Biomaterials: An Introduction to Materials Science – Material Defects

Material defects can be due to one of two causes: Chemical Impurities, or Physical imperfections.  Physical imperfections can be further broken down into the following

• Point Defects
  • Vacancy atoms
  • Interstitial atoms
  • Substitutional atoms
• Line Defects
  • Dislocations
• Area Defects
  • Grain Boundaries

Area Defects

When we talk about materials, we are generally referring exclusively to solids.  As engineers, we are expected to create these solids in specific shapes for specific applications.  To be able to mold and control the shape of the solids, engineers will solidify a molten material.  This happens in three steps.

1. nuclei form
2. crystals grow from the nuclei
3. the crystals grow until they meet each other; forming the grain structure

Grain structures can become problematic at grain boundaries.  A grain is defined as a domain of matter that has the same structure as a single crystal. 

Grain boundaries are where crystals of different orientations meet.  Note that grain boundaries are still single phase interfaces because the only difference between the two crystals are the orientations.

An example of an area defect is seen below:

 

We can see that each grain contains the same overall pattern and crystal structure.  However, they each meet at a different angle and orientation, thus causing gaps in the structure.

Line Defects

Line defects are the result of dislocations.  Dislocations are exactly as they sound, and are the result of atoms in the crystal structure being dislocated, from their (wait for it….) original location.

The classic analogy is: Imagine a stack of papers.  Now insert a half sheet of paper.  The resulting stack is dislocated from the point of insertion and half of the stack will be higher than the other half.

There are two types of dislocations: edge and screw.

Edge Dislocation

The classic analogy I described to you earlier (stack of paper) was actually an example of an edge dislocation.  It’s really self explanatory

Screw Dislocation

A screw dislocation will result in a screw formation of the planes of atoms.  It’s a lot harder to visualize, but the best way I can explain it is to imagine a slinky.  When the slinky is completely compressed, we can think of it as a simple cylinder.  If you stretch it a little, then all planes in the slinky will dislocate a little bit in a circular manner causing a spiral staircase, screw like thing.

So in the image above, a boundary of the material is “cut” and pushed downward.  That downward pushing causes a spiral just like in the slinky.  The point of “cut” is referred to as the screw dislocation.

Dislocation Density: the total dislocation length / unit volume.

Dislocation deformations whether from screw or edge result in slip.  Slip is any sort of plastic deformation corresponding to the motion of a dislocation in response to a shear stress.

The slip plane is the plane of dislocation.  In the diagram above the slip plane is the middle of the tetris block.  The slip direction is the direction of movement that results from the dislocation. 

Point Defects

Point defects can be any one of the following:

• vacancy atoms
• interstitial atoms
• substitutional atoms

Vacancy Atoms

Vacancy atom point defects are due to a missing atom.  The planes of atoms surrounding the vacancy deform due to things like creating the lowest entropy.

Interstitial Atoms

Interstitial atoms point defects are when an atom inserts itself into the crystal structure of a material, and disrupts the structure.

Substitutional Atoms

A substituational point defect is due to an atom of a crystal structure being replaced by a completely different type of atom.  Thus there is an impurity in the material.  This different composition may result in a completely different structure.