TL;DR: The PDT Premium Vitrified at 285 grit is a very hard, high concentration CBN stone. Surface finish and cutting action appears to be a bit better than on it’s even coarser brother. Feedback is very high, while most EP stones and some resin stones show better surface and higher material removal rates.

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 received this stone from https://www.uksg.tools/ – they seem to have a massive selection and availability of PDT Sharpening stones! Thanks UKSG – lovely to supply me with some material for the blog! <3

We’ve previously had the 225 grit stone on the blog which I bought with my own money – check the review here!

This time, we are taking a look at their second coarsest PDT Premium Vitrified CBN stone, specifically the 285 grit, which is somewhere in the range of 60-80 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 is a black one, and a noticeably higher concentration than on the metal bond CBN stones. This looks promising! Some reflective white particles are visible. Let’s see whether we can differentiate these in the SEM:

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 larger particles – some appear very smooth, almost metal like in their morphological appearance, some show clear brittle cleavage – typically a hint towards abrasive contamination (or added ceramic fillers!).

Vitrified bonds are made from a wild mix of different ceramics – typically some feldspars, clay, glasses and other ceramic components. These are fired at high temperatures, which fuses the individual components together – hence the name vitrified. Naturally, these vitrified bonds are slightly porous, very hard and strong. These are all characteristics that are very advantageous in precision CNC grinding applications. Moreover, they are very resistant to chemicals and heat, which is also nice during high speed grinding. Now, I personally find it a curious choice to advertise vitrified as the solution for hand sharpening. The characteristics that make it so suitable for high speed grinding, make it less suited for sharpening – the pores, which usually transport chips and coolant, will clog and load more easily than on a dense stone. The high hardness of the bond will impair self sharpening, and lead towards burnishing from the bond itself. The typically brittle characteristic is not that much of an issue in this exact stone – I managed to drop it from about 1m height while unboxing it out of the ghastly blister, and it survived with no damages!

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. Surprisingly, some titanium is also visible – this could be the “metal like” particles we have seen before. I am unsure why titanium is added – probably to add toughness to it?

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 here shows quite a bit of structures worth analysing. First, we can see that the apex is actually folded at about 60° towards us. It is quite wide (consisting out of several grains that are starting to loose cohesion!), and gives the resemblance of heavy burr formation.

I think this is one of the reasons why this stone is so hyped – typically, the burr formation during sharpening is detected by stroking perpendicular to the edge with the thumb.

Secondly, we can see in the surface along the bevel that a lot of prows are formed on the surface. A prow is a plastic deformation of the surface – if a metal workpiece is not cut cleanly, but the cutting edge pushes the material to form what amounts to a micro burr, it forms to a small, melted looking feature. As light is refracted in these, the surface turns very matte – something that can be seen on blades sharpened with these stones.

While using the stone, there is a lot of friction, which is also very constant. Because the stone is very hard, it is easy to differentiate the contact point.

I found this stone to be pleasant to use – feedback because of friction and hardness is very nice. Comparing it to slightly coarser stones I’ve previously tested on this blog – for example the TSPROF Alpha 120 or the Atoma F140, I find material removal and apex width to be much inferior. Compared to the 225 grit stone, this stone definitely refines the apex to a thinner one. It’s not too bad, and definitely nicer than the 225 grit one. It’s also absolutely lovely on the feedback – I could see this being a fantastic benchstone, followed by some excessive stropping or a finer resin stone.

Overall, I am a bit disappointed: I hoped that this will become a very long living stable in my sharpening kit, used for reprofiling knives. Unfortunately, because of the bad surface finish, low speed and wide achieved apex width, I don’t consider this to be a very good 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.