Ceramic Structures

Ceramic Glass

Ceramics with an entirely glassy structure have certain properties that are quite different from those of metals. Recall that when metal in the liquid state is cooled, a crystalline solid precipitates when the melting freezing point is reached. However, with a glassy material, as the liquid is cooled it becomes more and more viscous. There is no sharp melting or freezing point. It goes from liquid to a soft plastic solid and finally becomes hard and brittle. Because of this unique property, it can be blown into shapes, in addition to being cast, rolled, drawn and otherwise processed like a metal.

Glassy behavior is related to the atomic structure of the material. If pure silica (SiO₂) is fused together, a glass called vitreous silica is formed on cooling. The basic unit structure of this glass is the silica tetrahedron, which is composed of a single silicon atom surrounded by four equidistant oxygen atoms. The silicon atoms occupy the openings (interstitials) between the oxygen atoms and share four valence electrons with the oxygen atoms through covalent bonding. The silica atom has four valence electrons and each of the oxygen atoms has two valence electrons so the silica tetrahedron has four extra valence electrons to share with adjacent tetrahedral. The silicate structures can link together by sharing the atoms in two corners of the SiO₂ tetrahedrons, forming chain or ring structures. 

A network of silica tetrahedral chains form, and at high temperatures these chains easily slide past each other. As the melt cools, thermal vibrational energy decreases and the chains can not move as easily so the structure becomes more rigid. Silica is the most important constituent of glass, but other oxides are added to change certain physical characteristics or to lower the melting point.

Ceramic Crystalline or Partially Crystalline Material

Most ceramics usually contain both metallic and nonmetallic elements with ionic or covalent bonds. Therefore, the structure the metallic atoms, the structure of the nonmetallic atoms, and the balance of charges produced by the valence electrons must be considered. As with metals, the unit cell is used in describing the atomic structure of ceramics. The cubic and the hexagonal cells are most common. Additionally, the difference in radii between the metallic and nonmetallic ions plays an important role in the arrangement of the unit cell.

In metals, the regular arrangement of atoms into densely packed planes led to the occurrence of slip under stress, which gives metal their characteristic ductility. In ceramics, brittle fracture rather than slip is common because both the arrangement of the atoms and the type of bonding is different. The fracture or cleavage planes of ceramics are the result of planes of regularly arranged atoms.

The building criteria for the crystal structure are:

• maintain neutrality
• charge balance dictates chemical formula
• achieve closest packing

A few of the different types of ceramic materials outside of the glass family are described below.

Silicate Ceramics

As mentioned previously, the silica structure is the basic structure for many ceramics, as well as glass. It has an internal arrangement consisting of pyramid (tetrahedral or four-sided) units. Four large oxygen (0) atoms surround each smaller silicon (Si) atom. When silica tetrahedrons share three corner atoms, they produce layered silicates (talc, kaolinite clay, mica). Clay is the basic raw material for many building products such as brick and tile. When silica tetrahedrons share four comer atoms, they produce framework silicates (quartz, tridymite). Quartz is formed when the tetrahedra in this material are arranged in a regular, orderly fashion. If silica in the molten state is cooled very slowly it crystallizes at the freezing point. But if molten silica is cooled more rapidly, the resulting solid is a disorderly arrangement which is glass.

Cement

Cement (Portland cement) is one of the main ingredients of concrete. There are a number of different grades of cement but a typical Portland cement will contain 19 to 25% SiO₂ , 5 to 9% Al₂O₃, 60 to 64% CaO and 2 to 4% FeO. Cements are prepared by grinding the clays and limestone in proper proportion, firing in a kiln, and regrinding. When water is added, the minerals either decompose or combine with water, and a new phase grows throughout the mass. The reaction is solution, recrystallization, and precipitation of a silicate structure. It is usually important to control the amount of water to prevent an excess that would not be part of the structure and would weaken it. The heat of hydration (heat of reaction in the adsorption of water) in setting of the cement can be large and can cause damage in large structures.

