Sliding metals show fluid-like behaviour

Researchers at Purdue University took high speed video of metal sliding over each other and discovered a strange swirling fluid-like behaviour not normally associated with solids.

This research might have major implications for the gun industry. Guns are essentially a collection of sliding metal parts. If this fluid behaviour can be reduced in the barrel, maybe the barrel life can be significantly extended.

The reader who emailed us about this research pointed out that this may be one of the pricinples behind the Blish Lock phenomenon that was exploited in the Thompson Autorifle and the original Thompson SMG. John Bell Blish discovered that “certain dissimilar metals will resist movement with a force greater than normal friction laws would predict” from observing naval gun breeches which unlocked when firing weak charges but not heavy charges.

Thompson Autorifle. Drawing from Hatchers Notebook 2nd Ed, 1957, p. 45

The Thompson Autorifle was just about the simplest fullpower semi-automatic rifle ever made. It’s action was not much more than a rotating bolt which resisted being unscrewed when the barrel pressure was high but would unscrew when the pressure decreased. It was a very clever invention but required a lot of lubrication and ejected brass violently (the latter being a common problem with all delayed blowback rifles).

[ Many thanks to Frank to the tip. ]

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • Jake Barnes

    I think the largest application will be in pre-treating billets prior to extrusion processes to avoid excessive die wear.

  • bbmg

    “If this fluid behaviour can be reduced in the barrel, maybe the barrel life can be significantly extended.”

    You could always go smoothbore…

    • MrMaigo

      The AMR also fires a sabot. Just a sabot alone would decrease barrel/rifling ware.

      • bbmg

        Indeed, this is true of the APDS rounds fired by the Phalanx CIWS system for example, and if I’m not mistaken the 30mm rounds fired by the A-10’s GAU-8/A gatling also have synthetic driving bands to reduce wear.

  • Tim

    Didn’t the Blish effect turn out to be negligible, leading the US Army to develop a Thompson that was straight blowback?

    • LiamHaslam

      You are right, by adding a few ounces it could easily be converted to straight blowback. The Blish lock made for a more expensive and complicated gun.

  • S O

    Solids also behave like fluids in shaped charges. The (copper) liner is being squeezed forward and reshaped without turning liquid (no matter how many people suppose otherwise – experiments have settled the issue by observing that the different parts do not fusion as a fluid would have done).

    Very high pressures are clearly beyond a human’s practical experience of physics. That’s why some things that happen under high pressure sound so ‘wrong’ to us.

  • Denny

    This is not necessarily surprise for those aware of properties of metals: they have natural pre-ponderance to selfadhesion (especially those with similar or identical microstructure). The better the surface finish, greater the adhesion force appears to be. In case of bullet moving thru the bore are in place couple of other phenomena. One is that typically bullet is larger than bore; other the fact that speed of bullet in in excess of material capability to react and to copress. This makes the “wave” so much more pronounced.

    So, every time you fire a shot, you are effectivelly ‘ripping off’ particals from barrel bore. This is in addition of material being ‘buned out’ in process of flame. Burning out is most intense couple of inches in front of chamber, material pick-up closer to muzzle where bullet speed reaches maximum velocity. The partial solution to avoid bore material pick-up was demonstrated on projectiles with plastic drive band.

    I commend editor for bringing up item related to underlaying principles, not just ‘cosmetics’ of guns. Guns are just one of many branches of mechanical engineering which share same concepts.

    • S O

      “One is that typically bullet is larger than bore; other the fact that speed of bullet in in excess of material capability to react and to copress.”

      Actually, almost no bullets travel through rifles barrels at more than 1,200 m/s, while speed of sound in steel is in excess of 3,000 m/s, almost 6,000 m/s in common steels.

      Only rail guns come close to such muzzle velocities, and AFAIK they aren’t supposed to have a contact between projectile and barrel.

  • delarrn

    What is the direction of travel in the picture? Is the top piece moving from left to right, with the lower metal being drawn to its underside? Or is the top piece moving from right to left and creating a bow wave in front of it? I’m guessing the former, and so created by the vacuum created by the wake of the upper piece (as opposed to friction in the latter case). In which case, the strengthening of the force would make sense when the speed is increased as it will generate increased forces in the vaccum.

  • gunslinger

    ohh… nerdy scienc-ey stuff. moar plx!

    seriously, neat article. I’d love to hear more of the science behind different firearm properties and such.

  • RocketScientist

    This behavior is one common to most poly-crystalline solids (and of course most amorphous solids). As you can see from the high-lighting most of the dislocations are occurring at the grain/layer boundaries where cohesive forces are weakest. This is actually a pretty good illustration of a concept that a lot of engineering students have at tough time grasping, that the shear strength of most common ductile engineering materials is significantly lower than their ultimate tensile (or obviously compressive) strengths, and that in most cases the failure mode, on the micro-scale, is shear induced, regardless of the macro loading conditions being tensile or compressive. I’m not really sure that the phenomenon described here is necessarily responsible for the ‘Blish principle’ but who knows. Friction is one of the fundamental areas of engineering we have a limited understanding of. Most of the math dealing with it is basically the result of empirical observation: we can closely predict the friction characteristic between glass and rubber because we’ve observed that interaction a million times and characterized it pretty accurately, not because of a complete understanding of the fundamental principles involved. Kinda like the mechanic who can rebuild a carburetor with the best of em, but couldn’t explain Bernoulli’s principle and doesn’t understand latent heat of vaporization. Steve, if you could post a link to (or e-mail me?) the source article I’d be interested to read.

