A ceramic knife is a knife

Knife

A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

 made out of very hard and tough ceramic

Ceramic

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

, often zirconium dioxide

Zirconium dioxide

Zirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2...

 (ZrO₂; also known as zirconia). These knives are usually produced by dry pressing zirconia powder and firing them through solid-state sintering

Sintering

Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...

. The resultant blade is sharpened by grinding the edges with a diamond-dust-coated grinding wheel

Grinding wheel

A grinding wheel is an expendable wheel that is composed of an abrasive compound used for various grinding and abrasive machining operations...

. Zirconia ranks 8.5 on the Mohs scale of mineral hardness

Mohs scale of mineral hardness

The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...

, compared to 6 to 6.5 for hardened steel, and 10 for diamond

Diamond

In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

. This very hard edge rarely needs sharpening.

Zirconium oxide

Zirconium oxide is used due to its polymorphism

Polymorphism (materials science)

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...

. It exists in three phases: monoclinic

Monoclinic crystal system

In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. They form a rectangular prism with a...

, tetragonal

Tetragonal crystal system

In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...

, and cubic

Cubic crystal system

In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....

. Cooling to the monoclinic phase after sintering causes a large volume change, which often causes stress fractures in pure zirconia. Additives such as magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

, calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 and yttrium

Yttrium

Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...

 are utilized in the manufacture of the knife material to stabilize the high-temperature phases and minimize this volume change. The highest strength and toughness is produced by the addition of 3 mol% yttrium oxide yielding partially stabilized zirconia. This material consists of a mixture of tetragonal and cubic phases with a bending strength of nearly 1200 MPa. Small cracks allow phase transformations to occur, which essentially close the cracks and prevent catastrophic failure, resulting in a relatively tough ceramic material, sometimes known as TTZ (transformation toughened zirconia).

Properties

Ceramic knives will not corrode in harsh environments, are non-magnetic, and do not conduct electricity. Their resistance to strong acid and caustic substances, and their ability to retain a cutting edge longer than forged metal knives, make them a suitable culinary tool for slicing boneless meat, vegetables and fruit. Since they are brittle they may break if dropped on a hard surface, and cannot be used for chopping through bones, or frozen foods, or in other applications which require prying, which may result in chipping or catastrophic failure. Several brands now offer a black colored blade made through an additional hot isostatic pressing

Hot isostatic pressing

Hot isostatic pressing is a manufacturing process used to reduce the porosity of metals and influence the density of many ceramic materials. This improves the material's mechanical properties and workability....

 (HIP) step, which improves the toughness.

Issues

Ceramic knives may present a security problem as ceramics are not seen by conventional metal detector

Metal detector

A metal detector is a device which responds to metal that may not be readily apparent.The simplest form of a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field...

s. To hinder misuse of concealed knives many manufacturers include some metal to ensure that they are seen by standard equipment. Ceramic knives can be detected by extremely high frequency

Extremely high frequency

Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low infrared light, also referred to as terahertz radiation...

 scanners (e.g. millimeter wave scanner

Millimeter wave scanner

A millimeter wave scanner is a whole–body imaging device used for detecting objects concealed underneath a person’s clothing. Typical uses for this technology include detection of items for commercial loss prevention, smuggling and screening at government buildings and airport security...

s) and X-ray backscatter scanners.

Suppliers

Ceramic blades have become increasingly popular over the last decade, especially due to the lightweight and limited need for sharpening. Kyocera, Asahi and Tivosan are three of the more established ceramic knife brands. Newer design focused brands include Bodum, Edge of Belgravia and Mastrad. Some of the traditional steel knife manufacturers, such as Victorinox, have also launched ceramic knife ranges.