A few ceramic knives

[kwakster]

A small black ceramic backlock folder without any markings (Böker ?) i did in 2013.
This well made knife had practically no edge and no point to speak of when i received it, and even opening mail with it was difficult.
I used the Tormek SB-250 Blackstone graded with diamond to around 1000 grit for freehand reprofiling, followed by 3 Paper Wheels coated with 15, 6.0 and 3.0 micron diamond compounds for refining & polishing.
The new edge fits in the 30 degrees inclusive slot on my Tormek angle gauge, is slightly convex, and can shave the hair on my leg in both directions.









Specs:

Length open: 4.72 inches (12,0 cm)
Length closed: 2.76 inches (7,5 cm)
Blade material: black zirconium oxide
Blade length: 1.57 inches (4,5 cm)
Blade thickness: 2,0 mm
Handle material: Ivory Micarta & Nickel Silver bolsters
Weight: 61 grams

[Beaker]

Nice sharpening job!  Very even bevels.  Thanks for sharing the excellent photos.

[kwakster]

A niece of mine is an avid amateur Chef (but not yet a knife afi), and this black ceramic knife (a rebranded Kyocera) is one of her favorites in the kitchen, as due to the tougher blade material the edges can be made thinner and thus the knife cuts better than a white ceramic version.
Longtime use however (not always on a suitable cutting board) plus storing it unprotected in a drawer between a bunch of steel bladed knives had blunted the edge to a point next to unusable, complete with quite a lot of (micro) chips and a broken tip.

This is the knife as it was when i received it.
(when you click the pictures 2 x you can see the chips clearly)









This is the knife after sharpening.
I reprofiled the rather bad factory edge to an ever so slight convex edge of ~25 degrees inclusive, and the sharpness is just hairwhittling (only towards the root, not to the point)
It easily slices single layer toiletpaper (torn apart 3-layered version) and a tomato of course.
Removing the chips and setting the new bevel was done with a Tormek T7 fiited with an SB-250 Blackstone (black silicon carbide), refining & convexing with a Paper Wheel coated with 15 micron diamond compound, and semi-polishing with a second Paper Wheel coated with 6 micron diamond compound.










Specs:

Overall length: 11.0 inch (28,0 cm)
Blade length: 5.8 inch (14,8 cm)
Blade thickness: 1,84 mm
Blade type: black ceramic / saber-hollow
Thickness behind the edge: 0,4 mm

The first clip shows the slicing of a piece of standard 3-layered toilet paper, for the second clip i peel off 1 layer and slice the remaining 2 layers, and the third clip shows the slicing of just the remaining single layer of toilet paper:

http://www.youtube.com/watch?v=Ubg_drYHoKE

http://www.youtube.com/watch?v=hUkKfwGFogY

http://www.youtube.com/watch?v=RSkxPA9BK8E

Slicing a tomato:

http://www.youtube.com/watch?v=R3-A0SMcIbw

[3D Anvil]

That's an awesome result, kwakster.

I just finished shooting a video of my first attempt at ceramic sharpening.  It was a mid-grade Kyocera utility knife.  Average of five BESS readings at start was 302.  I sharpened at the factory angle (13 dps) on a worn 1000 grit CBN wheel, edge trailing. 

After going for a while I wasn't sure if I was removing chips or adding new ones, so I stopped.  At that point, the average BESS reading was 268, so bit of an improvement, and the readings were less variable than before.

Next, I honed at the same angle on a Ken Onion with blade grinder, using a leather belt with 6.5 micron diamond.  After that I was getting scores in the 120-135 range.

Next, I repeated with a leather belt with 2 micron diamond, which brought the scores down to the 70-85g range! 

Everything I did after that made my scores worse.  That consisted of honing on 1 micron and then .25 micron diamond, which brought the score up to 153, average of five readings. 

I subsequently went back to 6.5 micron and then 2 micron, which got me under 100 again.

Video should go up on YT in the next day or two.

[3D Anvil]

[tgbto]

Wow, that's a very nice video ! Very informative, and lots of food for thoughts.

All in all, I think it confirms how differently ceramics behave wrt steel. I don't know if you have close-up pics of the "in between" states (after CBN, after 6.5µ, etc.) ?

The fact that you got many small chips eventually shows clearly that what's at work here is microfracturing of the ceramic. It got chipped away by the hard CBN crystals.
By the way, as mentioned in other ceramics-related posts, my feeling is that edge trailing will increase the risk of having this macro/meso chips along the edge. The force applied by the crystals when sharpening edge trailing tends to pull material away from the blade, ie creates tensile constraints which ceramics tolerate notoriously poorly. I would also think angle, pressure and belt slack are also important when honing.

My two cents at explaining why 1 and .25µ don't do much or even worsen BESS score : their RMS height compared to the leather belt is small so their effect on the ceramic bevel is negligible, but the leather belt microchips the edge in the same fashion that a wooden cutting board can produce a dent in the ceramics if you hit it slightly sideways on the edge of the board. You again fracture it by creating a tensile constraint, you do not grind it.

If I had access to your hardware, I'd probably give a shot at:
- CBN edge leading with low pressure until there are no dents
- 2 microns diamond leather belt only, no 6.5, no 1, for a long time, with little pressure and at the exact angle you shaped it with CBN. Or leather wheel with 2µ. This is no honing in the sense that you won't remove any burr, you're just trying to find the sweet spot for microchipping it in the smoothest fashion, along the sides of the bevel, not at the apex.

The average grain size of the ceramics sure plays a key role in how fine an edge one might expect anyway, as the material will fracture along those grain lines.

[tgbto]

And to be even more verbose : I don't think ceramics follow the Wootz rule of "the lower the angle, the better the edge retention as long as the edge does not collapse" (I quote from memory).

So BESS score might be totally irrelevant to real-world cutting performance of ceramic knives*, unless one is willing to spend more time shaping the knife than using it for cutting.

* Probably not so related to steel knives' middle-term cutting performance either, but hey ...

[3D Anvil]

Thanks, tgbto.  It does makes sense to try edge leading next time.  I'll say that I've become wary of trying to assign reasons for the results I get after spending time at the Science of Sharp website.  I think a lot of times we're just fitting a theory to match our results.  I'm more inclined just to try a bunch of different things, see how they work, and roll with what seems to work best.  I'll leave the whys to the guy with the scanning electron microscope.