Sapphire Blade Ramble

[sortitus]

This story goes a while back. Until about a year ago, I worked at a jewelry store. Part of my job was to clean the parts of the store that had hardly been glanced at in about fifty years, and one of the perks was that I could ask to take almost anything I found home. In one pile on a box that was 90% polishing wheel cloth scraps and polishing dust, I found some awesome stuff. One, my current main watch. Two, an 8oz. bag of 18k casting gold, stamped and with a receipt from 1955. My employer didn't let me keep that one. Third, and the point of this story, a sapphire watch crystal.

My boss let me take the crystal, so that day I came home hoping to test out just how tough sapphire is. I cut papers with the edge of the crystal, but it wasn't really sharp. No surprise there. I scratched my pocketknife and failed to scratch my mother's diamond. Normal tests. About a week after I got the crystal, I rediscovered it on my desk and was playing with it when I dropped it on the cement in the garage. I expected it to chip, but it was unharmed when I checked it over. Naturally, that made me wonder just how impact resistant the crystal was. I figured that other than for testing hardness, the stone would be fairly useless to me.

Anyway, I grabbed a screwdriver, hammer, and some safety goggles. I placed the crystal flat on the concrete and swung the hammer as hard as I could onto the crystal. The hammer head was dented, but that's all that happened. I tried the same experiment with the screwdriver focusing the force with similar results. I positioned the crystal on our steel vice, straddling the gap, with the screwdriver focusing the blow again. I figured that I might as well break the persistent bugger and get some entertainment for my trouble, but, as you may have guessed, the crystal survived, leaving the vice with some marks and the screwdriver a bit dull-headed.

I still have the crystal, and rediscovered it most recently last night. My brother and I got to talking (he was there when I duplicated the experiments to show that I wasn't crazy) about the use of a stone that hard and shatter-resistant. Being the crazy fantasy reader I am, my first thought was a sapphire sword. Making an entire blade of saphire would be impractical, but having a nicely rigid metal I-beam core would be decently light and lend some semblance of financial practicality to the project. (Says a broke college student. )

Sapphire can be made to be very sharp, would be resistant to dulling, etc. Basically all of the characteristics of a ceramic blade, but a little harder and infinitely cooler. The only Google result of any usefulness I got for any variation of corundum/sapphire/aluminium (tri)oxide blade/knife/sword is here: bob-basset's livejournal. The link's in Russian. Here's Google's translated version: woo, language!

The discussion last night included crysknifes, glass daggers used by Mistborn, and the RPG series Elder Scrolls' weaponry (Glass weapons are made of green volcanic glass, Ebony and Daedric are made of obsidian. The Daedric dagger from Morrowind looked sweet.). These were just the examples that sprung to mind, but I'm sure there are many other stories that use transparent stone weapons.

To sum up, sapphire is surprisingly strong, and I'm impossibly long-winded and nerdy. I could ask a synthetic sapphire manufacturer about a 6-8" knife blade made to my specs for mounting on a metal rail. Watch crystals are pretty cheap, but a custom job like that wouldn't be. Ah, if only I had money to burn for ridiculous projects like this. I guess it's better (more practical and inexpensive) than my original idea for a micro-serrated diamond blade edge. A super high-tech sword would also be awesome for its capacity to baffle future archeologists.

[Eerongal]

Yeah, sapphires and rubies (or rather, the corundum they're made from) has a hardness of 9 on the Mohs scale. Max being 10, which is where diamonds fall. Would probably make a fantastic, although impractical, weapon.

[Chaos]

Hmmm, I'm pretty sure there's a story in there somewhere

Wouldn't a completely sapphire sword be prone to breaking? I could be wrong, but just because something is hard doesn't necessarily mean large quantities of it could stand up to the impacts of combat. Sure, the sapphire itself wouldn't be harmed, but it would need a more flexible core to work.

That is, if you could even get a sapphire like that.

[ryos]

IOW, ceramics are brittle. Metals can take a fair amount of elastic deformation until turning to plastic deformation and finally breaking entirely. Ceramics hardly deform at all before breaking. This has a couple implications for our hypothetical sapphire blade:

• An entirely sapphire blade would likely not notch or dull, but would instead accumulate stress fractures and quickly break.
• On a metal blade with a sapphire edge, your sapphire inlay needs to be very carefully designed to allow the metal blade to bend and flex without breaking the inlay. I'm not sure of the best way to go about this.

This may be why I've never seen or heard of anyone actually doing something like this, even though, on the surface, it sounds brilliant.

[Peter Ahlstrom]

Okay... The obligatory quote from Aether of Night.

Bright rose crystals grew from his Aether, encasing his fist in a gem-like sphere of Amberite.  The crystals grew quickly, lengthening from his fist, crawling and growing outwards in a thin spike.  He held his hand forward as the crystals grew upon one another, forming outward into a blade several feet long.  The weapon looked like rosy glass or a multi-faceted piece of quartz, but it was far stronger than either.

