ceramics

Ceramics are the hard, heat-proof, often brittle materials: pottery and porcelain, brick and tile, the white insulators on power lines, and high-tech versions such as the alumina in spark plugs or the zirconia in dental crowns. Chemically they are compounds of a metal (or a metalloid like silicon) with a non-metal such as oxygen, nitrogen, or carbon - for example alumina (Al2O3), silica (SiO2), and silicon carbide (SiC).

Their bonding is ionic and/or covalent - strong, directional bonds that lock atoms firmly in place. That is why ceramics are hard, very stiff, keep their strength at high temperature, resist chemical attack, and act as electrical and thermal insulators. But those same rigid bonds do not let atoms slide (dislocations cannot move easily), so a ceramic cannot deform to relieve a stress - instead a tiny crack runs and it shatters. It is brittle. Alumina's modulus is around 380 GPa, stiffer than steel, yet a dropped porcelain cup still breaks.

Here is the honest part about strength. A ceramic's strength is dominated by its worst internal flaw - failure starts at the biggest crack or pore - so nominally identical parts break at very different loads. A single 'strength' number is therefore misleading; engineers use Weibull statistics to describe the scatter and design conservatively. Glasses are close cousins: the same oxide chemistry, but non-crystalline (a frozen-liquid structure).