APSC 278 - Ceramics

what are applications for ceramics?

• glasses (glasses, glass ceramics)

• clay products (structural clay products, whitewares)

• refractories (clay, nonclay)

• abrasives

• cements

• ceramic biomaterials

• carbons (diamond, graphite, fibers)

what is a ceramic?

• “burnt material”

• the desirable properties of ceramics are typically developed through a high-temperature heat treatment process known as firing

what is the composition of ceramics?

metallic + nonmetallic, two nonmetallic elements, larger multi-ion atoms

what are the bonds in ceramics?

• range from purely ionic to purely covalent

• could have a combination of both bonding types, with the degree of ionic character depending on the difference in electronegativity between the atoms

what is the AX type of ionic ceramic crystal structure?

• metallic ions are cations (A) and nonmetallic ions are anions (X)

• equal number of cations and anions

• can form several crystal structures, each named after a material that exhibits that structure

• eg.: NaCl, CsCl, and ZnS

what are the A_mX_p and A_mB_nX_p types of ionic ceramic crystal structure?

• if the charges on the cations and anions are not the same, a compound can exist with

  • A_mX_p formula, e.g.: AX₂: CaF₂, ZrO₂, and ThO₂

  • A_mB_nX_p formula (A and B are cations) eg.: BaTiO₃

what are silicates?

• covalent ceramics composed primarily of silicon and oxygen

  • eg.: soils, rocks, clays, and sand

  • basic building block: (SiO₄)⁴ tetrahedron

  • one Si atom with an O atom bonded to each orbital

  • each O^- ion is left with a single negative charge

  • equal separation and distribution of the 4 O atoms in space (tetrahedron shape)

  • different silicate structures exist that are the basis of commercial glasses

what is the simplest silicate?

silicone dioxide (silica / SiO₂)

• 3D network, where the corner O atoms in each tetrahedron are shaped by adjacent tetrahedra

• if tetrahedra are arrayed in an ordered manner, a crystalline structure is formed

• can also be a non crystalline or amorphous solid (glass) with atomic randomness

potential silica crystalline structures

what are network modifiers?

oxides that have been added to modify the SiO₄^4- network to form inorganic glasses

what is Na₂O (sodium oxide, “soda”)?

a network modifier that reduces the glass viscosity and melting point, allowing the glass to be formed at lower temperatures

what is CaO (calcium oxide, “lime”)?

a network modifier that strengthens the glass structure and improves durability

what is Al₂O₃ (aluminum oxide)?

a network modifier that increases hardness and scratch resistance

what is K₂O (potassium oxide)

a network modifier that creates compressive stresses

what network modifier makes up 90% of all commercial glasses?

soda-lime (sodium oxide - calcium oxide)

what is carbon as a covalent ceramic structure?

• carbon exists in two allotropic forms: diamond and graphite

• in graphite:

  • C atoms are located at corners of interlocking regular hexagons that lie in parallel (basal) planes

  • strong, covalent bonds between carbon atoms on the base planes

  • weaker van der waal bonds between layers

what are example applications of carbon as a ceramic structure?

• lithium-ion battery anodes

• solid lubricant

• carbon fiber composites in aircraft structures

• smartphones and laptops

what are advantages of properties of ceramics?

• high melting point (excellent thermal insulator)

• high creep resistance

• high elastic modulus

• high hardness and wear resistance

• excellent compressive strength

• chemically inactive (resistant to most acids, alkalis, and organic solvents)

• high electrical resistivity

what are disadvantages of properties of ceramics?

• low ductility (very brittle)

• low fracture toughness (~0.5 MPa m^0.5)

• potentially difficult to manufacture

specific strength vs specific stiffness plot for ceramics

classification of ceramic fabrication techniques

what are the steps in the particulate forming process?

1. powder

2. forming

3. drying

4. firing

what is the first step of the particulate forming process?

powder

• mill (grind) and screen constituents to obtain the desired particle size

• minerals and materials like clay, silica, ect. extracted from the earth and ground into powder

what is the second step of the particulate forming process?

forming

• mix the powder with water or keep dry

• press or cast the powder into the desired shape

• resulting part is still soft and pliable

what is the third step of the particulate forming process?

drying

• remove the residual water before firing

what is the fourth step of the particulate forming process?

firing

• fire at high temperatures for vitrification or sintering of powder

what are the three main ways to shape ceramic powder into the desired form in the forming stage?

1. powder pressing

2. hydroplastic forming

3. slip casting

what is powder pressing?

