(05-22-2018, 11:25 AM)EOU Wrote: There's been a discussion going on over in KG's EDGE STABILITY IN BUTCHER’S AND KITCHEN KNIVES concerning ceramic knives and edge rolling etc. We borrowed a couple of inexpensive Kitchen Aid from an unsuspecting wife's knife drawer and brought them to the office for test in the SET instrument. Here are the two ceramic paring knives:
The long one measured 335 and the short 382. There was a fair amount of edge sharpness disparity along both edges but that is what they measured at the two test points. We don't think that these edges had seen much, if any, previous use.
We performed our standard Phase I test procedure (5 up and back passes over the edge) and then measured again.
Our 335 now measured 326 and our 382 now measured 376. There is no obvious reason why the measurements would go down other than to chalk it up to the vagaries of this particular test. One thing appears obvious though and that is that ceramic edge don't roll. Here's a picture of one of the edges after the test:
We tried sharpening one of the ceramic blades with a ceramic belt just to illustrate a point and we were successful in that regard because we were unsuccessful in doing so. It simply demonstrates how hard these blades really are relative to steel. We've got a couple of better quality ceramic knives headed our way and we're going to order some belts that will conquer ceramic. Then we'll test how sharper ceramic edges fair in the SET. We don't think that they'll roll but perhaps the standard SET piston design will damage (chip) the edge.
In the meantime, KG is going to put some ceramic knives to test in a meat processing facility and that is going to be where the rubber meets the road.
Let me briefly recollect why ceramic blades are harder and brittle then steel blades.
When iron atoms are grouped together to form a piece of iron or steel, the outer electrons of iron atoms leave their parent atoms and become an electron cloud. The mobility of valence electrons makes it possible that iron atoms can slide past each other when the steel is deformed. Generally we can say that metallic bonds are delocalised.
Ceramic blade is usually made of zirconia which is an ionic compound of zirconium and oxygen atoms (ZrO2). The ionic bonding of atoms is much stronger than metallic bonding. On the Mohs hardness scale ceramic blade has some 8.5 while hardened steel has 7.5 to 8 and diamond has 10.
Ionic bonding in ceramics results in high hardness and high chemical inertness. On the other hand this strong bonding accounts for low ductility and low tensile strength. Most ceramics are poor conductors of electricity because they do not have free electrons like metals.
By adding small amount of yttrium oxide we get stable zirconia, material known as Transformation Toughened Zirconia (TTZ). TTZ is relatively tough ceramic material and should not undergo catastrophic blade failure.
Jan