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Ceramic knife
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 is 8.5 on the Mohs scale of mineral hardness, compared to 5.5 for normal or stainless steel〔http://www.rockrollers.com/features/hardness.html〕 and 7.5 to 8 for hardened steel and 10 for diamond. This very hard edge rarely needs sharpening.
== Zirconium oxide ==
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 1,200 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).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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