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 stone is the “sister” one to the one review in part 10. It’s from the Ukrainian company „PT.tools“ (also known as PDT or Poltava), which are a manufacturer of abrasive tools. It uses a bronze bond, and the grit chosen (2.5 µm) is perfect for polishing, according to the manufacturer. This is a rare CBN superabrasive stone.

Maybe a quick detour on the nature of CBN:

Diamond, as a super-abrasive is used because it’s the hardest material we can manufacture in large quantities. It’s super hard, but suffers from one major issue: it’s the meta-stable configuration of carbon, and not only is it susceptible to high temperature graphitisation, it also experiences chemical wear in a number of materials, first and foremost in steel.

CBN is the abbreviation for cubic boron nitride – funnily enough, it doesn’t look cubic 🙂 check out some more SEM pictures on this blog entry.

It has the same crystal latice structure as diamond. But what’s that structure?The crystal structure is called fcc = face centered cubic, a cube with atoms on the corners and faces of the cube.

A representation of the fcc crystal lattice.

For the “diamond” or CBN structure, two of these lattices are intersected into each other, at a shift of 1/4 of the lattice cell:

“diamond” lattice structure, which is two identical fcc cells intersected at 1/4 of the lattice.

Moreover, CBN has the same bond between the atoms as diamond – a so called “sp3” orbital. This is a very strong chemical bond. The amazing hardness of CBN and diamond stem from this bond mechanic, the interatomic distance and crystal structure. While diamond clocks in at 10000 HV in the hard direction, CBN is more around 4500-5000 HV. But unlike diamond, CBN experiences no chemical wear when machining steel, which made it the abrasive of choice in commercial, precision CNC grinding of hard steels. CBN generally is more expensive by weight than the same amount and grade of diamond abrasive.

Optical micrographs of the PT CBN 2.5 µm stone. Instrument: Leica Emspira.

The stone is virtually indistinguishable to the naked eye from the previously reviewed diamond stone of the same grit. The stone is very firm, showing a dark grey colour, that slightly reflects reddish/bronze coloured when the light hits it. Under the microscope, a very even structure is visible. Individual grits are near impossible to make out, because the bronze binder is so reflective.

Let’s take a look under the scanning electron microscope!

SEM micrographs of the PTD CBN 2.5 µm. Instrument: Zeiss GeminiSEM560.

Even at 2kx magnification, it’s hard to make out the CBN. The stone is dominated by large SiC particles and the metallic bond. But fear not, avid reader, I’ve zoomed in far and found a CBN grain for you:

There you have it! A sole grain in specification of the size stated by the manufacturer, and quite a bit of smaller debris, likely from the dressing of the stone. Around the grain, some cracking is visible. Even though it is nearly fully embedded, no good wetting (covering of the metal matrix) is achieved, giving it a loose appearance. EDS analysis reveals a couple more grains, but the bond is dominated by SiC embedded into it. The SiC particles are about one order of magnitude larger than the CBN abrasives.

EDS analysis of the PTD CBN 2.5 µm. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

Zooming in a bit further, the CBN really becomes visible:

EDS analysis of the PTD CBN 2.5 µm. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The surface height maps are very similar to the diamond stone with the same matrix:

Instrument: Bruker Alicona µCMM, 50X objective lens, 3×3 FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%).

The ISO 25178 parameters show a slightly coarser and rougher surface than on the diamond stone. I would consider this to be larger than within the process variation of a commercial flat grinding process.

ISO 25178 parameters of the PTD CBN 2.5 µm.

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 blade finished with the PTD CBN 2.5µm. Deep, regular scratches are visible that were created by the stone, but also fine, smearing of the material. The apex is completely dulled. At least there is no burr 🙂

The surface finish left by this stone was abhorrent. After just 2 passes, the whole surface turned matte and super dull, with lots of visible scratches. The SEM pictures show this very clearly. It is even worse than the similar sized diamond stone. The blade tested to a value of >300 BESS.

The verdict for this stone is very similar to the equally sized diamond stone with copper bond. At this concentration, with these many large SiC particles, this isn’t a very good stone. I don’t believe that copper-bronze metal bonds at this particle size are very suitable for non-CNC based sharpening. At larger grain sizes, and with a large increase in grain density, I could see this as a fantastic, very tough, long lasting sharpening stone.

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.