Module 3,4,5 Flashcards

Question

What are the properties of Carbon nitriding

Answer 1

The steel part is case hardened by immersion in a bath of molten sodium cyanide (NaCN) or potassium cyanide (KCN) and heated with a torch to above the A1 for several minutes. Both carbon and nitrogen are absorbed into surface from the cyanide. The steel part is normally quenched immediately and subsequently tempered to the desired toughness and hardness. The cyanide salts are poisonous when mixed with acid, which often prompts manufacturers to find other alternatives. This method does, however, produce a shallow case depth relatively quickly (just a few minutes).

Answer 2

Stainless steels or corrosion resistant steels (CRES) obtain corrosion resistance by being able to form an adherent chromium oxide layer (passive layer) on the surface.

Answer 3

An environment not containing oxygen. Elements are present which interfere with the oxide layer. Chromium is not in solution so cannot react with the oxygen in the environment. An environment that removes the oxide film (very high flow).

Answer 4

ferritic, austenitic, martensitic

Answer 5

11 to 27% Chromium, 0.2% C max (low C)

Answer 6

The Cr (which is BCC) stabilizes the BCC structure so that ferrite is present at all temperatures up to the melting point (i.e., this material is not allotropic so does not have a phase change). The lack of allotropism makes these grades not quench hardenable. As with other BCC lattice materials, ferritic stainless steels have low toughness at low temperatures (i.e., they have a T15).

Answer 7

furnace parts, valves, cutlery, etc.

Answer 8

10.5 to 18% Cr, up to 1% C (high C)

Answer 9

The carbon counteracts the chromium and allows austenite to form at high temperatures. Carbon is an austenite stabilizer. Due to the high carbon and chromium contents, these steels have high hardenability (are easily quench hardened to form martensite). The combination of martensitic structure and the formation of chromium carbides make these grades very wear resistant and hard A problem with these grades is maintaining a minimum of 10.5% Cr in solid solution to obtain the corrosion resistance (i.e., if the chromium is tied up with carbon to form carbides, it is not able to react with oxygen to form the protective oxide layer). Martensitic stainless steels tend to have lower corrosion resistance than ferritic or austenitic stainless steels

Answer 10

surgical tools, knives, valves, springs.

Answer 11

12% Cr (minimum) for corrosion resistance 7 to 22% Ni stabilizes the austenite phase at all temperatures. Higher Cr alloys needs more Ni in order to get a fully austenitic structure. C must be as low as possible (most contain 0.08% C max. although some may contain up to 0.25% carbon for creep resistance). The most common CRES is 18/8 (AISI 304) containing 18% Cr and 8% Ni. They are many variations of 304. For example, AISI 316 is 18/8 but also contains 2% Mo for extra chloride pitting resistance for surgical applications.

Answer 12

Highest general (uniform) corrosion resistance which makes it the most common of all stainless steels. Double the cost of ferritic stainless steels due to the high nickel content. The Ni and Cr give good strength from solid solution strengthening. Very formable even though they are very strong (FCC). Very tough at low temperatures (FCC so there is no T15). Non-magnetic (FCC). Prone to chloride SCC (stress corrosion cracking) as shown in Figure 11. Ferritic stainless steels are superior where chlorides are present. Extremely high work hardening rates. The austenite is metastable at room temperature. When cold worked, the austenite can transform into hard martensite, in addition to normal work hardening. During machining (e.g., drilling) deep cuts must be used to cut below the work hardened surface from the previous pass. When cutting CRES with a band saw, damage to a single tooth can cause excessive work hardening of the CRES which causes a chain reaction when the next tooth dulls. With good band sawing practice, a blade will cleanly cleave the CRES. The band will cut quickly, accurately, quietly and lasts a long time.

Answer 13

high corrosion applications such as high temperature or oxidizing acid storage. It is NOT suitable for reducing acids, such as H2SO4 or other low oxygen applications.

Answer 14

Precipitation Hardened Stainless Steels (PH CRES), Duplex Stainless Steels

Answer 15

17% Cr, 7% Ni, and 4% Cu, where the copper is added to allow for precipitation hardening.

Answer 16

High strength steels. The part can be machined in the soft condition and then precipitation hardened to increase the strength. These steels are not quench hardenable. Cu (or Al or Ti) forms extremely fine precipitates when the temperature is raised to approximately 500C (precipitation hardening). As the precipitates grow they strain (stretch) the space lattice and block movement of dislocations which increases the strength. The advantage of PH over martensitic stainless is that there is no quench which causes distortion. PH parts will have much better dimensional tolerances.

Answer 17

Aircraft parts, high strength valves.

