TL;DR: The PDT Premium Vitrified at 1100 grit is a very hard stone with an impressive CBN concentration. Unfortunately, the performance is subpar – a hesitant guess points towards the vitrified binder (which is at a similar hardness as most steels) rubbing and burnishing alongside the actual abrasive. The result is a matte, dull-ish edge. With some stropping, this would make for a sharp knife, but I think there are much better alternatives on the market.

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 another PDT. The Ukrainian company is pumping out different sharpening stones like there’s no tomorrow – which is very good for this blog, as there’s always something very interesting to look at! I bought this stone from https://www.uksg.tools/ – they seem to have a massive selection and availability of PDT Sharpening stones!

We’ve previously had the 225 grit stone on the blog which I bought with my own money – check the review here! We also took a look at the 285 grit stone – check it out here!

This time, we are taking a look at their fine(st?) PDT Premium Vitrified CBN stone, specifically the 1100 grit, which is somewhere in the range of 15 µm. Let’s take a closer look:

Optical micrographs of the PDT Vitrified CBN 225 grit stone. Instrument: Leica Emspira

The optical microscope shots show a very regular appearance. The CBN grit can barely be made out, as it’s starting to become really small. The stone itself has some weak translucent quality to it under the microscope light – cool!

SEM micrographs of the PDT Premium Vitrified CBN 225 grit stone. Instrument: Zeiss GeminiSEM 560.

The CBN grain is very easy to make out on this stone – a large number of blocky, cubic grains! It’s a very high concentration, and a decent mixture between the binder and the grains. But also, there are some much larger particles – block, flakey and some are remelted. Overall, the appearance is dominated by the larger grains, and the CBN kind of takes a step back here – on other stones I would mistake it for a binder!

If you want to read more about vitrified bonds, check out the previous two reviews on this stone series.

Let’s look at the chemical composition! I am very curious, what this bond is made out of. 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.

We can see that there’s a good amount of (in purple) Boron and Nitride -this is what our superabrasive cubic boron nitride consists of. The binder itself features mostly Al, Si, O and some Carbon. This is the formulation of Feldspar (M(Al,Si)4​O8), where M typically is some trace element such as Na, Ca or Ba. We can identify some Na in our EDS analysis – I think it is safe to conclude that the vitrified bond used here is therefore mainly feldspar. There’s a bit more Si and O than would be needed – but as feldspar significantly lowers the melting point of silicates, the re-melted zones we can see in the SEM micrographs probably are silicates (SiO2). Feldspars (around 700 HV) are softer than silicates (roughly 1000 HV). For comparison, 60 HRC can be converted to around 700 HV.

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 PDT Vitrified CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

The surface hear shows clear signs of smearing the material around. Some much larger scratches are also visible – I would guess that this stems from the SiO2 particles, that are several times larger than the CBN in this stone. The apex is refined further, compared to the 285 grit stone. Overall, it is still very wide and slightly rounded over.

Feedback on this stone was smoother than on the other stones. I’m unsure what to say here – compared to a similar grit resin stone, the surface and apex are definitely nothing to brag about. I think this suffers from the same problem as all “very fine” and at the same time “hard” stones do – the binder is working alongside the actual abrasive, and we do not get the pure, cutting power of the superabrasive. Compared to the 285 grit stone, I like this one even less. I think there’s fantastic alternatives on the market – I won’t be using this stone in the future.

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.