This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer. Note: this review is for my own product and in certain countries can be considered as advertisement. Therefore: beware, WERBUNG!

Review

Today’s sharpening stone is something pretty new to me. You might have gotten the very correct impression that I am a huge fan of diamonds. I firmly believe, and I think it is starting to come through when looking over the reviews in this blog, that diamond seems to result in a superior cutting action, which is in part because it’s a superabrasive. Super, as in super hard.

There is one other abrasive that can be considered such – and that is CBN. CBN has a similar crystallographic structure such as diamond, is much softer (about half the hardness), BUT: is chemically inert to the steels we are sharpening. I wrote a bit more about CBN here when I had the first CBN stone on the blog, check it out here. There’s a lot of myths going around CBN, and you can find many CBN stones on the market. I personally love CBN when high speed grinding in my dayjob – but have never found a decent, pure CBN stone for purchase that would allow me to explore their interaction with a cutting edge in detail. All commercial stones I’ve had on the blog so far had massive amounts of either SiC or Al2O3 as a filler in them. Hence I set out to make one myself, where the pure effect of CBN in handsharpening can be observed!

Today’s triplet of stones is a new product line which I call “experimental series”. Experimental as in: I do not think that these stones will outperform my Scientific Sharpening Stones. I actually would be surprised if they have a higher performance. But I am unable to test them in every condition, every steel and also – sharpening is a very subjective thing. Maybe some people will love the edge produced by this. I think some of my avid readers might be interested in trying this out – and become the scientist themselves through their experiment! 🙂

These stones are produced with the same principles as my diamond stones are: very pure, no filler, homogeneous grain distribution both in size and location (aka: no agglomeration). You could say, these are identical to my diamond stones, but feature CBN. How much CBN? Well, so much that they are fully black, without any colouring in them:

A set of Dr. Marv’s experimental series CBN stones – grain size: 30 µm, 15 µm, 5 µm.

This gives the stones a wonderful, cool look, black but sparkly:

The 30 µm Dr. Marv’s Experimental Series CBN stone, right after dressing, before the first use.

Let’s take a look at the composition and appearance of these stones:

Optical micrographs of the stones: First two pictures: 30 µm, Second two pictures: 15 µm, last two pictures: 5 µm. Instrument: Leica Emspira

Let’s take a look under the SEM – stone by stone. For this, I’ve taken both images of the powder used, but also broke a stone in half to enable us to look at the cross section:

SEM micrographs of the 30 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

SEM micrographs of the 15 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

SEM micrographs of the 5 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

I think this experiment can be considered a success at this point! The stones show a uniform distribution of CBN grains, with no agglomeration and a very decent concentration!

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus. For these CBN stones, they are also applied in their natural progession: so the blade that is sharpened with the 15 µm CBN stone is sharpened beforehand with the 30 µm CBN stone. As I consider these a “set”, it is only natural to use them to prepare the whole bevel.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

Let’s start of with the 30 µm stone again:

SEM micrographs of the edge finished with the 30 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

The surface shows a more matte, scratched appearance. There are a couple of deeper scratches, but they are evenly distributed. Zooming in, one can see a folded over (towards the observer) wide burr. Between the deeper scratches, heavy prow and burr formation in the apex plane can be identified – typically a sign of burnishing and not cutting. Near the apex, a couple of cracks are visible, albeit small.

Followed by the 15 µm stone:

SEM micrographs of the edge finished with the 15 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

The 15 µm stone shows a more refined apex, but the edge is a bit toothy now. I find this very interesting! I could image that this edge, when either stropped or further optimized via a fine diamond stone could give you a fantastic working edge.

And finally the 5 µm stone:

SEM micrographs of the edge finished with the 5 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

There are some weird, spidery structures near the apex on the 5 µm stone. I took this one out 3 times, cleaned it and cleaned it again. I’d say these structures, which look like heavy plastic deformation really are there. Most curious!

Let’s compare the edge in their progression: 30 – 15 – 5 µm:

A look at the morphological appearance under the optical microscope:

Optical micrographs of the edge. First two pictures: 30 µm, second two pictures: 15 µm, last two pictures: 5 µm CBN stone.

I have to say – I’m quite surprised. The surface created by the 5 µm stone is superb – a nice, glossy reflection! I have a lot of very expensive swiss CBN grinding tools at my dayjob, and I can’t produce such a surface with those. Seems like purity and good particle distribution really are key to fancy finishes!

Reflection on the 5 µm finished surface!

But if you compare the 5 µm CBN with my 5 µm diamond stone:

Comparison between the 5 µm CBN (first/left picture) and the 5 µm diamond (second/right) picture stone.

I feel like the diamond stone just… left clearer edges. More refined apex. Less burr and burnished prow formation.

I’m intrigued. My suspicion that CBN cuts less clean than diamond seems to have a first data point. I’ll revisit this in a bit with some other steel, and also some deeper look into how CBN behaves while cutting metal.

If you want to experiment with these stones – they will be available beginning of december in a very limited, individually numbered run. Just…don’t expect the same level of performance I promise from my diamond stones!