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.

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.

TL;DR: The PDT Premium Vitrified is a very hard, high concentration CBN stone. Surface finish and cutting action doesn’t really life up to the hype around these stones. Nevertheless, they can be used for reprofiling a knife bevel. 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 bought this stone at https://www.uksg.tools/ – they seem to have a massive selection and availability of PDT Sharpening stones!

This time, we are taking a look at their PDT Premium Vitrified CBN stone, for this part of the series the 225 grit, which is somewhere in the range of 80-100 µm. This is a super hyped stone, as vitrified is a very high-tech, modern bond for sharpening tools. They have garnered massive attention and were often demanded for a blogreview – so let’s dig a bit deeper!

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! Let’s take a look in the SEM:

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

Inside the SEM, the CBN grains are actually harder to make out – as we loose the comfort of colour! But because of the large grain size, as well as the high concentration, it’s easily visible that this stone has a lot of abrasive particles. The binder in between is slightly porous, and consists of a flakey, sometimes melted, sometimes very dry and particulate mass. Before we dig into the chemistry, it’s time for “Dr. Marv explains…” – in this case, I should probably state what a vitrified bond is, and why it typically is considered a high-tech matrix for abrasives.

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 Ba 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 here shows quite a bit of structures worth analysing. First, we can see that the apex is actually folded nearly 90° 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. Now, I could see why this stone is percepted to create an ultra quick burr – because this gianormous plastic deformation of the apex will be detectable near instantly. But I think this is a misconception – I wouldn’t consider something this big to really be a burr – not in the sense of it forming the apex. I think this would actually get you started too early on the next grit of your progression, ultimately leading towards duller edges.

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. 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.