• can be used for both clay and non-clay compositions (such as metals)

• dry process, no water added

what is hydroplastic forming?

• when mixed with water, clay is highly plastic and may be molded without cracking

• yield strength is low but sufficient enough to permit a form ware to maintain its shape during handling and drying

• extrusion is the most common technique

  • a stiff plastic ceramic mass is forced through a die orifice having the desired cross-sectional geometry

• eg.: brick, pipe, ceramic blocks, tiles

what is slip casting?

• for clay based compositions

• slip poured into a porous mold, water is absorbed from the slip into the mold, and leaves behind a solid layer on the mold wall, and the thickness depends on time

what is solid casting?

repeated slip casting process until the entire mold cavity becomes solid

what is drain casting?

• terminating the slip casing process when the sold shell wall reaches the desired thickness by inverting the mold and pouring out the excess slip

• as the cast piece dries and shrinks, it pulls away from the mold wall and the mold may be disassembled and the cast piece removed

what happens during the drying and firing process?

• as water is removed and drying progresses, the interparticle separation decreases and shrinkage occurs

• if drying is occurs too quickly, the sample may warp or crack due to non-uniform shrinkage

what are outcomes from heat treatment between 900 - 1700°C?

1. sintering

2. vitrification

what is sintering?

• occurs when a firing piece below the melting temperature (no liquid is formed) and where the piece has been powder pressed (no added water)

• the particles coalesce due to an overall reduction in surface area due to surface energy

• gaps between particles are also reduced

microstructural changes that occur during firing in a powder compact

SEM micrograph of after sintering a powder compact

what is vitrification?

• the gradual formation of a liquid glass that flows into and fills some of the pore volume

• temperature at which the liquid phase forms is lowered by the addition of fluxing agents

• fused phase flows around the remaining unmolten particles and fills in the pores through capillary action

• shrinkage also occurs during this poress

• upon cooling, this fused phase forms a glassy matrix that results in a dense, strong body

SEM of fired porcelain

how do glassy materials solidify compared to crystalline materials?

glass becomes more viscous as temperature decreases, and there is no specific temperature at which liquid transforms to solid, as with crystalline materials

what is needed to form distinct crystallographic structures involving SiO₄^4- tetrahedra

1. extremely slow cooling - time for the tetrahedra to organize into the complex crystal structures

2. nucleating agents are added - assists in forming the initial crystallization sites (phosphorus and boron can do this for glass)

3. silica can exist in a supercooled liquid below melting temperature

explain the specific volume vs temperature behaviour of crystalline and non crystalline materials using the graph (T_g = glass transition temperature, T_m = melting temperature)

• crystalline materials solidify at the melting temperature

• for cooling of a supercooled liquid, a change in slope occurs at T_g (glass-transition temeprature)

• below T_g, the material is considered a glass

• above T_g, it is considered a supercooled liquid (cooled below its conventional T_m)

• during heating, if the material is in crystalline solid form, it melts when it reaches the T_m

what is the effect of temperature on viscosity?

viscosity increases with decreasing temperature

what is the strain point?

• the temperature at which the viscosity is v = 3×10¹³Paꞏs

• the glass will fracture without plastic deformation under this temperature

what is the annealing point?

• v = 10¹² Paꞏs

• sufficient atomic diffusion to remove residual stress

what is the softening point?

• v = 4×10⁶ Paꞏs

• maximum temperature at which glass can be handled without significantly deforming it

what is the working point?

• v = 4×10⁶ Paꞏs

• the glass is easily deformed

what is the melting point?

• 10 Paꞏs

• the point where glass is considered liquid

what are thermal stresses?

• internal stresses developed during cooling from high temperatures

• arise from different cooling rates and thermal contraction between the surface and interior'

• ceramics are cooled slowly to avoid thermal stresses

what is thermal shock?

when the material is weakened or fractured due to high thermal stresses

how can thermal stresses be reduced if they are already present?

1. heat the glass to the annealing temperature

2. cool slowly to room temperature

what is thermal tempering?

heat treatment that induced compressive residual stresses at the surface to increase the strength of glass

how does thermal tempering work?

1. the glass is heated above the glass transition temperature but below the softening point

2. glass is rapidly cooled using air jets or sometimes an oil bath

3. the surface cools faster and becomes rigid first

4. the interior cools more slowly and remains plastic for a longer time

5. as the interior cools and contracts, it pulls against the rigid surface

6. results in compressive stresses at the surface and tensile stresses in the interior

room temperature residual stress distribution over the cross section of a tempered glass plate