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 episode, Part 1 is about the Fällkniven DC3. Part 2 is about the DMT mini W7C

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.

Today’s sharpening stone is actually one in two conditions. I recently aquired a TSPROF K03 Pro Hunter sharpening system. After DIY-messing-it-up twice to make a similar system and loosing the wish to continue sharpening knifes over the past decade, I decided it’s time to go for another round in the why-is-this-not-electric-powered. Or maybe my friend Roman Kasé bought one, and I generally try to imitate the master of steel as much as I can when it’s about sharpness. 🙂

The Blitz F1000 is a galvanic bound diamond grinding stone. Yes, we are noticing a pattern here. Why are there so many galvanic bound stones? Generally, because they are dirt cheap to make. Let me share the process by which you create galvanic bound stones with you: First, you take a metal substrate that is conducting. You sprinkle some diamonds on top. Then you immerse it in a (typically blue) solution containing nickel-ions, apply some voltage for a couple of minutes. The first growth of the electro-deposited nickel matrix starts away from the stone, as that one functions as part of your anode-cathode system. After a short amount of time, you remove it, brush off the excess diamond (only the submost layer will stick if you time it right!), transfer it into a second bath of nickel-solution, and continue the electrodeposition for a couple more minutes. The whole process is ghastly unhealthy, energy intensive and cheap enough that even in Germany companies are producing grinding media this way.

Now, the TSPROF F1000 is the “finest” of the galvanic bound diamond stones in the TSPROF set of 5 stones. The grit, according to the manufacturer is 1000, which should be somewhere in the range of 17 µm. This is already very fine and quite difficult to make on galvanic bound grinding media.

Optical micrographs of a brand new TSPROF Blitz F1000. The scale bar is visible in the lower right corner. Measurement Instrument: Leica EMSPIRA.

Optical micrographs show a smooth, regular surface with slight dents and fractures along the circumference and the edges. This doesn’t hurt the function, and I think the corner might also be from me, using it to scratch in a part number on a blade I was sharpening…

The real magic is revealed inside the SEM, as usual. Unfortunately, 6″x 1″ large grinding stones don’t fit into the desktop SEM we have, so the “big one” has to come to the rescue. The upside for you: pictures are so much better. This is taken on the Zeiss GeminiSEM560, a ultra high resolution field emission gun scanning electron microscope, featuring a nano-twin lens that combines the magnetic and electrostatic field into the last lens. If you get really close, this beast has sub-nanometre resolution across the whole voltage spectrum. Resolution improves over the desktop model by nearly 3 orders of magnitude. It’s likely the most expensive SEM you can buy, and probably the first time one of these sees a knife grinding stone 🙂 The magnification is defined identically to our other SEM (polaroid standard comparison), so you can easily cross reference with previous (and future) blog entries from the other SEM.

SEM Micrographs of the surface morphology of the unused TSPROF Blitz F1000 sharpening stone. Microscope: Zeiss GeminiSEM560.

The SEM micrographs reveal quite the even spacing between grains. Some are embedded very far, whereas others are peaking out massively. In my professional opinion, this is the result of someone who has a very decent workflow in preparation (spacing), but struggles with the very fine grain size. Grain size distribution is pretty even, and the grains are very sharp and flat ones. This is one hell of an abrasive stone. Good thing it’s meant to do abrasion!

White light interferometry height map of the diamond surface. Instrument used: Zygo Nexview NX2, Objective Lens: 10X. Stitched overview of 4×4 images.

White light interferometry looks a bit weird at first glance. In the SEM pictures, the grains weren’t this densely packed together. It shows large regions with higher and lower parts, but the difference between these should be measured in low single digit µm. This is likely height variations of the matrix we are detecting here! Zooming in a bit into the overview reveals the actual grains:

White light interferometry height map of the diamond surface. Instrument used: Zygo Nexview NX2, Objective Lens: 10X.

Fortunately, the ISO 25178 parameters are, while dependent on the area you select, pretty robust. Once you capture around 40 roughness creating events in every direction, your parameters don’t change a lot, and we can get away with a single parameter table this time.

Unsurprisingly, this is the smoothest stone with the lowest numbers so far. Even if you were to directly “imprint” the surface of this stone onto your knife, your surface roughness would be just above 1 µm. That’s often a challenge in steel for mediocre milling machines. The edges made by this stone are, while not glossy, already very fine, sharp and shiny.

