Why can metals be scratched and develop cracks and yet not catastrophically fail? The reason is that metals can slide along slip planes to break the crack up. Take a look at the following video showing schematically how a crack in a metal becomes a blunted crack and a void, which can effectively stop the initial crack from growing and catastrophically failing (fracture). This is in contrast with the case of ceramics (in this case, glass). As we have mentioned before in this class, the atoms cannot easily slide past one another. This is due to the fact that in a ceramic we have predominately ionic bonding, which results in positive and negative ions alternating. So, if a row of atoms attempts to slide past the next row of atoms this would move positive ions towards positive ions and negative ions towards negative ions. That is typically too costly from a free energy point of view. Instead of stress caused by the crack being relieved by slipping, the crack keeps growing, usually to fracture, as shown in the following (1:13) animation.
Let's imagine that we have a surface crack in a metal. Metal atoms can easily slide along what are called slip planes to blunt the crack. So let's take a look at how this might occur. So I have a slip plane in a metal, and the crack as you see parts of the metal shift over stretching out rather than having the crack propagate all the way through the metal and thus having a fracture. So you end up with a blunted crack and a small void in this case.
Now let's look at a surface crack in a ceramic. Unlike in a metal, the atoms of the ceramic cannot move easily past one another. So instead of the material blunting the surface crack as occurs in metals, in a ceramic the stress from the crack ends up concentrated at the point of the crack. This can lead to the material fracturing as shown in this video.
So, can anything be done to prevent cracks in ceramics from growing out of control? One method is to put the surface of the glass under compressive stress (we will discuss this further in the next section). When you do this, you are building in a stress to help you with a property of the glass. This is different from annealing glass. In the case of annealed glass, the glass is heated, but not melted, and residual stress is allowed to release.