the mixed ionic-covalent bond

What actually holds a ceramic together? Atoms can share the glue of bonding electrons in two ways. In an ionic bond one atom hands electrons to another, and the two resulting charged ions (a positive cation and a negative anion) stick by electrical attraction, like magnets. In a covalent bond two atoms share a pair of electrons in the space between them, like two hands gripping the same rope. Ceramic bonds are almost always a blend of the two.

The blend is set by how greedy each atom is for electrons, its electronegativity. A big electronegativity difference (a metal handing electrons to oxygen, say) makes a mostly ionic bond; a small difference (silicon and carbon are close cousins) makes a mostly covalent one. Pauling's rough rule estimates the fraction of ionic character from that difference. So MgO is about 70 percent ionic, SiO2 roughly half and half, and SiC about 90 percent covalent, a genuine spectrum rather than two boxes. (The bonding field works this out with numbers; here we need only the idea.)

This one fact, strong directional localised bonds rather than a metal's free-electron sea, sets the whole personality of ceramics. Strong bonds are hard to break, so ceramics are hard, stiff, and refractory (high-melting). Electrons pinned to atoms cannot carry current, so ceramics insulate. And because the bonds are rigid and directional, atoms cannot slip past one another to accommodate a load, so instead of bending a ceramic cracks: it is brittle. Every later property in this subject traces back to this bond.