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 stone is a novel one for this blog. It’s from the Ukrainian company “PT.tools” (also known as PDT or Poltava), which are a manufacturer of abrasive tools. It uses a bronze bond, and the grit chosen (3/2 µm) is perfect for polishing, according to the manufacturer.
Let’s take a look under the microscope!
Optical micrograph of the PTD diamond 3/2 µm. Instrument: Leica Emspira.
The stone is very firm, showing a dark grey colour, that slightly reflects reddish/bronze coloured when the light hits it. Under the microscope, a very even structure is visible. Individual grits are near impossible to make out, because the bronze binder is so reflective.
For a better look, I’ve put the stone into our ultra high resolution scanning electron microscope.
SEM micrographs of the PTD Diamond 3/2 µm. Instrument: Zeiss GeminiSEM560.
The bond is very typical of a dense, highly sintered bronze bond. At the topmost surface, some plastic deformation of the matrix is visible, in deeper recesses some porosity from sintering, but generally speaking this is one dense bond! I typically encounter such tools in my dayjob for precision grinding of glass. The super low concentration of the abrasive also highlights this.
EDS analysis confirms what the manufacturer stats -small diamond grains in a bronze binder. The larger particles visible appear to be Silicon carbide, I’d guess embedded from the dressing process?
EDS analysis of the PTD Diamond 3/2. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
Under the focus variation confocal microscope, a relatively smooth surface, dominated by the metal binder and dressing process is visible. This stone will likely create an immense amount of cutting pressure.
Instrument: Bruker Alicona µCMM, 50X objective lens, 3×3 FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%).
The surface parameters do mirror this finding – a smooth stone with a relatively low surface roughness, and generally dense material ratio (Sdc).
ISO 25178 parameters of the PTD Diamond 3/2 µm.
In order to evaluate the sharpening performance 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 blade finished with the PTD Diamond 3/2 µm. Deep, regular scratches are visible that were created by the stone.
The sharpening result of this stone was abhorrent. My regular preparation with my own, DrMarv Scientific Sharpening stones leaves a near mirror finish, with a super high gloss at 10 µm. Only by varying the light, some very, very fine scratches can be made out. With the PTD stone, even after just 2 passes, the whole surface turned matte and super dull, with lots of visible scratches. The SEM pictures show this very clearly – with carbide matrix fractures near the apex, and prow formation. It barely did shave, but was easily felt that it’s more tearing and less cutting.
I’m unsure what the issue is here. I would guess that the low concentration and embedded larger SiC particles, combined with the very hard binder mar the surface of the blade. From my professional day job, dressing such a bond is very difficult, requires a quick dressing spindle and low engagement. Nothing that is done easily or cheaply. While grinding with such bonds and concentrations on a milling machine, immense cutting pressure and heat is generated. I wonder why it is made with such a fine grain. If the concentration was bumped by a factor of 10, and large grits were used, it would likely be a fantastic, very long lasting sharpening stone, if the manufacturer was able to integrate some self sharpening properties.
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.
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 stone is a very famous, well known brand. Naniwa is a Japanese brand, that has been making sharpening stones for over 60 years. Their homepage says they deal in all things abrasive. I like that! The stone I have bought is from the chosera line, with 5000 grit. It is, according to diverse homepages, an alumina-oxide stone with a magnesia binder. Let’s take a closer look:
Optical micrograph of the Naniwa chosera 5000. Instrument: Leica Emspira.
It has some marbeling to it, with a fine and smooth surface. Zooming in, individual particles and grains become visible. For a closer look, as usual, we take a look in the SEM!
SEM micrographs of the Naniwa Chosera 5000 stone. Instrument: Zeiss GeminiSEM560.
At low magnifications, the stone appears to have a smooth cover above the abrasive grit, covering about 80% of the surface. Zooming in further, one starts to identify cubic, small abrasive grits, but also that the covering “film” actually consists out of an uncountable amount of sub-µm particles, that are slightly rounded and longish. This is an interesting stone! To identify this, we will employ EDS analysis – check out this segment of the blog to understand the SEM metrology better. I mentioned before, that things get… tight once you employ the full suit of sensors. Because this stone is non-conductive, but we need a lot of acceleration voltage to get a reliable EDS reading, I’ve employed our low vacuum mode. With the aid of a small orfice, it is possible to increase the chamber pressure, but leave all sensors functional. For this, a diode-type BSD sensor with the small aperture is inserted pneumatically into the chamber. The EDS sensor meanwhile is shaped like a pen, coming in from the other side. To show you how unbelievably tight and confusing everything gets, I snapped you a shot of the chamberscope:
View from the chamberscope. The stone is visible diagonally from lower left to upper right. The EDS sensor is the pen shaped object coming from the right upper corner. The low-vacuum aperture sits below the pyramidal pole piece, and has been inserted from the left side of the picture. Instrument: Zeiss GeminiSEM560.
