The Character of a Material—Material Properties Profiles
When selecting a material for a specific application, one needs to consider it's entire character—it's material property profile. Each material has this: the full collection of all materials properties. It is this profile that determines the materials performance in a specific application. And, if you remember all the way back to the first chapter of this text, it is this profile that helps us classify a material. Materials of a specific class have similar property profiles!
First, there are two types of materials properties: intrinsic materials properties and extrinsic materials properties.
Intrinsic materials properties are inherent to the material itself. This would include a physical property like density or a mechanical property like tensile strength. These properties are directly dependent on the structure of the material — for example, if I knew the positions of the atoms I can compute the density of the crystal, full stop. Extrinsic materials properties are dependent on external factors. An example is the price of a material. This has little to do directly with the structure, and depends more on some external influence like scarcity, geopolitics, or perception. Extrinsic properties are not lesser than the inherent ones, indeed things like price may be the most important selection criteria!
Mechanical Properties
We've seen many of these properties already in this text: there are mechanical properties. A more complete list list can be found here. Those marked below with an asterisk we haven't discussed, but you are now at the point that you could extend your understanding by looking them up!
- Tensile strength
- Elastic modulus
- Hardness*
- Ductility
- Toughness
- Fatigue limit*
- Loss coefficient*
Electrical Properties
We've covered some of the electrical properties below (more here), such as:
- Electrical resistivity or conductivity
- Electron/hole mobility
- Carrier type and concentration
- Band gap
- Permittivity
Thermal Properties
We've only done a bit on thermal properties, but we may consider:
- Coefficient of thermal expansion
- Melting point
- Thermal diffusivity/conductivity
- Heat capacity
Other Properties and Extensions
And you may also consider many other properties, acoustic, radiological, chemical, magnetic, optical, etc. These are intrinsic properties.
You can also have multifunctional or coupled materials properties that are classified in two or more of the classes mentioned above! These include piezoelectric materials (which can produce a voltage under an applied force) or photoelasticity, which describes how optical properties change when a material is in a state of mechanical deformation.
Extensive properties are interesting themselves. We mentioned price above, but you need also consider environmental properties like $\ce{CO2}$ footprint (which can, of course, change with energy needed for production or energy source used in production), water usage footprint, or even perceived color — which depends on human perception.
Our task in materials selection is to determine which aspects of a materials property profile are important, and to select the best material for the job based on its character. In the next section, we'll explore how to do this using the Ashby Diagram.