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.
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.
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.’
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.
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.
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.
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.