The EDS analysis reveals the chemical composition:
EDS analysis of the Naniwa Chosera 5000. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
There’s a fascinating mix of chemical elements up there. The old truth “you can find the whole periodic table in ceramics” hold’s especially true. Very interesting is the large particle, with flaky composition. Let’s zoom in a bit more on that one:
SEM micrograph of a flaky particle, found in the Naniwa Chosera 5000 stone. Instrument: Zeiss GeminiSEM 560.
Elemental analysis of the flaky particle. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
This is really fascinating! we can pick out what we expected – Mg, O, Al, but also massive amounts of silicon. This is the moment where I am really happy, I haven’t spend more time studying minerals, because I don’t even want to imagine what this could be. Nevertheless, I went on a literature deep dive for you folks. There’s a couple of possibilities this could be – a paper I found had something similar to the flakes we are seeing, but was writen by geologists1. They stated that the flakes they are seeing could either be: Anortit – CaAl2Si2O8; Albit – NaAlSi3O8; and Paligorskit – (Mg,Al)5(Si, Al)8O20(OH)28H2O. The nice thing about geology is, pretty much every mineral has decent SEM pictures online. Albit2 has a matching appearance3, but the excited reader might now ask, where is our Sodium (Na) at this point? Well, it’s pretty exchangeable with Calcium (Ca), which we found in our spectrum. I have no idea what this is called, and I am quite unsure at this point what we found. Structure wise, it should be a triclin crystal latice, and it consists out of the chemical elements Si, Mg, O, Ca. If anyone has studied geology and wants to supply the solution here, reach out. If not, I now declare this to be something like Albit. It doesn’t really matter – it just shows they sinter these stones at quite the high temperature, and there’s cool structures hidden in the microcosmos! All of these oxides are of similar hardness – quite a bit harder than steel, not much harder than carbides.
Let’s take a look at the surface composition!
Instrument: Bruker Alicona µCMM, 50X objective lens, 3×3 FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%).
It’s a smooth stone, without a large amount of bearing surface. Roughness is not exceptionally smooth, but there are also no super deep recesses. This is a finely made stone, and if you scratch along the surface with your fingernail, there’s nothing catching it. The feedback, because of the relatively rough surface is noticeable. This stone doesn’t glitch over your knife edge.
ISO 25178 parameters of the Naniwa Chosera 5000 stone.
In order to evaluate the sharpening performance 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 stone was only splashed with water, not soaked (in accordance with the website I bought it from).
SEM micrographs of the blade finished with the Naniwa Chosera. A slight cross hatch pattern is visible, which stems from me changing hands during the sharpening. The edge is burnished at some parts, whereas other parts show fractures. Overall, the appearance diminished a bit compared to the 10 µm resin stone.
The edge shows no burr, but some cracking near the carbides, brittle failure of the edge and larger scratches are visible. I think the stone struggled a lot with the very hard (65 HRC!) high carbide steel, but also the fact that on a guided system, you can only splash it with water – it doesn’t build up a slurry. Seriously? I think this might be a fantastic stone for freehand sharpening and lower hardness steels. On this guided system, with the minimum amount of water I was able to apply, I am not a super large fan.
I will try to revisit this stone in the future with a softer steel.
References:
Paper: Cvetković, Vesna & Purenović, M.M. & Jovićević, Jovan. (2006). Change of water electrochemical characteristics in contact with magnesium enriched kaolinite-bentonite catalyst substrate. CHISA 2006 – 17th International Congress of Chemical and Process Engineering. ↩︎
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:
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.
Review Today’s sharpening Stone is a triplet of stones. These are from a German sharpening shop called “Schleifjunkies”. The stones are advertised under the name “resinbond”, and according to the manufacturer create high gloss and super sharp edges in minutes, not hours. Ok! Let’s take a closer look 🙂
The stones are well finished on the top surface, with a smooth, green surface. The side is raw and appears to be saw or maybe beam cut? They are actually glued down to the holder with some flexible foam tape, allowing for some flex between stone and aluminium holder:
Let’s take a look at each stone under the optical microscope.
