Notes On Material Science
A High Level Overview of the Most Important Materials Used in Everyday Engineering
The other day, I was reading about the Space Shuttle Challenger developed by NASA in the early 1980s. It was a symbol of America's technological prowess. This reusable spacecraft had flown nine successful missions, carrying satellites, conducting crucial scientific experiments, and inspiring a nation. Yet, on January 28th, 1986, tragedy struck. A mere 73 seconds after liftoff, with millions watching in horror, the Challenger disintegrated, consumed by fire. All seven crew members, including school teacher Christa McAuliffe, were lost.
The cause for this catastrophe was a benign rubber O-ring. The circular seal, designed to prevent hot gases from leaking within the shuttle's solid rocket boosters, had lost its elasticity due to unusually cold weather.
During launch, the compromised seal gave way, allowing a jet of flame to escape. This flame impinged directly on the external fuel tank, causing a catastrophic structural failure that doomed the Challenger and its crew.
To me, it was a chilling reminder of the devastating consequences that can result from an incomplete understanding of materials. The O-ring, composed of a rubber-like material, was not intended to function at such low temperatures. Its failure shows how crucial it is to grasp not only a material's typical behaviour but also how those properties change under diverse conditions.
As an engineer, I have spent countless hours designing systems and components. But being heavily involved on the software side of things has afforded me little to no knowledge of the substances from which they were made. How could I truly innovate or optimise my designs when I lack a grasp of the fundamental building blocks themselves?
This epiphany ignited the article that I have written for this week. I want to get a deeper understanding of materials and share it with my readers, most of whom are engineers themselves.
I realise that given the constraints of an ‘article’, this shall be a high level discussion and while this won't make us all materials scientists, it will equip us to think more critically about the metals, alloys, superalloys, polymers, and ceramics at our disposal. Afterall, a base level understanding is better than having no understanding at all.
Understanding on an Atomic Level
If we want to grasp the true nature of a material – its strengths, weaknesses, and potential behaviours – we need to understand them on an atomic level first. It's at this minuscule level that the fundamental properties of a material emerge.
The way individual bricks are stacked and mortared together determines the overall characteristics of a wall. Similarly, the arrangement and bonding of atoms within a material dictate its macroscopic properties, i.e., strength, flexibility, reactivity, conductivity...the list goes on.
For example, consider iron. Pure iron is relatively soft and malleable. Its atoms form a crystal lattice structure where they can easily slide past one another. But introduce a tiny amount of carbon, and suddenly you have steel. Those carbon atoms nestle within the gaps of the iron lattice, hindering the movement of iron atoms, and drastically increasing hardness and strength.
By studying materials under the atomic microscope (both literally and figuratively), we can unlock a lot of information. We can see how atoms are arranged, whether in orderly lattices or more random structures. This influences traits like strength and ductility. The types of chemical bonds between atoms, strong or weak, directional or non-directional, determine things like a material's melting point, reactivity, and electrical conductivity.
Conversely, even the tiniest of imperfections in the atomic arrangement can profoundly impact a material's behaviour, dictating its susceptibility to cracks, corrosion, or other failures. Understanding the atomic structure and the characteristics we observe macroscopically allows us to tailor materials for specific needs.
What is a Bond?
Every material has constituent atoms that are bound together. This binding force, known as a chemical bond, dictates the material's most fundamental properties, from its strength and hardness to its ability to conduct electricity or resist corrosion. By understanding these bonds, we gain the power to predict material behaviour and engineer structures with specific functionalities.
Metallic Bonding
Imagine a vibrant social gathering, where instead of clinging to just one or two close friends, individuals move freely, interacting with everyone around them. That's the essence of metallic bonding. In metals, the outermost electrons, known as valence electrons, shed their allegiance to any single atom and instead roam freely throughout the entire metal lattice. This creates a delocalized "sea" of electrons with a positive charge from the remaining metal ions (atoms that have lost electrons) balanced by the negative charge of the electron sea.
The key characteristic of metallic bonding is its non-directional nature. Unlike some other bonds, metallic bonds aren't formed by the precise overlap of specific electron orbitals between atoms.
