CBN can abrade ceramic knives *micrographs inside*

Sadden

Sadden

Joined
Dec 19, 2011
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So earlier this evening I received a Skype from Ken asking if I had ever concluded that CBN could in fact abrade ceramic knives. This was a project I started awhile ago but lost some wind when I realized that the knife I was working on was heavily chipped out from the atomas , making further work a moot point. I had been planning on revisting this topic earlier but I just have not had the time. Now I have a wider (mostly coarser) range of diamond films , and CBN.

Back to this evening. Ken inquired regarding the issue so I told him to give me 5 minutes and grabbed the scope. Here is the factory , used and abused edge on a Stone River ceramic knife, I managed to find a spot big enough without chips to take a picture.


So I grabbed a fresh roo strop , loaded her with some 45u CBN and made a few strokes.

Looks pretty conclusive to me , those are indeed scratches from the CBN , theres actually two visible sets of scratches , one near the edge of the edge and one occupying 95% of the bevel , initially I was too steep so I used lighting to check my contact area and made the necessary adjustments.

So with that in mind here what it looks like after 5 minutes of stropping. Ignoring the fugly edge of the edge for now (I will need to go -much- coarser in order to fix all the damage this thing has) we can see that I relatively quickly removed all the factory scratches , and cutting performance , wow , it can actually cut paper now , something the knife could not do 5 minutes ago.
 
Any idea on other abrasives that work (mainly stropping compounds)?

Definitely cool that a ceramic blade can be stropped with the right material, though. That was the one nothing that worried me the most.
 
Diamond works. And it's no surprise that CBN works, it is right behind diamond in hardness. Nice photos.
 
I was under the impression one could use silicon carbide wet/dry as well, though consensus seems to be diamond on a powered wheel is best.
 
Silicon carbide does indeed abrade ceramic, i use the SB-250 Blackstone on my Tormek to reprofile ceramic knives.
 
Almost all ceramic knives are made of zirconia (zirconium oxide), which has a Vickers Hardness (VH) of about 1300. That's not as hard as other abrasives, let alone cubic boron nitride (CBN).

fQebu5I.gif

QDIx5NO.png


Sources:
http://www.gordonengland.co.uk/hardness/ehe.htm
http://cadenceinc.com/technology-design-center/materials-coatings-information/zirconia-ceramic/
 
Wait, so AlOx is harder? Looks like I gotta test this. I doubt it would abrade much, but not having to get special belts to do ceramic would be nifty.
 
The chart is approximate, because there are slightly different forms of each ceramic. For example, the zirconia in ceramic knives is not pure zirconia; it is often "transformation toughened" zirconia with other elements added in (such as yttrium) not to mention some fancy chemistry and stuff for the transformation toughening. Plus stuff like microstructure (size of crystals in polycrystaline versions), and allotropes/polymorphism matter.

You can find articles on the above stuff in wikipedia.
http://en.wikipedia.org/wiki/Allotrope
http://en.wikipedia.org/wiki/Polymorphism_(materials_science)
http://en.wikipedia.org/wiki/Fracture_toughness#Transformation_toughening

btw, personally, I found ceramic knives really super hard to grind properly. I think that probably aluminium oxide, silicon carbide, are probably not hard enough to be useful in practice (will scratch the ceramic, but they are too darn slow!). I think I would recommend CBN and diamond, although I haven't had much experience sharpening ceramic knives.

A nice popular-science description of transformation toughened zirconia (and its use in ceramic hammers) is in Eberhart's book (highly recommended):
_Why Things Break: Understanding the World By the Way It Comes Apart_ by Mark Eberhart (2007)
http://www.amazon.com/Why-Things-Br...d=1395359461&sr=8-1&keywords=why+things+break

The Coors company (yes, the beer company) had a branch that manufactured technical ceramics, including ceramic hammers. Why ceramic hammers? For when you absolutely cannot afford to create a spark when hammering (say, around explosive and/or flammable materials). So the question is, how do you make a ceramic hammer that does not shatter? See eberhart's book for the answer. The same method is used in ceramic knives. Eventually, the ceramic division of Coors spun off into their own company.
http://www.coorstek.com/
 
dkb45 said:
Wait, so AlOx is harder? Looks like I gotta test this. I doubt it would abrade much, but not having to get special belts to do ceramic would be nifty.
Click to expand...

Hardness (relative), from most-hard diamond at the top, descending to least hard:

Diamond
CBN
Silicon Carbide
Aluminum Oxide
Chromium Oxide ('green')

Aluminum oxide abrasives are a wide-ranging category; some are harder than others, depending on manufacturing specifics. So, it's possible that some versions of AlOx abrasives in hones/wheels/belts could also be somewhat harder than some versions of 'ceramic' blades. Others may not be.

As previously mentioned, even if a given abrasive is somewhat harder, it may still not be the best choice. In powered/driven applications, most abrasives will be subjected to extreme heat, which can radically alter or degrade the abrasive's properties, such as hardness. A just-slightly-harder abrasive may be too slow to grind, which generates more heat. Even diamond is inferior to CBN in this regard, because diamond is known to break down at temperatures above 800°C (1475°F). This is primarily why CBN is chosen as an alternative to diamond in powered grinding applications, because it remains much more stable at very high temperatures.


David
 
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Diamond is only inferior to CBN if you're talking about grinding ferrous materials under high speed conditions. By hand on a free hand setup diamond is superior. At high speed on materials that don't contain iron diamond is superior. Diamond just can't be used on steel or iron at high speed, because the carbon in the diamond will migrate right into the steel or iron, and kiss your abrasive wheel or belt goodbye.
 
I tested it, and AlOx can, in fact, abrade ceramic. It is terribly slow going, but it did give me a *slightly* sharper edge. I would never use it to take out chips, but it will do something if you have nothing else.

Chalk up another useless fact to my wall of wasted brain cells!
 
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