Optical micrographs of the SJ Resinbond 6 µm stone. The scale bar is visible in the lower right corner. Instrument: Leica Emspira.
Quite a bit of colour is visible at higher magnifications. Green, translucent green (typically diamond), black, and some reddish-orange colour. I think this is going to be a very interesting stone to look at under the SEM.
Optical micrographs of the SJ Resinbond 3 µm stone. The scale bar is visible in the lower right corner. Instrument: Leica Emspira.
Optical micrographs of the SJ Resinbond 1 µm stone. The scale bar is visible in the lower right corner. Instrument: Leica Emspira.
The two finer stones show about the same colour – the translucent green particles do shrink in size though, most notably from 6 to 3 µm. I can’t really tell any difference in size on the other particles.
The stone was cleaned with IPA in an ultrasonic bath, rinsed and then blow-dried with compressed, ultra pure nitrogen gas before getting put into the SEM.
SEM Micrographs of the SJ resinbond 6 µm stone. Instrument: Zeiss GeminiSEM560.
The stone is a mix of 3 different, easily identifiable grains. There are larger, above 10 µm grains all across the surface, in a low conecntration. there is a higher concentration of smaller, blocky, fractured grains as well as a notable amount of rounded grains, that have some molten look to them. Between the grains, some areas are binder (matrix / resin) dense, whereas others are dense agglomerations of grains.
SEM Micrographs of the SJ resinbond 3 µm stone. Instrument: Zeiss GeminiSEM560.
The 3 micrometre stone shows the wide spread of grains, and also their diverse size:
There are some 10 µm sized grains, that are very long and narrow, interspersed with some more blocky, rounded grains that I suspect will be the diamond. On the upper left corner, one can make out the molten droplets in fine detail. These are also a bit lighter colour – typically a sign that they consist of a heavier element than the surroundings. I took a quick peak at the 1 micrometre stone, which looked nearly identical to the 3 micrometre one, but didn’t go through the trouble of recording the images, as I prefered to focus on finding out all it’s secrets – especially the 3 different grains that are visible! For this, I did energy dispersive x-ray spectroscopy (EDS) to create elemental composition maps over the SEM picture.
EDS analysis of the Schleifjunkies 3 micrometre resinbond stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
The EDS analysis brings some clarity to this! Let’s take a closer look at the elemental mapping ,and what we can deduce from this.
The stone has some large, blocky, molten looking red areas, which are carbon rich. This is the resin used to bond the particles together. The smaller, red grains are also mainly carbon – most definitely the diamond grain. SJ seems to have used a more blocky, smoother grain shape here.
THe green grains are silicon, but by comparing the carbon intensity map, we can see that they also consist of carbon. This is silicon carbide, at about 3 times the size of the diamond grains. Silicon carbide is quite hard (2400-3000 HV, depending on the type), which is why it is often used as an abrasive on it’s own. The use in resin bond stones is typically to make the bond harder. The purple grains are actually copper – which explains the reddish grain we could make out in the optical microscope pictures. Copper is added to industrial resin grinding wheels to improve heat conductivity, and while this makes a lot of sense at high cutting speeds, and if your abrasive is alumina oxide (corundum) or SiC, diamond has a much better heat conductivity, and it’s the first time I’m seeing this on a diamond grinding bit. Frankly, here it can only be either a cheap filler, or the manufacturer took the same mix they use for AO grinding wheels and just added diamond. Trace amounts of chromium can be detected, as well as some oxygen, matching particles with the silicon map, so I’d suspect that the rare, white-ish particles we have seen in the microscope are SiO2 (quartz) particles.
Let’s take a look at the surface roughness and appearance.
3D height map of the 6 µm SJ resinbond stone. Instrument: Bruker Alicona µCMM, 50X objective lens, singe FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%). 2nd picture: area extract to show the grain.
The surface, similar to the SEM picture, has large, very smooth sections, where the grain is still covered in a bit of resin, and also irregular, smaller sections with voids and recessed grains. This matches the view from the SEM quite well.
ISO 25178 parameters of the 6 µm SJ resinbond stone.