    • Icchan

      Here’s the link I sent him. Physical Review Letters’ 9/7 issue has the paper published in it, and there’s an abstract as well as links to the paper itself at that site.

      I wasn’t sure as to the Blish lock, and I know that the Thompson’s use of it had borne no fruit, but he was also trying to adapt an apparent effect in a much larger weapon to something that much smaller. It may well be that, if he were right, the .45 action was insufficiently large compared to the energies and sizes of the warship rifles. Whatever benefit his idea had may have worked in the larger weapons, but didn’t scale to the smaller hand-held ones.

      I honestly don’t know – but even still, understanding how metals work under very high friction is certainly interesting to the community. Better bullets, better barrels, better operating parts could yet come of it; understanding is a resource that can never be overvalued.

    • S O

      The shear issue can easily be explained with tensile strength and some basic mechanical formulas. Just look at the Mohr circle.

      Besides, metallurgy knows that distortions of the grid require shear forces to move. Shear forces and their effects are all over the place in mechanical engineering. You learn this stuff en masse during your 2nd year in engineering studies.

      • RocketScientist

        Perhaps a more accurate statement might be: “You ARE SUPPOSED TO learn this stuff en masse during your 2nd year in engineering studies.”

        As someone who has taught and/or TA’d these courses (Mechanics of Solids, Machine Design, Adv. Materials Science) I am constantly surprised how many students who are good engineers have a hard time with this. Every semester I dreaded teaching Mohr’s Circle, Lame’s Ellipsoid and Cauchy’s Quadric. Guaranteed late nights of tutoring for a week or 2. Only thing worse was the one year I taught Applied Elasticity, which covers the DERIVATIONS of all the 3-D coordinate transformation matrices/eqn’s. Ugh. And to think I considered a career in academia 🙂

      • S O

        I was referring to engineering studies in Germany…

      • W

        rocket scientist, thank you for reigniting my university-induced post traumatic stress disorder 😉 Im going to go cry now.

        seriously. Those are things I havent heard from in a long, long time.

        Somebody has to teach them. Im too busy having fun shooting instead of bucking down on the science behind it.

    • RocketScientist

      After reading the paper, it’s actually pretty interesting. The really neat part is their modeling seems to indicate that the smaller-scale geometry changes (the small cracks/folds created, not the large bulge ahead of the indenter) which result in the creation of slivers/platelets is primarily a result of different grains of metal exhibiting different plasticity (while keeping identical elastic behavior). This variation in plasticity seems closely linked to grain size (no surprise, as grain size is a major determinant of most polycrystalline solid properties). I’m interested to see where this research goes. It’s applicability to firearms may be limited for the time being though… they could only create this phenomenon when there was a sharp angle between surfaces as shown. Luckily for us, most of the sliding contact in our toys is between two (mostly) parallel lubricated surfaces. The better understanding of metallurgy and surface properties this could lead to would certainly pay dividends down the road though.

      • Icchan

        It was the cracking and minor splits that caught my eye the most, as they seem to be the earliest onset of potential fail points. Such things would also increase friction via deformation, putting undue wear on parts – I’m wondering if there might be some way to mitigate that, an additive to the metal blend during the alloy-making process that would help prevent that kind of splitting. Make the steel slightly stretchier on the micro scale, to prevent a future macro fracture.

        Mostly I’d see this happening along slide and bolt raceways; the front face of the bolt’s carrier interacting with whatever guide it uses (as well as general friction) could be made to last longer – if. But hey, if we can see an effect, we can probably do something about it. So…what WOULD be good ways to keep the steel from microfracturing or filling them in afterward?

    • John Doe

      Who ever said all gun owners are dumb rednecks can shoot themselves now. +1 to you.

      • W

        on a serious note, the science behind firearms cannot be even imagined in the anti-gunner’s fiercest wet dream.

        I have a unwavering admiration and respect for the brains that actually crunch the numbers for firearms design and ballistics.

    • Zincorium

      Are you familiar with amorphous metals? And if so, do you think they’d alter the dynamic being discussed, due to the lack of crystalline grains?

  • Vincent

    Too bad there’s no bar saying ‘this many nanometers’ or something.

    • RocketScientist

      I went ahead and downloaded the full text of the paper (not just the abstract that was linked to in a previous comment) and they have size scales on the photo, it looks like it was cropped out of the ones above. According to the scale they provide, the distance from the point of the indenter to the top of the ‘mound’ of substrate is approximately 600 um (microns, aka 0.0006 meters, or about 0.024 in, or 600,000 nm since you asked about nanometers).

      • Icchan

        If the measurement’s correct (no doubt it is) then those fractures are on the order of three or four thousandths of an inch deep. Not insignificant, certainly.

        And for the layman and hobbyist gunsmith like me, this is really interesting stuff!

  • ThomasD

    My WAG is that efforts to reduce such behavior may instead result in increased galling.

  • RocketScientist

    If anyone is curious, the full text of the article can be found in .pdf format here:

    While the language will be a little obtuse to some, it’s really worth taking a look jsut for the pictures. They have some REALLLY cool SEM (Scanning electron microscope, 1200x power magnification) images of the surface showing the mounding, and platelet/slivers it leaves in its wake. really neat looking stuff. Or maybe I’m just a nerd 🙂

  • Mike Knox

    Jeez, you look at a potato long enough, you’ll think there’s an entire recosystem in there. Well, more or less there is.

    This is just micro fluid dynamics in static materials, just the same way as dragging your finger along some clay and the deformations behave like fluids. Except at different relative ratios of time and energy.

    As the celing one told us, you humans think too slow in some spectrums of knowlege..