Nitride Ceramics

Nitrides combine the superior hardness of ceramics with high thermal and mechanical stability, making them suitable for applications as cutting tools, wear-resistant parts and structural components at high temperatures. TiN has a cubic structure which is perhaps the simplest and best known of structure types. Cations and anions both lie at the nodes of separate fcc lattices. The structure is unchanged if the Ti and N atoms (lattices) are interchanged.

Ferroelectric Ceramics

Depending on the crystal structure, in some crystal lattices, the centers of the positive and negative charges do not coincide even without the application of external electric field. In this case, it is said that there exists spontaneous polarization in the crystal. When the polarization of the dielectric can be altered by an electric field, it is called ferroelectric. A typical ceramic ferroelectric is barium titanate, BaTiO₃. Ferroelectric materials, especially polycrystalline ceramics, are very promising for varieties of application fields such as piezoelectric/electrostrictive transducers, and electrooptic.

Phase Diagram

The phase diagram is important in understanding the formation and control of the microstructure of the microstructure of polyphase ceramics, just as it is with polyphase metallic materials. Also, nonequilibrium structures are even more prevalent in ceramics because the more complex crystal structures are more difficult to nucleate and to grow from the melt.

Imperfections in Ceramics

Imperfections in ceramic crystals include point defects and impurities like in metals. However, in ceramics defect formation is strongly affected by the condition of charge neutrality because the creation of areas of unbalanced charges requires an expenditure of a large amount of energy. In ionic crystals, charge neutrality often results in defects that come as pairs of ions with opposite charge or several nearby point defects in which the sum of all charges is zero. Charge neutral defects include the Frenkel and Schottky defects. 

A Frenkel-defect occurs when a host atom moves into a nearby interstitial position to create a vacancy-interstitial pair of cations. A Schottky-defect is a pair of nearby cation and anion vacancies. Schottky defect occurs when a host atom leaves its position and moves to the surface creating a vacancy-vacancy pair.

Sometimes, the composition may alter slightly to arrive at a more balanced atomic charge. Solids such as SiO₂, which have a well-defined chemical formula, are called stoichiometric compounds. When the composition of a solid deviates from the standard chemical formula, the resulting solid is said to be nonstoichiometric. Nonstoichiometry and the existence of point defects in a solid are often closely related. Anion vacancies are the source of the nonstoichiometry in SiO_2-x,

Introduction of impurity atoms in the lattice is likely in conditions where the charge is maintained. This is the case of electronegative impurities that substitute a lattice anion or electropositive substitutional impurities. This is more likely for similar ionic radii since this minimizes the energy required for lattice distortion. Defects will appear if the charge of the impurities is not balanced

Polymer Structure

Engineering polymers include natural materials such as rubber and synthetic materials such as plastics and elastomers. Polymers are very useful materials because their structures can be altered and tailored to produce materials 1) with a range of mechanical properties 2) in a wide spectrum of colors and 3) with different transparent properties.

Mers

Mer –    The repeating unit in a polymer chain Monomer –   A single mer unit (n=1) Polymer –   Many mer-units along a chain (n=103 or more) Degree of Polymerization –   The average number of mer-units in a chain.

A polymer is composed of many simple molecules that are repeating structural units called monomers. A single polymer molecule may consist of hundreds to a million monomers and may have a linear, branched, or network structure. Covalent bonds hold the atoms in the polymer molecules together and secondary bonds then hold groups of polymer chains together to form the polymeric material. Copolymers are polymers composed of two or more different types of monomers.

Polymer Chains (Thermoplastics and Thermosets)

A polymer is an organic material and the backbone of every organic material is a chain of carbon atoms. The carbon atom has four electrons in the outer shell. Each of these valence electrons can form a covalent bond to another carbon atom or to a foreign atom. The key to the polymer structure is that two carbon atoms can have up to three common bonds and still bond with other atoms. The elements found most frequently in polymers and their valence numbers are: H, F, Cl, Bf, and I with 1 valence electron; O and S with 2 valence electrons; n with 3 valence electrons and C and Si with 4 valence electrons.

