I for one would like to hear the TTZ explanation, particularly from the book cited. I knew there was something besides zirconia and alumina in there, but couldn't remember what. The old ceramic knives may have been stabilized with just the alumina, as the newer ones are considerably tougher from what I've seen.
To stay on topic, I do not have a steel per se. I have been using the spine of another knife. I got the idea from reading about African game guides that steeled their blades on the spines of other knives while hunting. I do have a little bit more information. I have cut considerable amounts of cardboard with a 1095 blade that was not hardened at all. The edge just rolls, and can be steeled back many times before the actual edge "wears" off. There are other problems that make this type of blade impractical, but if all you need to do is cut soft material, it will work surprisingly well. I have also tested my Kudu, and after cutting about 45 feet of cardboard, it was brought back to close to it's original sharpness with just a few passes on the spine of another knife. Even the simple stainless in the Kudu cut 4 to 1 the amount of cardboard an unhardened blade did before needing to be steeled, illustrating the importance of strength, not just wear resistance in an edge.
Hi me2,
Sorry, I meant to give a brief account of how transformation-toughened zirconia works, but I had wanted to re-read Eberhart's book just before doing so. But I misplaced my copy of his book... It's somewhere at home, so I'm a bit irked. ^_^;
In any case, here's a brief description by myself, from memory alone, so it may have some minor errors.
The first thing is to remember how a crack propagates, say during fracture. One of the critical ideas is _stress concentration_. You can think of stress concentration as being vaguely like a lever or wedge: Although, overall, you only applied a certain amount of force, the stress in the sample may not be uniformly distributed. In fact, like a lever, the force (stress) can be multiplied. This is okay, since force is not energy, so it's not like we are violating any laws of conservation. Engineers have round that stress (force) in a sample can be concentrated/magnified at sharp features. For example, in the passenger aircraft, the de Havilland Comet, it was found that it's square windows tended to concentrate stress at the corners.
https://en.wikipedia.org/wiki/Fatigue_(material)#de_Havilland_Comet
Roughly speaking, the sharper the corner, the more likely it is to cause stress concentration. This is why a crack tends to grow: The tip of the crack is basically a corner so sharp, it has an angle of almost zero degrees inclusive. The stress concentration can be enormous. As a result, the material at the very tip of the crack is so stressed, it breaks, which means the crack grows. The crack then continues to progressively grow, which often leads to a break all the way through the sample.
In particular, brittle materials require very little energy to grow the crack. We should be careful to distinguish between force and energy. It make take a lot of force to break a material, but relatively little energy. Let's consider glass as an example. It is known and verified that pure, 100% defect free glass has a tensile strength greater than steel. That is, the _force_ necessary to break glass (under tension) is greater than that of steel. In contrast, the _energy_ required to break steel is much larger than for glass. This is the difference between strength (force) and toughness (energy) of materials, which we discussed earlier in this thread.
So if glass is actually stronger than steel, why do we experience it as so fragile in every day life? There are two reasons, and both are related to fracture toughness. Consider a crack in glass, even a microscopic crack so small you wouldn't be able to see it. This crack has a very sharp tip, so sharp that the stress concentration is enormous. Even applying a very weak force to the glass, the stress concentration will focus and magnify the force to such a degree, that at the tip of the crack, the tensile strength of glass is exceeded. So the crack will grow. Next, glass has very low toughness; this means that as the crack grows, it consumes very little energy. (This is in contrast to ductile materials like copper steel, which tend to be much tougher. See the earlier discussion in this thread for more details and references. Also see the post by me2 about the different types of toughness.)
So glass is brittle for two reasons: First, it easily forms very sharp cracks that grow by stress concentration. Next, growth of cracks requires almost no energy in glass (low toughness). The result is that glass shatters easily and instantly. So _in practice_ less force is required to break glass than steel. But this is due to micro-scratches and defects in almost all glass you will encounter. If you actually made a completely defect-free sample of glass, it would support more tension than steel. This amazing fact has actually been demonstrated experimentally. In J. E. Gordon's book, he mentions a material scientist who would go around with a box that had a virtually perfect sample of glass, which he would then bend to a ridiculous degree. Bending of the sample introduces compressive stress on the inside of the bend, and tensile stress on the outside of the bend. Given a sample of a specific thickness or geometry, the amount you can bend it before breaking is a crude indicator of how strong the material is. The problem is, the demo sample of glass had to be kept inside an ultra-clean box at all times. Even just touching the glass surface could introduce micro-scratches and micro-defects into the surface. After that, well, it would probably just snap like ordinary glass.
So if we wanted to make a ceramic hammer, how could we overcome this brittleness? One strategy is to find a way to impede the growth of cracks. But how? There are two strategies I've heard of for this, although there are probably many others (I'm not a material scientist, so I don't know what I don't know). In both strategies, we will try to destroy the stress-concentration at the tip of the crack.
The first strategy is increase the ductility of the material. Very ductile materials will undergo plastic deformation when the stress exceeds their strength. Generally, though, ductile materials are softer because otherwise the wouldn't undergo microscopic flow when over-stresed. A good examples of ductile materials include copper and gold; these soft metals can be folded, squished, or stretched without failure. This plastic movement can occur at the tip of a crack: if the crack tip were to just flow plastically, then the sharp corner is stretched out into a blunter, rounder tip. Once the sharpness of the crack tip is gone, so is much of the stress concentration. The result is, the crack stops growing. The crack can be forced to grow more, but that requires a larger force to overcome the loss of stress concentration. Furthermore, the plastic deformation of the material requires energy (just try "ripping" a sheet of copper plate; you will see a zone of deformation and stretching on both sides of the tear. This zone is sometimes called a "plastic zone" and these deformations require a lot of energy. Ductile materials tend to have large plastic zones.). So in ductile materials, growing a crack requires _more_ force and _more_ energy.
