Ceramic Blades

Ceramic kitchen knives are relatively inexpensive and are claimed to “stay sharp” longer than traditional steel knives.  For this study, I purchased some “name brand” blades made by the Kyocera Japan, and some inexpensive “Gibson” brand knives imported from China by Bed Bath & Beyond, as well as a very inexpensive knife from my local Dollar Store.  All of these blades were made of white zirconium oxide (zirconia) ceramic.

The images below show the “factory sharpened edge” of one of the Gibson knives.  All of the blades studied displayed similar characteristics, with an apex width in the range of 1-2 microns (about 20 times wider than a typical razor or box cutting blade).  The apex is typically flat with roughness resulting from the underlying crystal structure. Individual crystal grains are clearly visible at the highest magnifications.  One or both of the apex shoulders typically displays sub-micron keenness although with an obtuse angle.  It appears that the cutting performance of these blades can be attributed to the geometry of these shoulders and to the sub-micron roughness of the apex, rather than to simple geometric (triangular) keenness.  Anecdotally, the factory edges on these knives were all quite effective at slicing with a draw but clearly lacked the keenness required for push-cutting paper.

Gibson_factory_01

Edge-view SEM of the factory sharpened edge on a ceramic kitchen knife.

Gibson_factory_04

Cross-section view SEM of the factory edge on a ceramic kitchen knife.

All 3 brands were fabricated from Zirconia ceramic, toughened with the addition of Yttria, Hafnia and Alumina.  All were formed from micron-scale crystalline grains.  The Gibson and Dollar store blades contained very similar concentrations of additives (suggesting a common manufacturing source), while the Kyocera contained about half the amount of of Yttria of the other two.  Comparison of the grain structure (from just a single spot and sample of each) displayed subtle differences.  (This is clearly not intended to be a comprehensive or systematic comparison).  The Kyocera ceramic was the finest grained with obvious grain sub-structure.  The Gibson ceramic had the most voids, particularly at the triple-point intersections of the crystals.  The Dollar Store blade had similar grain dimensions, but also contained a small number of aluminum oxide grains (dark in the images).   There is no obvious deficiency in the blade material to warrant the rather ominous warning on the Dollar Store blade packaging “Hand wash only. Wash and dry immediately. Do not Soak.”    However, the presence of aluminum oxide grains may suggest the material did not meet quality control standards and also explain how it ended up in a dollar store.  Whether these observed differences will correlate to variation in performance and/or “sharpness” longevity is beyond the scope of  this investigation.  I would expect that there are differences in the “toughness” of the various blades – for example, how easily they will break if bent too far or dropped on a hard floor.  Anecdotally, I did not observe any obvious difference in the cutting or sharpening performance of the three blade samples.

Gibson_grain_structure_

Cross-section of a Gibson ceramic blade showing the zirconium oxide grain structure.  The grains are mostly sub-micron in size with one 2-3 micron size grain visible.  Small voids are observed a the “triple points” (where 3 grains meet) and a few larger voids are also observed.

Kyocera_grain_structure

Cross-section of a Kyocera blade showing the zirconium oxide grain structure. The grains are mostly sub-micron in size with subtle sub-structure (twinning, etc). Few voids are observed.

DS_ceramic_factory_06

Cross-section of the Dollar-Store blade showing the zirconium oxide grain structure. The grains are mostly sub-micron in size with a few voids between grains. The dark grain is aluminum oxide, one of the compounds added to toughen the ceramic.

DS_ceramic_factory_10

Side-view of the zirconium oxide Dollar Store blade. The dark spots are aluminum oxide – I would speculate that if the ceramic had been correctly processed, these should have dissolved into the zirconia matrix.

Ceramic knives are purportedly difficult to sharpen, and in the following images I document a few attempts with conventional hand sharpening to help understand the reason for this reputation.

In the first example, the blade was sharpened with a DMT extra fine (1200 grit) diamond plate.  Although the blade material was efficiently ground away, a keen apex never formed.  Instead, the apex continually broke away leaving a 3-4 micron wide ‘flat.’

Gceramic_DMTEF_10

Cross-section view of a ceramic blade sharpened at 40 degrees (inclusive angle) with a DMT EF diamond plate.

Gceramic_DMTEF_13

Edge-view SEM of a ceramic blade after sharpening with a DMT EF diamond plate. The individual zirconia crystals are visible at the apex.

In the second example, I sharpened a Kyocera vegetable peeler blade with a very fine hone, a 16k Shapton Glass stone. This was much more effective than the Diamond plate above.

