Ceramic knives
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 4.5 for normal steel and 7.5 to 8 for hardened steel and 10 for diamond. This very hard edge significantly reduces the need for 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 magnesia, calcia and yttria are used 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).
Ceramic knives are substantially harder than steel 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 better suited for slicing boneless meat, vegetables, fruit and bread. Since ceramics are brittle, blades may break if dropped on a hard surface although better manufacturing processes have reduced this risk. They are also unsuitable for chopping through bones, or frozen foods, or in other applications which require prying, which may result in chipping. Several brands now offer either a black-coloured or a designed blade made through an additional hot isostatic pressing step, which increases the toughness.
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