Notes of Ceramics

Engr 270 AA -- Materials Science CERAMICS

Ceramic materials are inorganic nonmetallic materials which consist of metallic and nonmetallic elements bonded primarily by ionic and/or covalent bonds. Traditional ceramics - clay is the primary raw material Engineering ceramics - typically consist of pure or nearly pure compounds such as aluminum oxide, silicon carbide and silicon nitride.

Typical Properties: 1. good thermal and electrical insulators -- due to absence of “free” electrons 2. many ceramics are transparent -- due to absence of “free” electrons that absorb photons 3. high hardness and brittleness; low ductility and toughness 4. weak in tension and strong in compression -- due to brittleness 5. can undergo loss of strength over time -- static fatigue 6. can crack/fracture due to sudden changes in temperature (thermal shock)

Applications: 1. bricks, tiles 2. porcelain used in electronics 3. glasses and lenses 4. refractories -- high temperature resistant materials 5. Silicon carbide (SiC) used in high temperature gas turbine engines 6. protective coating -- enamels

Main Categories: A. Crystalline ceramics -- with crystalline atomic structure B. Glasses -- noncrystalline C. Glass-Ceramics -- initially formed as glasses and then recrystallized

Crystalline Ceramics: 1) Silicates -- based on SiO2 - Ex. cement, pottery, clay, porcelain, bricks - Si and O are very plentiful (together make up 75% of the earth’s crust) making silicates inexpensive. 2) Nonsilicate oxide ceramics - Ex. alumina (Al2O3) - used for electronic packaging magnesia (MgO) - used as refractory uranium dioxide (UO2) - nuclear fuel zirconia dioxide (ZrO2) - metal substitute 3) Nonoxides - Ex. silicon carbide, SiC - used as an abrasive on tools and also as reinforcing fiber in composite materials silicon nitride, Si3N4 - used as a metal substitute in gas turbine engines. With ceramic engine parts, the engine can run at much higher temperature and with better efficiency. However, higher temperature lubricants are needed.

Crystal Structure: - at least two elements --> more complex than those of metals

1 Coordination number -- the number of equidistant neighbors to an atom or ion in a unit cell crystal structure; ex., in NaCl, coord. # = 6 since equidistant Cl- anions surround a central Na+ cation. Radius ratio -- ratio of the radius of the central cation to that of the surrounding anions; determines coordination number and coordination geometry (Table 12.1). Stable ceramic configurations -- when anions surrounding a cation are all in contact with the cation. Critical or minimum radius ratio -- when all surrounding anions just touch each other and the central cation; determined by pure geometric considerations

Common Structures a) AX-Type: rock salt structure, cesium chloride, zinc blende b) AmXp-Type: fluorite c) AmBnXp-Type: perovskite

Glasses:

Noncrystalline silicates (SiO2 is the main component) containing other oxides, notably CaO, Na2O, K2O, and Al2O3. Constituents are heated to an elevated temperature above which melting occurs, and then cooled to the rigid condition without crystallization. Glass Transition Temperature -- the center of the temperature range in which a non-crystalline solid changes from being glass-brittle to being viscous. Glasses have special properties such as transparency, hardness at room temperature,, and excellent resistance to most environments which make them important for many engineering applications.

Glass-Ceramics: Glasses with an overall crystalline structure Fine-grained material produced by proper high-temperature heat treatment Properties: - low coefficient of thermal expansion (resistant to thermal shock) - relatively high mechanical strength (fewer pores) - higher thermal conductivities

Mechanical Properties of Ceramics: 1. Brittleness Brittleness of ceramics is due to their ionic and covalent chemical bonds. Covalent Bonds are very strong directional bonds. Due to the directional nature of its bonds (involving sharing of electrons), the material can only change shape by breaking bonds. This leads to brittle fracture due to the separation of electron-pair bonds without their subsequent reformation. Ionic Bonds result in limited number of slip planes. Family of planes that result in ions of the same charge being in contact tend to separate and hence do not allow slip. Cracking occurs at boundaries and subsequent brittle fracture occurs. 2. Strength Weak in tension and relatively strong in compression! Failure of most ceramic materials at room temperature usually originates at the largest flaw. Mechanical failure occurs mainly from structural defects (surface cracks, voids, inclusions, and large grains) produced during processing.

2 Ductile materials have the ability to deform plastically in the vicinity of a crack tip to redistribute high stress distributions. Thus once cracks starts to propagate, unstable growth happens rapidly. ---> Tensile strength of ceramic material is LOW. Compressive stresses tend to close (not open) the cracks and consequently does not diminish the inherent strength of the material. Hence, ceramics are stronger in compression than in tension. 3. Toughness

Fracture toughness: KIC = Y σf π a , where Y = dimensionless geometric factor

σf = fracture stress a = half the size of the largest internal flaw. KIC is low for ceramic materials. 4. Static Fatigue -- Loss off strength over time at room temperature without cyclic loading ; due to chemical degradation -- Water attacks SiO2 at the surface, creating cracks. 5. Creep - deformation with time Sliding of adjacent grains along the grain boundary. Ex. Old windows - glass flows, the thickness of glass is greater at the bottom of the pane than at the top.

Thermal Properties of Ceramics: 1. Low thermal conductivities and are good thermal insulators - due to the strong ionic-covalent bonding; High heat resistance and high melting points: used as refractories - materials that resist the action of hot environments. 2. Thermal Shock Resistance Heating or cooling results in an internal temperature distribution. Thermal stresses may be established as a result of thermal gradients. For ductile materials, thermally induced stresses may be relieved by plastic deformation. For brittle materials, rapid cooling may cause thermal shock since the induced stresses are tensile.

Thermal Shock Resistance parameter TSR:

σf k TSR ≅ E α

where σf = fracture stress k = thermal conductivity E = Young’s modulus of elasticity α = coefficient of thermal expansion

Maybe improved by modifying properties; ex. ordinary bottle glass has a coefficient of -6 thermal expansion of approximately 10 × 10 /°C. Reducing CaO and Na2O contents while at the same time adding B2O3 to form boro-silicate (or Pyrex) glass will reduce the coefficient of thermal expansion to about 3 × 10-6/°C.

Optical Properties:

Transparency and Color:The absence of free electrons makes glasses transparent. In metals, the conducting electrons absorb photons (visible light) based on the energy level of the electrons. The wavelengths of light absorbed gives the metal a particular color. To obtain color in glass, introduce a metal ion which will absorb selected wavelengths of light.

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