Super Hard vs. Soft Edges
Or Are Very Hard Edges Really That Brittle

Cutting material for the second test was again aluminum. However, this time I had to use thicker aluminum, which was yet another disc,~93mm in diameter, but considerably thicker compared to the ones used in test #1, the thickness was 0.22mm vs. 0.14mm in the first test. As it turned out, that was a very significant difference, and even if the change is in sub-millimeter range, percentage-wise it is almost 60% increase in thickness. Knives tested in this session: Kershaw Shallot 1840CBZDP, ZDP-189 steel 63-64HRC; William Henry B12 Spearpoint, ZDP-189 steel 67HRC; The same Calphalon Paring Knife, you already know what it is, sort of :). Now, the reason I've used the same Calphalon knife is that a) I have real shortage of the soft knives, in fact I have just two at the moment, and the second is involved in another long running experiment, which currently is a secret ;) I'll see how it goes, and post the results when it's over; b) I got curious, if it was my use of light/moderate pressure when steeling, during the first experiment was the culprit with the edge restoration problems, so I've decided to use higher pressure on the steeling rod this time, if I still failed to restore the rolled edge. And finally, as in the test #1, Call 3 test knives had 0.25µm finished edge, Calphalon exactly at 15° per side angle, Kershaw shallot and William Henry B12 both are slightly below 15°, but let's assume they are also 15° per side. If anything, that gives slight advantage to Calphalon again. Not that it helps in any meaningful way though.

The second experiment ended up being much shorter. Because of the edge on calphalon. It got damaged so badly after the first cut, there was no way to restore it with steeling or stropping, or even with 1000 grit ceramics. This type of result was not really anticipated, so I'll use a different format to describe the test.

First test knife was the Shallot from Kershaw. I choose it because of its lower specified hardness 63-64HRC. If indeed ZDP-189 was as chipping prone as some believe, then lower hardness was safer bet, although 63-64HRC is not low hardness in general, by no means, not even to me, even though I specifically collect and use very high hardness knives. Anyway, I made a single cut, about 80mm(~3") long. First thing I noticed immediately, the difficulty of making the cut, compared with previous test. Even if the aluminum disc was larger, and I had better leverage, and the increase in thickness was about 60%, the difficulty of cutting was far greater. Another problem became the knife getting stuck in the aluminum sheet, because it was so much wider, 93mm vs. 25mm in the first test, the blade was getting stuck in the metal and I had to twist and pry the knife to get it free again. All that added unexpected level of loads and testing to the experiment - lateral loads, and in one of the worst form at that. In general, knife edge, as a metal can take vertical loads(compression) quite well, it is the lateral(bending to sides) loads that kill the edge fast and easy. It's much easier to bend metal to the side than to damage it by compressing vertically. Simply put, you can bend a long nail using your hands, but the same nail will go through wood and metal being hammered. Strictly speaking, as a human beings, none of us will be able to cut holding perfect 90° angle, thus inducing zero lateral loads on the edge, no one is that strong and precise, plus the mechanics of the muscle play their role. On top of that, irregularities in the cutting medium can induce lateral loads themselves, i.e. material being softer on one side or having small cavity, or less density. Plenty of reasons, but twisting itself increases those lateral loads hundreds of times.

I was a little hesitant in the beginning, but then I figured, worse comes to worst, I'll chip and scratch the steel in a few places and then, ok I'll spend another hour or more restoring it. There was another reason I've continued with testing under those conditions, I did plan to use twist and pry method with all three knives involved in this test, although I was planning that on a wood. It's a quite good test of the edge strength, stick the edge about 1mm deep in the dry wood and twist it to the side. Strong edge survives without any ill effects, however weak or fragile ones will either deform or chip, depends which is it. So, be advised and use with caution, you may damage your knife!!!

As it turned out, my fears were not really justified. After cutting through the disc, I've examined the edge very carefully. No reflections from the edge, I couldn't see or feel any rolls with my fingertip, so I went with further examination, under the microscope. The fact that I had to use the microscope to find the damages was very encouraging already. In the end, I couldn't find anything, but one microscopic chip, which I am not even sure wasn't there to begin with, I could've missed something as small during the initial exam, I wasn't really looking for chips that small at the beginning of the experiment. So, ZDP-189 at 63-64HRC did withstand the test with success and no chipping to speak of.

Next up, William Henry B12 spearpoint knife, which is officially rated at 67HRC. That's pretty much the upper limit for the ZDP-189 steel, I've heard only once someone claiming ZDP in 66-68HRC range, which still gives 67HRC median value, and for the record, Rockwell hardness itself maxes out at 69HRC. If it was supposed to chip, then the William Henry B12 knife was a better candidate for chipping. I've made the same 80mm long cut, through the same 0.22mm thick aluminum disc and examined the edge. Success again, or in other words failure to chip ;) William Henry B12 was easier in this case, thinner blade had less problems with getting stuck in the metal. In the end, two ultra hard knives, both with rather thin, ~30° inclusive angle edges, neither suffered any detectable chipping during the cutting process, and both were able to shave just fine after the cuts were made. For the statistics, I was using the ~25mm(1") long sections of the blade to make those cuts.

