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Nowadays heavy duty drilling and machining is often done with cutting tools fashioned from cemented carbides, which consist of a hard phase like Tungsten Carbide embedded in a "soft" metal like Cobalt. This gives a really tough material that withstands a lot of punishment. I'm wondering if swords made from such a material could best convential steel weapons and armor.

Here are some reasons why noone tried it before:

  • Cemented carbides were invented in the 1920s when swords were long obsolete
  • Their production is costly and requires powder metallurgy
  • A sword has some very challenging geometries for this method (especially the thickness)
  • You need actual diamonds to sharpen them (and depending on the pressed pellet you would need to remove a lot of material)

Now suppose an organization has the means and the will to make such weapons and swordfights were still a thing, would a warrior equipped with a cemented carbide blade have a distinct advantage over an adversary with steel weapons and armor?

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    $\begingroup$ Are you speaking of full plate armor combat with broadswords, or something a bit more nimble with foils? $\endgroup$ – 011358 smell Sep 16 at 12:16
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    $\begingroup$ The TLDR on this is that properly forged steel is still the best possible material for a sword that we (humans) know of; because you need an optimal combination of sharpness, edge retention, strength, and flexibility. Anything superior to steel in any one of those categories is almost always unworkably inferior in one of the others. $\endgroup$ – Morris The Cat Sep 16 at 13:41
  • $\begingroup$ Are you limiting yourself to the modern-day WC/Co cemented carbide system, or are other matrix and/or carbide materials acceptable? $\endgroup$ – Shalvenay Sep 17 at 2:29
  • $\begingroup$ It can be another kind of Carbide but as I understand the WC/Co combination is tougher than cermets for example. But I am completely open. $\endgroup$ – And Sep 17 at 9:23
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Steel swords were made by exactly this technic. Hard-tempered steel at the edge and soft elastic steel in the middle. That is why it was an art and good swords were so expensive.

As for cemented carbide - it is a bad material for swords for exactly the same reason why diamond is: it is fragile. It does not like hard impacts and can just shatter into pieces. Steel is the best material for such a thing as a sword even now. No one uses cemented carbides in an axe after all.

Adding cemented carbides would still be a good option, but only for the tip of a sword, for single use thrust through armor. It would be a really great advantage.

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    $\begingroup$ Not a sword tip, certainly. Swords are terrible things for penetrating decent armour. The spiky bits on a mace or warhammer on the other hand... $\endgroup$ – Starfish Prime Sep 16 at 13:02
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    $\begingroup$ A short dagger is what is normally used to penetrate armor. The mace and warhammer isn't so much to penetrate the armor, but to deliver blunt damage. You're delivering damage through the armor. $\endgroup$ – Nelson Sep 17 at 4:20
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    $\begingroup$ @Nelson that's not quite right. Typically, maces and warhammers (the sort of objects that in D&D would be mis-classified as blunt) came with sharp sticky-outy bits that were designed for puncturing armour. No one is able to summon up the kind of force needed to put a dagger through plate. Daggers were primarily used to stab through weak points, such as the gaps between plates, or areas that the plate didn't cover, such as the armpits. $\endgroup$ – Ingolifs Sep 17 at 4:40
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    $\begingroup$ @Ingolifs Not quite. The spikes or flanges on maces are primarily there to make the blows bite into the metal instead of glance off, as plate armours were of course shaped in such a way as to deflect attacks. They're also useful when you hit someone who isn't wearing as much armour. However, they wouldn't be expected to penetrate plate and probably rarely did - I suppose a two-handed bec de corbin might get through a thin plate part with its spike, but that wasn't the spikes' primary purpose. $\endgroup$ – Pahlavan Sep 17 at 4:57
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    $\begingroup$ @Luaan : true, but as the estoc and half-swording did historically exist, it means that there were occasions when swords were used against armor. Of course, by thrusting, not cutting. (and let's not forget the Mordhau: if your opponent wears an exceptionally good breastplate, you should not bother trying to pierce it, just grab your sword by the blade and hit him on the head with the hilt or crossguard) $\endgroup$ – vsz Sep 17 at 7:26
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Starting from near the bottom, you do not need diamonds to sharpen cemented carbide; it can be done with a so-called "green carborundum" grinding wheel (which is silicon carbide bonded grit). Specific, but quite a bit cheaper and more durable than diamond grit abrasives.

