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.

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 the DMT mini diamond-coated stone, specifically the “blue” medium coarse one. According to the manufacturers homepage, this is the “quick” solution to transform a dull knife to proper sharpness. It’s a diamond abrasive with 45 micrometre size. Apparently, it sharpens quicker because of the micronized monocrystalline diamond surface 🙂 *DrMarv smiles in marketing-speech*

Optical Micrograph of the diamond side. Note the “engineered surface”, aka massive diamond free areas that are recessed. Magnification and scale bar are visible on the lower right part of the image. Microscope: Leica Emspira

Immediately visible on the sharpening stone is the “engineered” surface structure. The very thin metal layer that is coated in diamonds is fixed to a blue plastik body, which is likely fiber reinforced to add stiffness. The circular cutouts are recessed. This allows for room for the swarf – likely a reason why these stones are very aggressive and useable without water or oil. Moreover, circular, large radius milling marks (I’d guess a large insert cutter or flycutter) is visible as periodic structures along the surface. The diamond coating on an optical level is very dense and coarse.

SEM Micrographs of the DMT W7C stone. The funky looking structures in the plastic recess is charge-up from the electrons, as the plastik is totally non conducting. Instrument: Thermo Fischer PhenomXL Scanning Electron Microscope

SEM pictures reveal a dense coating of diamonds. This is very close to what professional, manufacturing level galvanic coated grinding tools look like and is a statement to professional level galvanic organisation. Having many grits and a nice, dense coating means a decent lifetime, but also lot’s of kinematic active cutting edges. This is a diamond sharpening stone with a quick material removal rate. Grain distribution is pretty regular, but quite a bit larger than the advertised 45 microns. Grain shape isn’t very coarse or sharp. My guess here is that by increasing the grain size over the advertising, but reducing grain sharpness, a similar surface quality with longer lifetime is possible.

Energy dispersive x-ray spectroscopy (EDS) inside the scanning electron microscope show the diamond grain (C) as well as the galvanic binder around the grains (Ni). Instrument: Thermo Fischer PhenomXL Scanning Electron Microscope

Chemical analysis shows exactly what one would expect – diamond in a nickel binder from the galvanic process. Something noteworthy here is the extreme stick-out of the grain. This is one heck of a sharp tool. The downside of such a stickout is that grain retention is low, and even on this unused and brand new stone you can immediately identify some “impressions” in the nickel binder where grains previously were stuck but got lost along shipping / handling.

White light interferometry height map of the diamond surface. Instrument used: Zygo Nexview NX2, Objective Lens: 10X. Stitched overview of 3×3 images and 6×6 fields of view.

The surface scans from the white light interferometer show pretty much what was already visible inside the optical microscope: large, recessed circular areas, as well as the feedmarks from the manufacturing process, which create some waviness alonge the surface. periodicity of this waviness seems to be in the range of 0.2 mm, with an amplitude of around 10 micrometre. I think this won’t be noticeable on a hand-held sharpening stone, but could be felt as “vibration” on a guided system.

This is quite a bit coarser than the Fällkniven diamond stone we looked at in part 1 of this series. Sa and Sq are already in the double digit range, with a very large spread in Sdc visible. The grain profile isn’t very sharp, which is visible in the sub-3 value of the kurtosis (Sku).