Answer 18

Duplex Stainless Steels are a combination of ferritic and austenitic stainless steels.

Answer 19

There is sufficient nickel in these alloys to stabilize only half of the grain structure to austenite. The remaining structure is ferritic. This gives the superior general corrosion resistance of austenitic stainless with the stress corrosion cracking resistance of the ferritic. These alloys are somewhat less expensive than austenitic since the nickel content is lower. The alloy also has a higher yield strength which means the parts can be thinner (less metal) and lighter/cheaper. In some reinforced concrete applications, duplex steel rebar can replace plain carbon steel to reduce overall lifecycle costs.

Answer 20

Sensitization (carbide precipitation) refers to CRES becoming sensitive to corrosion. It can lead to intergranular corrosion or weld decay.

Answer 21

Sensitization normally occurs in the temperature range of 500 to 900˚C when carbon migrates to the grain boundaries and bonds with chromium. Since chromium is bonded to carbon it cannot bond with oxygen to form a passive layer. Sensitization will occur during slow cooling of the weld bead, which then will corrode in service.

Answer 22

500° C to 900° C

Answer 23

Use a low carbon stainless steel (below 0.03%). At this level there is insufficient C available to form significant amounts of Cr carbides. The most common type of stainless steel is AISI 304L. The L indicates a low carbon version of 304. Solution heat treat anneal. Apply a high temperature post weld heat treatment (1050 to 1125°C) to redissolve the chromium carbides and get the Cr back in solid solution followed by a rapid cool (1 minute to below 500 °C) to prevent them from precipitating again. Use the stabilized grade of stainless steel. Elements are added to the stainless steel that is strong carbide formers (called stabilizers). Nb (AISI grade 347) or Ti (AISI grade 321) tie up the carbon and prevent Cr carbides from forming.

Answer 24

Pitting corrosion is a common problem where elements or surface deposits are present that interfere with the oxide layer. Chlorine is often present in most waters as a dissolved salt. Chlorine is more reactive than oxygen, and some Cl will replace the O on the chromium. Chromium chlorides are locally removed since they are soluble in water. This leaves an extremely active anode location surrounded by a very large cathodic region. A large cathode to anode ratio accelerates localized pitting. Surface deposits (dirt, rust, etc.) can also locally prevent oxygen from getting to the surface, which prevents a chromium oxide layer from forming. This will lead to pitting.

Answer 25

Creep is the slow continuous plastic flow of a material under load usually at high temperature. It is characterized by sliding of the grain boundaries past each other.

Answer 26

it is a problem at temperatures above 550˚C for steels

Answer 27

The addition of carbide formers (such as Cr, w, or Mo) can increase the creep strength of steels, which can form stable carbide particles at the grain boundaries. These carbides lock the grain boundaries in position, preventing movement and creep.

Answer 28

Tool Steels

Answer 29

W Water hardening (low hardenability plain-carbon steel) is the least expensive. (e.g., W2) O Oil hardening contain small amounts of alloying (e.g., Cr, Mo, V, etc.) to increase hardenability). (e.g., O1) A Air hardening (higher alloy alloys air quench to be used for larger parts which minimize the potential for distortion and quench cracks). (e.g., A2) M Molybdenum is added to form MoC particles at the grain boundaries. High speed steel (M2) is used for cutting tools to retain hardness at high temperatures (creep resistant) T Tungsten is added to form WC particles at the grain boundaries. High speed steels used for cutting tools (creep resistant) H Hot work steels. Carbide forming elements are added for high temperature creep strength applications (such as hot forging dies).

Answer 30

HSLA steel (also known as micro-alloyed steel) are used for pipelines and structural applications.

Answer 31

HSLA steels are cheap (low alloy saves money), however they are tough (with reasonable strength and ductility) which makes them a good candidate for replacing some of the more expensive steels with higher alloying contents. Their alloying consists of low levels of carbon (approximately 0.03%-0.2%) along with additions of carbide and nitride formers such as Ti, Nb and V.