The biggest issue with carbide for something like a sword is brittleness. Carbide inserts in saw blades and machine tools are made of cemented carbide, and they're notorious for chipping and breaking on impacts (such as interrupted cuts). This can be managed in machine tools and saw blades by controlling feed rates, but you don't have that option with a sword. A hard parry is likely to leave you with an effect like that scene in one of the Highlander movies, where the sword just shatters like glass.

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  • $\begingroup$ You'll also notice that the entire saw blade or drill bit is not made of carbide, just little teeth where it contacts the material to be cut. In my experience, a tooth is much more likely to break off the saw blade entirely. $\endgroup$ – jamesqf Sep 16 at 18:50
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    $\begingroup$ @jamesqf the only solid carbide tools I'm familiar with are end mills, die grinder burrs, and planer knives. Making blades with inserts started as cost control, though. $\endgroup$ – Zeiss Ikon Sep 16 at 18:52
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    $\begingroup$ No doubt. But I was thinking of the case where the sword has carbide edges bonded to a steel core, similar to saw teeth. So rather than the whole sword shattering, the teeth break off, leaving you waving a blunt steel rod around :-) And of course in actual sword fighting (or at least in fencing, kendo &c) you spend a lot more time parrying the opponent's sword than striking him. $\endgroup$ – jamesqf Sep 17 at 5:45
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There is one thing to be aware of:

Many modern steels already work in a very similar fashion.

You will notice that tungsten and/or vanadium are often mentioned on tools or knives - these elements are so called carbide formers, and the carbides they form (as literal little lumps of carbide... controlling their size is much of the art of making such steels) are much harder than the surrounding steel matrix. Resulting in a material inheriting a mix properties from both soft matrix and hard carbide.

A steel with "coarse carbides" (big lumps) can be very very durable, but also limits how sharp and especially how easy it can be sharpened. Trying to grind a mishmosh of hard and soft particles smooth can easily end up with the harder particles being displaced - and the holes they leave being stuffed shut with worn off bits of the softer material.

Iron itself also forms carbides, however these are closer in hardness to the matrix steel itself. Steels solely relying on iron carbides and a harder-than-usual steel matrix are preferred for, say, a fine woodworking plane or sushi knife that you don't need to be super wear resistant but super sharp and hard.

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Unfortunately, there seems to be a significant amount of missing information in the OPs thought process about using carbide tools to cut through armor. A carbide tipped/toothed sword might look cool, but I seriously doubt it would work very well.

Disclaimer: I'm not a professional machinist, I only pretend to be one (sometimes) at night and on weekends. I don't have all the answers, but I know of plenty of problems.

Feeds and Speeds

When doing milling, cutting threads, or anything to do with machining metals, feeds and speeds are absolutely critical to the process. These calculations take into account the type of material you are cutting (by actually knowing what it is), the cutting speed (as accurately as possible), the size of the cutter, and even the angle of the cutting blade to the surface (baked into the type of machining as well as the tool used).

If you get it wrong, you risk damaging the tool to the point where it dulls or even breaks. With many machining tools, it won't even cut below a certain speed, it just wears the tool out prematurely by grinding the material away instead of cutting.

This is a science and machinists know it can go very wrong very quickly. "Wrong" being a damaged or completely broken tool. There is definitely a fair amount of "wiggle room" here, depending on your machine and tools, but you won't know how much you can "wiggle" until you are familiar with the tool, machine, and the material. With the material changing with every single opponent's armor, you're never going to be able to know what's safe until it's likely too late.

https://www.wisc-online.com/LearningContent/mtl8202/MLT8202.htm

Without knowing the exact material and hardness of the material you are up against, you immediately risk damaging your sword. Since no human can gauge their swing against a mobile attacker to inches per second or feet per second until it's already in motion and not with the accuracy needed, there's no way to know if it's appropriate to actually cutting the armor vs blunt force. There's also no way to get the correct cutting angle between the sword and the armor, since both are in motion and a change in surface curvature in the armor might cause the carbide to skip off, rather than cut. Since carbide is so brittle, skipping off amounts to a hard shock which can break it. Even sword made of steel can't be brittle, since they need to flex with every blow the take or give.

Even if you had a motorized chainsaw sword, you still aren't getting the feeds & speeds right, so instead of breaking off 1-2 teeth, you're breaking off 10-20, if not all of them.

Cutting depth

Milling processes need to be done with a specific cutting depth and chip removal. If you remove too much material at once, you also risk damaging the tool. If you don't remove the material fast enough, you risk cutting it again, preventing you from making a deeper cut in the armor.

This is managed partly by the tool as well as by the feeds & speeds mentioned above.

Tool pressure.

If you don't have the right pressure between the tool and the work piece, you risk breaking the tool. This is why machining machines usually have really tight tolerances and machinists talk about backlash (recoil arising between parts of a mechanism) and chatter (to vibrate rapidly in cutting) quite a bit, since they can negatively affect not only a cut but also damage tools. Both can cause excessive tool wear as well as not correctly cutting the material.

When using a tool freehand (sword) with a material that isn't in a vise or otherwise clamped down (opponent), you aren't going to get the correct tool pressure to get a good cut. Depending on circumstances, you may have too little pressure or too much. Too little and you don't get a cut, and too much damages the tool.

You also have to worry about how far apart and the size of your teeth. Uncommon knowledge says that you should have at least 3 teeth engaged with material, or you risk shearing them off. This goes for band saw blades made of spring steel, but I can imagine it works for just about anything.

Also, if your teeth are too close, you'll need more pressure to do more than scrape across the surface. Too far apart and you have too much pressure. If the spaces between the teeth are too shallow, they clog and prevent you from removing more material. However if they are too deep, you risk compromising the structural integrity of the teeth.

Again, this all depends on the material you are cutting as well as the speed and feeds you are trying to cut at.

Carbide inserts

Carbide also comes in inserts for tools, so when they get dull, you can swap them out easily and either sharpen or toss them. This is for mid to high grade milling tools, and they can cost quite a bit. This allows for better tool maintenance and longevity, since you can replace a single tooth, instead of scrapping the whole thing, or having to to a major repair in the field.

Replacing a broken tooth still isn't something you're likely to do in the middle of a battle, but rather something you'd do before/after a battle, since it still takes tools and time. You also don't want to be carrying around a 20 lbs. bag of replacements that can get in the way, wear you down, get sliced open, get cut off, or otherwise inhibit the battle.

https://en.wikipedia.org/wiki/Cemented_carbide#Inserts_for_metal_cutting

Conclusion

With all the things mentioned on this thread about carbide tools, it just doesn't seem to be a good idea for use with a sword. There's too many things that can go wrong and other materials that are better suited, as mentioned in other Answers.

Some of it can be mitigated with the carbide inserts, but not enough to make it more effective than the current steel used in swords. Adding the complexity of all the different considerations with speeds, feeds, tooth distance, tooth size, and more, it make this nearly an impossibly unsolvable math problem to avoid completely damaging your sword beyond use in a single encounter, let alone multiple opponents on a battlefield. This might work, marginally, in a duel, but not when potentially engaging with multiple enemy on a battlefield. If your sword isn't reliable enough, then it's basically useless for more than blunt trauma, which makes a bad sword.

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