Sapphire Swords and Other Things

[erisiamk]

In my WIP, sapphires are of significant spiritual and cultural value to one of the central races in the story. I've established some details about them being mining orientated in trade and a few other things, but I have a few questions to ask on how corundum, aluminium oxynitride, and other analogous materials could be used within their culture.

I'm aware that synthetic sapphire and aluminium oxynitride are used to make bullet-proof windows, and other tough transparent surfaces. One question I have is that, if the manufacturing industry was strong enough, would it be feasible to construct large vehicles (like starships) that used sapphire "panes" as viewports, and would there be a maximum size as to which the panes could be built before it would be structurally unsafe to use? I imagine that the apparatus for manufacturing these panes would be extremely large, but not completely impossible.

Second, would a set of armor incorporating sapphire plates into its design be usable? I imagine that due to the lack of cleavage planes, a sapphire plate of sufficient thickness would stand up well to melee weapons, but would this thickness make the sapphire plates prohibitively bulky? Furthermore, would its resistance to scratching, thermal effects, and chemical weathering be comparable to that of steel and other conventional materials? The plates would mostly be ornamental, but I'd like for them to not fall off or be significant liability to the user.

Finally, would a sapphire sword be viable at all, or would it simply undergo brittle fracture on impact? Again, I imagine that sapphire would perform much better than diamond in this context due to the lack of cleavage planes, but whether it would perform better than steel is questionable. Would a blunt weapon be more practicable, such as a mace or tonfa? Would they be able to block hits from other melee weapons? Let's assume that money isn't an issue for all of these questions.

Thanks in advance for your help; the sources I've found online seem to be a bit contradictory as to whether sapphire and similar materials have practical toughness or not.

 

[Brian G Turner]

Coming at this purely as an outsider, using sapphire for weapons and armour in a sci-fi would read as anachronistic to me.

There are far more suitable technologies in development at the moment - everything from carbon nanotubes and metallic polymers to aerogels - which are going to offer their own advantages.

If you wanted to argue the point that sapphire is especially important, then my immediate reaction would be to point out that diamond is widely regarded as stronger.

However, here's your escape clause - you could easily invoke futuristic technologies based on whatever science you want to imagine - with the caveat that if sapphire is especially important, then you could have sapphire molecules laced into whatever polymers, nanotech, or gels that you are using.

That way, sapphire is present in everything - just not solely used for construction.

Additionally it's worth noting that in human history, where a material was unavailable, then a suitable colour could be used to symbolically represent it - ie, the colour yellow was used to represent gold metal in anything from art to heraldry.

Hope that helps.

 

[Venusian Broon]

Hi erisiamk,

I found this on an interweb site....

Brittleness basically indicates how resistant the material is to plastic deformation. A very brittle material will, when placed under stress, break/fracture rather than bend. In the case of a sapphire crystal versus a glass crystal, the sapphire is considerably more brittle. As a result, a sapphire crystal is more likely to chip or crack than is glass counterpart if both are subjected to an equally hostile stress (banging, etc.).

So basically all three of your applications for sapphire could be viewed as...challenging

Yes the material is very hard, just not tough. Would be my guess!

For elements like body armour or swords my basic understanding is that such things need a nice composite mixture of ductily and hardess: ductile or flexible enough to stop any fracture and therefore keep the weapon/armour in one piece, hardness to keep the blade sharp. So I'd guess that sapphire does not lend itself as a good sword material, likely as it is to shatter. As for armour, it might help make a good ornamental cermonial suit, but probably not great for practical warfare.

I think having a sapphire mace would have the same brittle problem.

Another thing that would make such a weapon even less practical is: how do you repair damage? With steel and other metals, it is much easier to hammer out nicks and sharpen the blade. How would you do that with a crystal?

As for windows, I don't know. My limited understanding of 'bullet proof' windows was that they were, again, composite structures with some layers that were hard and others with a lot of give. So perhaps using sapphire as one of the layers in the structure?

 

[erisiamk]

Thanks for the advice guys! There is a fairly good escape clause for sapphire being so prevalent with the race, so using diamond over it probably wouldn't be very popular with them.

You are right about transparent armor being made of layers, so I will make sure to incorporate that where feasible.

On the brittleness debate, sapphire is obviously more brittle than glass, but to my understanding this just means that sapphire is more likely to fail in a brittle manner than glass, not that sapphire will fail under lower stress than glass. I've heard that sapphire generally has a higher fracture toughness than glass, and if i remember correctly that means it takes a higher stress for the fault to appear in the surface, hence the use in transparent armor.

I think that, given the discussion, the weapons would probably incorporate sapphire into their design, but the weapons would be composite with steel or similar ductile materials.

Repairing sapphire is also a good point, which leads to another question. Wikipedia says that sapphire is infusible. Does this mean that it's impossible to fuse sapphire at any temperature, or is it just prohibitively hard due to sapphire's high melting point?

The color symbolism is a good idea; the problem is that sapphires come in pretty much every colour of the rainbow depending on metal inclusions in the ionic structure, so it would be difficult to pull this off!

 

[Biskit]

Brittle materials and mechanical impact are never a good combination. Even when you use clever tricks to improve the strength, when they start to go, that is it. Brittle failure is a fairly catastrophic process.

On the really downside, little nicks and scratches can become disastrous flaws - anything that provides a stress concentration point can become a fracture. Depending on how your sapphire-clad warriors go out to play, throwing a good handful of diamond shards at them would be a good opener - just something to put a few surface scratches in place. Sapphire is supposed to be resistant, but that just means its better than other crystalline metal oxides on offer...

There is a classic materials science experiment - take a glass rod, draw it out to a thin cross-section and then bend it. If thin enough, you can achieve a quite amazing curvature. Now stroke a finger along one side and try again, bending it so that the stroked side is on the outside of the curve. The rod generally snaps very quickly, because that contact has triggered a whole load of minute flaws in the surface.

Materials such as tempered glass essentially make the material resistant (but NOT impervious) to these effects. So far as I am aware, you can not use these tempering tricks with a crystalline material. (Of course some clever ***** might have figured it out since I did this stuff for a living.)

As for melting/fusing sapphire, the melt point I found for it is 2400K, which is pretty high. When I (breifly) worked with silica, melting point 1830K, that needed an oxy-propane flame and UV safety glasses. Now to really put that in context, the glass research group I had contact with used platinum crucibles for melting their mixtures, and platinum melts at around 2041K. Handling molten sapphire would not be a trivial thing to do.

 

[Montero]

In our culture folks get excited about precious metals - gold, silver, platinum (for the modern man). This could be considered analagous to your positioning of sapphire in your society.
Precious metals were used as decoration, or for items which didn't take a beating. Gold chasing on armour, jewelry and gold or silver table ware.

It wasn't used for anything super-structural, because it just wasn't up to it and the man on the battlefield wanted steel.

If I were you, I'd invent a material that does everything you want and then imbue it with significance.

 

[Venusian Broon]

Repairing sapphire is also a good point, which leads to another question. Wikipedia says that sapphire is infusible. Does this mean that it's impossible to fuse sapphire at any temperature, or is it just prohibitively hard due to sapphire's high melting point?

Again only a guess - I only looked at metals on a very very small scale when I was having a go as a scientist - but I'd think that if you melted sapphire then you would probably destroy the crystalline structure hence convert into into Bauxite or some mineral-like form of Aluminum oxide.

A bit like diamond perhaps - the longest energy state of the carbon atoms actually is graphite (but diamond is not too far off). So diamond itself only forms in very highly specific places where the temperature and pressures are high enough where the diamond phase is the lowest energy structure. If you were to melt diamonds therefore - they'd convert to graphite if you couldn't control the pressure...

 

[Venusian Broon]

oh forgot about this!!, this'll work for sapphire swords, maybe:

The Aztec of course used obsidian in their swords, or more accurately wooden clubs inlaid with multiple obsidian blades: http://en.wikipedia.org/wiki/Macuahuitl

Easy to repair (if a singular blade falls out - just fix one back in) and I believe they were remarkably effective at cutting, slicing heads of people and horses and the such like.

I'm sure that sapphire could be fashioned into blades in a similar way.

I think the drawback to this weapon is that it's only a slashing, club-like thing - you can't stab like a metal swords or daggers which have good sharp points at the top.

 

[Mirannan]

Sapphire is aluminium oxide, Al2O3, with traces of other minerals forming the colour; the classic deep-blue sapphire is caused by titanium and iron impurities. When the mineral is chromium, you have a ruby instead.

Unfortunately, like all mineral crystals sapphire is rather brittle - I suspect it would make a terrible sword. Which isn't, of course, stopping its use as part of a structure mostly made of some sort of really tough material like nanotube-based. Or, of course, the weapon could be made of some such material that happens to look like sapphire.

As for sharpening sapphires - well, one method that comes to mind is to use industrial diamond dust to do the job?

 

[erisiamk]

oh forgot about this!!, this'll work for sapphire swords, maybe:

The Aztec of course used obsidian in their swords, or more accurately wooden clubs inlaid with multiple obsidian blades: http://en.wikipedia.org/wiki/Macuahuitl

Easy to repair (if a singular blade falls out - just fix one back in) and I believe they were remarkably effective at cutting, slicing heads of people and horses and the such like.

I'm sure that sapphire could be fashioned into blades in a similar way.

I think the drawback to this weapon is that it's only a slashing, club-like thing - you can't stab like a metal swords or daggers which have good sharp points at the top.

This seems like a good solution to the sword problem; it's closer to my initial ideas than a sword was anyways, so no problems there!

Although at this stage inventing another material (or perhaps simply imbuing special particles into the mix) would probably be the best way to solve the fusibility problem, which is the biggest issue I have atm. Perhaps a sapphire with inclusions of a macguffin metal element could be manipulated in a special way to maintain crystalline structure after fusion? Would this be a good enough justification?

 

[Venusian Broon]

Perhaps a sapphire with inclusions of a macguffin metal element could be manipulated in a special way to maintain crystalline structure after fusion? Would this be a good enough justification?

If you're going to go for some sort of alchemical macguffin, you might as well make your culture discover some sort of process that makes sapphire retain it's hardness but also have the ductile properties of steel - so that you can have your swords made entirely out of sapphire

(and then you can panel beat out the flaws and brittleness no longer becomes a problem!)

Your world, your rules!

 

[nubins]

If this is sci-fi miniaturising the production could help. Skies the limit I guess.. silksteel was a type of armour in an old game i played called Alpha Centauri - but as the name suggests it was just steel but spun like spider silk and crafted into shape to create incredibly strong but flexible material.

Warhammer 40k has a type of weapon for one of its super futuristic races called a Harelquin's Kiss - a nasty weapons that launches a diamond monofilament wire that, if it manages to get into the body (usually by isnerting the barrel into an orrifice) it then minces the insides of the unfortuante victim. There were also a number of other technologies based on diamond monofilaments.

It's the manufacturing process that determines the material, iron on its own, was not great at armour until someone worked out how to manufacture it into steel. Even steel had better and worse qualities depending on how it was manufactured.

So if you have nanoscale manufacturing, you can pretty get away with doing what you like, because who is to say there isn't some clever way on a molecular scale of accessing a saphires positive properties and manufacturing out its negative properties?