What are the benefits/disadvantages of using a sapphire window?

I see that some high end flashlights use a sapphire (as opposed to glass or acrylic) window. Aside from scratch resistance, are their any other benefits of using sapphire? And how resistant is sapphire to breakage when dropped (in a flashlight)?

Any disadvantages other than cost?

Thanks,

Yes. Sapphire's only superior feature in a flashlight is much better scratch resistance (Mohs' hardness: 9).
It costs more, it let's a little less light through and it shatters much more easily (hard stuff is more brittle).

If you don't want sapphire for the sake of owning a sapphire lens or if you don't need the extra scratch resistance, there's basically no reason to buy it. ;-)

Sapphire is probably a good idea on small keychain lights, where the lens is small and difficult to break, but where a non-sapphire lens would be likely to get scratched by keys etc.

Artificial sapphire is grown as a single crystal, unlike glass, which is an amorphous solid with no coherent structure. That makes it both harder to scratch and harder to break. It's not as transparent as premium-quality glass, though.

fyrstormer said:

Artificial sapphire is grown as a single crystal, unlike glass, which is an amorphous solid with no coherent structure. That makes it both harder to scratch and harder to break.

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Please provide a link supporting the claim that synthetic sapphire (Mohs hardness 9) is less brittle than (surface) hardened glass (Mohs hardness 6.5). I very much doubt this.

That's not the easiest information to find. This is the best I can do at the moment:

http://en.wikipedia.org/wiki/Fracture_toughness

Wikipedia said:

Here are some typical values of fracture toughness for various materials [EDIT: All units are in KIc​ (MPa-m1 / 2​), whatever that means.]

Metals
Aluminum alloy (7075)	24
Steel alloy (4340)	50
Titanium alloy	44–66
Aluminum	14–28
Ceramics
Aluminium oxide	3–5
Silicon carbide	3–5
Soda-lime-glass	0.7–0.8
Concrete	0.2–1.4
Polymers
Polymethyl methacrylate	0.7–1.6
Polystyrene	0.7–1.1
Composites
Mullite-fibre composite	1.8–3.3
Silica aerogels	0.0008–0.0048

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It was easy to find information confirming that corundum is more brittle than glass, yes, but brittleness just determines the likelihood of shattering versus plastic deformation once the fracture limit is reached. It's the fracture limit that determines how much bending force a material can withstand before it will be damaged by the strain.

If it helps, here's a page on the properties of sapphire (a.k.a. alumina or aluminium oxide):
http://www.roditi.com/SingleCrystal/Sapphire/Properties.html

Tensile strength at 20 deg C: 400 MPa
Flexural strength: 480 MPa to 895 MPa
Young's modulus: 345 GPa
Shear modulus: 145 GPa

For tempered glass:
http://www.aisglass.com/flat_tempered.asp

Tensile strength: 120-200 MPa
Young's modulus: 70 GPa

A comparison is made difficult by the fact that there are different manufacturers of tempered glass, but here's another site:
http://www.glazette.com/Glass-Knowledge-Bank-25/tempered-glass.html

Tensile strength: 65 MPa (note the difference between this and the previous link!)
Flexural strength: 120-200 MPa

Sapphire may be more stiffer and hence more brittle in that sense, but the higher tensile/flexural strength means it can take more force before it breaks, and in the end that's what we're interested in.

Side note: If we go solely on hardness, what do you expect will happen if we load both talc (1 on the Mohs scale) and sapphire? If I'm reading your logic right you'd expect the sapphire to fail first.

I have a swiss watch with sapphire glas, looks like new while my PA40 mineral glas is already scratched from one cleaning. Love sapphire glas

Here's another website I just found that discusses the relative benefits of corundum, fused-quartz glass, borosilicate glass, and soda-lime glass.

http://www.rayotek.com/techincal_info_glass_sapphire.htm

Thanks for those links. I still have doubts whether tensile or flexural strength are important parameters for a small flashlight lens.
I'd say that only in relatively rare occasions a foreign object will hit the flashlight lens directly. The majority of cases will be that the flashlight "just" falls onto the hard ground and the resulting G-forces are causing the shattering (-> brittleness ("a material is brittle if, when subjected to stress, it breaks without significant deformation (strain)")).
Or, by deformation of the flashlights bezel, the metal will "poke" into the lens and cause it to fracture. Don't know what kind of physical force measures the latter example but I would say usually there are no forces flexing or pulling your (~1") flashlight lens so these might not be the correct forces to compare the different materials by.

Bear in mind diamond is the hardest known material, but it is relatively brittle and can shatter.

