Comparing Polymers: Metal and Ceramics
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Ceramics are inorganic and nonmetallic materials formed from metallic and nonmetallic elements whose interatomic bonds are either ionic or mostly ionic. Many of the ceramics desirable properties are obtained usually by a high temperature heat treatment. Ceramics are made up of two or more elements. In a crystalline structure is more complex than that of metals. When the bonding is mostly ionic the crystal structure is made up of positively charged metallic ions, cations, negatively charged nonmetallic ions and anions. When the ions are bonded together the overall charge must be neutral. To have a stable system the anions in the structure that surround a cation must be in contact with that particular ion. There needs to be a ratio of the cation radius to the anion radius for the coordination and understanding of the structures geometry. If for example there is a lack of coordination, the cation would be incorrectly incased by the anions thus causing a collapse in its expected structural stability. There are many different types of structures exist for ceramics. One crystal structure is the AX type where there are an equal number of cations and anions. Another crystal structure that exists for ceramics has a different number of cations and anions but still has a neutral charge because the ions have different magnitudes of charge is called an AmXp structure. An AmBnXp structure has more than one type of cation, represented by A and B but only one type of anion. This type of structure is also seen in close packing of ions in metals. Imperfections occur in the crystal structure of ceramics very similar to metal structural defects. Defects can occur in each of the two ions of the structure. At any time there can be cation, anion interstitials, cation or anion vacancies. Most defects or imperfections occur in pairs to maintain the electroneutrality. A Frenkel defect is a cation vacancy and cation interstitial pair. When a cation and anion vacancy pair occurs they are called a Schottky defect. Ceramics can also have impurities in the crystal structure like metals. Figure 12.21 gives a schematic diagram of the Frenkel and a Schotkey defects (pg 435). In many cases ceramics tend to be very brittle which can lead to catastrophic failure with very few signs of fatigue. This is due to the fact that ceramics absorb very little energy before they fracture. When ceramics are subjected to a tensile stress, they almost always fracture before any plastic deformation takes place. Fracture occurs because of the formation and propagation of cracks perpendicular to the applied load. Ceramics have a greater ability to resist compression than tension. The modulus of elasticity decreases with more pores in the ceramic material. When there are many pores in the material they act as stress concentrators which expose the material to weak portion. However, ceramics are very hard and are good for applications where abrasive or grinding action is needed.
Most polymers are organic and are composed of hydrocarbons with interatomic forces that are represented as covalent bonds. Most polymers chains are quite long and very complex. These long molecules are made up of repeat units which are repeated along the chain. The smaller repeating unit is called a monomer. Polymers can be made up of a single repeat unit, called a homopolymer, or two or more different repeating units called copolymers.
Polymers generally have a very large molecular weight. These molecular chains tend to have many kinking, bending, and coiling along with entanglement with neighboring chains may occur. This causes the outcome material to be very elastic. Polymer chains can have side groups which cause different configurations based on which side and with what regularity they bond. They can present a level of crystallinity similar to the packing of the molecular chains to create an ordered atomic array. This crystal structure can be much more complex than metallic crystal structures. Defects in polymers also differ from those found in metals and ceramics. Defects in polymers are linked to the chain ends because they are slightly different than the chain itself and emerge from the segments of the crystal. Polymers are very sensitive to strain rate, temperature, and chemical nature of the environment. Different polymers can exhibit different stress strain behavior depending on the complexity of the molecular chain. Certain polymers display a level of is brittle where fracture occurs before elastic deformation which is very similar in the case of ceramics. Another type of polymers is very similar to metals where elastic deformation takes place first followed by yielding and plastic deformation. A third type is exhibited by elastomers which have totally elastic and recoverable deformation. Polymers generally have a lower modulus of elasticity and tensile strength then metals. Some Polymers can be stretched up to ten times longer than its original state where metals and ceramics cannot easily accomplish. Polymers exhibit viscoelasticity at temperatures between where elastic and liquid like behaviors are prevalent. Similar to metals and ceramics, polymers can experience creep. Creep is a time dependent factor due to deformation under stress or elevated temperature. In both ceramics and polymers, creep depends on time and temperature. Polymers may be ductile or brittle depending on temperature, strain rate, specimen geometry, and way of loading which is very similar to the properties of metals. Polymers are brittle at low temperatures and have somewhat low impact strengths. Polymers can experience fatigue under a repetitive loading. They are generally softer than metals and ceramics and unlike metals and ceramics, polymer melting occur over a range of temperatures instead at a specific temperature.
