Why are some materials brittle? Materials such as glass and ceramics are brittle because their atoms are held together by strong bonds that cannot slide past each other.
If you have ever dropped a jar of something on a hard floor, you will know that glass is very brittle and shatters when impacted. Or, if you have tried to post some ceramic mugs without sufficient bubble wrap and the intended recipient has received a box of ceramic shards, you will know that ceramics are also very brittle. Some materials are not brittle. If you hit a metal object with a hammer, the metal will bend and deform, but it won’t shatter. If you hit a wooden bowl, the bowl might crack, but it won’t shatter. What makes some objects brittle and others are not?
All of the objects in the world are made of joined together atoms. The atoms connect with each other using chemical bonds. They can connect with a covalent bond, which means that both of the atoms share an electron. Or they can connect with an ionic bond, which means they are connected by the electrostatic force of positively charged and negatively charged atoms. Sometimes these bonds can be much stronger than other times.
All materials that are brittle are also very hard. That is because the bonds between the atoms are very strong and they are very rigid. If you compress the material, the bonds don’t move. This is why materials like glass and ceramics are extremely hard and do not compress. If you compress a material like metal or wood, the bonds are much less rigid and the molecules can move closer to each other, allowing the material to compress.
These hard objects are very strong in compression, but they are not strong when under tension. They don’t have any elasticity. If an object is stressed, there are three things that can happen. 1. The bonds between the molecules can break and the two halves of the object will separate. This is what happens with glass. 2. The bonds between the molecules separate, but the molecules can still slide over each other and the two halves of the object are deformed, but they don’t separate. This is what happens with plastics. 3. The bonds between the molecules stretch, but they don’t break and they return to their original position. This is what happens with elastic, which is why this is called elasticity.
When glass and ceramics are stressed, the bonds between the molecules sheer and the material breaks cleanly into pieces. You can see that it has broken cleanly with no deformity because you can put the broken pieces back together and they fit perfectly. The fact that they can be compressed but they cannot be stressed makes them brittle. Glass and ceramics shatter when you drop them on a hard wood floor, while wood or metal would not. This is also because they are brittle materials. When you drop a wooden bowl or a metal bowl on the floor, the stress travels through the bowl and the energy travels through the bonds between the molecules. The bonds stretch with the energy, but they are able to move back again after the energy has passed because these materials are somewhat elastic. When your glass bowl or ceramic bowl hit the hard floor, the energy is transferred through them, but it is more than the bonds between the molecules can withstand and they break apart. The pieces fracture cleanly and generally in straight lines. Any imperfections on the surface of the glass or the ceramic bowl can create areas where stress gathers and builds up, increasing the likelihood that cracks will appear there. Wood and metal obviously can break, but this usually happens if there is some kind of weakness already in the material. The energy will accumulate around that point and sometimes break it.
When brittle materials such as glass and ceramics break, they do so with an audible sound. This is because the force that is imparting the stress on them makes all of the pieces vibrate at slightly different frequencies. The energy in the material has to go somewhere and it is released as sound. This produces a lot of different notes, which is why breaking glass sounds like a cacophony of sound. The strength under compression and the way they distribute the force can sometimes be useful and both glass and ceramics are used in bulletproof armor. And this is what I learned today.
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Sources
https://www.bbc.co.uk/bitesize/guides/zjgmn39/revision/1
https://en.wikipedia.org/wiki/Brittleness
https://www.glazingrefurbishments.co.uk/blog/why-does-glass-break
Photo by Romain de Tirtoff: https://www.pexels.com/photo/shattered-glass-against-sky-15117972/