The ability for molecules to form long chains is a vital to producing polymers. Consider the material polyethylene, which is made from ethane gas, C₂H₆. Ethane gas has a two carbon atoms in the chain and each of the two carbon atoms share two valence electrons with the other. If two molecules of ethane are brought together, one of the carbon bonds in each molecule can be broken and the two molecules can be joined with a carbon to carbon bond. After the two mers are joined, there are still two free valence electrons at each end of the chain for joining other mers or polymer chains. The process can continue liking more mers and polymers together until it is stopped by the addition of anther chemical (a terminator), that fills the available bond at each end of the molecule. This is called a linear polymer and is building block for thermoplastic polymers.

The polymer chain is often shown in two dimensions, but it should be noted that they have a three dimensional structure. Each bond is at 109° to the next and, therefore, the carbon backbone extends through space like a twisted chain of TinkerToys. When stress is applied, these chains stretch and the elongation of polymers can be thousands of times greater than it is in crystalline structures.

The length of the polymer chain is very important. As the number of carbon atoms in the chain is increased to beyond several hundred, the material will pass through the liquid state and become a waxy solid. When the number of carbon atoms in the chain is over 1,000, the solid material polyethylene, with its characteristics of strength, flexibility and toughness, is obtained. The change in state occurs because as the length of the molecules increases, the total binding forces between molecules also increases.

It should also be noted that the molecules are not generally straight but are a tangled mass. Thermoplastic materials, such as polyethylene, can be pictured as a mass of intertwined worms randomly thrown into a pail. The binding forces are the result of van der Waals forces between molecules and mechanical entanglement between the chains. When thermoplastics are heated, there is more molecular movement and the bonds between molecules can be easily broken. This is why thermoplastic materials can be remelted.

There is another group of polymers in which a single large network, instead of many molecules is formed during polymerization. Since polymerization is initially accomplished by heating the raw materials and brining them together, this group is called thermosetting polymers or plastics. For this type of network structure to form, the mers must have more than two places for boning to occur; otherwise, only a linear structure is possible. These chains form jointed structures and rings, and may fold back and forth to take on a partially crystalline structure.

Since these materials are essentially comprised of one giant molecule, there is no movement between molecules once the mass has set. Thermosetting polymers are more rigid and generally have higher strength than thermoplastic polymers. Also, since there is no opportunity for motion between molecules in a thermosetting polymer, they will not become plastic when heated.

• Types of polymers
• Commodity plastics
  • PE = Polyethylene
  • PS = Polystyrene
  • PP = Polypropylene
  • PVC = Poly(vinyl chloride)
  • PET = Poly(ethylene terephthalate)
• Specialty or Engineering Plastics
• Teflon (PTFE) = Poly(tetrafluoroethylene)
• PC = Polycarbonate (Lexan)
• Polyesters and Polyamides (Nylon)

Composite Structures

Components of Composite Materials • Matrix phase: bulk materials such as: Metals Ceramics Polymers • Reinforcement: fibers and particulates such as: Glass Carbon Kevlar Silicon Carbide Boron Ceramic Ceramic Metallic Aggregate • Interface: area of mechanical

A composite material is basically a combination of two or more materials, each of which retains it own distinctive properties. Multiphase metals are composite materials on a micro scale, but generally the term composite is applied to materials that are created by mechanically bonding two or more different materials together. The resulting material has characteristics that are not characteristic of the components in isolation. The concept of composite materials is ancient. An example is adding straw to mud for building stronger mud walls. Most commonly, composite materials have a bulk phase, which is continuous, called the matrix; and a dispersed, non-continuous, phase called the reinforcement. Some other examples of basic composites include concrete (cement mixed with sand and aggregate), reinforced concrete (steel rebar in concrete), and fiberglass (glass strands in a resin matrix).

In about the mid 1960’s, a new group of composite materials, called advanced engineered composite materials (aka advanced composites), began to emerge. Advanced composites utilize a combination of resins and fibers, customarily carbon/graphite, kevlar, or fiberglass with an epoxy resin. The fibers provide the high stiffness, while the surrounding polymer resin matrix holds the structure together. The fundamental design concept of composites is that the bulk phase accepts the load over a large surface area, and transfers it to the reinforcement material, which can carry a greater load. 

