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 – this time, their “silver bond” CBN stone, at 650 grit, which they state as 28/20 µm. It uses the superabrasive CBN. According to the manufacturer homepage, it is a “vitrified metal” bond. I strongly suspect, this is the case of using the marketing buzzword of “vitrified”.

Let’s take a look under the microscope:

Optical micrographs of the PDT Silver stone. Instrument: Leica Emspira

The stone is very silver in colour – not at all like the regular bronze coloured metal bond stones. We can also make out a decent amount of black CBN particles. Let’s further look into this under the SEM:

SEM micrographs of the PDT silver CBN stone. Instrument: Zeiss GeminiSEM 560.

We can spot some very large, darker particles, but also a lot of smaller grains, of which the majority is in the size of the abrasive. This matrix looks like a regular metal bond, no signs of vitrified bond are visible here. To be fair, one can start to be very picky about the classification here: Both a metal bond as well as a vitrified bond are typically created by taking a low melting point matrix, and raising the temperature to a point where the bond matrix starts to fuse together. On metal bonds, one would call this sintering, as the “vitrification” typically implies a glassy phase, that one does not achieve with metals. It is save to say, that this stone is not vitrified, but instead a sintered metal bond.

We can further verify this in the SEM via it’s chemical composition! 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 PDT Silver 650 grit stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

This stone is once again a colourful firework of different elements! Let’s dig into what we can see: The largest visible particle is some titanium (blue, uppermost corner, around 8%). Titanium is typically added to CNC abrasive bonds to make them tougher. We can see the CBN grains (B, red) which are distributed with a slight tendency to agglomeration all over the image. Moreover, there’s a bit of silver in the bond (about 1%), and a high tin content (24%) compared to the copper content (25%). This explains the silver colour of the stone! Overall, the addition of these elements makes the bond harder (higher tin content), quite significantly so! The downside is, it also increases the brittleness- which is probably why there are some SiC as well as the titanium particles. Overall, it looks to me like the mix between SiC and CBN is about 1:5, making this mostly (but not exlusively!) a CBN stone. Overall, I this will be a very hard, long lasting stone. I expect it to have quite some pressure and push the material more around than it is cutting.

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 from the spine towards the apex (edge trailing) 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.

The stone has a rough, high amount of feedback during sharpening. A homogenous, matte surface is the result. After a couple of strokes, the edge has a detectable burr or prow formed. Let’s take a look at this under the SEM:

SEM micrographs of the edge finished with the PDT silver 650 grit stone. Instrument: Thermo Fischer PhenomXL SEM.

We can see quite a few signs of burnishing and plastic deformation here: not only is the surface showing lots of small micro burrs/prow formations, but also the cutting edge shows a very visible (bent towards the viewing direction) burr/prow. It is at least a couple of microns wide. I would believe that this is overall a sign that the stone is not cutting very freely, but creating a lot of pressure through it’s dense, hard matrix. This helps with the quick formation of something that feels like a burr – but is more plastic deformed material. With some stropping, this will likely be refined and raised to be sharper, but it does not look like a well formed apex.

Optical micrographs of the edge created by the PDT Silver stone. Instrument: Leica Emspira

Overall, I think this is a very hard, durable stone. The results are homogeneous with few deeper scratches. A well formed edge with a very regular, low waviness bevel is formed. I did not really like the plastic deformation happening to the bevel, but can now understand the “hype” on the internet about this stone – after all, it creates something very much like a burr with just a few strokes! I would expect this stone to require regular rework to renew it’s cutting capabilities, for example by etching the bond to release new, sharp grains.