Sharpening a black ceramic kitchen knife

[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), and it easily slices single layer toiletpaper (torn apart 3-layered version)
(and a tomato of course)









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

A black (HIP) blade is made out of a black zirconium oxide and offers extra durability.
This type of blade goes through an extra firing process called a "hot-isostatic press," creating a tighter weave between the ceramic molecules, thus creating a tougher blade.
The white ceramic blade is also made out of zirconium oxide, but does not go through this expensive sintering process.

[Sparky415]

 

Great job on an impossible material 

And great pictures 

[jzmtl]

What did you use to sharpen it?

[Sparky415]

What did you use to sharpen it?

+1 Thats what I should have said 

[kwakster]

Removing the chips and setting the new bevel was done with a Tormek T7, 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.

[derekmac]

Wow, awesome job!  I wish I had that knife to use. 

[Top-Gear-24]

That's very impressive ... 

Wish I had those kind of sharpening skills  .

[Agonistes]

i've always prided myself on a decent razor edge but no way could i put such a spiffy mirror edge on something.  this is a beautiful job.

[jzmtl]

Ah, thought you must've used the tormek.

I tried sharpen a black ceramic knife I have and failed miserably, guess I needed equipments that cost 30x the knife did. 

[Syndicate]

I like the new mirror edge on the knife, I tried to sharpen my black ceramic knife on a diamond coated stone with little success.

[kwakster]

Here are a few YouTube clips showing the slicing of toilet paper with this ceramic knife.
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 a single layer.

[David]

Impressive!

[Threeme2189]

Wow!
Nice job!

I do like a sharp knife in the kitchen. It's just so satisfying.

[derekmac]

Ouch!!  I think I got a cut just watching those vids!   Very impressive job! 

[HankSolon]

Tormek T7

Something I would like to have....

I really don't like ceramic knifes, because of the properties (hard and brittle and not easy to get sharp again). But you showed very impressive what can be done. Great. 

[kwakster]

Just took the sharpness of this knife a small step further with the help of a third Paper Wheel coated with 3 micron diamond compound.
The edge can now whittle hair (or sever it immediately) towards the point.

[kwakster]

Of course it still remains to be seen how long this edge will last in real world use, and if 25 degrees is a suitable edge angle for this black ceramic material.

[kwakster]

Basically i start out with a new uncoated & unslotted Paper Wheel and coat the surface with a layer of light oil with tackifier in it, then let dry for at least 12 hours or until the surface looks dry.
Then i coat the surface with a thin layer of 3M diamond paste and immediately after with another layer of oil, rub it in & mix with a clean finger, then again let dry for at least 12 hours or until the surface looks dry.
I repeat the process regularly after several knives, as not all of this mixture stays on the wheel indefinitely.
After several layers of oil & diamond paste as well as sharpening with it the surface of the wheel develops a tacky dark grey layer which will hold the diamond particles noticeably better than in the beginning.

The diamond paste i use is made by 3M for the electronics industry (chip production) and consists of a light grey colored oily clay base mixed with diamond particles.
The mixing oil i currently use is a specialty grade used in the audio industry for turntables (certain types of thin chainsaw oil work as well), but it seems you can also buy tackifier separately and add it to your choice of light machine oil.
I haven't done this yet but plan to.
Both the tackifier and the drying of the oil help to keep it on the wheel, while the oil itself holds the diamond particles.

A Wheel with 15 micron diamond compound lets you sharpen ceramic knives that are just plain blunt or have only minor chipping.
I use this wheel also as a finisher on folders and smaller fixed blades with various wear-resistant steels like S30V, S90V, ZDP-189, CPM-M4 and M390, and it produces only a very tiny burr which is easily stropped off (i use the Tormek leather wheel)
For the removal of larger chips and the setting of completely new bevels on ceramic knives however you will need heavy machinery like for instance a Tormek T7 fitted with a SB-250 Black Silicon stone like i do.

A Wheel with 6 micron diamond compound lets you refine the edge on ceramic knives and gives you a very good sharpness, edge quality as well as bite.
I also use this wheel to finish certain larger bladed steel knives like for instance kitchen knives made from wear-resistant steels.
For what it's worth i never had a better edge on my early '60's Gerber kitchen knives which use hard chromed M2 high speed steel, and which can be notoriously difficult to sharpen by other means.

BTW: the best quality ceramic knives i have found to date are the black ones made by Kyocera: the Revolution series if you're on a budget, and the Kyotop series if you want to spend more.
As the black knives are also tougher than the white ones they can be made with thinner edges, which not only cut noticeably better but are also more easy to resharpen since you have less material to remove.
Reprofiling a rather thick white ceramic knife can take up to several hours, while a black knife reprofiles in about 30 to 45 minutes, both done with the Tormek T7 fitted with an SB-250 Black Silicon stone.

[kwakster]

Slicing a tomato with this black ceramic knife:

[kwakster]

Slicing the same kind of tomato with an older white ceramic knife i sharpened for testing purposes in my own kitchen: