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I am reading a novel set in the future. A planet has been found where there are extensive deposits of diamond. Presumably diamond is mined much like marble is here on earth (there is not a lot of detail on this).

A character has a knife where the blade/shaft is a single piece of diamond. In 2015 here on Earth you can purchase diamond blade knives, but they seem to be all steel (no diamonds per comment by @mart). There are some saw blades that have pieces of diamond attached to a steel blade.

Assuming large quantities of reasonably priced materials, would it be possible to design and manufacture a functional knife made from a single section of diamond (all but the handle)?

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  • $\begingroup$ actaully, the linked article is about a knife forged in a special way, not about one with actual diamonds attached to the blade. AFAIK diamond is applied to tools as a dust and is used as a grinding agent, not to form blades. $\endgroup$ – mart Feb 23 '15 at 13:14
  • $\begingroup$ I believe you are correct, updated question. $\endgroup$ – James Jenkins Feb 23 '15 at 13:20
  • $\begingroup$ Diamond microtomes (used frequently in biology to create ultra-thin microsections for electron microscopy) are closer to your presumed knife (they're 100% diamond along the cutting surface) rather than blades with diamond grit. $\endgroup$ – Nick T Feb 23 '15 at 16:19
  • $\begingroup$ Weapons are of course in a permanent battle with armor, and diamond weapons don't fare well against armor - especially if that armor uses diamond scales. Diamond flakes will be dirt cheap and dull many sharp stabbing weapons. $\endgroup$ – MSalters Feb 24 '15 at 0:31
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You could work pure diamond into something shaped like a knife, but it wouldn't make a very good knife. Diamonds are hard, but they're brittle. The blade would break long before its edge went dull, but this is arguably worse: you can resharpen a dull blade, but a broken blade (especially one not made of a material that can be melted and reforged) becomes essentially useless.

If this is for a story, then one possible subject for research might be the macahuitl: a swordlike weapon used by the Aztecs. The main body of this weapon was a simple shaft of wood with grooves running along its edge, but small blades of sharpened obsidian would be fitted into the grooves. A blade could chip or break, but the weapon itself would remain intact through most blows, while the scalpel-sharp obsidian could cut even better than metal (seriously: obsidian is dangerous stuff). Modern "diamond knives" work according to a similar principle: only the edge is made of diamond, while the main body is made of metal, or something else that's not so prone to breakage.

For the purposes of a story, I could imagine the diamond-knife concept being scaled up into a sort of "neo-macahuitl" that used diamond blades. The main body of the weapon could be made of metal, hard rubber, or any other material of sufficient strength, and then small diamond blades (all diamond, or diamond-edged metal) could be fitted into it. It might even be possible to make the blades replaceable.

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The first criterion would be to obtain a diamond, or a fragment of a diamond, large enough from which a knife could be made. Just because such a diamond has not been found on Earth doesn't mean it can't exist elsewhere. Years ago, astronomers found a diamond star, 4000 km across. Another source claims a "corpse of a star" 55,000 km across (five times Earth's diameter) made of carbon and oxygen "most likely crystalline in nature, like a diamond".

To shape a knife, with a combined handle, from a single diamond would require very careful grinding and polishing. This would require much effort and time, but it wouldn't be impossible. It's how gem diamonds are cut from raw diamonds. A lot a care would be required to prevent the diamond from shattering. It's a delicate procedure that requires skill, the right tools, the correct procedures and experience.

After the knife has been shaped, the knife would need to be polished; something that has already been done on a smaller scale.

One issue with a knife made entirely of diamond is how useful would it be and if it would shatter if it were dropped or if it struck another hard item with force, given that diamond is brittle.

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    $\begingroup$ Hardness != indestructible. I remember being very disappointed that my tungsten wedding band isn't bulletproof. $\endgroup$ – Doug Dawson Feb 23 '15 at 14:43
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    $\begingroup$ @DougDawson I know the feeling, how long did your wife wait before shooting at you? $\endgroup$ – MDMoore313 Feb 23 '15 at 16:41
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    $\begingroup$ @BigHomie, hahahaha, signed up on engineering.stackexcange just to upvote your comment $\endgroup$ – Nik Kyriakides Feb 23 '15 at 17:05
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Working a large chunk of raw diamond into a knife blade is theoretically possible. We could do it today on earth, although the process wouldn't be cheap.

The biggest problem I see is that while diamond will maintain a sharp edge, it is too brittle to make a useful knife blade except perhaps for specialty applications where the knife won't be subjected to other stresses on the blade, and absolute sharpness is really really important.

Other properties of diamond you might want to explore are that it is a very good conductor of heat, can withstand high temperatures, is transparent, and has a high index of refraction. I would focus on exploiting these characteristics in a science fiction novel where the material itself is cheap and abundant. There are a lot more interesting ways to use cheap diamond than making knife blades out of it.

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  • $\begingroup$ Diamond may withstand high atmospheres, but not in an oxygen atmosphere. $\endgroup$ – Dietrich Epp Feb 23 '15 at 15:34
  • $\begingroup$ @Diet: I don't see how the height of the atmosphere has anything to do with it. Our own atmosphere contains significant oxygen, and diamonds are unaffected by it, so your point makes little sense. Of course we can't know whether you consider our atmosphere "high" or not. $\endgroup$ – Olin Lathrop Feb 24 '15 at 14:27
  • $\begingroup$ That's a typo... I meant to write "high temperatures". Diamond will burn at high temperatures in the presence of oxygen. $\endgroup$ – Dietrich Epp Feb 24 '15 at 17:46
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Methods already exist for creating sizeable artificial diamonds from more common forms of carbon. Some companies even offer a service where you can send the ashes of a beloved family member or pet to be compressed into a diamond for jewelry. It's not unreasonable to believe that this process could be scaled up and specialized to form other shapes like a knife. That won't mitigate any of the material limitations other answers have already detailed though. It'll just make it easy to get a knife-shaped diamond.

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The answer is no, and the phrasing is key.

"assuming large quantities of reasonably priced materials" (are available)

Then yes, it would certainly be possible to manufacture a knife made from a single section of diamond. As @nick-t commented, Diamond knives exist and are used in ultramicrotomy, the sectioning of a substance into extremely thin slices for use in transmission electron microscopy. The shaft is not diamond in this case, and the diamond blade is surrounded by metal as a mounting bracket, but these are solvable differences which mostly exist for cost reasons. Assuming large quantities of gem-quality diamond are reasonably priced and available, no problem.

This, however, isn't solvable:

...a functional knife...

(emphasis added)

Diamond doesn't create a functional knife. Ceramic knives made of aluminum oxide or other very hard materials exist, you have likely used them. They are not good (compared to steel) as they exhibit microchipping almost invariably. The same problem vexes the users of glass knives and obsidian ones; diamond knives are intended to delay that problem, and do, but this is in the hands of professionals doing surgery, or in a machine. In a normal, out-in-the-world usage scenario, a diamond knife would fail near-instantly. The crystal lattice of the diamond, which gives the diamond its hardness, also causes this fragility; so the problem is inherent to diamonds.

CATRA testers are widespread in the knife industry, at least among larger manufacturers. They are also widely known to be inaccurate in testing some steels, and especially ceramics; the problem arises because a human cannot cut as precisely as a machine can, and cannot entirely prevent lateral forces. A diamond knife would no doubt score a wonderful result in CATRA testing, as do ultrahard steels; but in reality, they would not be functional.

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All material exchanges hardness for toughness. The harder it is, the more brittle. Some material can be treated or combined (alloyed) to make something with the desired traits, but a pure material is subject to the limitations of the material.

A brittle knife is not a functional knife. It is a knife shaped object. This is perfectly usable of a one-time (or short time) use until the fractures make it unusable. It is also perfectly fine for a wall hanging... would make an interesting art piece especially if lit well.

Shivs/spikes (long shards) made of diamonds could be used as throwing "knives". That would be a single-use weapon from diamonds.

It's just not a suitable material.

In my experience, the heat treating of an alloy (such as steel) is the best way to get a balance between toughness and hardness for a long-term use functional knife.

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