Metals are a material made up of metallic elements that are bonded metallically like common alloys. The electrons are not bound to any particular atom creating a matrix of ion cores surrounded by many electrons. They are very good conductors of heat and electricity where as ceramics and polymers are lacking. Polymers and metals are both ductile and are not that brittle though metals also exhibit a level of malleability. Ceramics are very brittle, they tend to fracture under a load which means they are lacking in ductility. Polymers are the softest material due to their complex structure, while ceramics are the hardest but are not very tough because they fracture before plastic deformation occurs. Polymers plastically deform very easily and have the smallest Young's modulus. Ceramics have the highest value because of their brittleness and never reach the point of plastic deformation because they would fracture first. The values of Young's modulus for metals fall between those for polymers and ceramics. These three materials have diverse structures and exhibit different levels of defects.
"Alloying," using the term in the broadest sense.
Simply an alloy is a metal compound that consists of 2 or more metal or nonmetallic elements. These combinations of metallic and non metallic elements ultimately create new compounds that in result display superior structural properties as compared to the elements by themselves. The type of alloy mixtures is highly dependent on the desired mechanical property of the material. Alloying can be applied to metals, ceramics and polymers where in each specific properties are desired.
One of the most desired properties of metal alloys is the hardenability. A material with a high level of hardness will resist deformation caused by surface indentation or abrasion while a material with a low hardness level will deform more easily under similar conditions. The main factor in a material's hardenability is its martensite (the rate which austenitized iron carbon alloys are formed when cooled) also content and is related to the amount of carbon in a material. With this application of alloying on metals, the material can exhibit greater strain and stress resistances as well as elasticity. These properties are favorable when dealing with construction and manufacturing processes.
A ceramic alloy is basically a fusion of a ceramic with of 2 or more metals. As seen in metal alloys, ceramic alloys can consist of impurity atoms in a solid state. In ceramic alloys an interstitial and substitutional states are possible. In an interstitial type, the anion has to be bigger than the impurity of the ionic radius. The substitutional impurity applies where the impurity atom usually forms a cation in the ceramic material thus the host cation will be substituted. Figure 12.23 provides a great visual representation of interstitial and substitutional types in a ceramic alloy (pg 437). Significantly, to properly achieve a solid state of solubility for substituting impurity atoms, the charge and the ionic size must be as the same as the host ion. If they were different it there would need to be some other way for the electroneutrality to be maintained within the solid. An easy way to do this is to create a formation of lattice defects of vacancies or interstitial of both ion types. Cobalt chromium is a perfect example of a ceramic alloy in which was designed to be used for coronary interventions thus because it does not degrade once placed in the human body.
Polymer alloys consist of two or more different types of polymers in a sense blended together. There are a variety of additives that can be blended or mixed in with the polymer to create the desired effect for the material. Polymer additives that support the modification of its physical properties are fillers, plasticizers, stabilizers and of course flame retardants. Fillers are generally introduced to a polymer, when a greater comprehensive strength and thermal stability is desired. Creating these types of alloys are very beneficial because they are generally very easy to create and use in their desired form. Plasticizers help improve the flexibility and toughness of polymers by reducing the hardness and stiffness of the material. They are often introduced to polymers that are generally brittle at room temperature. These additives are especially useful because they generally lower the glass transition temperature thus allowing the polymer to have a extent of pliability. Due to the fact that certain polymers are not resilient to environmental conditions, stabilizers are introduced. They provide stability and integrity against deterioration against the mechanical properties. The two most common forms of environmental deterioration are UV exposure and oxidation. A major concern with many polymers is that they are highly flammable. Flame retardants are introduced to such polymers to reduce the combustibility of the material by interfering with its ability to combust through a gas phase or initiating a different combustion reaction that generates less heat. This process will reduce the temperature that would eventually cease the burning process.
ESG 332 - R01
Exam #2 (Question #2)
Describe with reference to phase diagrams and dislocation theory, how precipitation age hardening can be achieved in aluminum alloys.
Generally aluminum is a metal with a low level of density compared to other metals. Due to this low level of density, it conducts electricity and heat better than copper. Aluminums just over 1200 degrees Fahrenheit which is comparably low to other metals. Due to these simple facts, it seems ideal to bond elements such as titanium, silicon, copper, zinc and other materials to magnify aluminums positive attributes. The process precipitation age hardening can amplify the alloying of aluminum. This process involves supersaturating a solid solution precipitating evenly dispersed particles on the aluminum. This will help stop the movement of dislocations within the metal structure. The basic concept of dislocation is the atomic misalignment of atoms in a linear plane. These atomic misalignments affect a whole series of atoms on a plane. The series of misalign atoms form a line called a dislocation line. There are two known types of dislocation called the screw and edge dislocation. Screw dislocation and edge dislocation are the primary types of dislocations but require a certain amount of each other to occur. By reducing the amount of dislocations can radically increase the strength in the metal. The process of alloying usually makes a pure material harder. The process of alloying is having one metal bond with impurity atoms from other materials to change its mechanical properties. An alloying process called solid solution alloying uses a solution to substitute bonds inside the metal. The limiting of dislocation movement is a major factor for alloying because it can be used to strengthen metals. Alloying metals with the precipitation hardening makes the strength of the new material stronger as the progress of the process is delayed. The reason for precipitation hardening is sought after is because of its abilities in making metals stronger.
