Define porosity and explain its effect on the mechanical properties of ceramics.

• A. Porosity increases strength and stiffness.
• B. Porosity has no effect on density or fracture toughness.
• C. Porosity reduces strength, stiffness, and density, and can lower fracture toughness.
• D. Porosity only affects thermal properties.

Answer: (C)

Porosity is the fraction of a ceramic’s volume that consists of voids or pores. In a ceramic, the solid skeleton carries most of the load, so introducing pores reduces the amount of material available to bear stress. That lowers stiffness (the effective modulus) because the average resistance to deformation drops when voids interrupt the load path. It also reduces strength for the same reason: pores concentrate stress around their edges and shorten the continuous ligament that must carry the load, making crack initiation and propagation easier. Since voids replace dense material with air, the overall density decreases. Fracture toughness, the resistance to crack growth, can also drop because cracks can more easily navigate around or through pores, decreasing the energy required to extend a crack. All together, porosity tends to reduce strength, stiffness, and density, and can lower fracture toughness, which is why that option best fits.