So now we can finally get to the strategy used in transformation-toughened zirconia. Notice that in a ductile material, the stress concentration could be destroyed by blunting the crack sharpness when the material flows under tension. What if we can get the material to "move" or "change" by a mechanism, other than plastic flow? This sounds kind of weird and exotic, and it is. But this is the key idea in transformation-toughened zirconia.
It turns out zirconia can exist in several forms. Carbon commonly exists both as graphite, and as diamond. Both of these forms are just carbon, but arranged differently. In material science, they call these different forms "allotropes". Just like carbon, zirconia can exist in several different allotropes (forms). I forget all the names for the different allotropes of zirocnia, and also what conditions and temperatures they occur in, etc.
But here is the basic idea: zirconia occurs in at least two allotropies, which I'll call A and B (just because I forget their technical names). Normally, zirconia occurs in form A which is less dense than B. This difference in density is key. Unfortunately, the denser B form only exists at unusual conditions, such as extremely high temperatures, so normally, you never see the B form of zirconia. But imagine what we could embed particles of B type zirconia inside a matrix of A type zirconia. This would kind of look like concrete, where the cement is the less dense A-type, and the aggregate rocks are the denser B-type.
If we had a crack moving through this material, the tip of the crack would eventually hit a particle of B type zirconia. Since the B type is denser, it could expand and revert to the A type. So instead of stress-concentration breaking the material at the tip of the crack, instead, the tip simply _transforms_ from the B allotrope of zirconia into the A allotrope. And in doing so, it _expands_ which literally clamps the crack tip shut. To grow the crack even further, one would have to apply enough force to force the crack tip through the particle at the tip of the crack. That would mean enough tension to overcome the compression of the particle which just expanded in volume (because it changed from type B to type A).
This amazing idea sounds very cool, but how would you even do it? But we just said that type B zirconia only occurs at high temperature; so how could we even embed particles of B in a ceramic at room temperature? And besides, since type B only occurs at high higher temperature, wouldn't it be _less_ dense than type A? Don't materials expand when heating?
This is where the unusual properties of zirconia come into play. Yes, materials tend to expand when you heat them, but this is mostly because the molecules jitter and more, but they stay in roughly the same position. But if you heated zirconia so much that it's molecules re-arranged themselves into a _new crystal structure_ (something like the change from body-centered-cubic to face-centered-cubic) then all bets are off. In the new crystal structure, the density of material can be different, possibly more, possibly less.
Okay, we see that actually, zirconia at high enough temperature _could be_ denser than room temperature zirconia due to a transformation from one allotrope to another allotrope. But if it only occurs at high temperature, then what use is it? Once again, another unusual property of zirconia comes into play: Chemists have discovered that adding in a small amount of other elements (such as yttria) can _partially stablize_ the B form of zirconia. So you take some normal zirconia, room temp, in the A form, and heat it up. It transforms into the denser B form of zirconia, and then you keep it there with some chemical additives (yttria is common). You stabilize it just enough so that at room temperature, the B type stays B. But in a way, it's unhappy: It _wants_ to transform back into the A type, but can't because of the yttria. So, if you don't over-stablize the B type, then if you were to put it under tension, that would be enough of a "kick" to let it snap back into the A form. This is what I mean when I say,"partially stabilized".
In an earlier post, me2 also points out that it takes some energy to convert the partially-stabilized B zirconia into the A form. This may increase toughness, which is the energy required to break the sample.
So in a modern ceramic knife that uses zirconia, you have a matrix of two allotropes of zirconia. The "cement" is regular zirconia, in the "A" allotrope. And embedded in that, are particles of "B" zirconia that have been partially stabilized with yttria (or other additives). A crack in the material propagates through the "A" type easily, but then the tip of the crack his a particle of "B" type zirconia. When this happens, the B-type zirconia expands as it transforms into the "A" type. This expansion stops the crack by literally clamping it shut!
So we are using three properties of zirconia:
(1) Zirconia has allotropes of different density.
(2) It is "easy" for one allotrope to change into another. (This is not true for carbon: it is _very_ hard to change graphite into diamond.)
(3) Using chemistry, we are able to control or partially-stabilize the transformation from one allotrope to another.
All of these properties (1),(2), and (3) are not only true of zirconia, they are practical in manufacturing. However, it is fairly rare! And this is why only transformation-toughened zirconia is used in almost all ceramic knives.
Sincerely,
--Lagrangian
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"What grit sharpens the mind?"--Zen Sharpening Koan
P.S. Here are two books mentioned in the above post:
_Why Things Break_ by Mark E. Eberhart (2004)
http://www.amazon.com/gp/product/14...&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846
_The New Science of Strong Materials_ by J. E. Gordon (2006)
http://www.amazon.com/Science-Materials-through-Princeton-Library/dp/0691125481/ref=pd_bxgy_b_text_c
Additional info can be found on wikipedia:
https://en.wikipedia.org/wiki/Fracture_toughness#Transformation_toughening
https://en.wikipedia.org/wiki/Zirconia
https://en.wikipedia.org/wiki/Cubic_zirconia
P.P.S. Please keep in mind that the "A" and "B" allotropes I mentioned actually have scientific names, that I forgot. The official names are in Eberhart's book and wikipedia. I made up the terminology "A" and "B" just for this post.
P.P.P.S. Goron's book goes into a concept known as _critcal crack length_, which is very interesting. Basically, cracks smaller than the critical crack length are unlikely to grow, but cracks larger than the critical crack length will grow. To keep the discussion simple, I didn't go into this. For details, see Gordon's book.