Aluminum alloys can have precipitation in a very specific way. Heat treatment occurs when one material is heated a supersaturated mixture at a specific phase and so two different phases can be present together. A precipitate forms in small pieces throughout the entire material. When the mixture is at its equilibrium, the forming process comes to an end. The small pieces of precipitate then diffuse together to form one large precipitate. This stage of the precipitate tends to weaken the materials fundamental structure. The small pieces of precipitate in the material make it harder for dislocations to move. When strength of the material diminishes due to the movement of the precipitate it is called overaging.
There are two things need for heat treatments to be applied. Figure 11.21 provides a graphical representation the relationship between temperature and composition for aluminum and copper (pg 402). The copper phase represented at a shows a supersaturated solid solution in aluminum while the compound that between the two elements is symbolized as ?. Interestingly the point M represents the max solubility point at certain temperature and composition in the material. Point N represents the solubility limit of a and (a + ?) L symbolizes the temperature needed for the solution to become a liquid. If a major amount of solute is made available in the solution, we would have a precipitation hardened alloy. The limit of the solubility curve vastly decreases in concentration as the temperature decreases.
There are two different ways precipitation can occur. One process is the use heat treatment where the solute can be dissolved to form a solid single phase solution. This method can be done by heating an alloy to a very high temperature. Figure 11.24 shows that the ? phase is blended into a phase (pg 404). Then the alloy is cooled where all that is left is a supersaturated a phase. Precipitation heat treatment the (a + ?) phase is heated to a specific temperature to allow the ? phase to precipitate. The alloy is cooled and the hardness of the alloy is determined by time. A logarithmic function a comparison with strength and time proves the dependence of temperature and strength.
ESG 332 - R01
Exam #2 (Question #3)
Describe what is meant by the term "glass transition temperature" and illustrate your answer from polymer and ceramic point of view.
Typically a glass transition temperature is where a noncrystalline form of a polymer or a ceramic is cooled and transforms from a super cooled liquid into a glass. A ceramic or a glassy material is a noncrystalline material that becomes increasingly more viscous when it is cooled. Due to the fact that glassy materials are noncrystalline there is no definite temperature when the liquid will transform into a solid. Though, it is also important to note that in noncrystalline materials the specific volume is dependent on temperature and will decrease with the temperature. The glass transition temperature displays a reduction in the rate at which the specific volume decreases with temperature. When the temperature is below this value, the material is in a ceramic from and directly above this point the material is considered a supercooled liquid. The glass transition temperature occurs in both glassy and semicrystalline polymers, but not in crystalline materials. As certain molecular chains in noncrystalline materials temperature drop due to lack of motion the glass temperature transition occurs. Basically glass transition is the time in which a steady transformation occurs from the liquid state to a slightly rubbery state and then to the final more rigid solid material. The glass transition temperature is the state in which the material goes from its rubbery to rigid state.
This transition can take place in both directions. As a polymer for example is cooled to a rigid solid, it can be heated and undergo the same transition in reverse. As the material undergoes all of these changes its properties change from state to state. Some materials can experience greater change include the stiffness, heat capacity, and the coefficient of thermal expansion for the material during this transition. The glass transition temperature also acts as a limit boundary for applications of polymers and polymer matrix like components. If this temperature is beyond the material threshold, it will no longer fit the desired properties the task had called for and the application would be useless. The molecules that had been frozen in place below the will both rotate and translate at the temperatures above. Molecular characteristics have an impact on the chain's stiffness and will in turn affect the glass transition temperature for the material.
Some molecular characteristics that can cause the chain's flexibility to be reduced and the glass transition temperature to increase that include bulky side groups on the molecular chain. Also these characteristics can affect polar atoms or groups of polar atoms on the side of the molecular chain, double bonds, and aromatic groups. The glass transition temperature will also increase as the molecular weight of the material increases. Branching also influences the of a material, many branches will decrease the chains mobility and increase, a lower density of branches will cause the to decrease as the molecular chains will have a freer range of motion.
