Sintered Ruby As A Replacement Material For The Sharpmaker Ceramic Triangles

The Deacon said:
MSRP for a set Spyderco's diamond sleeves for the Sharpmaker is which are described as the next hardest thing to diamond. At least one BF dealer h$80, as is the MSRP of their new CBN Sharpmaker rods has them for under $50. That ruby file you use for reference costs $68. Have to wonder how much less a pair of Sharpmaker rods made of that material could be sold for, or how much better they would be for a similar price.
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From the Spyderco website:

//QUOTE//Cubic Boron Nitride Tri-Angle Sharpmaker™ Rods
Cubic Boron Nitride (CBN) is a superabrasive that is second in hardness only to diamond. It offers superior abrasion resistance and thermal conductivity compared to conventional abrasives like silicon carbide and aluminum oxide, making it the preferred abrasive in many advanced industrial grinding applications. Now you can combine the advantages of this fast-cutting abrasive with the convenience and ease of use of Spyderco’s Tri-Angle Sharpmaker with these CBN-coated Sharpmaker™ rods 7" X 0.5" (178mm X 13mm)

Sold in a pair, 204CBN triangle rods fit into the 204 Sharpmaker sharpening system.

Use them dry, without oil, water or lubricants. To Clean: rinse in fresh water.//UNQUOTE// Bolding mine

If the description is really accurate, these rods have all the attributes I am looking for to replace the ceramic Spydie triangles. The only thing that disturbs me a little, is that the rods are only CBN-coated and not solid Cubic Boron Nitride.
 
Just did a little research:-- These cubic boron nitride rods are very coarse and only good for establishing a bevel or grinding out chips or dents. I can already do that with my much larger and easier to use diamond coated plates.
 
Obsessed with Edges said:
That '20,000' figure threw me, too. Might reflect different units of measure for the same Knoop Hardness test. I have seen references (quoted below) to Knoop hardness measured in different units, creating a different, but similarly arranged scale, with the same proportional differences between listed materials. I see the '7000' figure at the top for diamond more often, with Al[SUB]2[/SUB]O[SUB]3[/SUB] usually in the ~2000-2200 range.

Here's an example of a reference to a different Knoop scale, referenced to diamond's hardness @ 70,000 (versus 7,000) and AlOx proportionally measured @ 20,000 on that particular scale:



David
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Oh, you are right. Typically, the units for Knoop are in kgf/(mm^2), where kgf is the weight (downward force) of one kilogram in earth gravity. However, some people use N/(mm^2) which is Newtons per square millimeter. Well, since 1 kgf = 9.8 N, that explains the factor of about 10x. (Of course, the 9.8 factor is due to little g, the acceleration of falling objects in Earth gravity.)

Good to know! So the quote of 20,000 Knoop hardness is fine, but they need to say they are using N/(mm^2).
I think if you quote Knoop hardness without units, it is assumed to be kgf/(mm^2).

http://en.wikipedia.org/wiki/Knoop_hardness_test
 
Hmm... I'm still a little confused. It seems that "sintering" doesn't always mean what I thought it means. Here is a quote from the United Abrasives Manufacturing Association:
http://www.uama.org/Abrasives101/101Manufacture.html

"Sintering

"In some cases, it is desired to agglomerate smaller abrasive particles together into larger chunks. This is frequently done by sintering. In sintering, the abrasive is mixed with a quantity of another material which has a lower melting point than the abrasive. The mixture is then heated beyond the lower melting point. This causes the mass to adhere together. The sintered mass can then be formed into granules of the desired size and shape, and allowed to cool."

So it seems that sometimes "sintered" abrasives may use a binder, and sometimes it does not.

It's not clear to me, for sharpening stones, that "ceramic" is different from "sintered"? They seem, to me, to have the same meaning. Anyone got a reference for this? I'll also try to find some references.
 
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Lagrangian said:
Hmm... I'm still a little confused. It seems that "sintering" doesn't always mean what I thought it means. Here is a quote from the United Abrasives Manufacturing Association:
http://www.uama.org/Abrasives101/101Manufacture.html

"Sintering

"In some cases, it is desired to agglomerate smaller abrasive particles together into larger chunks. This is frequently done by sintering. In sintering, the abrasive is mixed with a quantity of another material which has a lower melting point than the abrasive. The mixture is then heated beyond the lower melting point. This causes the mass to adhere together. The sintered mass can then be formed into granules of the desired size and shape, and allowed to cool."

