What a ceramic knife is made of
Ceramic knives are made from zirconium dioxide (ZrO2), which is obtained by special processing of the mineral zircon. Zircon (ZrSiO4) is a material belonging to the class of silicic acid salt minerals, which was discovered by the German chemist M.G. Klaproth in 1789. Zirconium (Latin: Zirconium; denoted by the symbol Zr) is a material of the periodic system, with atomic number 40. It is a lustrous metal, silvery gray in color. It is highly ductile and resistant to corrosion. Zirconium compounds are widely distributed in the lithosphere. In nature, its compounds are known exclusively with oxygen in the form of oxides and silicates. Despite the fact that zirconium is a diffuse element, there are about 40 minerals in which zirconium is present in the form of oxides or salts. The most common in nature are zircon (ZrSiO4), baddeleyite (ZrO2) and various complex minerals.
Zircon is the most common zirconium mineral. It occurs in all types of rocks, but mainly in granites and syenites. In Hinderson County, North Carolina, USA, zircon crystals several centimeters long have been found in pegmatites, and crystals weighing several kilograms have been found in Madagascar. Baddeleyite was discovered in 1892 in Brazil. The main deposit is in the Posus di Caldas region of Brazil. The largest zirconium deposits in terms of size are located in the United States, Australia, Brazil, and India.
Raw materials for zirconium production are zirconium concentrates with mass content of zirconium dioxide not less than 60-65%, obtained by enrichment of zirconium ores. The largest volumes of zirconium production are concentrated in Australia (40%) and South Africa (30%). The main methods of obtaining metallic zirconium from concentrate are chloride, fluoride and alkaline processes.
Zirconium has been used in industry since the 1930s, but its high cost limited its use. Metallic zirconium and its alloys are used in nuclear power. Zirconium has a very low thermal neutron capture cross section and a high melting point. Another application of zirconium is alloying. In metallurgy it is used as a ligature. It is used as a deoxidizer and deazotator. Zirconium alloying of steels (up to 0.8%) increases their mechanical properties and machinability. In industry, zirconium dioxide is used in the production of zirconium-based refractory materials, ceramics, enamels, glasses. It is used in dentistry for dental crowns. It is used as a superhard material. Zirconium dioxide conducts current when heated, which is sometimes used to produce heating elements that are stable in air at very high temperatures. Heated zirconia is able to conduct oxygen ions as a solid electrolyte. This property is used in industrial oxygen analyzers and fuel cells. What distinguishes zirconium ceramics from other materials is its tremendous heat resistance and hardness, which is usually not less than 80 HRC. In addition, zirconium oxide is completely unreactive to most acids, alkalis and other active substances.
Zirconium oxide is obtained from zircon by chemical processing with additives. The resulting powder is mixed with additives. There are sintering additives, which affect the sintering characteristics and quality of the finished ceramics, and auxiliaries, which aid in molding. Accordingly, zirconia blanks are manufactured by various techniques. In particular, it is possible to alloy zirconium dioxide with oxides having a cubic crystal lattice. The most commonly used oxides for this purpose are oxides of the elements – calcium and magnesium, as well as metals – iron, manganese, chromium. In addition, zirconium oxide is often alloyed with aluminum oxide. The alloying oxides can change the color of ceramics from white to black (black color can also be obtained by special treatment). For example, this is used in the coloring of fianites – artificial diamonds based on cubic zirconium oxide.
Zirconium dioxide has a high hardness, which is measured using the Mohs hardness scale of materials. The hardness of zirconium dioxide on the Mohs scale is about 8.5 units, while the hardness of steel on this scale, depending on the heat treatment, from 4 to 7 units, corundum about 9 units, diamond 10 units. Thus, the material from which ceramic knives are made, in terms of hardness is close to diamond. Zirconium ceramics is also used in jewelry, in the aviation industry and mechanical engineering, in dentistry. Zirconium dioxide has more than 80 times the wear resistance of steel.
HOW TO MAKE CERAMIC KNIVES
The technical process of creating zirconium blades is as follows: obtaining alloyed zirconium oxide powders, preparation of press compositions and pressing, firing at high temperature (1350C+, in some cases up to 1700C), hot isostatic pressing at high temperatures and pressure.
The process of making ceramic knives is quite labor-intensive. To produce a ceramic blade, zirconium dioxide powder is first pressed under a pressure of 300 tons per square centimeter, then heat treated at temperatures of 1600-2000 degrees Celsius in special ovens for a long time (from two to six days). At the same time, zirconium dioxide crystals are sintered and the process of forming blanks is underway. The longer the product is kept in the furnace, the stronger it becomes. Depending on the specifics of the technological process, black or white ceramics are obtained. Black ceramics are made by adding a special black dye and keeping the workpieces in the kiln for a longer period of time, as a result of which they become stronger. The quality of ceramic knives varies greatly, as it is highly dependent on the technological capacity of the manufacturer, and the adherence to a complex technological process.
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PLUSES AND MINUSES OF CERAMIC KNIVES
The properties of zirconia ceramics depend significantly on the technology used to produce them, ranging from the purity of the initial zirconia powder, alloying system, powder granulometry, sintering regimes, etc.
In terms of mechanical properties, zirconium ceramics are significantly inferior to the most common steels, in particular, in terms of bending strength by a factor of about two, and in terms of impact strength by several times. This severely limits the versatility of ceramic knives. Because of their brittleness, most manufacturers urge not to use these knives for meat with bones, frozen foods, work on hard surfaces (glass, ceramic), etc. However, it should be noted that ceramics have unique properties that are superior to steel in terms of corrosion resistance and inertness to food.
CHARGING A CERAMIC KNIFE
Due to the fragile nature of the cutting edge, a ceramic knife requires quite large sharpening angles. On average, it is recommended to sharpen it to a full angle between 30-40 degrees. Sharp angles of 20 degrees or less are contraindicated for such knives, as the fragility of the cutting edge at this angle of sharpening becomes very high. Sharpening of ceramic knives is complicated by the fact that in the process is not formed burr and control of the angle must be maintained with the help of special devices, primarily electronic angle meter. Thus the manual sharpening of ceramic knives, without the use of sharpeners, requires an extraordinary, virtuoso skill from the sharpener.
Not all abrasives can handle sharpening a ceramic knife. Budget stones made of silicon carbide and aluminum oxide can’t handle these knives. The quality of the grinding powder and bond plays a key role here. The American Boride CS-HD sharpening stones are very effective in sharpening ceramic knives. The grain size of the stone should not be very coarse, in particular the Boride CS-HD for sharpening ceramics should be started with a 320 grit stone, because a coarser abrasive will cause cracks on the cutting edge. Obviously, the reason for this result is the very high quality of the silicon carbide powder and ceramic porcelain bond used in this American manufacturer’s products.
Electroplated and organic bonded diamond stones also perform well when sharpening knives. A little less active in sharpening them are elboron stones, which do not remove the zirconium layer as quickly as diamonds. However, all these abrasives are suitable for this sharpening and produce a good cutting edge condition.