Because the metallic bond isn't dependent on a specific arrangement of atoms, the electron sea can readily adjust as metal atoms slide past one another. This allows metals to be hammered into sheets (malleability) or drawn into thin wires (ductility). The freely moving electrons in the electron sea act like tiny highways for electrical current, making metals excellent conductors. Those same mobile electrons also efficiently transfer heat throughout the metal structure.
Ceramic Bonding
Ceramics take a decidedly different approach to bonding, often utilising a combination of two powerful forces, i.e., ionic and covalent bonding.
In ionic bonding, one atom donates an electron to another, creating oppositely charged ions, i.e., a positively charged cation (the electron donor) and a negatively charged anion (the electron acceptor). These opposite charges, like tiny magnets, pull strongly towards each other, forming a stable ionic bond.
In covalent bonding, atoms share pairs of electrons, forming strong, directional bonds. These shared electrons occupy specific regions of space called molecular orbitals, creating a very defined and geometrically arranged bond.
The strong, directional nature of covalent bonds makes ceramic material hard and unmalleable. The ionic component can make ceramics brittle. If enough stress is applied, the ionic bonds can break, leading to shattering rather than bending.
Conversely, a strong electrostatic attraction in ionic bonds and the defined sharing of electrons in covalent bonds make ceramics highly resistant to chemical attack or corrosion. In most ceramics, the tightly bound electrons are not readily mobile. This translates to poor electrical and thermal conductivity.
Polymers
Polymers consist of artificially designed, long, chain-like molecules known as macromolecules. They are built by linking together smaller repeating units called monomers, with the bonds between these monomers typically being covalent. The way that the individual polymer chains interact with each other plays a crucial role in defining the material's properties.
Weaker intermolecular forces allow polymer chains to slide past one another easily, resulting in flexible and bendable materials like plastics. Conversely, introducing cross-links between chains strengthens these intermolecular interactions, leading to stiffer and more robust polymers.
The weaker intermolecular forces compared to the strong covalent bonds within the chain mean that less thermal energy is typically required to disrupt the order between chains and melt or soften a polymer.
Uses of Metals
Metals have been the backbone of human civilization for millennia, and for good reason. These materials offer a compelling blend of strength, versatility, and conductivity that set them apart. The sheer diversity of metals makes them suitable for countless applications. Let's look at some major categories.
Ferrous Metals
Iron: The foundation of steel. Its abundance and strength make it ubiquitous in construction, tools, and machinery.
Steel: A hugely diverse category of iron-carbon alloys. By controlling the carbon content and adding other elements, we can produce steels with a vast spectrum of strengths, hardnesses, and resistances to corrosion.
Non-Ferrous Metals
Aluminium: Lightweight, corrosion-resistant, and ductile. Used in aircraft, packaging, and countless consumer goods.
Copper: Superb electrical and thermal conductor. Finds widespread use in electrical infrastructure, plumbing, and electronics.
Titanium: High strength-to-weight ratio and excellent corrosion resistance. Used where those properties are worth the high cost, like aerospace components, medical implants, and performance equipment. I will discuss Titanium at length in the following sections.
Alloys
Alloys are the backbone of modern engineering. They are formed by mixing two or more elements, at least one of which must be a metal. By carefully controlling the composition and processing of alloys, we can create new materials with properties far superior to those of their individual components. Now let’s have a look at some of the most popular types of alloys used in the industry.
Carbon Steel Alloys
The most commonly used in the industry, carbon steels derive their strength and hardness primarily by adding different amounts of carbon to iron.
Low Carbon Steels: Relatively soft and ductile, easily shaped or welded. Used in automotive bodies, structural beams, and countless everyday applications.
Medium Carbon Steels: Stronger and harder, with good wear resistance. Found in gears, axles, crankshafts, and similar machine components.
High Carbon Steels: Very hard and strong but brittle. Primarily used for cutting tools, springs, and high-strength wires.
Stainless Steel Alloys
Renowned for their corrosion resistance, stainless steels contain a significant amount of chromium (usually above 10.5%). Chromium forms a tenacious oxide layer on the surface that protects the steel. Different grades of stainless steel add other elements like nickel or molybdenum to further tailor their properties.
Austenitic: The most common type. Excellent ductility and corrosion resistance. Used in kitchenware, pipes, medical equipment, and chemical processing.
