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 made by the Ukrainian company “KDTU”. Their name apparently stems from a (previous?) joint venture with a japanese firm. They have a really cool instagram channel with some videos of their production – you should check it out, it’s rare to get a view into sharpening stone production! The review is about their “Hybrid CBN” series, where according to their webpage, nonferrous powders are mixed with resin to create the optimum sharpening stone! Let’s take a closer look under the microscope:

Optical micrographs of the KDTU Hybrid stone. Instrument: Leica Emspira

The stone has a reddish colour, similar to bronze. Lot’s of different abrasive particles are visible, with the most regular being black like one could expect CBN to be. Let’s dig deeper in the SEM:

SEM micrographs of the KDTU hybrid CBN stone. Instrument: Zeiss GeminiSEM 560.

There’s a lot going on here! We have some blocky, massive grains, which likely will be the CBN grains. Zooming in, there is some smaller, blocky grains, some powder and also a component to the matrix that looks a bit molten, which likely is the resin. Fortunately, we can identify on an atomic element level, what we are seeing here. 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 KDTU hybrid CBN stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

This is probably the most colorfull stone we ever had on the blog 🙂 It seems like it contains pretty much everything. Let’s look a bit more in detail on it:

If you look at the individual channels, one can make out the large grains as containing nitrogen (and boron, which for some reason didn’t fit on the first page). These are our CBN grains. Around these grains, and for this you have to look on the right side of the picture towards individual channels, you can make out nickel (Ni) and phosphorus (P). These are often used as a wet-chemical coating (think: like galvanic, but not galvanic. The coating is created from chemistry and not current!) that increases the surface area around the grains and improves grain retention. Then, we have a massive amount of silicon particles, which nicely overlap with both oxygen and carbon signals – so a mix between SiC and SiO2 particles is there. Moreover, there’s aluminium, copper, tin, titanium and some trace alkaline metals such as K, Ca. It’s quite the wild mix. Typically, non-ferrous powders such as copper are added to grinding wheels to increase thermal conductivity, which is especially important for high speed grinding of ceramics and glass. Abrasive powders such as SiC and SiO2, but also Al2O3 are often added to increase the hardness of the bond, while reducing the cost by lowering the super-abrasive content.

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.

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

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

The stone has a decent, aggressive material removal. The surface is completely reworked after just a couple of strokes. There’s quite a few very deep scratches, and a noticeable burr is created on the edge. The stones feel very hard, with a lot of vibration (feedback) during sharpening.

The SEM pictures reflect this: The surface has some deeper gouges (remember, this is a coarse stone!) with a large, bend burr. Unfortunately, quite a bit of cracking in the burr is also visible – because of the high hardness of the bond, and the large amount of harder particles that don’t cut, a lot of pressure is added to the sharpening action.

Optical micrographs of the KDTU CBN stone. Instrument: Leica Emspira

The stone quickly removes material, feels very hard and has a curious composition. I think this is a decent stone for reshaping your apex, but it falls short of my favourite re-shaping stone – the Atoma electroplated stones. I think that a lower percentage of non ferrous powders and a higher content of superabrasive would make this a super nice stone – the resin technology looks spot on.

Sharpening disclaimer: I use a standardised approach to sharpening, which basically follows how most manufacturer of guided systems tell you to use this system. I am very aware, that every stone could perform much better than this, in terms of sharpness, but I want a comparable approach. The sharpening segment mostly shows the material removal mechanism – is it burnishing? is it cutting? is the cutting pressure too high so that carbides crack? Is there massive burr or prow formation? The BESS value definitely doesn’t highlight the ultimate sharpening performance of the stone, but was an often requested information. Over time, this blog will show BESS values for different edge morphologies, but by the holy endmill – don’t read it as a „this is the max value this stone can achieve“. I would also suggest to familiarise yourself with the works of Immanuel Kant, it’s absurd I need to write such a disclaimer here.