So it seems that sometimes "sintered" abrasives may use a binder, and sometimes it does not.

It's not clear to me, for sharpening stones, that "ceramic" is different from "sintered"? They seem, to me, to have the same meaning. Anyone got a reference for this?
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I don't think all "ruby" stones are going to be equal. The ruby color is likely a byproduct of how the AlumOx is created. The description from Degussit:

Made in Germany from pure, sintered crystals of synthetic ruby. No bonding material is used, so these stones and files are extremely hard and resistant to changes in shape. This is particularly important for quality precision grinding and finishing.

In addition, since Degussit Ruby Stones and Files feature outstanding resistance to wear, they are more economical to use than other comparable tools.

Use dry or with Gesswein Stoning Oil to grind, touch up or debur steel and carbide parts. Also recommended for use on materials such as glass, quartz and porcelain.
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Also noticed the price tag for a combination 4-3/4"L x 2"W x 3/8"H is $340. An assortment of 5pcs - 10cm long stones goes for $290. SOme of this might be marketing hype but they've been selling this stuff for a while.
 
HeavyHanded said:
I don't think all "ruby" stones are going to be equal. The ruby color is likely a byproduct of how the AlumOx is created. The description from Degussit:



Also noticed the price tag for a combination 4-3/4"L x 2"W x 3/8"H is $340. An assortment of 5pcs - 10cm long stones goes for $290. SOme of this might be marketing hype but they've been selling this stuff for a while.
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Thanks! I think that answers my questions. :)
 
Some additional references I came across, that may be of interest:

(1) http://www.qtstools.com/document_library/help-articles/Ceramic Abrasives.pdf
(2) http://www.thegrindingdoc.com/files/ceramic-grit-wheels.pdf
(3) http://www.congresstools.com/catalog/categories/get-subcategory/?id=58
(4) http://americanmachinist.com/features/all-about-abrasives
(5) http://www.azom.com/article.aspx?ArticleID=1389

Summary:

I think my confusion is that "ceramic abrasive" is an incredibly broad term (technically, almost all abrasives like SiC, Al2O3, diamond, garnet, zirconia, etc. are ceramics). So it's not always clear what it means, since it can depend upon context.

For example, in the context of "ceramic sanding belts" and "ceramic grinding wheels", what do they mean? In these cases they are referring to so-called "micro-fracturing ceramic abrasives" which have a microstructure much finer than "conventional" Al2O3. I found references (1) and (2) about this. (1) is more of brief intro for users, like a Q&A, while (2) is more technical, like a research paper at a conference.

The Degussit sintered ruby benchstones that HeavyHanded mentioned are sold by www.CongressTools.com and other places. Sadly, most of these stones and files are only 4 inches long. See (3). And yeah, they seem expensive, but it appears they have a very good and well estabilished reputation.

Degussit's website on their sintered ruby:
http://www.degussit.co.uk/FGTs.htm

It seems there are many many different kinds of aluminum oxide in industry. Links (4) and (5) are brief overviews of some of the types. In (4) I learned that sometimes chrome oxide is added to Al2O3 to slightly increase the toughness of the abrasive. I assume that is what these sintered ruby abrasives are, since chrome oxide makes Al2O3 pink or red. In some other Al2O3 abrasives, they use titanium oxide and/or zirconia. See (4) for more details.

I think that pretty much satisfies my level of curiosity. I feel like I learned a lot, so thanks to everyone on the thread and the original poster. I'l be curious to hear about everyone's sharpening experiences with ruby abrasives.
 
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Excellent links; Interesting reading.

Spyderco ceramic triangles are usually 7" long.... There may currently be size limitations for quality shaped sintered ruby that limit the manufacture of lengths longer than 6". Yet the Sieger sharpening rods (actually tubes), come in lengths of up to 11" and are perfectly smooth and even. Their tubular shape may make it easier to achieve the extra length.
 
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Got my sintered ruby stone from Gesswein about an hour ago....4"X1"X3/8"....fine grit....double sided....weight 3 1/2ounces. I immediately used it dry, on my slightly dull EDC, a ZT0770CF, M390 steel. After 2 or 3 minutes, it shaved hair easily. Performance was similar to the Sieger sintered ruby rod. With the difference that it's harder to keep the angle with this small stone. It was also harder to take strokes the length of the stone. But the stone won't slip off the point of the blade, rounding it, like the Sieger rod has a tendency to do.