Martensitic: Very hard and strong, but less corrosion-resistant than austenitic. Ideal for cutlery, tools, and surgical instruments.
Aluminium Alloys
Aluminium on its own is relatively soft. But when alloyed with elements like silicon, copper, or magnesium, it becomes versatile and lightweight. Different series of aluminium alloys offer unique balances of properties.
Aluminium-Silicon Alloys: Excellent castability and corrosion resistance. Used in engine blocks, automotive parts, and other cast components.
Aluminium-Copper Alloys: High strength-to-weight ratio and machinability. Widely used in aerospace and structural applications.
Aluminium-Magnesium Alloys: Superb corrosion resistance and weldability. Common in marine applications, chemical storage, and architectural uses where corrosion is a major concern.
Superalloys
These specialised materials, often based on nickel, cobalt, or iron, maintain their strength and properties at incredibly high temperatures. They are indispensable for the hottest parts of jet engines and power generation turbines.
Titanium
I felt the need to dedicate an entire section to Titanium as it has rightfully earned a reputation as a wonder metal in many engineering circles. Let's explore its standout characteristics and the reasons behind its extensive use in demanding applications.
Key Properties
Titanium is renowned for possessing an exceptionally high strength-to-weight ratio. Compared to steel, it offers a similar strength while being significantly lighter. This makes it a material of choice when weight is a critical factor.
Titanium naturally forms a thin but incredibly tenacious oxide layer on its surface, granting it resistance to seawater, many acids, and other corrosive environments. This resistance makes it a favoured material in harsh environments where corrosion is a major concern.
Titanium is also one of the most biocompatible metals, i.e., it doesn't cause adverse reactions within the human body. It finds extensive use in medical implants like joint replacements and dental implants.
Titanium maintains its strength and functionality at temperatures where many other metals would weaken or creep. This is valuable in high-temperature applications within aerospace and chemical processing.
Applications
Where weight reduction and performance matter most, titanium shines.
Aerospace: From aircraft frames to jet engine components, titanium's strength-to-weight ratio and temperature resistance are unmatched.
Marine Applications: Given its excellent seawater corrosion resistance, titanium is used for ship propellers, rigging, and components critical for long-term reliability.
Medical Implants: Titanium's biocompatibility and its ability to integrate with bone make it ideal for orthopaedic and dental implants.
Sporting Goods: Where lightness and strength are paramount, titanium is often found in high-end bicycle frames, golf clubs, and other sports equipment.
Problems
Just like with any great material, there are trade offs to consider. Titanium does come with some downsides. For one, the cost. Titanium's extraction and processing are complex, making it a relatively expensive material compared to common metals like steel. Titanium can also be somewhat difficult to machine due to its hardness and tendency to gall (form buildups on cutting tools). Specialised machining techniques are often needed.
Alumina (Aluminum Oxide)
In the final part of the article, I would like to discuss some of the common ceramics. The trouble with ceramics is that they are less used in the technology industry due to their brittle, unmalleable state. But that doesn’t mean they don’t have any applications. To begin with, let’s talk about Alumina, scientifically known as aluminium oxide (Al₂O₃), is one of the most important technical ceramics in the world. It possesses a remarkable combination of properties that make it suitable for a variety of applications.
Key Properties
Alumina is renowned for its incredible hardness, making it highly resistant to scratching and wear. This quality makes it indispensable in abrasive and wear-resistant applications. It is also exceptionally resistant to attack from acids, alkalis, and most solvents. This chemical stability makes it suitable in demanding environments where other materials would quickly degrade.
Alumina has very high electrical resistivity, making it a superb insulator even at elevated temperatures. It dissipates heat surprisingly well for a ceramic, making it useful in applications where heat management is essential.
Many ceramics shatter if subjected to rapid temperature changes. Alumina, however, possesses surprisingly good thermal shock resistance, making it a valuable choice for use in high-temperature settings.
Applications
Alumina's hardness makes it ideal as a component in cutting tools, grinding wheels, sandpaper, and countless wear-resistant parts in industrial machinery. The electrical insulation properties of alumina, combined with its thermal resilience, make it perfect for the insulator portion within spark plugs. Alumina's chemical inertness and high-temperature stability make it a go-to for crucibles, lab equipment, and other components subjected to harsh chemical or thermal environments.
