Properties > Hard, Wear & Rust

When we look at any component, we need to know how it will react to the environment it is in.  This is a combination of the mechanical properties but also surface properties.  When you think about it, the surface of a component experiences a similar yet different life to the centre of a component.  In a jet engine, a component is exposed to very hot, dense air which can have fuel in it.  The outside of a component therefore has to deal with it in a way that doesn’t create a defect that could cause the component to fail.  This means we need the surface of a material to be able to handle slightly different conditions to the centre of a component.  To describe how well a material can handle this interface with their environment we have a set of specific terms.

 

Oxidation and Corrosion

Corrosion is where the elements in a material try to revert to their natural, more stable form i.e. into oxides, hydroxides, sulfides, etc.  Basically the elements want to return to the form they were in when they come out of the earth.  With something like steel, the iron wants to return to iron oxide and so we get rust.  This reaction happens as the Fe bonds with the oxygen (O).  In a jet engine we have very hot, dense air which means there is a lot of O and plenty of energy from the heat to drive the chemical process of corrosion.  If we then add in a bit of fuel this can drive other types of breakdown of the material.

Because metals and ceramics have different atomic bonds which give them their different properties, they also corrode differently.  Ceramics usually have ionic or covalent bonds.  Ionic is where atoms donate electrons to each other, and covalent is where they share electrons.  This means the electrons are pretty much all allocated and tied to atoms.  Metals on the other hand share their electrons between the group of atoms and so because these electrons are more free to move, it is easier for other elements like O to bond to them thus causing corrosion.  This is why ceramics are more corrosive and oxidation resistant in general (and also the reason behind properties like high strength, brittleness, etc.).

 

Friction, Abrasion & Wear

When we have a lot of very fast rotating objects like in a jet engine, how they handle the points of contact during rotation is important.  Each part of a jet engine that rotates is held in place by rollers, shafts, bearings, or a combination of all these things.  These move very fast and so generate a lot of heat, but also a lot of friction.  Friction is how much a material resists when another material rubs against it.  Friction happens as nothing is perfectly smooth and so some force is used to overcome these mini bumps.  If a material is abrasive then its little bumps will act like a grater and start to wear away the other material.

 

Hardness

When you hit something that is hard what happens?  I am guessing you imagined a scenario where you get hurt but the thing you hit is undamaged.  Hardness describes how much force a material can resist before permanently deforming.  Basically how hard would I have to poke something before I made a permanent indent (reminder: toughness is when we go from indent to failure).