In the beginning of this post, I teasered that this stone will be featured in two conditions – and the second is obviously, used! I’ve sharpened a grand total of 4 blades on it – 3 from M398 at 68 HRC, 1 in nitro-x at 64 HRC. This is quite the “hard” and demanding steel, but not a lot of used. I’ve. then repeated the metrology we see here, so we can see the initial wear of such a stone. At this point, let me mention that the stone is still perfectly fine and works just like it did new. But it gives a very nice first impression on what is happening during grinding.

The optical micrographs look pretty similar. The stone was cleaned with a steam cleaner, ultrasonic bath (ethanol, 5 minutes) and then blow dried with pure nitrogen gas.

Optical Micrograph of the Blitz F1000 in lightly used condition. Note: the corner didn’t magically reappear, I just own two sets of these stones. Microscope: Leica Emspira.

SEM micrographs are really interesting this time. You can immediately see a large amount of torn out grains, but also of massive, swarf induced scratches in the matrix.

SEM Micrographs of the used TSPROF Blitz F1000 stone. Not the massive scratches and plastic deformation. Instrument: Zeiss GeminiSEM560 Scanning Electron Microscope.

One can even spot some grains that have moved and ploughed along the matrix. Pretty cool shot!

White light interferometry height map of the diamond surface on the used stone. Instrument used: Zygo Nexview NX2, Objective Lens: 10X. Stitched overview of 4×4 images, detail view of 1 FOV.

The height map of the surface shows a bit more even distribution of high and low spots. In the detail view, the diamonds, and especially some missing are noticeable. Because this is a very fine stone, the parameter table is nearly identical to the first one:

Some initial wear has reduced the total height (Sz) as well as the material ratio (Sdc), but no significant changes.

Looking at the individual diamonds, I identified some without any wear, and some with. This is totally normal, as not all grains have contact with your material – only the ones sticking out have. This is why you typically dress a grinding wheel – to even out the surface. Now, the internet believes that you can’t dress galvanic grinding media, because it’s a single layer. I’ll let this stand for a later blog post. Let’s start with our detail peeking with a unused grain:

SEM micrograph of a single diamond grain. Note the very low accelerating voltage (500 V) and detector type (InLens). This not only reduces charging effects, but reveals all of the fine, intricate surface structures of the diamond. Instrument: Zeiss GeminiSEM560 Scanning Electron Microscope.

Compared to this undamaged grain, let’s take a look at one that is just barely used:

SEM micrograph of a single diamond grain, with initial wear visible at the topmost tip. Instrument: Zeiss GeminiSEM560 Scanning Electron Microscope.

You can clearly make out a very small section of the topmost part of the grain, where initial wear is happening. Wear on diamonds on steel is always chemically motivated, as some diffusion is happening. Nevertheless, the wear is abrasive in appearance. What is happening here is that typically, diamond is so hard, that abrasion shouldn’t be a major factor (10000 HV compared to 1000-1200 HV even for the hardest steels). This is because of the structure (NaCl lattice structure, two FCC lattice sells translated half a cell into each other), but also because of the bond between atoms – the so called sp3 hybridisation, that forms a very strong connection. By bringing the diamond in contact with steel, and applying energy (e.g. force and temperature), the sp3 bond is broken into a sp2 orbital, which is the one dominant in graphite. The current understanding is, that chemical potential and energy influx change the surface of the diamond to basically a graphite layer (and some much more complex changes, that would require a blog post on their own), which then can be abrasively removed (and some diffusion into the steel also happens).

SEM micrograph of a heavily used diamond grain. Note the very flat top plateau with clear directional abrasive marks. Instrument: Zeiss GeminiSEM560 Scanning Electron Microscope.

If this process continues, the grain is slowly flattened. This typically improves surface finish, as less micro edges and flatter large cutting edges create smoother surfaces. At a certain point, typically when the diamond grain flattening reaches it’s largest surface area, the diamonds are torn out of the galvanic bond. As most galvanic stones are single layered, the stone is then “done” and will be replaced. This is also the reason why a lot of manufacturers talk about “breaking in” and the stone becoming “finer” over use. Obviously, the grain size distribution is not becoming finer. You are just reducing outliers and flattening the cutting profile, so it appears finer. A side effect here is an increased cutting pressure, which reduces your “sharpening speed” if you keep the same pressure on the knife edge.