The stone surface roughness (Sq) is very low, with a nice and tight control on the height of the surface bearing material ratio (Sdc). The kurtosis (Sku) is quite high here, a result of the very flat sections in combination with the very steep walls leading down to the voids. This smooth stone will glide quite easily along a blade, while providing little feedback. The pressure applied is spread over a large area, reducing the force acting on every grain.
3D height map of the 3 µm SJ resinbond stone. Instrument: Bruker Alicona µCMM, 50X objective lens, singe FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%). 2nd picture: area extract to show the grain. 3rd picture. ISO 25178 surface data.
The 3 micrometre and 1 micrometre stone do not differ significantly in their surface parameters. I believe the surface of these stones is dominated by both the filler grains (SiC & copper), but also the breakouts above large nests of grains in combination with the dressing from the manufacturer.
3D height map of the 1 µm SJ resinbond stone. Instrument: Bruker Alicona µCMM, 50X objective lens, singe FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%). 2nd picture: area extract to show the grain. 3rd picture. ISO 25178 surface data.
The 1 micrometre resinbond stone has a line through the center of the height scan, sitting quite a bit above the rest of the surface area. Maybe a missed spot on the final dressing of the stone surface?
In order to evaluate the sharpening performance of these stones, 3 blades were sharpened. In order to evaluate the sharpening performance 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. One blade was prepared with the 6 micrometre stone, the 2nd with first the 6 and then the 3 micrometre one, the last with all three stones.
First, some pictures of the 6 micrometre stone:
SEM micrographs of the sharpened M398 blade. Finishing Stone: Schleifjunkies 6 µm. Instrument: Zeiss GeminiSEM560.
The 6 µm blade shows a slightly wavy edge. Some burr is visible, as well as some carbide cracking from the grinding pressure. Periodically, deeper scratches are visible.
Following are pictures of the 3 µm stone:
SEM micrographs of the sharpened M398 blade. Finishing Stone: Schleifjunkies 3 µm. Instrument: Zeiss GeminiSEM560.
The 3 micrometre stone left a smoother surface with lower scratches. Near the apex, some cracking and a ghost burr are visible. Some deeper scratches are visible, similar to the 6 µm stone. The stone didn’t remove a lot of material, and mostly burnished the surface, which also explains why no significant sharpness improvement was visible.
The 1 µm stone felt very dull. I spend more than 15 minutes just on that stone, with barely an improvement on the blade. Because of the low material removal rate, I raised the angle by 0.1°, so that the edge was leading and we could be sure that what we are later measuring was created by the 1 micrometre stone. The blade tested notably duller on my BESS tester.
Whenever I got a section to become smoother, a larger scratch appeared again. These deeper scratches are very similar to the other two stones. I would hazard a guess that it’s the SiC particles, which are similarly sized in all 3 stones.
SEM micrographs of the sharpened M398 blade. Finishing Stone: Schleifjunkies 1 µm. Instrument: Zeiss GeminiSEM560.
The blade got a bit smoother, but also rounded of the apex. The deeper scratches are very similar to the other two blades.
Optical macro shots of the 6 / 3 / 1 micrometre finished blade. Instrument: iphone 15 pro max with a 120x optical loupe macro addon. Note the improved surface finish, but general appearance of larger scratches.
I’m quite disappointed in these stones. I have two Schleifjunkies resin wheels for my Tormek T8, which do a better job. These stones feel to hard, with not enough of a bite. It feels like I am constantly burnishing the surface, and not removing a lot of material. The mediocre BESS tests and persistent scratches are of note here. I sharpened a much softer knife at 58 HRC with this, and had better results.
The stones tested between 85 and 95 shore D at random locations. I took 5 measurements per stone. The measurements were taken at the sidewall of the stone.
Lemma: Do sharpening stones become “finer” over time?
Looked at new and used galvanic grinding stones under the scanning electron microscope, comparing grain wear, swarf accumulation and tear out over time
Applied first principle thinking based on the metrology results from the stone to determine grain engagement, and then proofed theory via experiments:
Sharpened two edges, one with a brand new and one with a nearly used up stone
Compared visual apperance, but also took SEM pictures and 3D metrology of the blades, to compare roughness, spatial parameters and morphology
Results:
With continuous wear, the grains actually become wider, and more grains become active. This reduces the depth of cut, visible in the step height determination on the 3D surface data
The duller grain and lower engagement create a burnishing effect, reducing depth of scratches by >60%, lowering roughness by 27% and introducing a convex bevel and more pronounced, plastic burr at the apex. It also increases gloss. The result therefore looks like it is done with a finer stone, minus the sharpness and shape, which deteriorate.
