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 an artificial silicon carbide sharpening stone manufactured by BORIDE, a US company that specialises in polishing stones for mold work. This is the CS-HD type, according to the manufacturer a green silicon carbide rock that excels at polishing up to 63 HRC.

Let’s take a look under the microscope!

Optical micrographs of the Boride CS-HD 1000 stone. Instrument: Leica Emspira.

The manufacturer printed the grit size onto the stone, which is visible at large magnifications as black dots. I would expect this to be printed, as it looks like ink leaked into the small cracks of the surface. The overall stone material is a mix of different coloured particles.

In order to make out more details, and look into the chemistry of the stone, we will do electron microscopy analysis. If you want to know more about this, I’ve explained the techniques in detail here.

SEM micrographs of the Boride CS-HD 1000 stone. Note at high magnifications how the grains have grown into each other. Instrument: Zeiss Gemini 560.

The stone shows a nice grain size distribution, with very regular shape. Green SiC is generally harder, but more brittle than black SiC. The BORIDE stones at higher grits show that green colour, at this size it is more an off-white colour, which stems from the particle size. At higher magnifications, the tendency to solid-phase sinter becomes apparent: the grains are interconnecting, which looks like they are melting into each other.

EDS analysis of the BORIDE CS-HD stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The EDS analysis shows that this artificial stone is mostly SiC – the oxygen we see is probably surface oxidation, and the minuscule amounts of aluminium are probably aluminium oxide impurities. This is expected of every SiC, as typical purity for green SiC is 99%. No large, impure particles can be found, which speaks for proper abrasive hygiene at the manufacturer!

Instrument: Bruker Alicona µCMM, 50X objective lens, 3×3 FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%).

Analysing the surface via focus variation microscopy, we can see that the matte, smooth appearance is mirrored in the surface roughness. The stone is unremarkable, with no distinct material ratio or deep voids. This is also reflected in the ISO 25178 parameters:

ISO 25178 parameters.

Let’s take a look at how this stone sharpens a blade!

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 blade sharpened with the BORIDE CS-HD stone. Instrument: Thermo Fischer PhenomXL

The boride stone created a matte, homogeneous surface. Under the SEM, lot’s of micro scratches as well as some deeper scratches with burr formation are visible. Near the apex, small foil type burrs can be found.

The stone provided a very regular feedback – it felt like every position on it is identical, with a smooth, even friction feedback. The blade surface showed a lot of scratches, some very horizontal – I could imagine that this stems from me wipping down the blade and rubbing a particle across it. I would ignore those!

Optical micrograph of the sharpened blade. The fine micro scratches, but also larger and deeper scratches are easily visible. Compared to the 5 µm resin stone, the surface visibly deteriorated.

I think this is an OK stone. 1000 grit is not super fine, so I expected the surface to deteriorate. The stones are not super expensive, and I would guess with enough skill, they would present you with a fantastic edge. I just don’t see the appeal in a SiC based abrasive stone, when there are super abrasives out there like CBN or diamond, or fantastic natural stones like the yellow Belgian coticule.