Kyocera_peel_factory_01

Edge view of the factory edge on the Kyocera peeler blade.  The single bevel blade has a broad apex; however, the bevel side of the apex has a rather keen edge.  Revealingly, in this sharpening geometry, micro-chipping of the apex yields a relatively keen shoulder on the one side (the left side in this image).

Kyocera_peel_16k_03

The Kyocera blade sharpened with the Shapton 16k Glass Stone.  A apex keener than any of the factory edges was easily produced.  This result also shows that diamond abrasives are not necessary, the aluminum oxide abrasive in the Shapton stone effectively abrades the zirconia ceramic blade.

Traditional steeling with a ceramic rod is a disaster, actually reducing the keenness of blade.  Steeling relies on the high local pressure generated from the small contact area of a rod approaching at an angle greater than the bevel angle.  This high lateral pressure simply breaks away the apex faster than a microbevel is formed.

Gibson_factory_03

Cross-section view of the Gibson blade showing the factory edge with a 35 degree (inclusive) bevel angle (prior to steeling).

Gibson_factory_ceramicsteel_01

Cross-section view of the Gibson blade following an attempt at steeling the blade with a ceramic rod.  A micro-bevel is visible (the darker material is an organic contaminate); however, the apex is broken away leaving a 3-4 micron wide apex.

The following images demonstrate that stropping is a viable approach.  This result is not surprising, since stropping produces micro-convexity with modest lateral pressure at the apex.

Gibson_Shapton2k_02

Edge view of the Gibson blade after sharpening with a Shapton 2k Glass Stone.  The blade bevel was efficiently ground; however, the apex has broken away leaving an approximately 3 micron wide edge.

Gibson_sh2k_MMdenim_02

Cross-section view of the Gibson blade following stropping on a hanging denim strop loaded with Mother’s Mag polish (aluminum oxide).  The apex is effectively micro-convexed with increased keenness compared to the 2k-honed blade.

Noting that the grain structure is not evident in the side view of the ground bevel, it is apparent that zirconia is abraded by the aluminum oxide in the 2k stone without “tearing out” individual zirconia grains. However, at the apex, the blade consistently separates along grain boundaries resulting in sub-micron roughness.

Fine grit hones and strops are effective at thinning the ceramic apex because they produce less lateral force than do coarse hones.  Steeling (micro-beveling) is ineffective because of the large lateral pressure that is exerted on the near-apex.  The challenge in sharpening these blades with traditional (coarse) abrasives is that the apex continually breaks off as the bevel is ground, and a keen apex never forms. The burr that typically forms at the apex of a steel blade during sharpening is the result of flexibility of that thin metal. Since ceramic blades have no flexibility, a burr will not form.

The SEM observations show that the perceived sharpness of Ceramic knives is not a result of  geometrical “keenness” but instead derives from the texture of the underlying microstructure.  This texture is easily produced in the apex due to the susceptibility of the material to micro-chipping and the lack of flexibility.  Without malleability, there is no smooth blunting or mushrooming as we normally see with steel blades; instead the two shoulders of the apex  continue to maintain sub-micron level keenness as the blade wears.  The fact that the blade wears by separation at the boundaries of the sub-micron sized ceramic grains ensures that the apex surface maintains an abrasive texture as it wears.   Additionally, the abrupt geometry of the shoulders can be maintained as the apex flat broadens.   With use, the continual micro-chipping will expose a fresh (flat but rough) apex with relatively keen shoulders.  In this way the blade maintains a perceived sharpness.   As a result, the blade can continue to perform acceptably, even as the blade wears.  This explains the observation that these knives “stay sharp” for an extended period.

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23 responses to “Ceramic Blades

  1. About time! Going through withdrawals here. *Wink wink* lol. Out of curiosity, when you tried the diamond plate did you try both edge leading and edge trailing? And you mention that the Shapton stones cut the ceramic – but is this basically only going to be viable for finishing? Can they cut well enough to set a bevel? I have sharpened a few ceramic blades finishing up edge trailing with diamond (DMT EEF to finish and some light stropping with loaded leather) and this worked great – much keener than factory edges and able to push cut paper easily as well.

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    • I didn’t observe any difference between edge trailing and edge leading, but I only tried this on a couple of hones. There are a few ways to approach this, but fine abrasive followed by stropping or buffing looks like the best bet. I may do a part 2 to this at some point.