Actual Science and long version:
This is part of a series of blog posts, where I try to apply my professional knowledge on how chip formation and material removal happen to knife sharpening. I think this could also be called: debunking myths. Because this probably will ruffle some feathers, and is likely to be denied by some people, let me state firmly here: everything you will see in this post is real, and repeatable. Because it is breaking with common misconceptions, I have done the below experiments twice, to verify it. For clarities sake and readability reasons, I only include one dataset below.
Something you can see on a couple of manufacturer homepages, but also often on the internet, is that galvanic stones become “finer” over time. You generally find two seperate statements about this, with a small difference in language, but a large difference in sense. The first is, like stated above, that with wear, galvanic diamond or CBN stones become finer. The second version is, that they behave like a finer grit after some use. Sometimes, you are also warned about a break in period, where they are supposed to be super aggresive.
Let’s take a look at a galvanic stone. You can either jump into the full analysis of the TSPROF Blitz F1000, or just take a peek at the following gallery.
SEM micrographs of the unused Blitz F1000 vom TSPROF. Instrument: Zeiss GeminiSEM560.
We can make out some grains that are deeply embedded, but also some that are nearly sitting completely on top of the galvanic bond. A good question here would be: How many of these grains are actually cutting at contact with a flat surface such as a knife edge?
In literature, this is called the difference between statistical edges (e.g., all you see above in the picture), and kinematically active edges. In order to clear up this point, take a look at the following diagram. It shows a typical grain distribution in a galvanic stone – actually, a pretty good one already. 90% of the grains are within 10% of their diameter variation. If you plot a straight line through these, you will see that only 4 out of the 10 grains are actually cutting into this imaginary straight line. I’ve coloured them red at their intersection. I like the colour red. It’s a professional thing.
Illustration of a decent height distribution of similar grains in a galvanic binder. A portion of the grains above a certain horizontal line is coloured red, to show how few grains are typically active in the cutting process.
Now, if we have a moving edge, the situation becomes a bit more complex. Because now, we don’t have a level, horizontal line of engagement. That would be the case of you lay your stone on top of a plate of steel, and becomes kinematically much more like lapping, with different material removal processes. Instead, what we have is a flat piece of metal (the knife edge), being dragged along the stone, and either by gravity or your hands being pushed into the material. If we imagine for a moment, that friction, elastic deformation & rebound are nonexistent, we can imagine that every grain basically removes the material to it’s very topmost, highest apex. But because the blade is moving, the “drag” behind it is finite, and a new portion of the blade is pressed into the grains and again thusly removed. I’ve illustrated this below. Please note that obviously, this is a very small section of the blade, and I’m not suggesting you should in any way be sharpening to a single sided edge.
Illustration of the removed material from the 4 kinematic active edges. The blade movement and force vector are resembled by the black arrow.
Now, here’s a couple of things we can deduce from this: the first is, the amount of grains that are cutting metal is actually pretty low. Secondly, because of the complex movement, the grains create some kinematic roughness in the blade. Our surface is not created by the highest grain, but by several of those, leaving a “ragged” surface behind. Nevertheless, the last grain leaves a track on the surface. With some use, these grains will wear first. In the space between grains, we will accumulate debris and swarf:
SEM micrograph of a used TSPROF Blitz F1000. Some flattened grains and swarf stuck to the galvanic bond are visible. Instrument: Zeiss GeminiSEM560.
Now, what does a very used galvanic stone looks like? For your viewing pleasure, and in the pursuit of the adventure of sharpness, I’ve sharpened edge after edge until my arms were sore and the stone was removing basically nothing:
SEM micrographs of a very used TSPROF Blitz F1000. This one pretty much didn’t remove any material at all, any longer. Instrument: Zeiss GeminiSEM560.
If we focus on one of the really flat grains, we can see how much is worn away, and how much swarf is embedded deeply into the galvanic bond:
SEM micrograph of a VERY used TSPROF Blitz F1000. Instrument: Zeiss GeminiSEM560.
Take a look at the grain directly above the red Kern logo (lovely, ain’t it?): This is pretty much completely rubbed flat, and level & even with the surrounding bond.