[Chaos]

IOW, ceramics are brittle. Metals can take a fair amount of elastic deformation until turning to plastic deformation and finally breaking entirely. Ceramics hardly deform at all before breaking. This has a couple implications for our hypothetical sapphire blade:

• An entirely sapphire blade would likely not notch or dull, but would instead accumulate stress fractures and quickly break.
• On a metal blade with a sapphire edge, your sapphire inlay needs to be very carefully designed to allow the metal blade to bend and flex without breaking the inlay. I'm not sure of the best way to go about this.

This may be why I've never seen or heard of anyone actually doing something like this, even though, on the surface, it sounds brilliant.

Indeed, that's what I was thinking. Such a sword would break very easily.

Of course, if one could magically enhance a sapphire blade against stress fractures...

[Eerongal]

Hmmm, I'm pretty sure there's a story in there somewhere

Wouldn't a completely sapphire sword be prone to breaking? I could be wrong, but just because something is hard doesn't necessarily mean large quantities of it could stand up to the impacts of combat. Sure, the sapphire itself wouldn't be harmed, but it would need a more flexible core to work.

That is, if you could even get a sapphire like that.

yes, Sapphire, though quite hard, is actually very brittle. In fact, it's more brittle than glass.

[sortitus]

yes, Sapphire, though quite hard, is actually very brittle. In fact, it's more brittle than glass.

Which property is this you are referring to? Are you comparing sapphire to normal glass or one of various reinforced glass products? The Young's modulus and shear modulus of sapphire (glass stats in wiki pages, sapphire stats on sapphire page) are much higher than that of glass, as far as I can see. Same with tensile strength. I've crushed glass between a hammer and concrete before, while the same exercise with sapphire chipped the concrete and dented the hammer.

Zirconium carbide, the material used in most high quality ceramic knife blades, is much more prone to shattering than sapphire (esp. synthetic sapphire, which is usually almost completely flawless), so the durability would be an increase from what some people are used to. I'm inclined to believe that an intelligently designed sapphire blade would be more durable in every way than a steel blade of similar weight. Unfortunately, none of us (that I know of) have any special kind of education when it comes to this stuff. All I have is gemology training (which hardly says anything about material toughness) and my own experiments.

Indeed, that's what I was thinking. Such a sword would break very easily.

Of course, if one could magically enhance a sapphire blade against stress fractures...

• An entirely sapphire blade would likely not notch or dull, but would instead accumulate stress fractures and quickly break.
• On a metal blade with a sapphire edge, your sapphire inlay needs to be very carefully designed to allow the metal blade to bend and flex without breaking the inlay. I'm not sure of the best way to go about this.

The idea of the metal core was to design the metal to be lightweight while maintaining rigidity to prevent the sapphires from bending. As long as you don't plan on blocking with your edge, I think the blade would last a surprisingly long time (but with an entirely sapphire blade, you'd better plan on blocking with the edge if you don't want a shattered sword). My brother and I talked about two inch long sections of sapphire fixed to the metal core, with small gaps in between to allow the bending that is bound to occur. Of course, the metal would be recessed from the edge so as to not interfere with the cutting edge. The tip would need a lot more reinforcement, and we weren't sure that having sapphire on the point at all would be a good idea.

Yeah, sapphires and rubies (or rather, the corundum they're made from) has a hardness of 9 on the Mohs scale. Max being 10, which is where diamonds fall. Would probably make a fantastic, although impractical, weapon.

The hardness prize currently belongs to various man-made carbon-based materials, but for all practical purposes diamond is the hardest. Diamond is far less durable than sapphire, especially when ground to a point. It's a common occurrence for a jeweler to break a diamond by holding it too hard with tweezers. The force needed to securely grip a faceted diamond with tweezers is not very far from the force needed to shear a piece off the edge.

Another design issue is the angle of the blade. If it's too steep, it would make cutting through things very difficult. If it's too shallow, breakage is much more likely. *sigh*

[ryos]

Unfortunately, none of us (that I know of) have any special kind of education when it comes to this stuff.

Haha, guilty as charged. All I've got to go on is a bit of materials science that I picked up during my days as a ChemE major.

[The Jade Knight]

I know a good weaponsmith, if you need an expert opinion.

While he works primarily with plastic these days (for training weapons), he focuses on stress testing and the like a lot, and could probably give you some good pointers.

However, I don't expect he'd be much help with the gem-specific aspects of the weapon.

[sortitus]

I'd love an expert's opinion, though I doubt this will go anywhere beyond fantasy.

I had a thought today, inspired by Ee's comment about glass being more shatter-resistant than sapphire. What are the stats of bulletproof glass like, and how does sapphire compare? I seek obscure knowledge! I will amaze the ladies at classy parties with my knowledge of the tensile strengths of elements useless for structural applications such as talc, solid mercury, and aluminium (tri)oxide!

[Eerongal]

I had a thought today, inspired by Ee's comment...

I'm a springboard of ideas!