Characteristics of Ceramics

Abstract

Hardness of ceramics is normally expressed by the size of the indent made by pressing a diamond indenter against the ceramic surface. If the indenter has a square pyramidal shape, the Vickers hardness is expressed as P/S, where the load is P and the surface area of the indent is S, and the unit is GPa. The Knoop hardness is measured by a rhombic-based pyramidal indenter, but the Vickers hardness is more popular. When a solid is stressed (force/area), the solid is deformed in accordance with Hooke’s law, which is expressed as (stress) = (elasticity) × (deformation). Strictly speaking, an isotropic solid has two independent elastic moduli. For example, if tensile stress is applied to a bar, it elongates in the same direction as the applied force. The elasticity (Young’s modulus, E) is obtained by dividing the stress by the deformation (elongation/length). The other is Poisson’s ratio (ν) which is the ratio of shrinkage in the direction normal to the direction of the elongation (Fig. 5.1). The shear modulus (or modulus of rigidity) and the bulk modulus of elasticity that affect shearing stress and volume compression, respectively, are other elastic moduli, but they can be calculated using the Young’s modulus and the Poisson’s ratio. The elastic coefficient of a material is calculated by measuring the deformation by bending tests and tensile tests or by measuring sonic speed of a material. The values obtained are called the static (isothermal) elasticity and dynamic (adiabatic) elasticity, respectively. E and ν of ceramics are 100–400 GPa and about 0.2, respectively.