If we flatten the grains in our previous illustrations, and apply the same GEDANKENEXPERIMENT of a blade being dragged along it, it looks like this:
Illustration on how flattened grains are interacting with the material. For this, a horizontal line was drawn, but the removal of the material “simulated” via linear interpolation between the highest peak of a grain and the lowest portion of the next grain (kinematic distance between active grains).
Now, a good question here would be to ask: Dr. Marv, why did you draw the engagements of these grains so shallow? They were taking much deeper cuts beforehand!
For this, we will apply critical first principle thinking. When you sharpen, what you are doing is WORK. And I mean this in the physics sense of the word. Now, we will postulate that you know what you are doing, so you are not pushing unevenly. Whether the stone is worn or not, you apply the same force and speed to it, thus the same work. The work we are doing while grinding consists of several components: friction (which generates heat), plastic deformation (which removes material, generates heat), elastic deformation (“bounce back”) and shearing action. If you have a good lubricated stone (for example, with oil), and are moving slowly, friction and heat are not that big of an issue*. Elastic deformation is generally only a fraction of what is happening here. So, we could equal the work being done to material removal. If instead of only a few grains, all of which are taking a larger cross section out of the material, if you have more active grains, these will all have a similar cross section (maybe a bit larger, as we are moving towards their center and therefore maximum diameter), your actual depth of cut will diminish.
*authors note: total lie here. Friction ALWAYS is a super big issue, and actually becomes worse, the duller your grain is. But for our Gedankenexperiment, it doesn’t change a lot. Just makes it less dramatic than my drawing. Trust me. I’m a doctor and I draw pretty illustrations. Also, I have stuff to prove this.
The following illustration is showing the new, much smaller intersection from the beforehand drawn kinematic active grains.
Illustration, highlighting the kinematic active grains and their active cross section in red. To see how much the grains are worn down, their full shape is visible with low opacity.
Now, what is the result of this? To prove my point, the above theory and showcase this, I’ve prepared two edges. One, with a brand new Blitz F1000. One, with the very much destroyed one. The steel used is M398 at 63.5 HRC, the edge was prepared with progressively finer galvanic stones (150-220-400-800) at 17 DPS.
Let’s compare the result from an optical perspective:
Optical micrograph of the same edge prepartion with two different usage states. Left side: brand new galvanic stone, right side: very used up galvanic stone. Instrument: iPhone 15 Pro with a 120x macro loupe, hence the distorted picture on the right.
This is a pretty drastic difference, the blade on the right side is much smoother, shinier, even glossy. It looks like a finer stone made this! …wait, what?
Luckily, I have access to better microscopes than my phone. Let’s compare what these two edges look like in the scanning electron microscope:
SEM micrographs of a sharpened blade, 250x magnification (FOV: 505µm). First/left picture: new stone, right/second picture: used stone. Instrument: Thermo Fischer PhenomXL.
SEM micrographs of a sharpened blade, 500x magnification (FOV: 505µm). First/left picture: new stone, right/second picture: used stone. Instrument: Thermo Fischer PhenomXL.
Now, this is a stark difference. The new stone created a sharp, uneven morphology. Lot’s of micro prows, deeper scratches and general uneven surfaces are visible. The used up stone created a much smoother surface. There are some scratches, especially closer to the edge. The surface further from the apex is very smooth and rounded, giving it a burnished appearance.
SEM micrographs of a sharpened blade, 1000x magnification (FOV: 505µm). First/left picture: new stone, right/second picture: used stone. Instrument: Thermo Fischer PhenomXL.
At higher magnification, we can make out a sharper apex from the new stone, but also more debris, burr and carbide cracking. The apex from the used stones looks more rounded and dull.
Now, while the SEM is a tool that is fantastic in spotting small details, it’s spatial (in the direction of the beam) resolution isn’t super good. To further analyse what we are seeing, I’ve used our Bruker Alicona µCMM to record some 3D data of the blade apex. It’s a very expensive, optical coordinate measurement machine that uses the measurement principle of focus variation to record height data over a surface area. Why did I use that one and not the Zygo interferometer? Well, the lovely zygo is currently at an exhibition, so I had to make do with what we had *crys in millions of metrology equipment*.