The significance here lies in that there are numerous matrix materials and as many fiber types, which can be combined in countless ways to produce just the desired properties. These materials were first developed for use in the aerospace industry because for certain application they have a higher stiffness to weight or strength-to-weight ratio than metals. This means metal parts can be replaced with lighter weight parts manufactured from advanced composites. Generally, carbon-epoxy composites are two thirds the weight of aluminum, and two and a half times as stiff. Composites are resistant to fatigue damage and harsh environments, and are repairable.

Composites meeting the criteria of having mechanical bonding can also be produced on a micro scale. For example, when tungsten carbide powder is mixed with cobalt powder, and then pressed and sintered together, the tungsten carbide retains its identity. The resulting material has a soft cobalt matrix with tough tungsten carbide particles inside. This material is used to produce carbide drill bits and is called a metal-matrix composite. A metal matrix composite is a type of metal that is reinforced with another material to improve strength, wear or some other characteristics.

Composite Structures (continued)

Classification of Composite Materials

Since the reinforcement material is of primary importance in the strengthening mechanism of a composite, it is convenient to classify composites according to the characteristics of the reinforcement. The following three categories are commonly used.

1. Fiber Reinforced – In this group of composites, the fiber is the primary load-bearing component.
2. Dispersion Strengthened – In this group, the matrix is the major load-bearing component.
3. Particle Reinforced – In this group, the load is shared by the matrix and the particles.

Fiber Reinforced Composites

Fiberglass is likely the best know fiber reinforced composite but carbon-epoxy and other advanced composites all fall into this category. The fibers can be in the form of long continuous fibers, or they can be discontinuous fibers, particles, whiskers and even weaved sheets. Fibers are usually combined with ductile matrix materials, such as metals and polymers, to make them stiffer, while fibers are added to brittle matrix materials like ceramics to increase toughness. The length-to diameter ratio of the fiber, the strength of the bond between the fiber and the matrix, and the amount of fiber are variables that affect the mechanical properties. It is important to have a high length-to-diameter aspect ratio so that the applied load is effectively transferred form the matrix to the fiber.

Fiber materials include:

Glass – glass is the most common and inexpensive fiber and is usually use for the reinforcement of polymer matrices. Glass has a high tensile strength and fairly low density (2.5 g/cc).

Carbon-graphite - in advance composites, carbon fibers are the material of choice. Carbon is a very light element, with a density of about 2.3 g/cc and its stiffness is considerable higher than glass. Carbon fibers can have up to 3 times the stiffness of steel and up to 15 times the strength of construction steel. The graphitic structure is preferred over the diamond-like crystalline forms for making carbon fiber because the graphitic structure is made of densely packed hexagonal layers, stacked in a lamellar style. This structure results in mechanical and thermal properties are highly anisotropic and this gives component designers the ability to control the strength and stiffness of components by varying the orientation of the fiber.

Polymer – the strong covalent bonds of polymers can lead to impressive properties when aligned along the fiber axis of high molecular weight chains. Kevlar is an aramid (aromatic polyamide) composed of oriented aromatic chains, which makes them rigid rod-like polymers. Its stiffness can be as high as 125 GPa and although very strong in tension, it has very poor compression properties. Kevlar fibers are mostly used to increase toughness in otherwise brittle matrices.

Ceramic – fibers made from materials such as Alumina and SiC (Silicon carbide) are advantageous in very high temperature applications, and also where environmental attack is an issue. Ceramics have poor properties in tension and shear, so most applications as reinforcement are in the particulate form.

Metallic - some metallic fibers have high strengths but since there density is very high they are of little use in weight critical applications. Drawing very thin metallic fibers (less than 100 micron) is also very expensive.

