Blog: Science of sharp

[Mark76]

I happened to stumble upon a new blog: Science of sharp[/url]. It’s pretty good and has a lot of SEM (?) pictures to illustrate things. It also covers quite a few topics we’ve been discussing here.

One article in particular examines the effects of stropping[/url]. The pictures are supposed to show also the burnishing effect that has been the topic of much debate here. Only… I don’t see it. I left the author a comment asking if he could explain it a bit further. But if anyone here sees it, please elaborate on it!

Molecule Polishing: my blog about sharpening with the Wicked Edge

[wickededge]

Mark,

These are great, thank you so much for sharing htem! It’s intriguing to see his comments on the burnishing effects of stropping. It’s exciting to see some science behind that hypothesis of mine since it’s been somewhat controversial.

-Clay

[Josh]

Thanks for that mark!

Yeah I think Cliff hit it on the head… there is burnishing occurring at some level, but it’s not from the strops or stropping compound but rather from other steel particles (i.e. the “black stuff”) on the strops. but like you said Clay, it will be great to have evidence to back this up!

[Josh]

Clay, I’m sure you could get with the owner of that blog, who is a user here (ToddSimpson) and get some SEM imaging done in different edges if you were curious about anything… of course, it is always better to do it in house! 😀 hope your plans are coming along!

[wickededge]

Clay, I’m sure you could get with the owner of that blog, who is a user here (ToddSimpson) and get some SEM imaging done in different edges if you were curious about anything… of course, it is always better to do it in house! 😀 hope your plans are coming along!

Thanks Josh, I reached out to him.

-Clay

[wickededge]

Cliff’s observations about burnishing are very interesting. His suggestion that the metal filings are responsible for the effect seem very plausible.

I’m curious about the diamonds as well though. Under magnification, each scratch in a scratch pattern appear to be a trough, gouged out by the diamond as it is slid across the metal, ostensibly removing the bit of metal that fills the cross section. The remaining material must come off onto the strop or into the local environment. I imagine the diamonds begin the abrasion by first sliding along the surface of the metal until they engage the metal with enough force to overcome its yield strength, at which point they begin dislodging metal. Does the dislodged metal flow in front and to the sides of the diamonds or does it immediately come away and either load the strop/stone or fall away as fine particles? It seems a bit like plowing snow; the edges of the trough build up as material is deposited through the plowing process which may be why we see raised ridges at the edges of each scratch. If that’s the case, then it does seem like some of the material is flowing instead of being dislodged. I’m sure Phil Pasteur could answer it for us.

-Clay

[Todd Simpson]

It is possible that small amounts of impurities affect what is observed in the “clean” strop experiments.
Although, in this case the strop (Scrupleworks oil-tanned horse/ natural linen) I used is essentially new; I have only ever used it for these experiments.

Typically, the scratches and the swarf particles are much smaller than the grit particles, so I doubt they play any role during honing or stopping on abrasive-loaded substrates, unless they are transferred to a subsequent finer grit substrate.
For example, here is some swarf on a piece of 3-micron aluminum oxide lapping film:

And yes, scratches typically have a snow-plow-like build-up (from plastic flow) at each side.

[Gregg776]

Todd,
Thanks for the picture of the swarf removed by the aluminum oxide grit. The grit particle is actually a lot sharper than I would have expected since the basic crystal structure of aluminum oxide is octahedral as is a diamond. The curls of swarf remind me of chips from a lathe cut with a negative cemented carbide insert and chip breaker. The corrugations on the inside of the chips show how the metal exceeded its elastic limit.
It would stand to reason that grains of abrasive that are not sharp in the direction of cut are hard enough to still exceed the metal’s elastic limit and cause burnishing simultaneously with metal removal. (I’m not trying to imply that reality follows reason.) It also would seem that abrasive grains not held tightly on a substrate would tend to burnish more than if they were rigidly held in place i.e. a strop.
From what I understand, the burnishing material needs to be of similar hardness or harder than the material being burnished. From personal experience, chips form a lathe can be much harder than the base metal. It would be hard to prove, but the swarf may have a burnishing role in knife sharpening especially in situations where the swarf is allowed to build up like a strop or possibly to a lesser extent with a water stone.

