When to use a Silicon Carbide Stone and when to use Aluminum Oxide.

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Larry777

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Jun 24, 2019
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Hi everyone just new here.

I was wondering when do you use a Silicon Carbide stone and when do you use an Aluminum Oxide stone.

I think I read on this forum when I was just watching and hadn't joined that a lot of guys said to use a Silicon Carbide stone on high carbon steels.

And they also said for softer metals like Stainless steels to use Aluminum oxide because it finish's the surface better Stainless Steel.

Is this correct or do I have it wrong.
 
SiC is harder, so at the limit it can work better than alumina on harder steels, but a lot depends on bond strength and type. The softer kind of SiC stones I have are best saved for harder steels, as alumina stones cut just as fast (if not faster, because I can lean into them) but wear much more slowly. There are however hard vitreous bond SiC stones that wear much more slowly. In the realm of industrial grinding it seems like SiC is a specialty product, with most wheels being alumina or super-abrasive (diamond/cBN).
 
All my stone are SIC and am going to buy matrix diamond stones as money permits. I have knives from 420 to s110v and 3v,D2 etc. Have never found the need to add aluminum oxide stones. It's only on the upper end steel that the need for diamonds is needed, but it sure as heck makes them ALL easier.
 
I don't think high carbon / a non stainless needs a SiC stone. I take those and my kitchen knives to Alum. oxide stones. As the steel gets better say, around 440C I take those to a SiC stone. DM
 
Larry777 said:
Hi everyone just new here.

I was wondering when do you use a Silicon Carbide stone and when do you use an Aluminum Oxide stone.

I think I read on this forum when I was just watching and hadn't joined that a lot of guys said to use a Silicon Carbide stone on high carbon steels.

And they also said for softer metals like Stainless steels to use Aluminum oxide because it finish's the surface better Stainless Steel.

Is this correct or do I have it wrong.
Click to expand...

The SiC stone will do better on 'high carbide' steels in particular.

'High carbon' steels won't necessarily be an issue with most any stone type, if they don't have a lot of hard carbide content. By 'carbides', I'm talking about chromium carbides, vanadium carbides, tungsten carbides, etc., which add a lot of wear resistance to the steel, because these carbides are 2X or more harder than the 'high carbon' matrix steel itself. That effectively makes them harder to grind, and some stone types in aluminum oxide will begin to struggle, or work much more slowly, if the carbide content of the steel is pretty high.

As an example, a steel like D2 will grind much more easily on a SiC (silicon carbide) stone, because D2's large and abundant chromium carbides add enough wear resistance to significantly slow grinding on aluminum oxide or natural stones. SiC is about 25% harder than aluminum oxide. So, with some steels containing hard carbides beyond a given threshold of wear resistance, a big difference will be noticed in comparing grinding speed and efficiency between the SiC stone and the aluminum oxide stone. D2 is one of those threshold steels where the difference will be obvious, significantly slowing grinding on the aluminum oxide stone, whereas the SiC stone makes it seem as if D2's chromium carbides aren't even there.

A 'high carbon' steel is, by definition, any steel containing approximately 0.5% or more carbon. This includes virtually all cutlery steels designed to be heat-treated to sufficient hardness to take and hold a sharp edge. The carbon content is what makes the steel responsive (hardenable) to that heat treatment. Simple steels like 1095, 420HC, 440A, etc. are all so-called 'high carbon' steels by this definition. But these steels don't contain enough hard carbides to be an issue with most any sharpening stone, including natural (Arkansas) stones and aluminum oxide stones. Other steels like 440C, D2, ZDP-189, S30V, etc. are also 'high carbon' steels by that definition (and they contain even more carbon, well above the defined minimum). But they also have much more wear resistance afforded by much greater hard carbide content not seen in the simpler steels. That's where the SiC stone or a diamond hone will make a much bigger difference in grinding them efficiently, over something like an aluminum oxide stone or a natural stone (has no hope with these wear-resistant steels).

Beyond the ability to grind, given stones may be preferable for finish reasons too. For simpler stainless steels, this is where one might like a basic aluminum oxide stone, which finishes nicely without overkilling the task like a SiC stone might do. Same could be said for using an Arkansas stone on simple steels like 1095 or CV. All of this is subjective and comes down to preference of the user.
 
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So, it is possible to make a fine edge with just SiC stones, I guess.
 
Absolutely. I used to go around to a couple of friend's restuarants and sharpen their knives with just a couple of SIC stones. All of their employees could dull a knife while you watched. And they all got a very sharp very toothy edge back. It's all in the angle. Any angle will do - you just have to be able to repeat it consistently.
 