Answer 32

1) Steel is formed in 7” to 10” thick slab from continuous casting; 2) the slabs then reheat in a furnace at approximately 1200°C which allows dissolving all alloying elements; 3) the heated slabs then are sent to a roughing mill (hot rolling). The mill gives several passes with big reduction in thickness (down to 1"-2" thickness). During this process the material is subjected to significant amount of deformation and recrystallization. The Ti, Nb and V additions are very important at this stage. They form carbides and nitrides during cooling, which prevents grain growth and keeps the grains small for further refinement. 4) Afterward, the material is sent to finishing mill (hot rolling) which gives several smaller reductions into the final thickness (depending on application, as thin as 1-2 mm). In the finishing mill stage, little deformation is expected and the process is done mainly for precision and consistency. The next step in process is controlled cooling, in which water jetting on top and bottom of the material cools the skelp in a manner specific to the application. Cooling can be adjusted to change the microstructure and material properties of the final product. There are several banks of these water jets, and each can be changed individually to deliver more or less water for a faster or slower cooling rate. Before cooling, the material is still above 1000°C. In the next step, the material is cooled down to the desired microstructure, then brought down to 400-600 °C most of the time. For some specific microstructures, the material can be cooled to as low as 150°C or as high as 750°C. 6) Towards the end of the process, the thinned metal sheets are coiled at temperature to allow some microstructural changes to take place after coiling (this is the main difference between the final cooling temperatures above). This process may take several hours for cooling to complete. This process is primarily used to allow some precipitates to form in a controlled manner, but occasionally used for proper microstructure changes. 7) Depending on the required cold work based on specific applications, some additional compression and work may be applied on the materials afterwards for additional increase in strength