It also serves as a base (substrate) for integrated circuits and other electronic components due to its electrical insulation and heat dissipation properties. And as discussed before, certain grades of alumina, with exceptional purity, are used in hip replacements and dental implants due to their hardness, wear resistance, and biocompatibility.
Silicon Nitride (Si₃N₄)
Finally, let’s talk about Silicon nitride, an advanced ceramic material gaining increasing recognition across industries due to its impressive combination of toughness, heat resistance, and wear resistance.
Key Properties
Silicon nitride boasts some of the highest strength-to-weight ratios among ceramics and excellent fracture toughness, i.e., the ability to resist crack growth. It maintains its strength and shape across a wide temperature range and can withstand temperatures that would cause many metals or other ceramics to falter.
Its extreme hardness and toughness give silicon nitride exceptional resistance to wear and abrasion. Like many ceramics, silicon nitride resists attack from most solvents, acids, and corrosive environments. Silicon nitride handles rapid temperature changes very well, expanding its suitability for high-performance environments.
Applications
Silicon nitride's unique blend of properties makes it invaluable in a variety of fields.
Engine Components: High-performance bearings, turbocharger rotors, and other engine parts benefit from silicon nitride's combination of wear resistance, low density, and high-temperature strength.
Cutting Tools: Silicon nitride inserts in cutting tools provide enhanced wear resistance and allow for machining at higher speeds, especially when working with cast iron and other hard materials.
Aerospace: Silicon nitride's high-temperature capabilities and strength-to-weight ratio make it a material of interest for aerospace applications, from propulsion components to structural parts.
Molten Metal Handling: Silicon nitride excels in crucibles and other components designed to handle high-temperature molten metals thanks to its thermal and chemical stability.
Armour Systems: The hardness and impact resistance of certain grades of silicon nitride make it a viable component in advanced armour systems.
Note
Although it's a great emergent material, one of the main hurdles with silicon nitride is its complex manufacturing process. Producing dense and high-purity parts can be challenging and costly. However, advancements in processing techniques are gradually making silicon nitride more accessible for wider use.
Vibranium
Finally, I would like to end this article by discussing Vibranium, the material that Captain America’s shield is made of. It feels like a crime to discuss material science without discussing Vibranium!
Properties
Unparalleled Energy Absorption: Vibranium famously absorbs kinetic energy. Impacts, vibrations, and even energy blasts are absorbed into the material, making it nearly indestructible.
Energy Redirection: Vibranium can not only absorb energy but also redirect it in amplified blasts or other ways.
Lightweight Yet Immensely Strong: Despite being incredibly strong and resilient, Vibranium is surprisingly lightweight.
Unique Energy Signature: Vibranium possesses a distinct energy signature or resonance that enables advanced technological applications.
Applications
The real-world applications of vibranium are staggering!
Revolutionary Armour: Shields, body armour, and vehicle protection capable of withstanding immense forces.
Advanced Prosthetics: Lightweight, super-strong, and impact-absorbing prosthetics.
Energy Storage & Generation: Vibranium could store and release enormous amounts of energy, revolutionising energy sectors.
Aerospace Innovations: Lightweight structural components for aircraft and spacecraft, enabling unprecedented design freedoms.
Problems
Cool as it is, Vibranium does face some major challenges. It is FICTIONAL for one.
But keeping that banality aside, Vibranium is also exceedingly rare. It poses immense challenges in terms of obtaining and processing. As far as we know it is only available in Wakanda. And Wakanda is, well, NOT REAL.
There is also the finer point to consider that Vibranium's properties seem to contradict fundamental laws of physics as we understand them. The mechanisms for absorbing energy with such efficiency and without heating up are otherworldly!
Importance
You may wonder why I considered adding fictional material to a rather serious article. But there is a reason for it, aside from hyperbole and humour.
Vibranium serves as a reminder of the boundless potential of materials science. While we may never create a substance that completely defies reality, the pursuit of materials with extraordinary properties drives innovation and pushes the boundaries of what we believe possible.
Let me know if you'd like to imagine the technological impact of Vibranium-like materials on specific fields or how its existence might reshape whole industries! I would be happy to explore this in-depth.
Love the inclusion of vibranium at the end! 😁
Thanks. That was my favourite part as well!