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Today we’ll explore more about two of the three main types of materials that we use as engineers: metals and ceramics. We’ll discuss properties of metals, alloys, ceramics, clay, cement, and glass-ceramic materials. We’ll also look at the applications of our materials with microelectromechanical systems and accelerometers.

Correction: aluminum oxide is not rust; rust refers only to iron oxide. Aluminum does corrode and the process by which it corrodes has been referred to as 'rusting', even if it is technically not rusting unless the material is iron or steel.

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CC Kids:
Materials matter.

You wouldn’t want to make a stop sign out of candle wax or a bench out of chocolate. Neither would be able to hold up to the weather outside, and things would get quite sticky rather fast.

So if you’re going to create things and solve problems, you’re going to need the right supplies. The right materials. And two of the three main types of materials you’ll encounter as an engineer are metals and ceramics. [Theme Music] Engineering, like life, is about choices.

For example: some people decide to get braces to straighten their teeth. But choosing to wear braces isn’t as simple as a yes or a no: there are many different kinds to pick from. Two of the most common are braces with traditional metal brackets, and ceramic ones.

There’s not always an obviously correct choice: picking the right type of braces is all about weighing the pros and cons. That’s true whether you’re the person wearing them or the engineer designing them. So to make a good choice, you need to know more about each material.

As a budding engineer – and, like, a person in the world – you probably already have a decent idea of what metals are. They’re in our cars, our buildings, and many of the devices we use every day. And you may have already worked with ceramics, or at least have had them in your home.

Pottery is usually the first example that comes to mind, but you can also find ceramics in many other things, like bricks, windows, and even electronics. So metals and ceramics are definitely common. But working with them isn’t always easy.

Or simple. There’s a lot that has to happen before you get your final product! For example, imagine what it would actually take to make a set of brackets for braces.

Before making anything, you have to get the material that you’re using, which often starts out as ore that we mine. Ore is a type of naturally occurring rock that contains certain elements or minerals. Once you get ore, you can extract the elements you want from it.

Then, through processes like casting or forging, you can make that material into a more-usable bulk form. A common example is an ingot, which is usually a bar of a pure metal, like the bars of gold you see in old Westerns or in Skyrim. You can then take those bulk materials and form them into stock shapes, like sheets, tubes, or even powders.

That’s what you might find at your local hardware or home improvement store. But then, to get your final product, you need to turn those stock shapes into a preliminary version of your braces. Even once you have that, and make any necessary changes to the design, the braces still need to go through a cleaning process, be checked by quality control, and then packaged up before they can be used or sold.

That’s a lot of steps, and those are just the basic ones. In real life, there are even more. Not to mention the time and effort it took to pick the right materials, design the brackets, and make sure they fit well.

You wouldn’t want to get all the way to the end and then find out they’re just going to snap in half! That’s why it’s so important to understand the properties of the materials you’re using. Most metals share a few common properties.

They tend to conduct heat and electricity well. Many are malleable, or easy to shape without breaking, and ductile, or easy to stretch and deform. Put another way, materials that are made from metals are relatively strong and stiff, but also resistant to fracture.

This makes them good for building structures! Metals also typically reflect light well, which is what makes them shiny. That might not be the best if you don’t want your braces to be so noticeable.

It just depends on what’s most important to you – brackets that are less noticeable, or ones that are less likely to break. If you decide to use metal in a design, for braces or anything else, there are lots of different kinds to choose from. In fact, about three quarters of all known chemical elements are metals.

Common metals that we find here on Earth are aluminum – or aluminium as we say back home! – iron, potassium, and magnesium. But often, these metals aren’t used by themselves – they’re added to an alloy, a metallic material composed of more than one element, with at least one of those elements being a metal. Steel is a good example – it’s an iron alloy.

It’s primarily iron, but it also includes a bit of carbon mixed in. Sometimes there are other elements like chromium or nickel thrown in there too, depending on the type of steel. But why not just use the iron itself?

Why go through the trouble of making steel, or any alloy for that matter? It’s because alloys have different, and often better, properties than the individual elements they’re made from. For instance, steel tends to have better impact resistance than iron.

And it’s far more resistant to rust and corrosion – especially stainless steel. So you’d want to choose stainless steel over iron for projects where corrosion would be a problem, like for the exhaust pipe in a car or a replacement joint like an artificial hip. A big reason that metals and alloys are the way they are is because their chemical structure is more free-spirited.

The bonds within metals and metallic materials leave a large number of electrons that aren’t bound to particular atoms and can move from one atom to another. That’s known as the free-electron theory of metals. It’s like these free electrons can move as a group though the metal itself.