Answer 33

``` Due to the ductile to brittle transition effect for BCC ferritic alloy steels (see module 4 and the Charpy impact test handout), the mechanical components made of alloyed steels cannot be used at temperatures lower than -101ºC (-150ºF). However, an addition of 8.5 to 9.5% nickel into these alloyed steel associated with a proper heat treatment, allows these steels to withstand temperatures as low as -196ºC(-320ºF). As a result, these steels become suitable for cryogenic applications such as process piping materials used in gas liquefaction units. Recent research on the nickel-added cryogenic steels has shown that two microstructural mechanisms are at work to enhance the low temperature performance of these steels (which is accomplished by decreasing the ductile-brittle transition temperature). The first mechanism is based on grain refinement and the second mechanism is based on the introduction of a small amountmetallurgically stable FCC austenite (5 to 15%) into the ferrite BCC structure. These modifications can be achieved by special heat treatment mechanisms such as quenched-tempering (Q&T) and also double-normalizing and tempering (NNT). Addition of some other elements such as Manganese can also introduce similar cryogenic effects into ferritic alloyed steels. ```

Answer 34

Dual-phase steels microstructure consists of a ferritic matrix (85 to 95%) with addition of a secondary phase martensite formation (5 to 15%) within this matrix.

Answer 35

Presence of hard martensitic phase within ferritic matrix provides a greater strength for dual-phase steels as compared to other common types of high strength low alloy (HSLA) steels. Also, due to the presence of high volume of ferrite, the dual-phase steels are expected to have higher degrees of ductility and formability. A proper controlled cooling from the austenite phase or from another dual-phase steels (such as the ones with ferrite plus austenite structure) can produce ferritic-martensitic dual-phase steels. During the cooling process, some of austenite phase transform to ferrite before a fast cooling rate transforms the remaining austenite to martensite. Generally speaking, ferritic-martensitic dual-phase steels exhibit great strength, good ductility (uniform elongation) and good fatigue resistance which makes them ideal candidates to be used in many automobile industry applications such as body panels, bumpers and rear rails and other parts designed for shock absorption.

Answer 36

DP steels have low levels of carbon (0.06–0.15 wt.%), however the manganese content is higher in these steels (1.5-3.0%). Manganese is added to stabilize the austenite phase by solid solution strengthening in ferrite. Other elements such as Cr, Mo, V, Nb and Si are also added into these alloys for enhancing the mechanical and microstructural properties.

Answer 37

TRIP steels will transform plastically when under impact to provide higher combination of strength and ductility as compared to most of the commercially available alloyed steels.

Answer 38

TRIP steel are mainly composed of ferrite plus some amounts of other phases such as martensite, bainite, and retained austenite. During forming or impact of a TRIP steel, the retained austenite transforms to martensite resulting in a higher work hardening rate that persists to higher strains.

Answer 39

TRIP steels have higher carbon content than dual-phase steels. The higher carbon content in TRIP steels are added to stabilize the retained austenite phase at temperatures below the room temperature. The addition of silicon and aluminum helps with the formation of ferritic and bainitic microstructure. Aluminum and silicon allow sustaining the required amount of carbon within the retained austenite by preventing formation of iron-carbide precipitation in the bainitic phase. During an impact, the transformation of retained austenite-to-martensite absorbs energy which enhances the ability of the TRIP steels to withstand mechanical shocks. After the impact, the bulk of material still will be made up of ferrite, bainite or martensite, but a portion (or all) of the retained austenite will be transformed to martensite. Due to these properties, TRIP steels are being used in shock absorbing applications, such as body panels and bracing members in the automotive industry. TRIP steels also have been used for military armor purposes

Answer 40

Twinning-induced plasticity, or TWIP steels possess an excellent combination of strength and ductility as well as wear and corrosion resistance.

Answer 41

TWIP steels have austenitic FCC microstructure with a low stacking fault energy (SFE) that allows deformation twinning (a type of directional slipping within crystal structure of a grain). The twinning increases the strength by enhancing hardening and increasing ductility. These steels have superior mechanical properties at room temperature with UTS greater than 800 MPa and % elongation exceeding 100% of original length at the failure point. TWIP steels possess excellent high work-hardening characteristics which makes them ideal in applications where shock and impact is detrimental to the life of a mechanical component. Similar to TRIP steels, TWIP’s are mainly considered for shock absorbent applications such as car panels and bracing members.

Answer 42

TWIP steels chemical composition contains less than 1 wt% C, less than 3 wt% Si or Al but they have very high manganese contents (20 to 35 wt% Mn).

Answer 43

Properties that do not change significantly and are said to be fixed depending on the composition of the material. Plastic deformation, thermal treatments and small amounts of alloying have little effect on physical properties. Examples are density, melting point, stiffness (Elastic Modulus), thermal and electrical conductivity.

Answer 44

The response of a material to applied loads (or forces). These properties can change significantly for the same material depending on how it was processed (e.g. deformation and thermal treatments). Examples are strength, ductility, toughness, hardness, creep and fatigue.

Answer 45

1. plastic deformation (hot and cold working) 2. thermal treatment (heat treating) 3. grade selection (alloying)

Answer 46

Tensile Stress (Tension), Compressive Stress (Compression), Shearing Stress, Torsional stress (torsion)