3D height maps of the two measured edges. Left/first picture: new stone. Right / second picture: Used stone. Instrument: Bruker Alicona µCMM, 50X objective lens, singe FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%)
Now, one thing is immediately visible: The new stone created a very flat surface, with deeper, and very direction scratches. The used stone created a slightly rounded of (convex) surface, with smoother roughness. The scratches are directional, but there are some deeper ones at a steeper angle.
With this height data, we can start doing real analysis. First, let’s take a look at the width and depth of the scratches. For this, I’ve filtered the micro roughness via a gaussian filter with a cutoff of 0.8 micrometre:
New galvanic stone blade: Above: filtered surface to remove micro roughness and make step height determination easier. Below: Step height determination. Software used: Digital Surface Mountains Map. <3
and extracted a profile perpendicular through the scratch: Step analysis shows a width of 11 µm, with a maximum “height” (or depth) of 1 µm.
Comparing this to the used stone, with identical workflow (filtering, extraction of profile, step height determination):
Used galvanic stone blade: Above: filtered surface to remove micro roughness and make step height determination easier. Below: Step height determination. Software used: Digital Surface Mountains Map. <3
We arrive at a similar width (10.5 µm), but a much lower maximum height (depth) of 0.38 µm. So, without question we can answer a lemma put up at the beginning of this post: galvanic bound grinding stones do not become finer over time. But because the grains flatten out, their actual depth of cut and the grooves they are creating are more shallow and smoother.
Let’s take a look at the surface metrology data we can extract. For this, I’ve extracted an area of the scan, leaving out the very fine apex of the blade, as we have some measurement artefacts on this one (“batwings”, basically diffraction light at a sharp and burry edge). For those of you working in the manufacturing world, feast your eyes on the ISO 25178 parameter table. This is how you state roughness: clear identification of the workflow, filters and parameter settings used, together with a coloured heat map of the surface recorded.
Surface heatmap and ISO 25178 roughness (S-L) parameters of the edge ground with the new TSPROF Blitz F1000 stone.
Now, what can we extract from this? The quadratic surface roughness Sq is a super fine parameter to evaluate surface roughness, as it is also the “power” of a surface, and therefore directly proportional to how shiny you experience this. We’ve achieved a value of 0.28 µm here, which is about what I would expect from this grit of galvanic stone. The kurtosis (Sku) is around 3, which is where “sharper” profiles start. This means the surface is more of a zig-zag instead of a well rounded sine profile.
We have a low material ratio Smr (17%), so that means only a fraction of our surface is found at the top 1 µm of our height data. If this was a bearing surface (and during a cut it is!) it would have very few spots it would actually have contact with. The auto-correlation length Sal is the dominant spatial structure – here we can see that the fine scratches we see at a direction of 144° to the x-axis of our recording (compare parameter Std, texture direction) are spaced at 1.35 µm.
Surface heatmap and ISO 25178 roughness (S-L) parameters of the edge ground with the used TSPROF Blitz F1000 stone.
Comparing the used surface with the one above, we can see a significant improvement of the surface roughness Sq at 0.22 µm (27% lower), a much higher kurtosis (Sku, 4.45 µm). The material ratio Smr is also crazy high – 79% definitely point towards a “flattened”, e.g. burnished surface. The auto correlation length Sal didn’t really change, as did the texture direction (153° compared to 144°).
So, that’s it folks. I think this shows that galvanic stones do not become “finer” over time. Surface finish improves, as the grains are flattened, outlier grains are torn out or shattered, and a burnishing process begins. The shape of the blade seems to deteriorate – at least with my skill of sharpening and on a TSPROF K03.
The absurd amount on sharpening stones on the market should ring some alarm bells. The first is: there must be a lot of money in this. The second: what’s the difference between them? The third: which is the ideal one (for me)?
I’ve ordered and then analysed a couple of different grinding stones. This is probably going to become an ongoing series of blog posts, whenever I get new and exciting grinding stones. 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.
Fällkniven DC3 (diamond/ceramic whetstone)
According to the manufacturer’s homepage, this is a “diamond grit 25 micron, sapphire ceramic grit 5 micron”. Let’s take a look!
Optical Micrograph of the diamond side. Magnification and scale bar are visible on the lower right part of the image. Microscope: Leica Emspira
The diamond side is coated in TiN. Typically, this coating can be found on cheaper HSS tooling, as it’s quite hard (2400-2700 HV), but also slick and doesn’t let chips adhere. It’s a curious choice to put on a grinding stone, as the grit used here (diamond) is quite a bit harder – depending on the grain orientation, it clocks in at 10000 HV. It’s certainly nice looking though, and I’d postulate that this is the main reason it is applied to any sharpening stone.
