Question

1. Describe in detail structural imperfections commonly present in bulk ceramics.

Answer 1

Types of imperfections in solids

• Point defects exist by definition as a point (0 – dimensional) and include vacancies, interstitial atoms, and substitutional impurity atoms. These point defects are shown in the two figures below and will be discussed further in the reading.
• One-dimensional or linear defects are called dislocations.

• Bulk defects

  Three-dimensional macroscopic or bulk defects, such as pores, cracks, or inclusions. Voids — small regions where there are no atoms, and which can be thought of as clusters of vacancies. Impurities can cluster together to form small regions of a different phase. These are often called precipitates.
• Planar Defects or 2-D defects A Planar Defect is a discontinuity of the perfect crystal structure across a plane. Grain Boundaries. A Grain Boundary is a general planar defect that separates regions of different crystalline orientation (i.e. grains) within a polycrystalline solid.

Imperfections present in bulk ceramics

The relevant imperfection determining the mechanical properties of ceramics are point defects, or dislocations, or both.

The periodic nature of crystalline materials can be interrupted by imperfections. The major point defects considered in the chapter are vacancies and interstitials, which are responsible for some observed phenomena via diffusional exchange with atoms in their vicinity.

One such process relates to climb which is an essential process in creep phenomena. Edge dislocations are involved in the climb process which occurs by leaving the glide plane, either in the positive, or the negative direction. Point defect-atom exchange by diffusion is the basic mechanism. Although one can talk about point defect hardening, the important defects that determine the mechanical properties of materials are line defects, commonly known as dislocations (edge or screw character). Their presence in crystals is essential, because of the orders of difference between the theoretical and actual strength of materials. The presence of dislocations makes deformation easier by the application of smaller stress than would be required in their absence. Conservative motion of dislocations occurs by slip, whereas non-conservative motion is associated with climb. The strengthening of material is a consequence of retarding the motion of dislocations, either by their intersection, or by particles of a second phase or by grain boundaries. Closely associated with dislocations are partial dislocations which usually produce stacking faults when they form. Basically stacking faults are surface defects. The association of partial dislocations and stacking faults define the extended dislocation, which makes cross slip more difficult, thus strengthening the material against deformation.