Fundamentals of Ceramics

FUNDAMENTALS OF CERAMICS

 Ionicandcovalent bonding

Ceramics

Ceramics (ceramic materials) are non-metallic inorganic compounds formed from metallic (Al, Mg, Na, Ti, W) or semi-metallic (Si, B) and non- metallic (O, N, C) elements.

Atoms of the elements are held together in a ceramic structure by one of the following bonding mechanism: Ionic Bonding, Covalent Bonding, Mixed Bonding (Ionic-Covalent).

Most of ceramic materials have a mixed bonding structure with various ratios between Ionic and Covalent components. This ratio is dependent on the difference in the electronegativities of the elements and determines which of the bonding mechanisms is dominating ionic or covalent.

Electro negativity Ionic Bonding

Covalent Bonding

Ionic-Covalent (mixed) Bonding

Characterization of ceramics properties

Electro negativity

Electro negativity is an ability of atoms of the element to attract electrons of atoms of another element. Electronegativity is measured in a relative dimensionless unit (Pauling scale) varying in a range between 0.7 (francium) to

3.98 (fluorine).

Non-metallic elements are strongly electronegative. Metallic elements are characterized by low electro negativity or high electro positivity - ability of the element to lose electrons.

Ionic Bonding

Ionic bonding occurs between two elements with a large difference in their electro negativities (metallic and non-metallic), which become ions (negative and positive) as a result of transfer of the valence electron from the element with low electro negativity to the element with high electro negativity.

The typical example of a material with Ionic Bonding is sodium chloride (NaCl).

Electropositive sodium atom donates its valence electron to the electronegative chlorine atom, completing its outer electron level (eight electrons):

As a result of the electron transfer the sodium atom becomes a positively charged ion (cation) and the chlorine atom becomes a negatively charged ion (anion). The two ions attract to each other by Coulomb force, forming a compound (sodium chloride) with ionic bonding. Ionic bonding is non-directional.

Covalent Bonding

Covalent bonding occurs between two elements with low difference in their electronegativities (usually non-metallics), outer electrons of which are shared between the four neighboring atoms. Covalent Bonding is strongly directional.

Ionic-Covalent (mixed) Bonding

Ionic-covalent (mixed) bonding with various ratios of the two fractions (ionic and covalent) occurs in most of ceramic materials.

Degree of Ionic Bonding can be estimated from the following formula:

Where

I.F. - fraction of ionic bonding;

ΔE - difference in the electro negativities of the elements.

Characterization of ceramics properties

In contrast to metallic bonding neither ionic nor covalent bonding form free electrons, therefore ceramic materials have very low electric conductivity and thermal conductivity. Since both ionic and covalent bonds are stronger than metallic bond, ceramic materials are stronger and harder than metals.

Strength of ionic and covalent bonds also determines high melting point, modulus of elasticity (rigidity), temperature and chemical stability of ceramic materials. Motion of dislocations through a ceramic structure is impeded therefore ceramics are generally brittle that limits their use as structural materials.

Ceramics may have either crystalline or amorphous structure. There are also ceramic materials, consisting of two constituents: crystalline and amorphous.

Structure of ceramic materials

The following factors affect structure of ceramics:

Balance of electrical charges of anions and cations

Radius Ratio (rc/ra)

Where

rc - radius of cation;

ra - radius of anion.

Radius Ratio determines Coordination Number (CN)- the maximum number of anion nearest neighbors for a cation.The anion neighbors do not touch each other.

rc/ra = 0.225…0.414(SiO2) CN = 4

rc/ra = 0.414…0.732(SnO2, PbO2) CN = 6

rc/ra = 0.732…1.0(ThO2) CN = 8

Covalent bonding component, which tends to form tetrahedral coordination, may affect the Coordination Number.

•Ions structure are  packed  with  maximum  density.

Ceramic structures are classified and designated according to the pattern structures of several natural minerals:

Tetrahedral silica block (SiO4-4) may form various silicate structures:

Island and DoubleIsland Silicates

Single or two silica blocks, containing other cations, form Island (olivine) or Double Island Silicates (hemimorphite).

Ring and Chain Structures

Several (3,4,5,6) silica units join each other, forming a chain (orthopyroxenes, clinopyroxenes, asbestos) or closed ring (beryl).

Sheet (layer) structure

Silica units connect to each other, forming infinite sheet (micas, serpentine, chlorite,

talc).

Framework silicate

Quartz, cristobalite, and tridymite minerals are based on the framework silicate structure.

Silicates exist in two forms: crystalline and amorphous (glasses).

General classification ofceramics

There are various classification systems of ceramic materials, which may be attributed to one of two principal categories: application base system or composition base system.

Application base classification of ceramic materials

Tribology of ceramics

Characteristics of friction and wear of a ceramic material are determined by a combination of its bulk microstructure parameters, surface conditions and environmental factors (temperature, atmosphere pressure, etc.), lubrication conditions.

Effect of microstructure on tribological properties of ceramics

o Parameters of microstructure and their influence on friction and wear of ceramics o Manufacturing processes forming microstructure of ceramics

 Effect of surface characteristics on tribological properties of ceramics o Surface characteristics

o Methods of modification of ceramic surfaces  Effect of lubrication on tribological properties of ceramics