SEM Micrographs of the Fällkniven DC3 stone. It shows quite a large range of grain sizes. Instrument: Thermo Fischer PhenomXL Scanning Electron Microscope
SEM pictures show gritty, sharp diamonds. The range distribution of the visible grains (measured at their largest diagonal distance) ranges from 50 to 75 micrometre, with a strong weighting towards the upper end. The grit’s have a distinct checkered look to them – this is the coating, sticking to some parts of the diamond, and not adhering to others. It is very likely that the first use of the stone would remove the coating at any point that is in contact with a blade.
Energy dispersive x-ray spectroscopy (EDS) inside the scanning electron microscope show the coating (Ti, N), the diamond grain (C) as well as the galvanic binder around the grains (Ni). Instrument: Thermo Fischer PhenomXL Scanning Electron Microscope
In order to faciliate a better sense of depth and size, a surface scan was undertaken via white light interferometry. This creates a very high resolution height map – the Z resolution here is absurdly small, where’s the X/Y resolution (“spatial resolution”) follows the Abbe diffraction limited law.
White light interferometry height map of the diamond surface. Instrument used: Zygo Nexview NX2, Objective Lens: 10X. Stitched overview of 4×4 images.
We can see the typical galvanic bound height distribution – unevenly spaced grains with some very high outliers. This is the main reason that galvanic stones leave larger scratches and commonly a worse surface than a similar grain sized vitrified or resin bound stone.
ISO 25178 surface parameters of the Fällknives DC3 diamond side.
The ISO25178 parameters show a rough surface (Sa/Sq are the arithmetic respective quadratic surface roughness). Sz is the total height of the surface. Very indicative of the distribution is the parameter “Sdc”, which shows the range between the lowest 10% and highest 90% of the measured points. This is a good indicator how “even” the height distribution is. A perfect flat surface would have a value of 0 here, whereas a widely spread surface shows a wider range. It’s a usefull parameter to compare stones, but leaves out the 10% outliers at every end. Sku, the kurtosis shows how “sharp” the surface data is. Typically, a value below 3 is considered flat, whereas values above 3 are considered very sharp.
The other “ceramic” side shows a typical ceramic abrasive mix.
SEM images show a pretty uniform, surface with some large voids.
SEM micrographs of the surface morphology. A typical, sintered alumina oxide appearance with some foreign particles (darker colour in the BSD image) and large voids are visible. Instrument: Thermo Fischer PhenomXL Scanning Electron Microscope
While the void size is suprising, this certainly allows for some swarf build-up. 🙂 Some metal particles (bright white colour), but also some different abrasive grains (slightly darker grains) are visible. The detector used is a back-scatter detector. Here, besides the topographical contrast, one also has a contrast based on the weight of the element. The rule of thumb here is: the heavier the element, the brighter the returned pixel is. Pure metals are typically the brightest, whereas ceramics or diamonds are of darker colour.
EDS analysis of the chemical composition. The colour corresponds to the individual element, visible above the scale bar. Instrument: Thermo Fischer PhenomXL Scanning Electron Microscope
Chemical analysis show several large SiC grains, as well as Al2O3 grains. As sapphire is chemically Al2O3, just in a monocrystalline configuration, I think we have identified plenty about the compoistsion. Trace elements of metals and Calciumoxide (blue colours) are likely impurities from manufacturing.
White light interferometry height map of the ceramic surface. Instrument used: Zygo Nexview NX2, Objective Lens: 10X. Stitched overview of 4×4 images.
The whitelight interferometry surface map shows a relatively rough surface. large voids are visible, the range of height values doubles compared to the diamond size. On the other hand, the uppermost part of the surface shows a higher plateau region. The contact area likely is higher on this stone side. Sku, the kurtosis shows how “sharp” the surface data is. Typically, a value below 3 is considered flat, whereas values above 3 are considered very sharp. Here, a much lower value than on the diamond surface can be seen.
Combined with the low sharpness of the dull ceramics, a burnishing effect is expected, improving the appearance of a blade with very low effort.
ISO 25178 surface parameters of the Fällknives DC3 ceramic side.
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