Connor said:

Thanks for those links. I still have doubts whether tensile or flexural strength are important parameters for a small flashlight lens.
I'd say that only in relatively rare occasions a foreign object will hit the flashlight lens directly. The majority of cases will be that the flashlight "just" falls onto the hard ground and the resulting G-forces are causing the shattering (-> brittleness ("a material is brittle if, when subjected to stress, it breaks without significant deformation (strain)")).
Or, by deformation of the flashlights bezel, the metal will "poke" into the lens and cause it to fracture. Don't know what kind of physical force measures the latter example but I would say usually there are no forces flexing or pulling your (~1") flashlight lens so these might not be the correct forces to compare the different materials by.

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Dropping the light would probably be an example of compressive force, from the above links sapphire is still stronger. There would also be a flexural component depending on how it hits the ground.

The forces from poking the lens will definitely exert a flexural force on the lens. To make a example, get a sheet of paper, hold it up by two sides and poke it somewhere. The paper will bend (read: flex) around the point that you're poking it at. This is the same deal for any object being poked regardless of material or size, it's just that it's a lot more obvious with the sheet of paper. You can also test the same if the flashlight is dropped by holding the paper at whatever angle you choose and lowering it onto the ground. You'll see the paper buckle and/or bend, indicating that there is almost always a flexural component, and a compressive component as well for that matter.

Even when it starts to break, in crack propagation, as per fyrstormer's first link sapphire also has a higher fracture toughness than glass.

Note that while the definition of brittleness in your post is correct, it indicates nothing about the overall strength of the material. Silly putty is much more flexible than glass or sapphire, but I'd bet on the glass or sapphire withstanding more than the putty would, even taking the putty's deformation into account. Practically, I don't care about whether the material flexes or not, all I care about is that it can withstand the forces without breaking or going into plastic deformation.

I can't speak to it's strength or scratch resistance, but the sapphire window on my Aeon doesn't seem to "gunk up" as quickly as the lens on my T1A Titan or fenix L0P did.
All three were carried under my shirt, on a lanyard around my neck.
The lens size and bezel depth may have something to do with it, but the Aeon lens didn't really need wiping over when I wiped it over yesterday, the Titan and fenix needed wiping off once a week

DM51 said:

Bear in mind diamond is the hardest known material, but it is relatively brittle and can shatter.

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It can, yes, as can sapphire, but glass will shatter from much less force. The likelihood of the lense shattering seems less relevant to me than the force it would take to damage it at all, because unless you use soft plastic (or maybe optical silicone?) as lense material, any lense will shatter with a sharp impact. The failure mode of sapphire isn't something that can be avoided by choosing glass instead, except perhaps within a narrow range of rather unlikely circumstances. I've broken glass lenses just from over-tightening bezels, whereas I've dropped lights with sapphire lenses face-down on sharp gravel without any damage at all.

You know I have a hard time believing that sapphire is as brittle as everyone says it is. Specifically because I have 15mm synthetic sapphire gemstones, 15mm cubic zirconia, and 15mm cheapo glass ones. I bought them just to compare how each would look in some jewelry I made. Well after picking ruby I had those extra gems. So I decided to see how tough they were. I hit each one with a hammer, glass turned to a million tiny pieces and dust as expected, zirconia split into two pieces one large and one small, the sapphire was nearly indestructible I took a tiny chip off the point but after that it took several more hits just to knock off more chips. At least when it comes to directly hammering the gemstones sapphire was definitely the strongest and most shatter resistant.

I have to agree with fyrestormer that just because it's young's modulus is higher because it's more brittle doesn't mean it can't withstand higher impact. It just means as you reach it's breaking point it will just crack vs flex. But the fact that the fracture point is so high you're still less likely to break it. Basically any impact that can shatter the sapphire, probably would have done the glass in. This would be evidenced by sapphire's much higher ultimate tensile strength of 1900 MPa vs glass at only 33 MPa making sapphire 57 times stronger than glass.

Nice to know someone's actually done those tests in real life and that it backs up what fyrstormer and myself have been saying. At least synthetic gems are getting cheaper these days as well!

This is a great thread!

Might have to read it again just for kicks. Good info guys! :thanks:

Oh yeah if anyone is wondering, this was what I made. Ruby center, originally it was going to be round but I switched it to a square, yellow sapphire outside and sparkly CZ on the inside. It's actually quite large the ruby is 15mmx15mm.

I don't know much about jewelry, but it sure looks pretty.

Very interesting and informative posts - thx Th232, fyrstormer & qwertydude