Ceramic knife
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A ceramic knife is a knife made out of very hard ceramic, often zirconium oxide (ZrO2; also known as zirconia). These knives are usually produced by compacting zirconia powder using high pressure presses which apply a pressure of around 300 tons to produce blade-shaped blanks. These blanks are very brittle and fragile and can be shattered by a slight blow; special binders are used to retain the shape of the blank until the firing process. Like all ceramics these are consolidated into a dense and strong ceramic by solid-state sintering at approximately 1400 degrees Celsius for 5–12 hours in a high-temperature furnace. The result is a very hard and blunt blade which is then sharpened by grinding the edges with a diamond-dust-coated grinding wheel.
Zirconia is very hard; it ranks 8.5 on the Mohs scale of mineral hardness, compared to 6 to 6.5 for hardened steel, and 10 for diamond, giving a very hard edge that rarely needs sharpening. Ceramic blades can only be resharpened with a material harder than themselves; industrial grade diamond sharpeners are usually used.
Pure zirconium oxide is little used in industry due to its polymorphism. When heated from room temperature it goes through three phases: monoclinic, tetragonal, and cubic. Cooling to the monoclinic phase cause a large volume change, which often causes breakage. To alleviate this effect additives are utilized to stabilize the high-temperature phases and eliminate this volume change. Magnesium, calcium and yttrium are often used to stabilize the zirconium oxide; yttrium provides the best mechanical properties, yielding a characteristic mother-of-pearl appearance. The highest strength, and more importantly toughest, zirconia is produced with 3 mol% yttrium oxide yielding partially stabilized zirconia. This material consists of a mixture of tetragonal and cubic phase with a bending strength of nearly 1200MPa.
Ceramic knives will not rust, leading to their use by SCUBA divers. They are also nonconductive and nonmagnetic, which can be useful for bomb disposal operations. Their chemical inertness to both acids and alkalis and their ability to retain a cutting edge far longer than forged metal knives makes them a very good culinary tool for slicing and cutting through boneless meat, vegetables and fruits. Since they are very brittle they cannot be used for chopping, cutting bones or frozen foods or for any application which tends to twist the blade such as prying, which may cause the cutting edge to chip or the blade to break free from the handle. The tips of these knives are resistant to rolling and pitting, but may break if dropped.
Several brands offer a black blade made by an extra firing or sintering via hot isostatic pressing (HIP), which improves the toughness of the blade, the main shortcoming of ceramic knife blades.
Ceramic knives present a security problem as ceramics are not detectable by metal detectors. To hinder misuse of concealed knives many manufacturers include some metal to ensure that they are detectable by standard equipment. Ceramic knives can be detected by extremely high frequency scanners (e.g. millimeter wave scanners), although as of 2006[update] these scanners are not in widespread use.