Dispersion Strengthen Composites

In dispersion strengthened composites, small particles on the order of 10-5 mm to 2.5 x 10-4 mm in diameter are added to the matrix material. These particles act to help the matrix resist deformation. This makes the material harder and stronger. Consider a metal matrix composite with a fine distribution of very hard and small secondary particles. The matrix material is carrying most of the load and deformation is accomplished by slip and dislocation movement. The secondary particles impede slip and dislocation and, thereby, strengthen the material. The mechanism is that same as precipitation hardening but effect is not quite as strong. However, particles like oxides do not react with the matrix or go into solution at high temperatures so the strengthening action is retained at elevated temperatures.

Particle Reinforced Composites 

The particles in these composite are larger than in dispersion strengthened composites. The particle diameter is typically on the order of a few microns. In this case, the particles carry a major portion of the load. The particles are used to increase the modulus and decrease the ductility of the matrix. An example of particle reinforced composites is an automobile tire which has carbon black particles in a matrix of polyisobutylene elastomeric polymer. Particle reinforced composites are much easier and less costly than making fiber reinforced composites. With polymeric matrices, the particles are simply added to the polymer melt in an extruder or injection molder during polymer processing. Similarly, reinforcing particles are added to a molten metal before it is cast.

Interface

1. The interface is a bounding surface or zone where a discontinuity occurs, whether physical, mechanical, chemical etc.
2. The matrix material must "wet" the fiber. Coupling agents are frequently used to improve wettability. Well "wetted" fibers increase the interface surface area.
3. To obtain desirable properties in a composite, the applied load should be effectively transferred from the matrix to the fibers via the interface. This means that the interface must be large and exhibit strong adhesion between fibers and matrix. Failure at the interface (called debonding) may or may not be desirable. This will be explained later in fracture propagation modes.
4. Bonding with the matrix can be either weak van der Walls forces or strong covalent bonds.
5. The internal surface area of the interface can go as high as 3000 cm2/cm3.
6. Interfacial strength is measured by simple tests that induce adhesive failure between the fibers and the matrix. The most common is the Three-point bend test or ILSS (interlaminar shear stress test)

We will consider the results of incorporating fibers in a matrix. The matrix, besides holding the fibers together, has the important function of transferring the applied load to the fibers. It is of great importance to be able to predict the properties of a composite, given the component properties and their geometric arrangement.

Isotropy and Anisotropy in Composites

1. Fiber reinforced composite materials typically exhibit anisotropy. That is, some properties vary depending upon which geometric axis or plane they are measured along.
2. For a composite to be isotropic in a specific property, such as CTE or Young’s modulus, all reinforcing elements, whether fibers or particles, have to be randomly oriented. This is not easily achieved for discontinuous fibers, since most processing methods tend to impart a certain orientation to the fibers.
3. Continuous fibers in the form of sheets are usually used to deliberately make the composite anisotropic in a particular direction that is known to be the principally loaded axis or plane.

Physical and Chemical Properties

Physical properties are those that can be observed without changing the identity of the substance. The general properties of matter such as color, density, hardness, are examples of physical properties. Properties that describe how a substance changes into a completely different substance are called chemical properties. Flammability and corrosion/oxidation resistance are examples of chemical properties.The difference between a physical and chemical property is straightforward until the phase of the material is considered. When a material changes from a solid to a liquid to a vapor it seems like them become a difference substance. However, when a material melts, solidifies, vaporizes, condenses or sublimes, only the state of the substance changes.

 Consider ice, liquid water, and water vapor, they are all simply H₂O. Phase is a physical property of matter and matter can exist in four phases – solid, liquid, gas and plasma.

Some of the more important physical and chemical properties from an engineering material standpoint will be discussed in the following sections.

• Phase Transformation Temperatures
• Density
• Specific Gravity
• Thermal Conductivity
• Linear Coefficient of Thermal Expansion
• Electrical Conductivity and Resistivity
• Magnetic Permeability
• Corrosion Resistance

Phase Transformation Temperatures

When temperature rises and pressure is held constant, a typical substance changes from solid to liquid and then to vapor. Transitions from solid to liquid, from liquid to vapor, from vapor to solid and visa versa are called phase transformations or transitions. Since some substances have several crystal forms, technically there can also be solid to another solid form phase transformation.Phase transitions from solid to liquid, and from liquid to vapor absorb heat.