The micrographs attached to this paragraph, #1 and $2 are Kershaw Shallot and William Henry B12 knives edges before testing, at 120x magnification. #3 is the edge, to be precise the most "damaged" section of the Kershaw Shallot, that's all I could find on the edge under the microscope :) And finally, #4 is what damage I found on the William Henry B12 knife after cutting. The "after" micrographs were taken straight after cutting, no edge restoration whateosver. Neither one needed any edge restoration anyway.

The final contender and the only representative of the soft steel side - Calphalon Paring Knife. After calphalon parer sustained pretty much irreparable damage, I had a bad feeling about this test. Well, to be precise and not to badmouth the poor parer here, irreparable damage means - irreparable, using simple sharpening tools generally speaking, that is steeling and stropping equipment, plus moderate grit sharpener, ceramic rod of 1000 grit in my case. Image #1 attached to this paragraph is a micrograph of the pristine edge on the Calphalon paring knife, 120x magnification, the edge is 30° inclusive angle, finished at 0.25µm.

I've proceeded with the same 0.22mm thick aluminum disc, attempting to cut the same amount of metal as with previous two knives, 80mm. I've marked the same 25mm(~1") long section on the blade and proceeded with cutting. About half way through things got real bad, I needed to use perhaps twice as much force to cut through the metal compared to the ZDP-189 steel knives, the blade was getting stuck, same twisting and prying to let it loose and finally I cut through the disc, although, the last part wasn't all that straight, due to all the forcing involved.

I didn't need a microscope, or even to hold the knife edge at an angle to sunlight to see the carnage incurred. The micrograph #2 represents general view of the edge carnage, about 30x magnification. Needless to say the edge was completely destroyed. The 25mm long section of the edge, used in the test, was completely rolled and badly deformed. I could see ripped out chunks of metal under the microscope. I didn't really know where to begin the edge restoration process. Ceramic stick with 1000 grit is quite abrasive, and had I started with that or any other low grit abrasive, it'd simply remove deformed metal. So, I've opted to start with smooth steel. At any rate, when the deformation is significant, it makes sense to realign the edge first and then proceed with sharpening, might save you quite a bit of a metal on the edge and sharpening work too.

So, I've pulled out my trusty borosilicate rod and proceeded. First, 10 strokes per side, using moderate pressure, no visible success. Then 20 more strokes with higher pressure, to literally push the bent out metal back in. Second run did improve overall state of the edge, but I could still feel and see the rolled edge. Obviously, when you can see the rolled edge with naked eye, there isn't much sharpness to speak of in that edge. For testing, I've tried to strop on the plain leather however, I quickly stopped after first stroke or two, the deformation was still quite significant, to the point, that it was scratching the smooth leather pretty bad. Next was the ceramic stick, and I've spent about 5 minutes with it, still no dice. I gave up at that point.

Micrograph #3 shows the edge before restoration and #4 micrograph is what I did achieve with all the stropping, steeling and minor sharpening. You can still see large roll on the edge next to the original, sharp edge. In the end, I did expect considerable damage to the edge, but I clearly underestimated the significance of the increased thickness of the aluminum disc. 60% increase in thickness caused way more damage to the edge, and much faster too, it was just 80mm this time, while in the first test I've cut few times more aluminum with the same calphalon knife and less damage was sustained in the end.

Coincidentally, another BF member - me2, was testing his knives on cardboard and aluminum as well. His results with similar knife, same class steel, around the same 56HRC, were a lot better compared to mine. I've asked about the details, since according to him, he could cut over 120" of aluminum sheet and still restore shaving sharpness, ok, just scrap shaving sharpness, using just the smooth steel. The mystery was quickly resolved once he stated that the aluminum he was cutting was from the standard soda can. I've checked on the net and to be absolutely sure, cut a can of soda in the middle myself and measured the wall thickness, it was just 0.07mm thick. Twice as thin compared to my first test, and more than 3 times as thin compared to my second test.

The difference in thickness alone, could have explained the longevity secrets of the softer steel in his tests, or the opposite, short lived edge life in my tests. On top of that, the edges he was using in his tests, were double(compound) bevel type edges, with 17° primary and 20° secondary bevels per side. In other words, instead of 30° edge angle, he used 40° edges for his test cuts. Although, in this case, I believe 99% of the difference can be blamed on increased thickness of the aluminum sheet used in my tests. To test that theory, I simply cut about 100mm of the same aluminum from that soda can, with 0.07mm thick walls using the same calphalon knife, with the portion of the edge that was still intact. 100mm long cut, at least it was longer than the previous test with the same knife, and after I made that cut, not only there was no visible damage to the edge, but it was still shaving sharp! Which is why I believe, majority of the damage came from the thick metal, and had I sharpened 40° edge on the calphalon, it is unlikely I would've achieved much better results.