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Materials and Manufacturing


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What is the primary difference between metals, ceramics and polymers?
Metals are characterized by free electons exhibiting good electronic conductivity whereas ceramics and polymers are not
What are bioceramics?
Bioceramics are ceramics that have biologic applications for use in the body.
What are the characteristics of ceramics for biologic applications?
Their relative inertness to body fluids, high compressive strength, and pleasing aesthetic appearance, have led use of cramics in dentistry as dental crowns, some carbons have been used as implants for blood interfacing applicaions such as heart valves.
Desirable properties of Bioceramics:
1. Non-toxic 2. Non-carcinogenic 3. Non-allergic 4. Noninflammatory 5. Biocompatible 6. Biofunctional for its lifetime in the host 7. Bioreliability
What are the different types of ceramics used in fabricating implants?
There can be characterized as non-resorbable(inert), bioactive or surface reactive (semi-inert) and biodegradable or resorbable (non-inert)
Give examples of a Bioactive Bioresorbable Bioceramic.
Most calcium phosphate based materials are bioactive. These include: Bioglass, hydroxyapatite, tri-calcium phosphate (TCP), amorphous calcium phosphates (ACP_. Recent clas of magnesium metals are also bioactive.
Give examples of inert Bioceramics.
Al2O3, ZrO2,Si3N4 and carbons are inert bioceramics.
What are apatites
Apatites are minerals which have the structural formula X3Y2(TO4)Z, where X and Y=Ca,Sr,Ba T=P Z=F,Cl,OH and O
What is the structural formula of hydroxyapatite?
What are carbonated apatites?
These are carbonates substituted on OH site (Type A) and phosphate site (Type B)
What are crystalline materials?
Characterized by a periodic arrangement of atoms or ions bonded to form a 3-D lattice
What are single crystal and polycrystalline materials?
Single crystals are made of severl unit cells of atoms or ions with no grain boundaries. Polycrystalline materials comprise several grain boundaries.
What is the difference between amorphous and crystalline materials?
Amorphous materials are supercooled from the liquid state to preserved the structure of the liquid. Crystalline materials contain periodic arrayof symmetric lattice points to form a 3-D symmetric structure. Crystalline materials contain grain boundaries while amorphouse materials ave only short range order.
What is the density difference between glass and crystalline materials?
Glasses have a higher molar volume and hence lower density compaed to crystalline materials.
What are hydrogels?
These are two phase materials containing water trapped between polymer chains.
What is the most characteristic property of hydrogels?
They swell in water
What is gene delivery?
This is the ability of nano-particulate materials to bind and package DNA to be inserted and delivered to the cell so tat the cell can ingest the DNA and express the functional protein to replace a non-functional protein
How do nanoparticles of calcium phosphate help in gene delivery?
These nanoparticles because of their reduced size (less than 50nm) they help to bind DNA since it has a negative charge and the particles have Ca which is positively charged. The bound DNA is then condensed during precipitation which results in complexes that have bound and packaged DNA to be endocytosed by the cells and the DNA is released for the protein to be expressed in the nucleus.
HOw can natural polymers be used as a gene delivery system?
The nanoparticles can be embedded in natural polymeric hyrogels and the entire composite system now called a genen delivery system can be implanted. The gels degrade and cells can uptake the nanoparticles resulting in expression of the protein for proper function.
How is gene delivery effective?
It can be used to treat a number of congenital disorders such as cystic fibrosis, Parkinson's isease, spina bifida, etc which are caused by a dysfunctional protein. If we know the gene associated with the specific protein, it can be delivered so that the nonfunctional protein can now be replaced leadng to proper function that was originally missing.
Can gene delivery be used for tissue engineering?
Yes, the delivery of genetic infrmation known to induce regeneration of various tissues can be delivered by using gene delivery agents which wll help in regeneration
What is biocompatibility?
this is the ability of a material to not induce any inflammatory response from the host cell when implanted inside the body
What is bioactivity?
This is the ability of a materials or system to induce known or unknown but non-toxic and non-inflammatory response when implanted inside the body.
What is biodegredation?
