Lecture # 8Structure and properties of ceramics

Application and processing of ceramics

Intended learning Outcomes:1- Structure of ceramic materials.2- Properties of ceramics and the crystal structure of them.3-Given the chemical formula for ceramic compound and the ionic radii of its component ions, predict the crystal structure.4- Impurities in ceramics.5-Mechanical properties of ceramics.6- Application and processing of ceramics.

CERAMIC CRYSTAL STRUCTURESceramics are composed of at least two elements, and often more, theircrystal structures are generally more complex than those for metals. The atomic bonding in these materials ranges from purely ionic to totally covalent; many ceramics exhibit a combination of these two bonding types, the degree of ionic character being dependent on the electronegativities of the atoms. Table 3.2 presents the percent ionic character for several common ceramic materials;

With regard to the first characteristic, the crystal must be electrically neutral; that is, all the cation positive charges must be balanced by an equal number of anion negative charges. The chemical formula of a compound indicates the ratio of cations to anions, or the composition that achieves this charge balance. For example, in calcium fluoride, each calcium ion has a 2 charge (Ca2), and associated with each fluorine ion is a single negative charge (F). Thus, there must be twice as many F as Ca2 ions, which is reflected in the chemical formula CaF2

The second criterion involves the sizes or ionic radii of the cations and anions,rC and rA, respectively. Because the metallic elements give up electrons whenionized, cations are ordinarily smaller than anions, and, consequently, the ratiorC/rA is less than unity

All in contact with that cation, as illustrated in Figure 3.4. The coordinationnumber (i.e., number of anion nearest neighbors for a cation) is related to thecation–anion radius ratio. For a specific coordination number, there is a critical orminimum rC/rA ratio for which this cation–anion contact is established (Figure 3.4),which ratio may be determined from pure geometrical considerations.

1-AX-TYPE CRYSTAL STRUCTURES

2-AmXp-TYPE CRYSTAL STRUCTURES

3-AmBnXp-TYPE CRYSTAL STRUCTURES

SILICATE CERAMICS:

Silicates are materials composed primarily of silicon and oxygen, the two mostabundant elements in the earth’s crust; consequently, the bulk of soils, rocks, clays,and sand come under the silicate classification. Rather than characterizing the crystalstructures of these materials in terms of unit cells, it is more convenient to usevarious arrangements of an SiO4tetrahedron (Figure 3.10). Each atom of siliconis bonded to four oxygen atoms, which are situated at the corners of the tetrahedron; the silicon atom is positioned at the center. Since this is the basic unit of the silicates, it is often treated as a negatively charged entity.

• Coordination # increases with Issue: How many anions can you arrange around a cation?

rcationranion

rcationranion

Coord #

< .155 .155-.225 .225-.414 .414-.732 .732-1.0

ZnS (zincblende)

NaCl (sodium chloride)

CsCl (cesium chloride)

2 3 4 6 8

Adapted from Table 12.2, Callister 6e.

Adapted from Fig. 12.2, Callister 6e.

Adapted from Fig. 12.3, Callister 6e.

Adapted from Fig. 12.4, Callister 6e.

COORDINATION # AND IONIC RADII

• Frenkel Defect --a cation is out of place.• Shottky Defect --a paired set of cation and anion vacancies.

Shottky Defect:

Frenkel Defect

Adapted from Fig. 13.20, Callister 5e. (Fig. 13.20 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.) See Fig. 12.21, Callister 6e.

DEFECTS IN CERAMIC STRUCTURES

• Impurities must also satisfy charge balance

• Ex: NaCl Na+ Cl-• Substitutional cation impurity

• Substitutional anion impurity

initial geometry Ca2+ impurity resulting geometry

Ca2+Na+

Na+Ca2+

cation vacancy

initial geometry O2- impurity

O2-

Cl-

anion vacancy

Cl-resulting geometry

IMPURITIES

• Room T behavior is usually elastic, with brittle failure.• 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials.

FL/2 L/2

= midpoint deflection

cross sectionR

bd

rect. circ.

• Determine elastic modulus according to:

E

F

L3

4bd3 F

L3

12R4rect. cross

section

circ. cross

section

Fx

linear-elastic behavior

F

slope =

Adapted from Fig. 12.29, Callister 6e.

MEASURING ELASTIC MODULUS

• 3-point bend test to measure room T strength.F

L/2 L/2cross section

Rb

d

rect. circ.location of max tension

• Flexural strength:

rect. fs m

fail 1.5FmaxLbd2

FmaxLR3

xFFmax

max

• Typ. values:Material fs(MPa) E(GPa)Si nitrideSi carbideAl oxideglass (soda)

700-1000550-860275-550

69

30043039069

Adapted from Fig. 12.29, Callister 6e.

Data from Table 12.5, Callister 6e.

MEASURING STRENGTH

• Elevated Temperature Tensile Test (T > 0.4 Tmelt).

• Generally,

time

creep test

xslope = ss = steady-state creep rate.

ssceramics ssmetals sspolymers. . .

MEASURING ELEVATED T RESPONSE

TYPES OF CERAMICS

FABRICATION OF CERAMIC MATERIALS