlites - volcanic rocks composed mainly of olivine and serpentine. Kimberlites occur in the form of tubular bodies ("explosion tubes"). Moreover, diamonds are extracted from the river and coastal-marine pebble placers, where they were carried out as a result of the destruction of diamond-containing kimberlite volcanic breccia. 

There is great internal stress inside diamonds since their formation took place under conditions of enormous pressures in the depths of the mantle of our planet and presumably 3 billion years ago they were brought to the surface. According to the shape of crystals, diamonds can be: planar, curved, deformed, elongated, stepped, polycentric, etc.

For several factors, the diamond has unique properties. At medium temperatures, it is chemically inert, and at high temperatures, it acquires chemical activity. When heated, the diamond dissolves in molten sodium and potassium nitrate and soda. In molten alkali carbonates at 1000-1200 degrees, the diamond turns into carbon monoxide. Certain metals, such as iron and nickel, partially dissolve the diamond at a temperature of more than 800 degrees.

In nature, diamonds are found in the form of individual crystals, their fragments, or polycrystalline aggregates. There are jewelry and technical diamonds. Jewelry includes diamonds of the correct crystal shape, transparent, without cracks, inclusions, and spots. All other crystals, as well as polycrystalline varieties, belong to technical diamonds. Technical diamonds are pre-processed to separate them by shape and size, as well as to isolate diamonds with higher strength properties. At the same time, diamonds are crushed, polished, heat-treated, and metalized.

Diamond hardness is 10 on the Mohs scale, the highest among all minerals. The microhardness of diamond is 93157-98648 millipascals (mPa), while corundum is 20200 (mPa), topaz is 1399 (mPa), quartz is 981 (mPa). However, the diamond has anisotropy of hardness, that is, on different faces and in different directions, the hardness is slightly different. This is connected with the structure of the mineral.

Turning to the abrasive properties, it should be noted that the average wear resistance of diamond is several times higher than the wear resistance of boron carbide and silicon carbide. If we take the abrasiveness of diamond for 1, then the abrasive capacity of boron carbide will be 0.5-0.6, and silicon carbide 0.2-0.3. The modulus of elasticity (in mPa) of a diamond is 88254 (boron carbide has about 294180, silicon carbide 357919, hard steel alloy up to 588360). The diamond can deform when exposed to the processed material. In this regard, when processing various materials with diamond, the specific pressure and temperature are several times lower than when using other abrasives. The bending strength of a diamond is low – 206-490 mPa, which is three to four times less than that of a hard alloy of steel, the compressive strength of diamonds is on average 1961 mPa, which is half the average compressive strength of a hard alloy of steel. The compression ratio of diamond and the compressibility modulus is four times less than iron.

Diamond is an almost perfect abrasive that can effectively handle any steel, work quickly, and not significantly pollute the workplace. It is slowly worked out and slowly glared. At the same time, it should be noted that rough diamonds on a galvanic bond can compete with other abrasives at a price too.

Synthetic diamonds

In addition to natural diamonds (designation A), today synthetic diamonds (designation AC) are also actively used. Synthetic and natural diamonds have the same properties: the same crystal lattice, density, hardness, etc. They differ only in the shape of the grains, surface qualities, strength, and fragility.

Today, there are three main technologies for producing synthetic diamonds: HPHT-diamonds, CVD-diamonds, and PCD - diamonds. HPHT methods - the use of high pressure and high temperature, requires the use of multi-ton presses that can develop a pressure of up to 5 hPa at 1500 °C. Chemical vapor deposition (CVD) is a method in which a plasma of carbon atoms is created over a substrate, from which the atoms gradually condense to the surface to form a diamond. This technology allows you to obtain diamonds of different geometric sizes. Also, there are technologies for creating semi-crystalline diamonds (PCD).  In PCD tools, diamond segments are bonded using high-temperature soldering on a carbide substrate. PCD elements are produced by sintering micron powders of synthetic diamonds to bond the particles in a process characterized by high temperature and pressure. This material is produced using cemented carbide firing, and when cobalt is added, a sintering process occurs. During this process, the metal from the hard-alloy carbide substrate penetrates between the diamond grains, ensuring their adhesion.

The types of synthetic diamonds are very diverse. Synthetic diamonds ASO, ASR, ASV, ASK and ASS (rus. АСО, АСР, АСВ, АСК, АСС ) are produced in sizes from 0.04 to 0.63 mm, micro-powders - ASM and ASN (rus. АСМ, АСН) - in sizes from 1 to 60 microns.* Grains of ASO diamond have the lowest strength, ASS — the highest. Depending on the grain size, ASS diamonds have a strength 1,3—2 times greater than that of natural diamonds. The performance properties of synthetic diamond grinding powders depend on the shape of the grains, the nature of their surface, and mechanical strength. The most developed surface is the ASO diamond grains, and the smoothest is the ASS.

CBNs

CBN is a superhard material based on the cubic β-modification of boron nitride or cubic boron nitride. Other names: borazon, kubonit, qingsongite, kiborit. In hardness and other properties, it is close to diamond and has a hardness of 10 on the Mohs scale. The chemical formula is BN. Cubic boron nitride obtained for the first time in 1957 by Robert Ventham (Robert H. Wentorf Jr.) for the General Electric company. In 1969, the trademark "Borazon" was registered. In the USSR, cubic boron nitride was synthesized in 1960 at the Institute of high-pressure physics and was named "Elbor" (Leningrad borazon).

Cubic boron nitride is a synthetic superhard material that is close to diamond in hardness but has higher heat resistance. It is a chemical compound of two elements — boron (43.6%) and nitrogen (56.4%). It has a crystal lattice with almost the same structure and parameters as a diamond. The color may vary from white (colorless) to black. Synthesized crystals are divided into different grades. Synthesis can occur in the lithium-boron-nitrogen system or the magnesium-boron-nitrogen system.

CBN is almost as hard as diamond. According to this parameter, it is 3-4 times higher than the hardness of traditional abrasives, and has significantly less wear of CBN grains during grinding and retains their sharpness for a long time. Another important property of CBN is its resistance to temperatures. Grain surface oxidation begins at 1000-1200 °C, as opposed to 600-700 °C for diamond. These grinding temperatures are instantaneous and occur only under very harsh grinding conditions. Therefore, CBN grain is very little to wear out from thermal loads. CBN also has a high chemical resistance. It does not react with acids and alkalis and is inert to almost all chemical elements that are part of steels and alloys. The main advantage is the inertia of CBN to iron, while diamond dissolves well in iron, which is the reason for the intensive wear of diamond wheels when grinding steels. CNB abrasives can withstand very high processing speed and high temperature. As well as diamonds, CBN is best worked on steels with a hardness of more than 55 HRC.  Wear of CBN on softer steels will be faster since the soft steel pulls out the abrasive grains and leads to their rapid wear.

CBN as the abrasive material has a series of advantages when sharpening knives. It preserves the sharpness of the grain for a long time (high wear resistance), withstands high thermal loads, does not require the use of water or oil, and is slowly glared. The production technology allows you to synthesize boron nitride of any size, so CBN blanks are suitable for rough sharpening and finishing of the knife.