Some musings on materials science
I’m working on writing up a longer post on a energy conservation experiment we did earlier in the year, but along the way, I’ve come up with a question that’s left me somewhat puzzled. How do we explain the fact that a steel spring flexes and returns to its original position when pressed, while a glass plate, when compressed often shatters, and a piece of clay deforms.
This seems like a very basic question in Materials Science, but for me, the answer has always been “atomic structure”—molecules in steel are very tightly arranged in a crystalline structure, while those in glass is much more disordered, and clay is even more disordered. It wasn’t until some researching today that I even realized that modeling clay is made up of a mixture of minerals (mostly aluminum silicon oxides), oils and waxes.
I get that the large scale crystalline structure of steel can explain its flexibility—the bonds between atoms each act like tiny springs, and thanks to the regular arrangement of atoms in the material, it is easy to distribute a load on the surface by compressing each bond” by only a tiny amount (springs in series add).
In glass, it’s much more difficult, since there’s no large scale structure. Following a line below a load doesn’t follow a regular arrangement of atoms and bonds, instead there can be places where the bonding changes quite rapidly between regions, and thus the same support force might require a much larger deformation than a neighboring region. This deformation can sometimes be large enough to actually break bonds between molecules, and can lead to a cascading failure as suddenly there are now fewer neighbors bonded to share the stress, causing the stress to increase on each neighbor, possibly past the breaking point.
Clay seems very similar to glass, in that an amorphous arrangement of regions of minerals oils and waxes. The major difference between clay and glass seems to be that when one of these failures occurs that would lead to cracking, the faille can often be contained since the the oils and waxes are fairly fluid, and can allow regions under stress to slip past one another, deform, and redistribute the load in a way that doesn’t lead to a cascading failure or shattering like glass.
Once the load is removed in these three cases, the steel returns back to its original shape, so all of the energy stored in compressed bonds is released to other forms, thus explaining its highly elastic nature. In deforming the clay, some bonds end up permanently deformed due to the shifting of regions, so clay is very inelastic.
One other interesting thing I discovered is that it is possible to make glass more impact resistant, and the way this is accomplished is by replacing the smaller sodium ions in the glass with larger potassium ions. This stresses the top layers in the glass in such a way that they are better able to withstand compressive stress. I’m not quite sure I follow this—is part of what is being accomplished here is that we are making sure that the top layers have more organized large scale structure? I think this is true, but there must also be more to it. Or are the larger potassium atoms better able to act as springs—bonds between them must be stretched further in order to break?
I’d love any feedback to help me flesh out misunderstandings here.