Answer 47

Tensile Stress (Tension) causes the material to be stretched.

Answer 48

Compressive Stress (Compression) creates a pushing or crushing force.

Answer 49

When a material is subjected to a shearing stress, the force is attempting to cut through the material at right angles to its length.

Answer 50

Torsional stress (torsion) causes the material to be twisted. The resulting stress is shear.

Answer 51

The change in the shape of the material is called strain. Strain =Change in Length/Original Length % Strain = Strain x 100% Strain is a ratio of the amount of deformation compared to the original size and has no units although it may be expressed as a percentage

Answer 52

The tension test gives the greatest amount of information about the static mechanical properties of a material. This test also may be used to determine the stiffness (Elastic Modulus) which is a physical property. The Tension Test applies a tensile load to a part. The load is applied gradually (a static load as compared to the much faster dynamic load) and is increased until the material fails. The test is a destructive test.

Answer 53

Stress is the amount of force distributed over an area. Stress =Load/Original Area = lb /square inch=psi

Answer 54

Gauge length (original length) is the distance between two marks punched into the reduced section?

Answer 55

f the applied stress is low, metals behave elastically, that is, they return to their original length when the stress is removed. Wherever close tolerances must be maintained, (as in almost all applications) the stress must be kept in this lower range to prevent permanent (plastic) deformation from occurring. Most metals have very small elastic strain before plastic deformation occurs. Polymers, such as an elastic band, have very high elastic deformation (high resilience).

Answer 56

The yield strength of a material is the stress to cause a permanent change in shape. Yield Strength = Yield Stress = Upper Yield Load/Original Area

Answer 57

resistance to scratching (scratch hardness - Mohs) resistance to abrasion (wear hardness) resistance to filing, cutting or drilling (machinability) resistance to indentation (indentation hardness)

Answer 58

ease of use part can be used after the test (unlike in the tensile test) other properties can be estimated (such as tensile strength)

Answer 59

Mohs, Brinell, Rockwell, Vickers Pyramid hardness, and Knoop hardness

Answer 60

The Mohs scale is the original hardness test and one that is still used today for geological samples (ores). This scale uses the scratch resistance of 10 minerals going from the softest (talc) to the hardest (diamond). While the Mohs scale gives the largest range, it does not provide the resolution of the Brinell or Rockwell scales.

Answer 61

The Brinell (HB Reference ASTM E10) uses a mass of 500, 1500 or 3000 Kg on metals while polymers may use much lower loads. Brinell tests use a standard Tungsten Carbide 10 mm diameter ball for 12 seconds.

Answer 62

The Rockwell (Reference ASTM E18) is very easy and quick to use since the hardness can be directly measured on a dial gauge (the gauge has 100 divisions; each division represents 0.002 mm depth). The deeper the indenter goes, the softer the material. Also, the Rockwell produces a very small indentation which causes less damage to the surface compared to the Brinell Test.

Answer 63

The Vickers Pyramid Hardness (HV) Test results in hardness numbers nearly identical to the Brinell test even through an inverted square pyramid is used (Brinell uses a 10 mm diameter ball). The standard test uses a mass of 5 to 120 Kg.

Answer 64

The Knoop Hardness (HK) is similar to the Vickers except the indenter is a trapezoid giving a long, narrow diamond shaped impression. This shape is often useful for micro hardness testing of individual phases or individual grains.

Answer 65

Toughness is defined as the work or energy to fracture a material; the higher the energy, the tougher the material. Toughness = Energy = Work = Force X Deformation

Answer 66

1. The most common method is to define the transition as the temperature where some critical value of energy (CV) has been absorbed. For most steels, T15 would be the temperature where at least 15 foot-pounds (i.e. 20J*) of energy was absorbed. For most common applications this is enough toughness to preventfracture. For higher impact, 30 foot-pounds may be chosen for the transition temperature (T30). 2. Another method is the “Nil Ductility Temperature” (NDT). The NDT occurs at the lowest temperature where there is still some ductility and the fracture appearance curve becomes 100% cleavage (e.g. ~99%). As the temperature drops, this is the temperature where there is virtually no ductility. In other words it will be a brittle fracture and very little energy will be absorbed (low toughness) since there is no deformation. 3. The TAV is the temperature where the energy is midway between the high energy and low energy shelves. It can also be determined as the temperature where the highest slope is attained. 4. The most conservative is the minimum temperature to maintain 100% ductile (0% cleavage) fracture in the sample (FPT in Figure 19b). (FPT; Fully Plastic Transition) 5. Another way that the transition temperature can be specified is to determine where the fracture shows 50% ductile and 50% brittle fracture. (50% FATT; Fracture Appearance Transition Temperature)

Answer 67

Low carbon content; steels that contain more ferrite (less of the brittle Fe3C) are tougher than medium carbon steels which contain more pearlite. : Manganese and nickel (FCC metals) are commonly added to lower the transition temperature in steels. Increasing carbon content tends to raise it since it decreases the ductility. : Killed steels (Deoxidized with silicon or aluminum) increases toughness by removing interstitial oxygen.

Answer 68

Cold working will reduce the steel’s ductility and the amount of deformation that can occur. Work (which is toughness) will decrease since it is directly related to the amount of deformation. Cold rolled steels have lower toughness compared to annealed steels.

Answer 69

Fine grains

Answer 70

Higher stress concentration factors (Kt)

Answer 71

