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.

Review

Today’s sharpening stone is the finer brother to our last instalment – the Naniwa Diamond Pro 3000 grit.

It is a very pretty stone – an unusual blue colour with a finely bead-blasted aluminium base:

Let’s take a look under the optical microscope!

Optical micrographs of the Naniwa Diamond Pro 3000 stone. Instrument: Marvscope

The stone shows a very irregular appearance in the optical microscope. We can differentiate between (probably) the diamond in green clusters, a blue phase that likely is the coloured binder, some grey-ish phases as well as a couple of very, very red spots. Intriguing!

Let’s take a closer look in the SEM:

SEM micrographs of the stone. Instrument: Zeiss GeminiSEM 560.

The stone shows, quite similar to it’s caorser brother, a standard phenolic bond. The grain adhesion seems to be similar. There are some larger particles visible, but also a lot of micron or sub micron particles.

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 stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The EDS reveals what I already suspected in the optical micrograph – mixing in this stone isn’t particularly well. There are large areas, where the friable Na-Al-F compound where I suspect it is the mineral cryolite (sodium hexafluoroaluminate) absolutely dominates. It seems to be much finer than on the 600 grit stone, but the finer powder clumped together and created some hollow voids where the surface is littered with it. Moreover, we can make out quite the agglomeration of diamond – instead of an even spread, multiple nests of diamond can be made out. There is also again a large amount of SiC, but very, very fine.

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. As this is a benchstone, I differ from the usual process by using a Katocut Nowi Pro to keep the angle constant. 2 blades are sharpened, one in 65 HRC M398, one in 59-60 HRC Nitro V.

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

Let’s start with the harder steel – the M398 blade:

SEM micrographs of the M398 edge finished with the stone. Instrument: Zeiss GeminiSEM 560

The blade shows a much refined apex compared to the 600 grit stone. The bevel has gotten much smoother, we can easily identify the carbides. Nevertheless, we can still a lot of random direction marring of the surface – very likely caused by the rolling grain accumulating on the stone. I would guess that this is by design – it emulates the feeling of natural japanese stones, where a slush of abrasives builds up, and of course speeds up the stone. Nevertheless, it’s not as clean as firmly bound abrasive, which is reflected in the optical images:

Optical micrograph of the M398 bevel. Instrument: Marvscope

The WLI measurement shows a curious wave like structure parallel to the apex – I’d guess that this is caused by the abrasive slurry?

3D surface height map of the M398 Bevel. Instrument: Zygo NewView 9000, Objective Lens: 20X. Metrological filter chain: LS-Plane to orient data, cutoff 0.1/99.9 percent to remove outliers.

The bevel itself is quite smooth, with the surface roughness parameters as follows:

Sa	0.0471	µm
Sq	0.0635	µm
Ssk	-0.19	–
Sku	4.740	–

ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.08 mm (gaussian). No F operation besides LSQ leveling.

Let’s take a look at the NitroV edge:

SEM micrographs of the NitroV edge. Instrument: Zeiss GeminiSEM 560

Again, similar to the 600 grit stone, the surface is slightly less polished, with deeper scratches. The optical micrograph shows a very matte, but uniform appearance with a few random deeper scratches:

Optical micrograph of the NitroV bevel. Instrument: Marvscope

The WLI measurement shows again this very curious double wave-structure close to the apex.

3D surface height map of the NitroV Bevel. Instrument: Zygo NewView 9000, Objective Lens: 20X. Metrological filter chain: LS-Plane to orient data, cutoff 0.1/99.9 percent to remove outliers.

The surface roughness parameters have improved, but are a bit worse than on the M398 blade:

Sa	0.0671	µm
Sq	0.0875	µm
Ssk	-0.2729	–
Sku	3.920	–

ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.08 mm (gaussian). No F operation besides LSQ leveling.

The stone itself still has a surprising amount of feedback for it’s 3000 grit rating. It leaves the bevel with a very matte, diffuse appearance, which is at least to me quite appealing, but a far stretch from a mirror finish. I feel like it is held back by it’s mediocre mixing and the large amount of filler particles. The result is a refinement of the blade, both in surface roughness but also apex width, and I would guess that with some medium duty stropping, one could get a very good edge of this.

Considering that the price of this stone is quite high in my home country of Germany. Unlike the 600 grit, I don’t really like this stone, and I think there are much better alternatives out there.