This explains why some metallic materials are so good at conducting heat and electricity – especially simpler metals. Their electrons can more easily carry that energy from one place in the metal to the other. So, metals have a lot of positives.

But they definitely aren’t right for everything, which is where something like ceramic might come in. Ceramics are compounds that aren’t completely metallic, but they don’t have carbon atoms bonded in a way that would make them organic, either. You can define glass materials the same way, so a lot of people include them in the same general category.

The main difference between glass and other ceramics is in their chemistry. Ceramics are usually crystalline, which means their molecules are more ordered, whereas the molecules in various types of glass are more random. But no matter how they’re organized, the molecules in both ceramics and glasses are made up of at least one metallic element, plus at least one non-metallic element.

They often include oxygen, nitrogen, or carbon. Common examples include aluminum oxide, better known as rust, and silicon carbide, which can form a very hard material that’s used to make everything from car brakes to bulletproof vests. And, of course, there are the more traditional ceramics that most people think of, like cement, or compounds made up of of clay materials, like porcelain.

In general, ceramics are relatively stiff and strong and often very hard. In this way, they’re similar to metals. But they’re also extremely brittle and can fracture very easily.

Which doesn’t sound too good if you’re gonna have them on your teeth for a few years. Ceramics are also usually insulators, so they don’t conduct heat and electricity well – although there are some that do. And they tend to be more resistant to extreme temperatures and harsh environments than metals and polymers are.

But while most ceramic materials have similar properties, there are differences. We could spend hours going through all the different types of ceramics out there, but there are a few major ones. Glass-ceramic materials, or materials whose chemistry falls between a glass and other ceramics, are relatively strong and don’t melt easily when heated.

They also tend to be biologically compatible. And they make good insulators, which is why they’re commonly used in electronic packaging applications. Plus, they can be completely transparent.

Perhaps one of the biggest appeals of glass-ceramic materials is how easily you can fabricate them. You can use conventional glass-forming techniques, and you end up with a product that’s nearly pore-free, which is great for handling liquids. So glass-ceramics are really useful!

But sometimes you want something a little simpler. Like clay. Clay is inexpensive and commonly found in nature.

You can often use it as-is after mining it, meaning that you don’t need to waste time or resources on refinement. And it’s super easy to work with. If you’ve ever taken a pottery class, you know that clay and water, when mixed in the proper proportions, forms a material that’s easy to shape.

Put that through a kiln, and you’ve got something great for mugs and dishes! For something a little sturdier, you might try using cement. It’s mainly used as a binder to help hold concrete together, which means you can find it everywhere from roads, dams, and buildings, to more decorative applications like patios and staircases.

One big benefit of using cement is that it can set at room temperature. Imagine if you had to break out some big flamethrower every time you wanted to pave a new road! So yeah, you could say cement’s properties make it an important part of our lives.

Ceramics and metals also show up in places you might not expect them, like with microelectromechanical systems, or MEMS. These are miniature smart systems that use tiny sensors to collect information by measuring things like mechanical, thermal, chemical, or optical properties. They then use the information to make decisions that tell devices called microactuators to do something, like move a fluid or redirect a beam of light.

A major application of MEMS these days is something you probably know pretty well: the accelerometer. For example, there’s one in your phone to detect its movement. And cars have accelerometers that trigger airbags to deploy during a crash.

Compared to older and more conventional airbag systems, ones that use MEMS are smaller, lighter, more reliable, and quite a bit cheaper to produce. And with something as important and life-saving as an airbag, those are big benefits! So, there’s a lot to consider when choosing between materials.

For a set of braces, metal brackets tend to be more durable and are better at correcting severely unaligned teeth, while ceramics tend to irritate the mouth less and aren’t as visible. And then there are your personal preferences, which also matter. Other factors, like time, resources, and design limitations, might also influence your designs in the field.

It’s all about finding materials with the properties that make them the best fit for want you want to accomplish. So today we learned more about two of the three main types of materials that we use as engineers: metals and ceramics. We saw the properties of metals and how we often use them as alloys in our designs.

Then we went over ceramics and the importance of clay, cement, and glass-ceramic materials. Finally, we talked about the applications of our materials with microelectromechanical systems and accelerometers. I’ll see you next time, when we’ll learn about the third main type of material that we’ll encounter as engineers: polymers.

Crash Course Engineering is produced in association with PBS Digital Studios. If you’d like to keep exploring our world, check out Eons and go on a journey through the history of life on Earth. From the dawn of life, to the “Age of Dinosaurs”, to the end of the most recent Ice Age.

Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people. And our amazing graphics team is Thought Cafe.