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. The stone for this review was supplied for the manufacturer free of charge. The manufacturer had no influence on this review and did not see it before publication.
Review
Today’s sharpening stone is a vitrified one with “a very high diamond concentration“. It’s the Cheefarcut Vitrified “Home” stone, which comes in 400 and 1000 grit size (a double sided one), and this review is about the coarser 400 grit side.
Let’s take a look under the optical microscope!
Optical micrographs of the Cheefarcut 400 vitrified diamond stone. Instrument: Leica Emspira
The stone is a nice, green colour. The large diamonds are easily made out – but there’s a lot of grains in there, and not all of them sparkle! Let’s take a closer look under the SEM:
SEM micrographs of the Cheefarcut 400 vitrified diamond stone. Instrument: Zeiss GeminiSEM 560.
I find this very fascinating. Vitrified stones we’ve had so far in the blog usually had a very dense matrix, whereas this one is very porous. I’d even go so far as to say it’s not fully “vitrified”, meaning the degree of glassification is not very high. We can make out numerous grains here, but not all of them look like they are diamond. Inter-grain connection is very low and thin.
Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.
EDS analysis of the Cheefarcut 400 vitrified diamond stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
We can make out that this is a typical vitrified bond, with large amounts of Mg, Al, Si and Na in it. The diamond concentration is highlighted by the carbon channel, and we can see that there’s a really decent concentration of diamonds in the stone, albeit particle size control does not look very good and there’s a slight tendency for agglomeration. This becomes more visible when zooming out to a larger FOV:
EDS analysis of the Cheefarcut 400 vitrified diamond stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. As this is a benchstone, I’m using a Katocut Nowi Pro to sharpen the blade and an exact angle and remove the human error. Two blades are sharpened – one is a custom heat treated M398 (65 HRC), one is a commercially available Nitro-V Blade (60 HRC), which shows the stones behaviour in two wonderful steels near the opposite ends of the spectrum of knife steels.
The edge is then analysed in the electron microscope for breakouts and morphological appearance.
The stone itself is curious to use. On the first stroke, it feels very much like a ceramic stone – there’s a lot of friction, which often gets labeled as feedback. Something I found curious is that on some movements, the stone feels a bit like rubber, and the blade starts vibrating and making noises, similar to a piece of chalk starting to jump over a whiteboard, just at a low frequency.
Obviously, I used very little pressure, but as it is a coarse stone, some pressure is applied. During the sharpening action, a lot of debris builds up on the stone – some of it is clearly identifiable as material from the stone, some is the swarf from our blade. Compared to other vitrified stones, this feels much more like chalk, and less like a fully sintered/bound bond.
Let’s take a look at the blade in M398:
SEM micrographs of the M398 (65 HRC) edge finished with the stone. Instrument: Thermo Fischer PhenomXL SEM.
The SEM pictures show a nicely formed apex for such a coarse stone. The edge is very toothy, but at that grain size this is expected behaviour. What I find very curious is that the surface is marred with a lot of structures. Most of these can be contributed to plastic deformation – such as burrs, prows and even some voids. I would guess that this stems from free, rolling grains.
The optical micrographs show a rough surface on the bevel, where the defects are clearly visible as matte structures:
Optical micrographs of the M398 (65 HRC) edge finished with the stone. Instrument: Leica Emspira.
Next, let’s take a look at the Nitro-V blade! It felt pretty much the same while sharpening, but a quicker material removal / swarf build up was noticeable.
SEM micrographs of the Nitro-V (60 HRC) edge finished with the stone. Instrument: Thermo Fischer PhenomXL SEM.
In the SEM, a large folded over portion is visible. It is a bit thicker than what I would call a burr – this is once again something that feels very much like a burr to your finger, but is actually plastic deformation of the whole cutting edge, and not yet the sign of a formed apex.
Optical micrographs of the M398 (65 HRC) edge finished with the stone. Instrument: Leica Emspira.
This is also visible under the optical microscope. The surface finish of this blade is much better though – a curious result!
The bad surface finish in M398 made me turn on the BSD sensor of the SEM. This specific sensor detects not so much the topograhpy of a sample, but instead gives us “elemental contrast”. here, the brighter regions are heavier elements, whereas darker areas are lighter elements. Surprisingly, a large number of dark particles embedded themselves into the blade material!
SEM Micrographs with the BSD detector (showing elemental contrast), highlighting small diamond particles that embedded themselves into the blade near the apex. Instrument: Thermo Fischer PhenomXL
EDS analysis shows these particles to be carbon, so most likely diamond:
EDS linescan over one of the embedded particles. Instrument: Thermo Fischer PhenomXL.
This is quite fascinating to me. The stone, as mentioned before has the tendency to develop a little bit of a slurry, very similar to a natural stone. I’ve not yet sharpened a blade on a slurry of diamonds – but this is a very fitting explanation for the structures we see on the blade. The rolling diamond is creating the large prows and plastic formations visible on the surface – it’s no longer a “grinding” tool by it’s technical definition (path constrained abrasive), but turns into a lapping (only force constrained abrasive) stone – or a mix in between.
Overall, this was an interesting and novel approach to a vitrified stone. It’s by far the cheapest vitrified stone on the market, and it contains a large amount of diamonds. The result is very unlike what is shown on the manufacturers homepage – there’s not really a fine burr formed, but instead the whole bevel pushed over. Furthermore, the stone, while pretty thick itself, wears itself down to form the slurry. This is something I’m not used to on ceramic or vitrified stones – they typically don’t show any apparent wear. Just like with everything, there’s an upside to this: the stone constantly self sharpens, and there’s no time for debris to get struck. Just rinse under running water and it becomes pretty clean again.
Would I recommend this stone? Probably not. The manufacturing looks to me like it is of insufficient quality – mixing, sintering and overall composition sure are points that could and should be improved. Then again, it’s a very cheap stone for what it is – thick, lots of diamonds and the start of what could be called a vitrified bond.
Especially the embedding diamonds will make subsequent sharpening actions more difficult, and the surface finish left is abysmal compared to the material removal rate. I think going for an ATOMA F400 is the better choice if you are chasing perfection and want a solid foundation for further sharpening. Nevertheless, I will be watching (and testing!) very closely what this new Cheefarcut company comes up with.