Crosslinks can occur in glassy polymers and can affect, they cause the reduction of motion and therefore increase. If there are too many crosslinks occur in the material, the molecular motion would be so limited that glass transition may not occur. It can be understood that many of the same molecular characteristics which affect the glass transition temperature also affect the melting transition temperature. The two are affected in such a similar manner that is usually somewhere between 0.5 to 0.8 times the melting transition temperature. Figure 15.19 demonstrates this mathematic relationship (pg 548). Both ceramic and polymers have a glass transition temperature. A glass can be referred to by several different names; such as vitreous solid, an amorphous solid or glassy solid. An amorphous solid has the mechanical properties of a solid, but does not have long range molecular order where they are in motion at a very slow rate that it be considered rigid for regular purposes. When glassy materials have been supercooled below the glass transition temperature they will take on characteristics similar to those of a crystalline solid. This solid will become rigid with an increased hardness and will be more brittle. However, if a glassy material is heated to above its glass transition temperature it will become softer and many of the intermolecular bonds will break allowing the material to flow at an increasing fluid viscosity. A polymer below the glass transition temperature is more rigid, but as it enters its glass transition phase, the material becomes more rubbery as its viscosity increases. The polymer can enter its glass transition at a lower temperature when critical factors that usually affect the motion of the molecules in the material are not all present.
When molecular weight of a polymer increases, the glass transition temperature will also increase. Many factors that increase the the rubber gasket would not do its job properly.
Polymers can exhibit the following structures: amorphous, semi-crystalline and crystalline. Describe these structures and explain how the mechanical properties may be influenced by these structural forms for a polymer of the same chemical formula.
Polymers can develop amorphous, semi-crystalline and crystalline structures of the same chemical formula. Polymers can exist as liquids, semi solids, or solids related to the crystal structures respectively. However each of these structures exhibit a variety of different mechanical properties. The crystallinity of a polymer depends on the intermolecular secondary bonding which will heavily influence the extent of any mechanical property of the polymer.
The tensile strength, elastic modulus and compression strength of a crystalline structure will be stronger than a semicrystalline structure and significantly stronger than amorphous type structure.
For a crystalline structure the molecular chains of the polymer are tightly packed together in an organized atomic group which take up space and will affect the polymers mechanical properties. These crystalline structures are heavily influenced by the glass transition temperature. Also the isomer and chemical formula lays out crucial factors that will be very important in the formation of the bulk material structure.
From certain large bulky functional groups there becomes an impending hindrance that will inhibit the movement capability of a molecule. This process will increase the energy requirement for any phase change. The outcome of this process is a greater transition temperature. This new temperature transition will increase the chances for the formation of a crystalline structure. The reason for this is and time span before the material becomes a disorganized liquid and requires a longer time for the molecules to arrange themselves properly. When polymers have many branches the weaker the material will be, even though crystalline structures are stronger than less ordered materials. Figure 15.18 demonstrates the change in these structural states when specific volume and temperature are compared (pg 546). Pure polymers have a very small melting point ranges and bond strength. Doped polymers and polymer alloys will generally have wider melting point ranges. The process of branching will decrease the strength of a polymer, which would continuously decrease the melting point temperature. Though, the act of branching on heavily dense branches will decrease molecule mobility. Also within this process the molecular weight is affected as well.
ESG 332 - R01
Exam #2 (Question #4)
How are T-T-T and C-C-T diagrams used to design heat treatment schedules for plain carbon steels.
Time-Temperature-Transformation or T-T-T and continuous cooling transformation or
C-C-T are used for heat treatment schedules for plain carbon steel. T-T-T are commonly known as an isothermal transformation diagrams can show the change of different phases at certain temperatures. C-C-T can be used to calculate percent transformation against the logarithm function through time.
The use the isothermal transformation and continuous cooling transformation diagrams can be used to develop a heat treatment for plain carbon steels. These diagrams will support the understanding of carbon steels through phase diagrams. When a structure is heat treated, its cooling process helps retain its structure. This process can be analyzed through T-T-T. Figure 10.13 displays a graphical representation of temperature against time with a third dimension with the percent of the steel alloy transformed to pearlite (pg 326). The understanding of a rapid cooling alloy sully depends on the understanding and application of heat treatment. It is understood that isothermal transformations do not change in temperature but continuous cooling transformation diagrams do. C-C-T and T-T-T display the same dimensions but over a larger spectrum of time and temperature. Figure 10.28 shows different forms of steel alloys (pg 338). A material that has been cooled to a temperature slightly below its eutectoid temperature, and isothermal transformation is maintained for an extended period of time, interestingly it cannot be depicted on T-T-T diagrams in spheroid forms.
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