Using the abrasive belt grinder, bevel the top and bottom edges (to prevent the sample from cutting your fingers and from scraping the grit off of the abrasive papers).

Answer 72

Keep the pressure on the center of the sample. Apply pressure on the forward stroke only (none on the return stroke). This prevents rounding of the surface during each grinding stroke. Always grind in one direction until all of the previous scratches are removed, then give the sample 3 times as many grinding strokes to remove the work hardened subsurface. Rotate the sample 90 when the sample is moved to the next finer grit. Grind the sample using successively finer abrasive papers in the following sequence. The 80 & 120 papers would be used to quickly remove the very heavily work hardened surface when a rough cut (e.g. band saw) has been used to remove the sample.

Answer 73

It is extremely important that the sample and your hands are cleaned in water prior to each polishing stage to prevent grit carry over. Diamond paste lubricated with lapping oil may be substituted for the alumina. Rough polish the sample using the 5 micron alumina (0.005 mm Aluminium Oxide) in water. Move the sample opposite to the direction of the wheel to give even polishing in all directions. Clean the sample and finish polish with 1 micron alumina. Clean the sample in water (You may rub the surface with your fingers; but No paper since it is abrasive and will scratch the steel). Remove water from the sample by rinsing with alcohol followed by the blow drying. The surface should be a mirror finish with no scratches at low power magnification.

Answer 74

Immerse and agitate the specimen in the etching solution for the recommended length of time. For general structure of plain carbon and low alloy steels etch for 5 to 20 seconds in 2% Nital (2% concentrated nitric acid in Isopropyl alcohol). Harder steels etch faster. Different alloys require different etchants it, for example stainless steel needs to use electrolytic oxalic acid.

Answer 75

Very high stiffness (modulus of elasticity). For example, aluminum would have to be three times larger to give the same deflection as steel in a bridge. Strength. Quench hardenable, which allows the steel to be formed in a soft condition and then hardened. Lower material cost.

Answer 76

In non-ferrous alloys, the major element is no longer iron which significantly changes the mechanical and physical properties. These alloys contain <50% Fe, however, iron may still be present. The base metal in non-ferrous alloys may be copper, aluminum, nickel, titanium, etc.

Answer 77

1. Good corrosion resistance (e.g., nickel, copper have low maintenance costs) 2. High electrical and thermal conductivity (e.g., aluminium, copper) 3. Good formability and machinability (e.g., aluminium, copper have reduced fabrication costs) 4. Specific strength (e.g., aluminum, titanium) 5. Appearance (e.g., copper, anodized aluminium for architectural applications) 6. No ductile to brittle transition (e.g., aluminium, copper, nickel do not lose toughness at a low temperatures)

Answer 78

1. Increase the material strength (e.g., by precipitation hardening) 2. Relieve internal residual stresses (e.g., stress relief) 3. Restore ductility after cold work (e.g., recrystallize the structure) 4. Obtain a uniform structure (e.g., reduce the effects of segregation or coring by using a homogenization anneal)

Answer 79

Precipitation hardening (age hardening or aging)

Answer 80

Step 1:Heat the steel to dissolve carbon in the single-phase austenite (FCC structure). Step 2:Quench to form martensite (stretched BCC structure), which is a very hard and strong phase, but lacks toughness. Step 3:Temper to restore some toughness and ductility (strength is reduced).

Answer 81

Step 1:Solution Heat Treatment - The material is heated into the single-phase region where all of the alloying elements are dissolved and a homogeneous structure is obtained Step 2:Quench - Rapid cooling of the material from the single-phase region into the two-phase region. At this stage the material is very soft and very ductile. The alloying elements are retained in the solid solution obtained in the first stage. This is not in equilibrium. Step 3:Aging (precipitation hardening step) occurs when the metal is heated slightly. This allows atoms to move and an extremely finely dispersed intermetallic precipitate forms within the matrix. These particles retain a coherent bond and stretch the atomic bonds far into the surrounding matrix . These particles also are very hard. The yield strength and hardness increases significantly since slip cannot occur easily due to stretched atomic bonds along the crystal planes. This is similar to putting sand on ice and to prevent slip.

Answer 82

Overaging is a condition that occurs when the temperature is raised too high or held for long period of time. The second phase will increase in size to a point where they are no longer coherent with matrix. As the particles get larger, the stretch on the atomic bonds become so high that fracture (cracks) occurs around these particles. This causes both the strength and ductility of the material to decrease

Answer 83

The temperature at which precipitation hardening takes place determines the maximum final strength of the alloy. Natural aging occurs near room temperature so the precipitate will be much finer and very closely spaced compared to artificially aging. Much longer times are needed to increase the hardness (as shown in Figure 5). Theoretically, natural aging, if given enough time (years), can get higher strengths then artificial aging.

Answer 84

Aluminum is extracted by electrolysis from bauxite ores (alumina). Aluminum alloys have an FCC structure, which means that they have high ductility and can be easily formed into wrought products. Aluminum alloys also have a low melting point (approximately 660° C), and so are easily cast to final shape. Commercially pure aluminum contains a minimum of 99% Al. As alloy additions are made, the electrical and thermal conductivity decreases along with the ductility, but the strength tends to increase. While many aluminum alloys have a yield strength of similar to structural steels, aluminum has a low modulus of elasticity (E), which is approximately 1/3 that of steel. To obtain the same stiffness, the aluminum has to have a cross section three times larger than a similar steel member.

