IIRC Boker says it takes special equipment to sharpen ceramic knives.
EDIT:
Boker forum on Bladeforums Last post, last line.
"you cannot sharpen it yourself, it takes special grinding equipment.
Thanks, Terry "
Yup, industrial grade diamond sharpening wheels...
A ceramic knife is a knife made out of very hard ceramic, often zirconium oxide (ZrO2). These knives are generally 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 which can be shattered by a slight blow and 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 degree Celsius for 5-12 hours in a high temperature furnace. The result is a very tough and blunt blade which needs to be sharpened to get the desired cutting edge. The blades are 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. However, when sharpening is needed, they cannot be resharpened the same way as steel blades, which are often sharpened with a ceramic whetstone. To sharpen the edge of a blade a material harder than the one that is being sharpened is required, and ceramic knives are usually sharpened with industrial grade diamond sharpeners.
Pure zirconium oxide on the other hand is restricted from broad industrial use due to its polymorphism. This means from room temperature to high temperatures it goes through three phases: monoclinic, tetragonal, and cubic. The volume change on cooling to the monoclinic phase is associated with a large volume change, which often causes failure of the component. To alleviate this effect, additives are utilized to stabilize the high temperature phases to eliminate this volume change. Magnesium, calcium and yttrium are often used to stabilize the zirconium oxide for which yttrium provides the best mechanical properties yielding the charactersitic 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 rigid they cannot be used for chopping, cutting bones or frozen foods or prying open things, which may cause the cutting edge to chip off or the blade to break free from the handle. The tips of these knives are resistant to rolling and pitting but may break when dropped to the ground.
Several brands also offer a black blade made by an extra firing or sintering via hot isostatic pressing (HIP). This process turns the ZrO2 into zirconium carbide (ZrC). The transformation to zirconium carbide improves the toughness of the blade, the key limitation to using ceramics in knife blades.
Ceramic knives present a conceptual problem to the security industry since ceramics are not picked up by metal detectors. To solve this problem, many manufacturers of non-military knives include a quantity of metal in each knife to ensure they are detectable with standard equipment. Ceramic knives can be detected by extremely high frequency scanners (i.e. Millimeter wave scanner), although (as of 2006) these scanners are not yet in widespread use.
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