Ceramic knife and how to sharpen it

The first historically known tools that resemble knives in their properties are flakes of obsidian-nuclei. I.e., products made of volcanic glass were used by the ancestors several hundred thousand years ago. And having come a long way in metallurgy, humanity returned to the use of ceramics at the end of the twentieth century. In 1985, the Japanese company Kyocera began manufacturing knives made of ceramic-based on zirconium dioxide. These knives were the result of the most advanced technologies at that time. To date, these knives have become very widespread, at an extremely low price.

WHAT CERAMIC KNIVES MADE OF 

Ceramic knives are made from zirconium dioxide (ZrO2), obtained as a result of special processing of the mineral zircon. Zircon (ZrSiO4) is a material belonging to the class of minerals of silicic acid salts, which was discovered by the German chemist M. G. Klaprot in 1789. Zirconium (lat. Zirconium; denoted by the symbol Zr) - in a periodic system, with the atomic number 40. It is a silver-gray shiny metal. It has high plasticity and is 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. Although zirconium is a dispersed element, there are about 40 minerals in which zirconium is present as oxides or salts. In nature, they are most widely distributed: zircon (ZrSiO4), baddeleyite (ZrO2), and various complex minerals.

Zircon is the most common zirconium mineral. It is found in all types of rocks, but mainly in granites and syenites. In Henderson County (North Carolina, USA), zircon crystals several centimeters long were found in pegmatites, and in Madagascar, crystals weighing several kilograms were found. Baddeleyite was found in 1892 in Brazil. The main deposit is located in the Posus de Caldas region (Brazil). The largest deposits of zirconium are located in the United States, Australia, Brazil, and India.

The raw material for the production of zirconium is zirconium concentrates with a mass content of zirconium dioxide of at least 60-65%, obtained by enrichment of zirconium ores. The largest production volumes of zircon are concentrated in Australia (40%) and South Africa (30%). The main methods for obtaining metallic zirconium from concentrate are chloride, fluoride, and alkaline processes.

In industry, zirconium began to be used since the 30s of the XX century, but the high cost limited the volume of its use. Metal zirconium and its alloys are used in nuclear power engineering. Zirconium has a very small cross-section of thermal neutron capture and a high melting point. Another area of application of zirconium is alloying. In metallurgy, it is used as an alloy. It is used as a deoxidizer and a devastator. The alloying of steels with zirconium (up to 0.8%) increases their mechanical properties and workability. In industry, zirconium dioxide is used in the production of refractory materials based on zirconium, ceramics, enamels, and glasses. It is used in dentistry for the production of dental crowns. It is used as a superhard material. When heated, zirconium dioxide conducts a current, which is sometimes used to produce heating elements that are stable in air at very high temperatures. Heated zirconium dioxide can 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 their colossal heat resistance and hardness, which is usually at least 80 HRC.  Besides, zirconium oxide does not react at all with most acids, alkalis, and other active substances.

Zirconium oxide is obtained from zircon by chemical treatment with additives. The resulting powder is mixed with other additives. There are agglomeration additives that affect the sintering characteristics and quality of the finished ceramics, and auxiliary materials that contribute to the formation of the product. Accordingly, the billets of zirconium dioxide are made by various methods.  In particular, it is possible to alloy zirconium dioxide with oxides having a cubic crystal lattice. The most commonly used for these purposes are oxides of elements-calcium and magnesium, as well as metals-iron, manganese, chromium. Also, zirconium oxide is often alloyed with aluminum oxide. Alloying oxides can change the color of ceramics from white to black (black color can also be obtained with special MAINTENANCE conditions). For example, it is used in the coloring of phianite – artificial diamonds based on cubic zirconia.

Zirconium dioxide has a high hardness, which is measured using the Mohs hardness scale. 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, is from 4 to 7 units, corundum is about 9 units, and diamond is 10 units. Thus, the material from which ceramic knives are made is close to diamond in hardness. Zirconium ceramics are also used in jewelry, in the aviation industry, mechanical engineering, and in dentistry. The wear resistance of zirconium dioxide exceeds steel by more than 80 times.

HOW TO MAKE CERAMIC KNIVES

The technical process of creating zirconium blades is as follows: obtaining alloyed powders of zirconium oxide, preparing 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 time-consuming. To produce a ceramic blade, zirconium dioxide powder is first pressed at a pressure of 300 tons per square centimeter, then subjected to heat treatment at temperatures of 1600-2000 degrees Celsius in special furnaces for a long time (from two to six days). At the same time, the sintering of zirconium dioxide crystals takes place and the process of forming billets is underway. The longer the product is kept in the oven, the stronger it becomes. Depending on the characteristics of the technological process, black or white ceramics are obtained. Black ceramics are made by adding a special black dye and keeping the billets in furnaces for a longer time, as a result of which they become stronger. The quality of ceramic knives differs greatly from each other since it depends on the technological capacity of the manufacturer, and on the observance of a complex technological process.

The properties of zirconium ceramics depend significantly on the technology of its production, starting from the purity of the initial zirconium powder, the alloying system, the granulometry of powders, sintering modes, etc.

In terms of mechanical properties, zirconium ceramics are significantly inferior to the most common steels, in particular, in terms of bending strength by about two times, and in terms of impact strength by several times. This greatly limits the versatility of ceramic knives. Due to their fragility, most manufacturers do not recommend using these knives for meat with bones, frozen food, work on hard surfaces (glass, ceramics), etc. At the same time, it should be noted that in terms of corrosion resistance and inertia to products, ceramics have unique properties that surpass any steel.

A ceramic knife, due to the fragility of the cutting edge, requires quite large sharpening angles. On average, it is recommended to sharpen it at a full angle within 30-40 degrees. Sharp angles of 20 or fewer degrees for such knives are contraindicated since the fragility of the cutting edge at this angle of sharpening becomes very large. Sharpening ceramic knives is also complicated by the fact that the process does not form a burr and the angle control must be maintained using special devices, primarily a digital protractor. Thus, the hand sharpening of ceramic knives, without the use of sharpeners, requires from the sharpener extreme, masterly skills. 

Not all abrasive materials can cope with sharpening a ceramic knife. Low-cost stones made of silicon carbide and aluminum oxide can not cope with ceramic knives. The quality of the sanding powder and bond plays a key role here. American sharpening stones Boride CS-HD show themselves in sharpening a ceramic knife very well. The gritness of the stone should not be very rough, in particular, Boride CS-HD for sharpening ceramics should start with a stone of 320 grit, since a coarser abrasive will lead to the formation of cracks on the cutting edge.  Obviously, the reason for this result is the very high quality of silicon carbide powder and ceramic porcelain bond used in the products of this American manufacturer.

Also, good results when sharpening such knives are shown by diamond plates on a galvanic bond and diamond blanks on an organic bond. Slightly less active in their sharpening CBN blanks, which remove the zirconium layer is not as fast as diamonds. But all these abrasives are suitable for such sharpening and allow you to get a good cutting edge.