This is the ability of a material to undergo chemical reaction when implanted insde the host body such that it breaks down into molecules soluble or ions that are elimnated by the body from the urine, feces or s trapped by the liver in a safe manner. The breakdown enables host cells to function using the aterial as a template while it breaks down without compromising the function of the cells.
What kinds of defects are typically seen in solid materials?
Typical defects run from point defects which include vacancies, interstitials, anti-site defects. Other defects include line defects such as dislocations both edge and screw dilocation and planar defects which are stacking faults.
How do defects affect materials property and manufacturability?
The presence of defects acts as centers for pile up of dislocation. As a result it increases the strength of a material making the mahcining difficult. Dislocations also serve to trap ions and impurities which affect the optical and electronic proprties particularly causing non-radiative emision as well as increasing the electronic resistivity. On the other hand, if the dislocations tend to be glissile then they can slip easily which can hel the plastic flow inducing forming, shaping and molding characteristics.
Wha is the difference between true stress and engineering stress?
True stress is force divided by instantaneous area while engineering stress is the force divided by the original rea.
What is the difference between true strain and engineering strain?
Engineering strain=change in length/original length True Strain=Integral dl/l
What characteristic changes happen when a material is at the ultimate tensile strength region?
The material undergoes significant change or reduction in cross sectional area and hence the reduction in area more than compensated for the reduction in force causing the enineering stress to decrease while the true stress continues to incrase. This phenomenon is called necking.
What is the implication of SSA?
This implies that smaller the particle large is the SSA implying higher activity and chemical reactivity of the material.
What is the principle of powder metallurgy?
Powder metallurgy involves sintering of materials that are in the form of powders to begin with. The steps involved are: 1. Powder are collected and separated into fines of the same size. 2. The sam size powders are then collected and mixed wth an organic binder such as cellulose acetate. 3. The powders containing the binder arethen pressed into a pellet by uniaxial press. 4. The cold pressed pellt is then heat treated at a temperature of 1/3Tm to form a sintered dense body
Whatare the fundamental mechanisms involved in sintering?
The basic principle is lowering the surface energy by processes such as grain boundary migraton, evaporation and condensation and diffusionacross th grain boundary and surface includig lattice to form low energy state by lowerin gthe surface energy and surface area. The contact points of cold pressed pellets are enhanced resulting in the formation of necking regions that increase in area to form dense objects.
What is casting?
This is the process by which a liquid material is placed into a mold and allowed to cool to take the shape of the mold. This is an old prcess common in metals, plastics and glass making.
What are the limitations of casting?
Poor dimensional acuracy and surface finish for some processes; e.g. sand casting, safety hazards to workers due to hot molten metals, and environmental problems due to hot gas evolution
What are the different components of a sand mold?
Pouring cup, downspure, runner, riser, core, cope, drag
What are expendable molds made of?
Expendable mold processes--Uses an expedable mold which must be destroyed to remove casting Mold materials: sand, plaster, and similar materials, plus binders
What are permanent mold processes made of?
Uses a permanent mold which can be used over and over again to produce many castings. Made of metal (or, less commonly, a ceramic refractory material)
What are cores generally made of in sand castings and what is the use of cores?
Cores are made of sand. The purpose of cores are generall to define the interior geometry of a part to be cast.
What is the user of the Riser?
reservoir in the mold which is a source of liquid metal to compensate for shrinkage of the part during solidification. The riser must be designed to freeze after the main casting in order to satisfy its function.
What does the mold constant depend on?
Mold constant Cm depends on: Mold material, Thermal properties of casting metal, Pouring temperature relative to melting point, Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using same mold material, metal and pouring temperature, even though the shape of the pat may be quite different.
What does the Chvorinov's rule predict for melt casting and about the sand molds?
A casting with a higher volume-to-surface area ratio cools and solidifies more slowly than one with a lower ratio. To feed molten metal to main cavity, TST for riser must be greater than TST for main caxting. Since mold constants of rier and casting will be equal, desigtn the riser to havea larger volume-to-area ratio so that the main casting solidifies first. This minimizes the effects of shrinkage.
What is the temperature range that needs to be followed while working with glasses?
What is the glass transition temperature?
It is the temperature at which the glass softens and begins to flow.
How is the melting point of glass lowered? Explain the process.