[Todd Simpson]

Greg,

The aluminum oxide grit is crushed, so there is no apparent crystal structure, unlike Chromium oxide or Cerium oxide, for example.
I have a collection of images of different abrasives that I can put together…
That particular grain is also an unusual shape and I suspect that is why there is a collection of swarf at that location.

I have no doubt that burnishing and abrasion occur simultaneously on either a stone or an abrasive loaded strop. Also, for the reasons you give, a diamond plate is much more abrasive than a soft water stone.

I have spent some time looking at “used” slurry, even to the point of it turning black with swarf, and (maybe) surprisingly the metal particles are very few a far between compared to the abrasive and binder particles. In fact it can be very difficult to locate a single metal particle.
Todd.

[Mr.Wizard]

I have spent some time looking at “used” slurry, even to the point of it turning black with swarf, and (maybe) surprisingly the metal particles are very few a far between compared to the abrasive and binder particles. In fact it can be very difficult to locate a single metal particle.
Todd.

Why does so little metal make the slurry black?

[Gregg776]

Todd:
The aluminum oxide grain in the picture is still sharp even after removing a lot of metal for its size. That’s the kind of edge we are all looking for on our knives.
As far as crystal structure I was imagining how diamonds are made and the process should form more or less basic crystals that would be more likely to be sorted for size rather than crushed smaller. Aligning and evenly placing diamond crystals on the surface of a steel plate for a sharpening stone is tricky at best and then keeping them attached is another problem; I would like to know how it is done for quality products.
Your analysis of used slurries could be very interesting (not to mention a lot of work). You mention that it is hard to find intact metal particles in the slurry. A simple gold pan may help concentrate the heavier stuff while washing away the lighter stuff. Tape a couple of rare earth magnets to the bottom of a plastic gold pan near one side with opposite polarity. Any electrically conductive particles within the field will be slowed down and tend to drop to the bottom. Anything with iron or nickel will be attracted to a greater extent.

Mr. Wizard:
I have no proof, but I imagine that the slurry turns black due to oxidation of the metal particles along with heat produced at the point where they are torn away. They are so small with a lot of surface area and during their removal virgin metal is exposed and ready for any available oxygen to combine and considerable heat on a molecular level is produced by the separation of the swarf molecules from the blade. The smaller swarf pieces are also being bent and compressed causing a lot of internal heat.
I remember seeing a large vertical turret lathe taking 10mm deep cuts with 2 mm feed on a 2 meter diameter cast steel part. They were using negative carbide inserts (where the actual cutting edge was pointed slightly away from the feed) and no coolant. Near the parting line the chips being removed were dull red hot and they almost immediately turned dark gunmetal blue-black. The black in this case is a ferro-ferric oxide Fe3O4 which is the same as mill scale on hot rolled steel. It is has almost ceramic hardness and caused shovels to actually wear out. The chips looked very similar to the picture Todd posted; one side smooth and shiny and the inner surface bunched up.
The same processes are happening on a smaller scale when sharpening a knife.

[Todd Simpson]

Why does so little metal make the slurry black?

For natural waterstones (Coticule, Jnat, etc) with grit in the range of 2-3 microns, the swarf particles from the abraded surface will mostly be in the 50-100nm range. A few larger “edge chips” will also be present.
A 50nm (0.05 micron) iron particle will be black, so you only need a single “layer” of such particles to turn the slurry completely black.

In these images, heavier elements (iron) will be bright (white):
as-formed, translucent slurry:

well-worked, dark gray slurry:

edit: 50-100nm particles are not resolved at these low magnifications. I present these images only to show how few large (micron-sized) swarf particles are produced. Also note there is “background” iron in the initial slurry (and in the stone).

I normally allow the slurry to “settle” to remove the silicate flakes and generally observe the slurry to turn yellow (to iron oxide) after a few days.

[Author]

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