Glass vs Aluminium

Jan 6, 2018

While I was writing about the Jet Black iPhone 7 in my review and comparing it with the current glass back 2017 iPhones have, I thought it would have been interesting to see the two materials from an engineering perspective.
I am to compare the Jet Black Aluminum back the iPhones 7 have and the glass featured in iPhones 8 and iPhone X. I've only had my hands on the 7, but I should be able to do some educated guesses on the other phones rear casing. What I'm focusing on it's just coloring techniques and surface hardness, if you are going to question impact resistance Aluminium is the clear winner: metals are maleable, glass is fragile (at least at room temperature).

Aluminium

Aluminium (or a Aluminum if you prefer, chemical symbol Al) is a metal, number 13 in the periodic table. Known for its lightness, it has a good advantage that shares with other metals: workability. Metals are great to be shaped. You can forge them, laminate them, drill them, carve, etc. They also get oxidised or corroded in a real world usage though, but here comes an advantage of Aluminium: if you force oxidation in a controlled way, Aluminium oxide acts as a protective layer for the virgin metal it has below. This is something you can't achieve with steel - for example - and if you control it properly, usually in galvanic baths, you can make Aluminium oxide grow in hollow columnar pattern. These microscopic pores can then be impregnated with a dye which becomes part of the material. For longtime Apple affecionados, this technique was the one used originally in iPod Mini to give them those nice colors.
Color is thus embedded in the metal, but this doesn't mean it can't be scratched off. It surely has gone deeper than a mere paint or surface deposition, but we are always talking about some tenths of millimiter here. Another advantage anodization gives is that the Aluminium oxide you make is not a metal anymore, it's ceramic! This means that even if hardness per se isn't increased much, abrasion resistance is.

Glass

While Aluminium is a metal, Glass is a ceramic amorphous solid. What this means is that glass doesn't feature that nice and ordered way of stacking atoms both metals and cristalline solids have. Glass has the structure of a frozen liquid and this gives a couple of advantages: the surface is chemically homogeneous* , it's transparent and the transition between liquid and solid phase isn't abrupt, but rather continuous. Liquid glass becomes more and more viscous as temperature drops until viscosity is so high you can consider it being a solid. So fare so good, but what are the advantages with respect to Al?
Glass can be made to be hard, much harder than Aluminium, and the way this is achieved is through atom swapping. I am going to dive a bit in solid state chemistry here: buckle up and follow me. Glass is a mesh of Silicon and Oxygen atoms linked together. Strong covalent bonds maintain these atoms close to one another and are the ones that make glass so rigid, it breaks if dropped. But if glass had only Si-O bonds, it would need to be melted at temperatures above 2000°C, so there's a trick. If you put single or double bonded atoms in glass - like Sodium (Na) or Calcium (Ca) - the network is interrupted in some areas and the melting temperature drops significantly below 1400°C. Other manufacturing aspects improve too, like lower viscosities and longer workability times which are all advantages when it comes to the forming of glass. All this known, here we begin to do some magic! Let's assume we have a glass containing Silicon and Oxygen as the main structure formers and Sodium as fondant. The structure is in equilibrium with no stresses. Now, let's put this glass in a molten Potassium (K) salt bath, what will happen? Because of concentration gradients Potassium actually enters the glass in its most outer layers taking the place of Sodium. And because K is larger that the Na atoms it swaps, the glass surface becomes a little compressed. This is what makes the whole difference. Because of this compressive status, glass surface has become "more tightly packed" and thus harder. It is called "Chemical tempering" and what this increased hardness yields is a much, much higher resistance to scratching. The way you can look at it, is that is glass who scratches others now, the hardest material has become it.

Final Words

All this said, there are few more details worth mentioning. As I wrote up there, Glass is infinitely more fragile than Aluminium, reason being that whereas glass has those strong yet brittle covalent bonds, Aluminium's atoms float in a "sea of electrons" which allows them to move and slide. For the same reason metals can be shaped, they can also accomodate big deformations before breaking.

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