This is done by adding alkali metal additives which act as a flux and lower the melting point by breaking the network forming oxygen ions.
How can glasses be made chemically at room temperature?
This isdone by a process known as sol-gel which involves polymerizaion and condensation reactions following hydrolysis of compounds known as metal alkoxides. These are salts of metals with alcohols.
What are the different glass shaping techniques
Spinning--similar to centrifugal casting of metals Pressing--mass production of flat products such as dishes and TV tube faceplates Press-and-Blow: production of wide-mouth containers such as jars Blow-and-Blow: production of smaller mouth containers uch as beverage bottles and incandescent light bulbs Casting--large items such as astronomical lenses that must cool slowly to avoid cracking
What is lyophilization?
Equivalent of freeze-drying: Process of isolating a solid from solution by freezing the solution and subliming the solid under vacuum Exploits the P-T diagram of liquids
Why is annealing needed while processing glasses?
This step is needed to reduce and eliminat shrinkage strsses that arise since there is reduction in volume as the glass is cooled during shaping. This results in volumetric stresses that result in cracking.
What is tempering?
Heating to a temperature somewhat about annealing temperature into the plastic range, followed by quenching of surfaces, usually by air jets. When the surface cools, they contract and harden while interior is still plastic. As the internal glass cools, it contracts, putting the hard surfaces in compression. Tempered glass is more resistant to scratching andbreaking due to compressive stresses on its surface. Product: Windows for tall buildings, all-glass doors, safety glasses.
How are windshields for automobiles made?
When tempered glass fails, it shatters into many small fragments. Automobile windshields are not made of tempered glass, due to the danger posed by this fragmentation. Instead, conventional glass is used: it is fabricated by sandwhiching two pieces of glass on either side of a tough polymer sheet. Should this laminated glass fracture, the glass splinters are retained by the polymer sheet and the windshield remains relatively transparent
What optical properties of glasses are important?
Glass is transparent and has optical properties that are unusual if not unique among engineering materials. For applications requiring transpraency, light transmittence, magnification and similar optical properties, glass is likely to be the material of choice.
What is the flat glass process?
Molten glass flows onto surface of a molten tin bath,where it spreads evenly across the surface, achieving a uniform thickness and smoothness--no grinding or polishing is needed. Commonly used for producing sheets of smooth finished glass.
How is color induced in glasses?
Coloris obtained by adding certain additives that result in absoprtion of cerain wavelengths of visible light producing color.
Wht is the principle that cause the additives to produce colorin glasses?
These additives are primarily transition metal oxides that contain unfilled d level electron. When the transition metal ion comes in the field of the neighboring oxygen, there is destabilization of the d levels causing splitting and as a result there is absorption of certain wavelengths of the incoming visible radiation. This is known as Ligand Field Splitting.
What are the typical deformation processes?
Bulk Deformation: Rolling, Forging, Extrusion, Wire and Bar Drawing Sheet Metalworking: Bending, Deep drawing, Cutting, Miscellaneous Processes
During metal forming, what regin of the deformation process is exploited?
Plastic region of stress strain curve is primary interest because material is plasticall deformed. In plastic region, metal's behavior is expressed by the flow curve: sigma=K*epsilon^n where K=strength coefficient an n=strain hardening exponent. Flow curve based on true stress and true strain.
Define flow stress. /
Flow stress=instantaneous value of stress required to continue deforming the material Y=K*epsion^n where Y = flow stress which is he yield strength as a function of strain
What s the average flow stress?
Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest Y=(K*epsilon^n)/(n+1) where Y is the average flow stres and epsilon is the maximum strain during deformation stress
What are the temperature effects on K and n?
For any metal, K and n in the flow curve depend on temperature: Both strength (K) and strain hardening (n) are reduced at higher temperatures In addition, ductility is increased at higher temperatures
What are the three temperature ranges in metal forming?
Cold, Warm and Hot Working
How is strain rate defined?
Strain rate in forming is directly related to speed of deformation V Deformaton speed V=velocity of the ram r other movement of the equipment Strain rate is define: epsion=v/ wher epsilon is the true strain rate and h is the instantaneous height of workpiece being deformed

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