Answer 85

Good formability/workability to reduce fabrication costs. High strength to weight ratio (approximately 1/3 the density of steel). Good corrosion resistance. Good electrical and thermal conductivity (100% better than copper per Kg).

Answer 86

Aluminum forms a thin, tightly adherent surface oxide layer similar to chromium in stainless steels. This acts to prevent further reaction of the environment, thus providing corrosion resistance. Some acidic, basic or moist environments prevent oxygen from forming at the surface, and this can lead to severe corrosion. The corrosion resistance can be enhanced by surface treatments such as anodizing or cladding. Pure aluminum (1000 series) has higher corrosion resistance than the alloys because the alloying elements will reduce the amount of aluminum oxide at the surface

Answer 87

Anodizing is an electrolytic oxidation process that thickens the protective surface oxide film to improve the corrosion and wear resistance.

Answer 88

Corrosion resistance; Aluminum oxide is very unreactive and protects the underlying metal by preventing contact with the environment. Abrasion resistance; the hard ceramic coating provides good abrasion resistance. The coatings can be from 5 microns to 100 microns thick. The tightly adherent coating provides excellent abrasion resistance, but it does tend to be brittle. If a point load is applied, then the yield strength of the underlying base material is exceeded and the coating will crack and flake off. Anodizing is not suitable for bearing surfaces since it is rough and will abrade the other surface. Coating can be coloured. Dyes can be absorbed in the pores of the oxide and provide coloured finishes. Cladding (Alclad) gives the corrosion resistance of pure aluminum with the strength of strong alloy. This is done by placing a pure aluminium plate onto an alloy and then hot rolling. The heat and pressure of hot rolling pressure welds the two plates together. Industry is switching over to casting pure aluminum directly onto the alloy surface, which creates a better bond. High strength aluminum alloys tend to have poor corrosion resistance compared with pure aluminum but these composites have the good corrosion resistance from the pure Al outside layers. Care must be taken to make sure the thin (<0.005”) outside layer is not scratched otherwise the high strength aluminium will be preferentially corroded (anode).

Answer 89

When lithium (Li) is added to aluminum alloys, there is a significant increase in the modulus of elasticity. Up to 4% Li can be added and each % Li results in a 3% decrease in density and a 6% increase in the stiffness (modulus of elasticity). These alloys have good fatigue crack propagation resistance, and the combination of lightweight with increased stiffness makes them good alloys to be used in aerospace applications.

Answer 90

1. Alloying. 2. Heat treating (annealing and some are precipitation hardenable). 3. Hot and cold working.

Answer 91

High thermal and electrical conductivity. High ductility (good formability) due to FCC structure. Good corrosion resistance. Non-magnetic (some alloys may become slightly magnetic when cold worked).

Answer 92

The principal smelting impurity of copper is oxygen (0.05 - 2.0% oxygen), which is removed to create an oxygen free high conductivity (OFHC) grade (electrical wire). The International annealed copper standard (IACS) is used to compare the relative conductivity of various alloys. The addition of alloying elements tends to reduce the electrical and thermal conductivity of most alloys. Since aluminium has better conductivity/$, it is used for most industrial (high voltage and current) applications. Copper is used for mostly residential since it has better creep and oxidation resistance.

Answer 93

Erosion resistance. Corrosion resistance. Strength. Price (because Ni is approximately 4 times the cost of copper).

Answer 94

These alloys tend to have the highest strength to weight ratio of commercial metals, however they are relatively brittle due to their HCP crystal structure. They are characterized as having poor wear, creep and fatigue properties, as well as poor corrosion resistance. Since magnesium is very anodic on the galvanic series, it is often used as cathodic protection anodes where high voltage is required. Environments in which magnesium alloys are particularly prone to corrosion include saltwater and salt air. These materials are often enameled or lacquered for improved corrosion resistance. They are used where strength to weight considerations are paramount.

Answer 95

Zinc is primarily used for galvanizing (i.e., metal coating) steel. The two galvanizing processes are hot dip (steel is dipped into a molten bath of zinc and a thin coating is applied) and electrolytic plating. Electrolytic plating produces a smoother surface but is more expensive. The zinc coating acts as a sacrificial anode (i.e., it corrodes preferentially to the steel) and even if the steel is exposed, corrosion resistance remains high until all the zinc is depleted. Even though zinc is a very reactive metal if it does form a protective oxide layer that prevents excessive corrosion. Sometime we use the high reactivity of Zinc to make cathodic protection anodes for steel structures. Zinc based alloys are also often used for die castings due to its low melting point. During the galvanization process for steels, the presence of silicon (a reactive element) in the steel improves quality, corrosion performance and appearance of the coating as well as providing a thicker zinc coating on the surface of steel. Silicon usually is added during steel manufacturing to remove oxygen from the molten steel. These steels are called silicon-killed steels and have a silicon content greater than 0.14 wt%, however for a thick zinc layer, silicon-killed steel with a silicon content in the range 0.15-0.22 wt% are preferred. For corrosive environment silicon content greater than 0.22 wt% is recommended.

Answer 96