 The phase transition temperature where a solid changes to a liquid is called the melting point. The temperature at which the vapor pressure of a liquid equals 1 atm (101.3 kPa) is called theboiling point. Some materials, such as many polymers, do not go simply from a solid to a liquid with increasing temperature. Instead, at some temperature below the melting point, they start to lose their crystalline structure but the molecules remain linked in chains, which results in a soft and pliable material. The temperature at which a solid, glassy material begins to soften and flow is called the glass transition temperature.

Density

Mass can be thinly distributed as in a pillow, or tightly packed as in a block of lead. The space the mass occupies is its volume, and the mass per unit of volume is its density.

Mass (m) is a fundamental measure of the amount of matter. Weight (w) is a measure of the force exerted by a mass and this force is force is produced by the acceleration of gravity. Therefore, on the surface of the earth, the mass of an object is determined by dividing the weight of an object by 9.8 m/s² (the acceleration of gravity on the surface of the earth). Since we are typically comparing things on the surface of the earth, the weight of an object is commonly used rather than calculating its mass.

The density (r) of a material depends on the phase it is in and the temperature. (The density of liquids and gases is very temperature dependent.) Water in the liquid state has a density of 1 g/cm³ = 1000g/m³ at 4^o C. Ice has a density of 0.917 g/cm³ at 0^oc, and it should be noted that this decrease in density for the solid phase is unusual. For almost all other substances, the density of the solid phase is greater than that of the liquid phase. Water vapor (vapor saturated air) has a density of 0.051 g/cm³.

Some common units used for expressing density are grams/cubic centimeter, kilograms/cubic meter, grams/milliliter, grams/liter, pounds for cubic inch and pounds per cubic foot; but it should be obvious that any unit of mass per any unit of volume can be used.

Specific Gravity

Specific gravity is the ratio of density of a substance compared to the density of fresh water at 4°C (39° F). At this temperature the density of water is at its greatest value and equal 1 g/mL. Since specific gravity is a ratio, so it has no units. An object will float in water if its density is less than the density of water and sink if its density is greater that that of water. Similarly, an object with specific gravity less than 1 will float and those with a specific gravity greater than one will sink. Specific gravity values for a few common substances are: Au, 19.3; mercury, 13.6; alcohol, 0.7893; benzene, 0.8786. Note that since water has a density of 1 g/cm3, the specific gravity is the same as the density of the material measured in g/cm3.

The Discovery of Specific Gravity

The discovery of specific gravity makes for an interesting story. Sometime around 250 B.C., the Greek mathematician Archimedes was given the task of determining whether a craftsman had defrauded King Heiro II of Syracuse. The king had provided a metal smith with gold to make a crown. The king suspected that the metal smith had added less valuable silver to crown and kept some of the gold for himself. The crown weighed the same as other crowns but due to its intricate designs it was impossible to measure the exact volume of the crown so its density could be determined. The king challenged Archimedes to determine if the crown was pure gold. Archimedes had no immediate answer and pondered this question for sometime.

One day while entering a bath, he noticed that water spilled over the sides of the pool, and realized that the amount of water that spilled out was equal in volume to the space that his body occupied. He realized that a given mass of silver would occupy more space than an equivalent mass of gold. Archimedes first weighed the crown and weighed out an equal mass of pure gold. Then he placed the crown in a full container of water and the pure gold in a container of water. He found that more water spilled over the sides of the tub when the craftsman’s crown was submerged. It turned out that the craftsman had been defrauding the King! Legend has it that Archimedes was so excited about his discovery that he ran naked through the streets of Sicily shouting Eureka! Eureka! (Which is Greek for “I have found it!”)

Substance	Density (g/cm3)
Air	0.0013
Gasoline	0.7
Wood	0.85
Water (ice)	0.92
Water (liquid)	1.0
Aluminum	2.7
Steel	7.8
Silver	10.5
Lead	11.3
Mercury	13.5
Gold	19.3