Edging Along

Ceramic knives have amazing edges.  By amazing I mean sharp with long term edge retention.

Basic folding ceramic pocket knife
Ceramic knives are typically made from zirconium dioxide (AKA zirconia).  Many manufacturers produce these blades through dry-pressing followed by firing of powdered zirconia. 

The resulting blade has a hardness of 8.5 on the Mohs scale of mineral hardness.  At the top of the scale sits diamond with a 10 while steel ranks 4.5 for normal steel  This means a ceramic knife blade is significantly harder than steel edges.  So the resultant blade stays sharper longer.  The downside is it is less elastic and more brittle than a steel edge.

These knives are typically designed for food preparation like slicing boneless meat, vegetables, fruit and bread.  The non-reactive nature of the zirconia means it will not be affected by acid foods like lemons, apples or tomatoes.

Like many minerals, zirconia undergoes several phase transitions which can weaken the blade.  Minerals like calcium, magnesium and/or yttrium oxides can be added to stabilize the blade.  These produce a white ceramic.  A black-colored blade results from adding a hot isostatic pressing step, which increases the toughness.

The blade is electrically non-conductive and non-magnetic so it isn’t seen by metal detectors.  You’ll often find small ceramic blades sold as part of a self-extraction kit hidden in your clothing in those countries when kidnapping is a national sport. 

The factory new edge looks like this:

up close with a zirconia blade
I’m guessing each of these defects are about 200 um in length.  Could be from having blades in contact with each other  at the factory.
new zirconia knife edge
Looking straight down on the edge, even at +63 X magnification, the edge is difficult to find.
But even here small defects from manufacturing can be found.  While very tiny, they could be beginning of larger ones. 

After enough years your edge will have sections like this:

Used zirconia knife edge
Seven year old edge,the big defect looks about the length as the new edge, but deeper. 

Dirty Harry once said a man has to know his own limits.  With care the blade will last for years before sufficient damage forms to the blade edge making the knife unusable.  The edge photographed to show you these defects still has years of cutting left in it.

used zirconia knife edge
Despite the ‘dull’ spot this edge has plenty of life left.

Last word, don’t try sharpening it yourself.  Even with fine diamond powder, any uneven pressure will only cause more damage.  Many of the manufacturers have a sharpening program you can take advantage of.