For a very fine grit stone, the sintered ruby works super fast. Cleanup with soap and water is under 1/2 minute. I wanted to carry it in my pocket but the edges and corners are too sharp. I don't want to lose useable surface by rounding them off with a diamond plate, so I'll make a pouch for the stone out of nylon webbing.

For anyone interested, I held the stone lengthwise between thumb and forefinger of my right hand and the knife in my left. I stroked the stone toward the edge only. I'll probably get a larger double sided stone from Gesswein in medium grit.
 
One of the stones came in. It's not as fine as my Gesswein fine ruby, but seems well made and seems to cut carbon steel almost like my black hard Arkansas. Provides good feedback. The stone is only 3/8" wide and 4" long, but I like it well enough that I ordered a bigger one. My suggestion is to look on Ebay and buy one of the inexpensive ruby stones. I think you'll be pleasantly surprised.
 
Glad to hear it didn't disappoint you. I'll keep checking Ebay. As of now they have some that are 200mm (almost 8") long.
 
Lagrangian said:
Thanks! I think that answers my questions. :)
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The Spyderco triangles are sintered aluminum oxide, particularly the white ones. The brown ones have some additives for color and are coarser. Ruby is also aluminum oxide with some chromium in it to give it the red color. Sapphire is aluminum oxide with an impurity (which I can't remember) to make it blue. The downside to sintered stones is they do not have a way to replace worn cutting surfaces, as the bond is quite strong and new particles are not easily exposed. In power grinding, as in industry, this is not the case. The surfaces/particles can fracture and form new sharp cutting surfaces. For finishing stones, this isn't really a problem. I have been using my fine Sharpmaker stones for over a decade with no noticeable loss in results. Most likely, the sintered ruby stones are not 100% dense, though it's possible. The Spyderco ceramic stones are likely not 100% dense either.

"Ceramic" is a generic term normally, and can include anything from pottery to advanced SiC whiskers. The carbides in steels can be considered ceramics. The word sintered can, evidently, also be considered generic. I tend not to think of it that way, but I hardly get to define the term. Sintering, as I learned it, is a diffusion based process that is used on a variety of materials, including powdered metals of various kinds.

Even Spyderco has used the notion of "synthetic ruby" in promotion of it's fine and UF sharpening products, though I don't think they do anymore. While it's certainly possible the Sieger and other "ruby" stones are actual synthetic aluminum oxide with the chromium impurities, it's possible they are also just colored that way. I would be curious if the original poster could tell a difference between a knife sharpened with the Sieger rod and a crock stick, assuming he didn't know which was which.
 
To add on to me2, what he says is essentially correct. The official definition for ceramic will differ across textbooks, but basically any compound that is neither a metal nor a polymer is a ceramic (but it must be a compound). The Spyderco stones are indeed alumina, and they are sintered, as is mentioned on their product description (The stone is shaped then kiln fired to temperature of 3000 degrees F (1649°C)). There is in fact no other way to produce most ceramic monoliths, as they are incredibly difficult to melt.

Sintering is a term used to describe the production of materials, normally ceramic but also metals, where it is heated to just below its melting point. At these temperatures diffusion is very fast and densification of the powder into a monolith is driven thermodynamically by the reduction of surface energy. Sometimes a very high pressure is also applied. The binders that are referred to in text isn't simply a glue as it would seem, they are specially selected ceramics with a slightly lower melting point that aids the sintering process. The process is then called liquid-phase sintering, and is preferred because it allows sintering at lower temperatures and thus finer microstructures. Gesswein's stones do seem to be purely solid-state sintered. The liquid-phase sintered alumina is more likely to be 100% dense than the solid-state one, but I don't think it really matters.

With this information I'm going to have to take the same stand as me2, which is slightly skeptical. It is possible that the liquid-phase additives in Spyderco's alumina will cause it to feel different from Gesswein's, but this is quite unlikely. The additives only affect properties like its toughness and strength, whereas abrasion resistance isn't really affected as the block is still essentially mostly alumina. The liquid-phase is rarely more than a percent or two. I'm inclined to think that the OP prefers the ruby rods because of a psychological effect, or that the ruby rods simply have a different surface finish from Spyderco's that might make it easier to clean.
 
It's interesting that the skeptics have never even used a ruby stone. I have been heavily involved in the cutlery business since 1973. I can tell you that the ruby stones cut differently than other stones, and they don't wear. Even the cheap Chinese ones I recently got work very well. I'm amazed that people will spend hundreds of dollars on waterstones (which wear out) but scoff at these ruby stones. There is nothing psychological about it.
 
