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A ceramic knife is a knife made out of very hard and tough ceramic, often zirconium dioxide (ZrO2; also known as zirconia). These knives are usually produced by dry pressing zirconia powder and firing them through solid-state sintering. The resultant blade is sharpened by grinding the edges with a diamond-dust-coated grinding wheel. Zirconia ranks 8.5 on the Mohs scale of mineral hardness, compared to 7.5 to 8 for hardened steel, and 10 for diamond. This very hard edge rarely needs sharpening.
Zirconium oxide is used due to its polymorphism. It exists in three phases: monoclinic, tetragonal, and cubic. Cooling to the monoclinic phase after sintering causes a large volume change, which often causes stress fractures in pure zirconia. Additives such as magnesium, calcium and yttrium 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).
Ceramic knives will not corrode in harsh environments, are non-magnetic, and do not conduct electricity at room temperature. Because of their resistance to strong acid and caustic substances, and their ability to retain a cutting edge longer than forged metal knives, ceramic knives are a much more suitable culinary tool for slicing boneless meat, vegetables, fruit and bread. 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 (HIP) step, which improves the toughness.
Ceramic knives may present a security problem as ceramics are not seen by conventional metal detectors. To hinder misuse of concealed knives many manufacturers include some metal to ensure that they are seen by standard equipment. Ceramic knives may be detected by extremely high frequency scanners (e.g. millimeter wave scanners) and X-ray backscatter scanners.