      Liked by 1 person

  2. Very Cool! I’m not quite sure what I’m looking at on the peeler pic. I know what I’m about to ask isn’t the focus of the study but what would you recommend as a sharpening protocol? Looks like the 16K Shapton glass finished with the Denim/Mothers strop.

    Liked by 1 person

    • The peeler initially has a single bevel (on the left) and I sharpen it with the 16k at the same angle on the left, and create a second bevel on the right. This looks like a good approach for a micro-bevel where we don’t need to remove too much material.

      Liked by 1 person

  3. Missed seeing your posts Todd, great to see this! Thanks for the research and analysis!
    I am curious if a ceramic blade could be sharpened on a belt… maybe when you get that Viel you can experiment around a bit 😉

    Liked by 2 people

  4. Thank you for sharing, very much appreciated. Just knowing that some common stones can touch up a ceramic edge is useful and the photo of the result with stropping speaks volumes. A very useful investigation. Again thanks.

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  5. Todd,

    Thank you ever so much for sharing this most valuable information; Haven’t seen the like of it anywhere else!

    A suggestion for possible future research: Surface conditioning and cutting action of fused ceramic abrasive stones as represented by Spyderco.

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      • Looking forward to reading your article on steeling. Would it be possible to determine a ‘perfect angle’ for a set bevel? In other words, exactly how much wider than the bevel for the best edge. Perhaps you know? Also pressure? Light is better but is there a perfect amount of pressure?

        I know theres a budle of variables, bevel angle, knife hardness and what hone is it ceramic, diamond or is it a f dick combi style steel.

        Just thought I would throw something to think about.

        Keep up the amazing work, truly remarkable information you have here.

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        • I’m not sure how we could determine the “best” angle since it depends on what we are cutting.
          My personal preference is sharpen knives at 30 degrees (inclusive). Apex angles below about 25 degrees seem to weak to be of any practical use. From there, I will use micro-convexity or a micro-bevel to increase the strength/stability of the apex.

          For steeling, it is critical to use an angle at least 10 degrees larger than the the sharpening angle to minimize the contact area. Pressure in sharpening/honing is mostly determined by the contact area rather than the force you apply. I will try to make this more clear in the steeling articles.

          Liked by 1 person

  6. Aloha Todd, thanks for sharing this amazing imagery, its thanks to you I bought my metallurgical microscope, still didnt satisfy the itch, but made me feel better for trying.

    Question: Since mothers mag polish is inexpensive and seems to work good, did you observe the micron grit size and variance? Apologies if you posted and I missed that detail.

    Thanks again for doing these amazing studies!

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    • Cliff, I looked at a variety of metal polishes and they were all fairly similar -mostly micron scale with a few 10-20 micron particles. So long as you use a substrate that can absorb the large particles, they all seem to behave comparably.

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  7. Arrived here after searching for a steel vs obsidian keenness SEM image comparison. Given how good the rest of your comparisons are I think you could do a better job than any I have yet found.
    I only use a basic 1000/6000 King stone for my knife honing, so am hardly your target audience I’d wager. I don’t even own a strop! Appreciated your work though.

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  8. So I was wondering if you knew how the leather works during stropping, not the effect on the blade. I want to know this because I’m wondering what characteristics I want from my leather in my strops. I am researching a lot about sharpening and I am trying to get into doing my own testing but before I test anything, I want to make sure it hasn’t been done before and the answer is somewhere to be found.

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    • I’ve never found any published research on the topic. Leather is not easily imaged with SEM, but the few samples I’ve analyzed had fairly similar surfaces. There are a variety of options for tanning, conditioning and compression – but that’s not within my areas of expertise.

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  9. Thanks for the reply Todd,

    Yeah Im still learning. I think I know what you mean contact area vs force in other words less pressure on coarse and slightly more on fine steel? Hmm I’ll just wait till you’ve finished your steeling articles lol.

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  10. Hi Todd!

    I really like your articles! Could you one day show the difference between strokes parallel to the edge, instead of edge trailing and edge leading? Since edge leading causes micro-chipping, and edge trailing can cause burr, I was wondering if parallel to the edge could be better, or if it would simply cause large session of the apex to chip.

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    • From the few times I’ve looked at it, edge parallel seems to be equivalent to edge trailing. One potentially downside is that parallel strokes can often create horizontal scratches near the apex that weaken the edge.

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  11. Great to see evidence based investigation. I have read recommendations to “strop” on a smooth and relatively hard surface, typically wood, instead of leather and using an appropriate compound (diamond paste??) If I read your work correctly you had the best results with stropping. Have you tried using a wood or other surface for stropping?

    Like

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