Edited:

Well it would seem I was wrong :) industrial ruby abrasives have about 2% chromium oxide added, which actually is a sweet spot that corresponds to a ~30% increase in fracture toughness of the alumina. It appears that this improvement preserves the surface via the reduction of microfractures, making the stones last longer.

The microstructure also changes drastically, becoming a mixture of large grains in a matrix of smaller ones, rather than a uniform grain size distribution. It's possible that this would affect the surface enough for it to cut better at the same measure of macro-roughness, though I'm not exactly sure why that would be the case. Maybe the larger grains jut out and concentrate the applied pressure, though this would imply a less uniform edge finish. Then again the length scale at which this happens may render it insignificant.

Interesting, I guess you learn something new every day. Colour me interested in ruby stones, though they're still a little expensive for my tastes. They are pretty though. I wonder if I can get a size that will fit on my edge pro. All the water does get messy.
 
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me2 said:
I would be curious if the original poster could tell a difference between a knife sharpened with the Sieger rod and a crock stick, assuming he didn't know which was which.
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Neither I nor anyone else could tell. What makes sintered ruby stones unique are ease of cleanup, exceptional cutting speed and long life. See posts #56 & #57.
 
Hi SPNKr,

Cool! That's consistent with the information I read (see links in my earlier post). In addition to chromium, apparently other additives are also used to increase toughness and modify grain structure (like titania).
http://americanmachinist.com/features/all-about-abrasives

btw, could you post the references for the specifics you mentioned? The 30% increase in fracture toughness is interesting, and I'd be curious to learn more about it in detail.

Sincerely,
--Lagrangian

SPNKr said:
Edited:

Well it would seem I was wrong :) industrial ruby abrasives have about 2% chromium oxide added, which actually is a sweet spot that corresponds to a ~30% increase in fracture toughness of the alumina. It appears that this improvement preserves the surface via the reduction of microfractures, making the stones last longer.

The microstructure also changes drastically, becoming a mixture of large grains in a matrix of smaller ones, rather than a uniform grain size distribution. It's possible that this would affect the surface enough for it to cut better at the same measure of macro-roughness, though I'm not exactly sure why that would be the case. Maybe the larger grains jut out and concentrate the applied pressure, though this would imply a less uniform edge finish. Then again the length scale at which this happens may render it insignificant.

Interesting, I guess you learn something new every day. Colour me interested in ruby stones, though they're still a little expensive for my tastes. They are pretty though. I wonder if I can get a size that will fit on my edge pro. All the water does get messy.
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Lagrangian said:
Hi SPNKr,

btw, could you post the references for the specifics you mentioned? The 30% increase in fracture toughness is interesting, and I'd be curious to learn more about it in detail.

Sincerely,
--Lagrangian
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Here is the paper: http://www.sciencedirect.com/science/article/pii/S0955221900000236. I don't know if you can access the full text though. If you're really interested I could download it and send it to you. Here's another report: http://www.sciencedirect.com/science/article/pii/S0025540810002011.

Basically the first paper found that addition of chromium caused a bimodal grain-size distribution. They think this is because the alumina particles adjacent to chromia particles take up a large amount of chromium due to a high diffusion coefficient at the surface in the early stages of sintering. This places the interface under greater coherency strain, because chromia and alumina have different lattice parameters which leads to a higher grain-boundary energy. The higher energy then causes these grains to grow faster than the rest. The bimodal distribution toughens the ceramic as the larger grains bridge across cracks, basically making them more difficult to grow. If you add more and more chromia then more and more of the alumina grains will grow quickly, eventually bringing you back to a unimodal distribution and you've lost your toughening mechanism.

The second one shows no toughness change when a uniform, fine grain structure was maintained. This is strong evidence for the toughening mechanism being crack bridging by larger grains from the bi-modal size distribution, rather than say, crack deflection or formation of a plastic phase. Interestingly I believe this ties directly to the degradation from sharpening, as larger grains in a matrix of small ones would presumably be less susceptible to pullout, whereas alternative toughening mechanisms that preserve the unimodal grain-size distribution wouldn't really affect the rate of pullout. In other words, I suspect it's not the toughening directly that causes the ruby stones to last longer, but the same microstructural features that cause toughening also preserve the surface. But then this is all conjecture on my part :)
 
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