Carborundum or Silicon Carbide was developed to sharpen Tungsten Carbide tooling at 72 HRC. The Corundum or Aluminum Oxide was not very efficient at that particular task. Al203 stones work fine on ZDP-189 which is hardened to 68 HRC.
When you get to Vanadium Carbide inclusions at 82 HRC, Cubic Boron Nitride or Diamond abrasives are in order. If your stone cannot abrade the Carbide in your blade, it can pull on it and maybe dislodge it.
 
miso2 miso2 ,I have a 400 grit SiC stone and Razor's Edge mfg. a 800 grit. So, yes you can find them fine. DM
 
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My Black Magic honing plates are a sintered green silicon carbide and have a maximum grain size of 12µm, with most of the grains being in the 3µm range. Because it's sintered, the finish produced is different than that from a vitrified stone. It closer to a fine or ultrafine Spyderco ceramic, but it's harder and is more friable than alumina, and so needs resurfacing less frequently, and the sharper shape of the grains makes it surprisingly fast-cutting for how fine it is. Like sintered alumina stones, the surface finish affects the edge finish produced, so it can be rendered finer or coarser by altering the grit size of lapidary diamond you use when refinishing the surface.

byxco-black-magic-honing-plate-4.gif


As far as when to use which, it really depends on a lot of the other variables are concerned. In industry SiC is usually used for grinding hard materials like various ceramic materials, glass, etc. while Al2O3 is usually used for ferrous metals. But those are almost always powered grinding tasks. And there are different grades of each material. In addition, the bond type, grit/bond ratio, density of the stone, and so on all impact the resulting finish, cut rate, optimum pressure to use, etc. So while there's a lot of science behind it, usually the best way to go about it is to just learn what advantages and disadvantages each variable has when altered and look for stones that strike the right balance of qualities for your intended application. That can be tricky when a lot of that info isn't made available. Most companies don't say a whole lot besides fairly generic statements that boil down to "it works great!" so there's a lot of trial and error involved in finding what works best for you. But understanding the theory behind it is helpful.

Basically, Al2O3 isn't quite as hard as SiC is, and the grains are more rounded in shape. Al2O3 comes in a wide range of grades, ranging from white (highest purity and most friable) to blue, various shades of ruby through pink, and "brown", which actually turns to a blue-grey during firing under most circumstances, which is the least friable. Different companies also have different specific formulations within those broad color classes. SiC is harder than Al2O3, and has sharper-shaped grains, but is also more friable. It comes in two major color classes: black and green. Black is the most durable type, with green being higher purity and more friable. Again, it has a lot of different specific formulations within those two color types. Al2O3 and SiC require different kinds of binders to hold them together properly, which is why under most circumstances you don't see the two stuck to one another in a dual grit stone unless they were glued together instead of vitrified as a single unit. There are even more kinds of binder variations than there are abrasive grain variations. There's a lot of science behind it, but it's a lot like culinary arts. You get better and more consistent results when you know the science involved, but there's almost limitless possibilities to how you put all the ingredients together to arrive at a final product.
 
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L Larry777 , one can read all kinds of info. on line. The 2 stone materials you ask about are very good to use for sharpening and will last you more than your lifetime. Go ahead and use them for every steel and get some first hand experience, before running out and buying some new stones. DM
 
Aluminum oxide and silicon carbide will effectively cut almost any steel, including high-vanadium steels, up to about 400 ANSI grit. Diamonds and other super-abrasives are only a requirement for high vanadium steels at grits in excess of 400 ANSI. And those high-vanadium steels generally give the greatest performance with a toothy edge, anyhow, because the biggest advantage to highly wear-resistant steels is usually in slicing tasks in dirty environments, not in push-cuts.
 
Exactly ^ which is what we have argued before here. Still, some are free to think you must have diamonds up to 1200 grit on all steels. DM
 
Like, I just don't see myself ever needing an 8k polished edge on S90V. Push-cutting tasks just aren't what that steel really excels at. It's not like it can't do it or anything, but rather that most push cutting tasks involve pretty low abrasive wear while most abrasive cutting is slicing work, and that's better done with a coarse to medium edge rather than ultra-polished.
 
Probably the hardness comparison between Silicon Carbide and Aluminum Oxide will be useful.
Of Course the hardness is just a part from the entire picture. Another important factors are sharpness of the particles edges (Silicon Carbide wins again) and particles durability (Aluminum Oxide/Alumina is the the winner here). I believe that FortyTwoBlades is correct and up to the 400 grit any of these stones will do the job - the relatively smaller blade carbides which are harder will not be cut, they probably will be pluck off. That shows that process behind the sharpening are enough complex (as FortyTwoBlades have mentioned).
carbide-hardness.jpg
 
Ah, yes, 18 post summary for OP from folks with experience. Good good. I'm glad for the different stones to build my own experience, besides, using just diamonds would be... variety is the spice, right? 👆
 
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