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 KDTU Silver CBN. According to the shop where I bought it, it features an “advanced hybrid bond enriched with tin and silver-based components, this stone features a distinctive silver-colored abrasive surface — the defining characteristic of the KDTU Silver Stones Series“

Alright. Sounds good. One thing that immediately stands out to me is the very dark colour of the stone. In a previous review, I quoted the manufacturer who said that discolouration is just something inherent to the stones, but the review proofed that it was a sign of bad mixing. I have high hopes that this stone is now better mixed, and not just coloured black to hide mediocre manufacturing.

Let’s take a look under the optical microscope!

Optical micrographs of the stone. Instrument: Marvscope

The stone shows a dark, grey/silver bond. We can immediately make out black CBN grains, which tend to cluster together. Moreover, there’s some streaks of a bronze coloured material visible as well. Honestly: looks pretty cool!

Let’s take a closer look in the SEM:

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

The SEM pictures show a fine, very dense bond. The abrasive particles are held tightly inside this bond. They are of a very square and blocky type.

The bond is surprisingly dense for a metal bond – most bronze bonds are a bit looser.

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.

In the EDS analysis, the dense bond is explained – it consists mainly out of aluminium, with some copper, tin, and zinc. The before-mentioned silver that gives this stone it’s name can also be found – in a sub 1% concentration. Silver often is used as either a flux or wetting agent to enhance grain retention. Moreover, we can find some silicon (Carbide?) particles, which are smaller than the CBN. Zooming out, one can see a decent amount of CBN, with an overall mediocre distribution. There’s some clustering, but at this stones grit this will not be a major issue.

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, edge trailing 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. Moreover, the same approach is repeated with a blade in NitroV at 59-60 HRC.

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 stone left a smooth bevel finish. We can see major signs of ploughing and burnishing – every track of the abrasive grain shows microburr and prow formation at the side. This is a clear sign of plastic deformation instead of clean cutting action. Zooming in to higher magnifications, we can see that the stone created enough pressure that pieces of the steel broke away or fractured near the apex. Nevertheless, the finish is very homogeneous and quite smooth for such a coarse stone.

This is confirmed in the optical micrograph, showing a very matte, silvery and homogeneous finish (do ignore the water drop stains on it – mea culpa maxima!)

Optical micrograph of the M398 bevel. Instrument: Marvscope

Which is further visible in the white light interferometer measurements of the bevel: a diffuse, marred surface:

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.

With the surface roughness parameters as follows:

Sa	0.3957	µm
Sq	0.5222	µm
Ssk	-0.608	–
Sku	4.854	–

ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.25 mm (gaussian). No F operation besides LSQ leveling.

Overall, this is a decent result in terms of surface finish and bevel appearance, but has not really refined the apex massively.

Let’s take a look at the NitroV edge:

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

The softer steel shows less cracking and damage near the apex, but we can also make out large, plastic deformed regions – some part of it has just bend over. Moreover, the surface morphology is once again showing the typical plastic deformation associated with burnishing.

Optical micrograph of the NitroV bevel. Instrument: Marvscope

The edge of the blade is quite ragged, even for this grit of finishing.

In the WLI measurement some deeper scratches can be made out:

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.

With a slightly higher surface roughness on this bevel:

Sa	0.4340	µm
Sq	0.5652	µm
Ssk	-0.2213	–
Sku	3.939	–

ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.25 mm (gaussian). No F operation besides LSQ leveling.

The stone itself is pleasant to use. It has a smooth, high feedback feel to it, and is very aggressive in the beginning. After some sharpening action, it loads and becomes a bit slower, but during my test not massively so. It cleans up with a bit of vigorous rubbing.

If I compare the results with the previous coarse CBN stone review from KDTU, I’d say it’s a small improvement, only detectable under advanced microscopes. Mixing still seems to be an area where KDTU struggles, like most abrasive manufacturers. To me, it feels like the harder bond is much tougher on the apex, but the overall feel in sharpening on this one has improved, as did the surface finish.

This brings me to my biggest issue with this stone & manufacturer:

The Ukranian manufacturers (PDT and KDTU) seem to be huge fans of CBN, which is only understandable seeing that during the soviet era, they were a major producer of CBN and still publish a lot of research on it via the V. Bakul Institute for Superhard Materials, a Ukrainian state founded research center for superabrasives. This is reflected in their marketing campaigns pushing these stones. Moreover, their product strategy seems to be the constant release of slightly improved abrasives, since I’ve had this stone 2 more lines came out. This is fine, but for me as a paying customer, it feels like I constantly need to open my wallet to get the latest and newest -and they all fall short of the competition, with no huge improvements.

There is a plethora of stones that outperform this one – both with aluminium oxide or diamond as an abrasive. While this stone is pleasant to use, I’d say there are better choices available.