|This article does not cite any references (sources). (June 2007)|
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 and 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 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 lighter 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 or catastrophic failure. Several brands now offer either a black-coloured or a designed blade made through an additional hot isostatic pressing step, which increases the toughness.
Sharpening & General Care
Unlike a traditional steel blade that benefits from regular honing and resharpening in order to keen a sharp edge, a ceramic knife will stay sharp and retain its cutting edge for much longer - up to 10x longer according to some tests. Although ceramic knife therefore does not need regular sharpening in the same way as steel, its blade edge will eventually degrade or chip and lose its cutting edge - especially if not protected or stored carefully. Avoid storing a ceramic knife in a drawer where it is likely to collide or knock against other knives or metallic objects - although hard, the ceramic blade is also brittle and particularly prone to chipping in such situations. Blade life can be extended by using a separate blade guard or storage box, and by using the correct type of cutting board. Always cut on a wooden or soft plastic board - do not cut directly onto glass, marble or similar materials or you will risk damaging the knife edge.
The inherent hardness of the ceramic material also makes it more difficult for the consumer to resharpen. A traditional sharpening or honing steel will not be effective - will be necessary to use a harder material such as a diamond grit grinding surface. Suppliers take different approaches. One popular supplier advises against re-sharpening, suggesting that ceramic knives can be considered as limited lifetime disposable items - Ceramic knives stay sharp very long and in general must not be [consumer] sharpened. Other suppliers encourage occasional resharpening and provide ceramic-specific diamond-based sharpening appliances which can be costly. A review of web articles reveals DIY solutions that have proved effective for resharpening ceramic knives:
- credit card sized diamond grit sharpening stone (using 1000 grade grit or finer) 
- compact micro belt sanders using diamond-grit based belt and honing guide
Use in the kitchen
Ceramic knives are particularly useful as kitchen knives for fine slicing and paring of fruit and vegetables as they do not react with food acids.
- http://kyoceraadvancedceramics.com/ceramic-advantage/kyocera-technology. “Recent tests show that the life of a Kyocera ceramic blade without resharpening is approximately 10 times that of a typical steel blade.” – The Cutlery and Allied Trades Research Association (CATRA), an independent testing organization.
- Care Instructions, LIDL Ernesto Ceramic Paring Knife, Version 06/2015. IAN 113349
- Care Instructions, LIDL Ernesto Ceramic Paring Knife, Version 06/2015. IAN 113349. "Risk of Injury! Do not sharpen the knife by yourself. Improper grinding of ceramic knives risks that small parts can splinter, which could be inhaled or ingested or cause other injuries. If necessary to sharpen the ceramic knife please contact a specialist. The blade should be sharpened with a diamond grinding tool (wet grinding) not with a sharpening steel or similar ordinary grinding tool"
- Media related to Ceramic knives at Wikimedia Commons