Titanium tends to be an expensive alloy. Although the ore is abundant, it is difficult to extract the metal since it is so reactive. Commercially pure titanium accounts for a quarter of use due to their high corrosion resistance, which is produced by an oxide layer. Titanium has excellent strength to weight ratio, erosion and corrosion resistance. The strength can be increased by alloying, and precipitation hardening is possible on some alloys. Beta (β) titanium has strength similar to steel and ½ of the density so may be used for aerospace applications. The strength of titanium alloys are not significantly reduced up to 480˚C, so they are often considered high temperature engineering alloys.

Answer 97

α (Alpha) Titanium alloys contain elements (e.g., Al, O, N, or C) which stabilize the HCP structure. These alloys are stronger than β alloys but more brittle. This makes them difficult to fabricate and so increases the costs. β (Beta) Titanium alloys contains alloying elements that stabilize the BCC structure. Mo and V lower the transformation temperature while Fe, Cr and Mn cause the transformation to be extremely slow. Beta alloys have some ductility so they can be both hot and cold worked. Cold working is difficult due to the extremely high work hardening rate. Alpha-Beta alloys contain sufficient stabilizers so they can be worked in the beta condition and then heat treated to give controlled decomposition to Alpha phase, which provides a higher strength. The most commonly used titanium alloy is Ti6Al4V although TiAl5V5Mo5Cr3 is becoming more popular due to its higher strength.

Answer 98

Compared to other alloys, this group of alloys has superior strength, erosion and corrosion resistance, particularly at high temperatures and reducing environments (e.g., sulfuric acid). One group of nickel-based alloys is Monel (UNS N04400 contains 67% Ni, 30% Cu), which is the best commercial alloy for corrosion and erosion resistance. This makes it useful for resisting metal loss in high velocity or turbulent corrosive environments. Addition of chromium produces Nichrome, which has very good oxidation resistance at high temperatures. Further addition of iron to the nickel and chrome increases the strength to produce Inconel(TM INCO). N06625 (Inconel 625) has further addition of Mo and Nb to give excellent pitting and crevice corrosion resistance.

Answer 99

Used in the chemical and food processing industries. Resistant to salt water, sulfuric acid, even high velocity high temperature steam. Used for steam turbine blades, pump impellers. Ornamental trim (a high polish can be achieved).

Answer 100

Refractory metals, such as Tungsten (W), Molybdenum (Mo), Niobium (Nb), and Rhenium (Re) are a class of metals extraordinarily resistant to heat (2200 to 2800oC), wear and corrosion. Because of their high melting point, components are never fabricated by casting. The process of powder metallurgy is used. Powders of the pure metal are compacted, heated using electric current, and further fabricated by cold working with annealing steps. They have poor low temperature formability. Superalloys (cobalt (Co)-based and nickel (Ni)-based alloys) can have operating temperatures of up to 1100oC (e.g., Hastelloy, Stellite, Inconel). Creep resistance is dependent on slowing the movement between grains along the grain boundaries. The cubic intermetallic phase [Ni3 (Al, Ti)] present in nickel and nickel-iron super alloys locks the grain boundaries. These very hard Intermetallic particles make these alloys difficult to machine and some have to be machined using EDM (Electric Discharge Machine).

Answer 101

Organic compounds, which make up more than 95% of all known compounds, differ from inorganic compounds in several general ways. For example, inorganic compounds do not contain carbon, except for carbonates, cyanides and some other relatively rare compounds. In organic compounds, carbon is bonded with only a small number of other elements. In order of their abundance, these are hydrogen, oxygen, nitrogen, halogens, sulphur and phosphorus; metals and other elements occur less frequently. When you use the term “organic chemistry,” you are really talking about hydrocarbons and their derivatives. Hydrocarbons contain only carbon and hydrogen atoms. Hydrocarbons are classified as alkanes, alkenes, alkynes and aromatic hydrocarbons

Answer 102

When the carbon-carbon bonds in a compound are all single (that is, each carbon is attached to four distinct atoms), that compound is said to be saturated. Alkanes (straight-chain, branched and cyclic) are saturated.

Answer 103

Organic compounds that have double or triple carbon-carbon bonds (alkenes, alkynes and aromatics) are said to be unsaturated. One example is vegetable oil (margarine), where the term polyunsaturated refers to multiple double bonds present in the fatty acid.

Answer 104

1. All saturated hydrocarbons are alkanes. All use the suffix ane. 2. The basic name for alkanes is determined according to the number of carbon atoms. The following prefixes are used for the first ten normal alkanes: meth, eth, pro, but, pent, hex, hept, oct, non, dec. 3. For normal and branched alkanes, the root name is that structure having the longest carbon chain. 4. Groups attached to the longest carbon chain are called substituent groups. Saturated hydrocarbon substituents are called alkyl groups and are named in a similar manner to the root name, except the ane suffix is replaced with an yl ending. Therefore, a two carbon substituent is called an ethyl group. 5. Number the carbon atoms in the longest chain, starting at the end closest to any branching (substituent group). When a substituent occurs at the same distance from each end, use the next substituent (if any) to determine the end at which numbering starts. 6. Using the appropriate name for each substituent, specify its position on the parent chain with a number. 7. When a given substituent group occurs more than once, attach the appropriate prefix (di for two, tri for three, tetra for four, and so on) to the substituent group. 8. The substituent groups are listed in alphabetical

Answer 105

1. Select the longest continuous chain of carbon atoms. If there is a double bond, then it is an alkene, and the ane is replaced with an ene ending., disregarding any prefix. 2. Identify the location of the “multiple” bond. Start at the end that will give the lowest number. 3. Substituents are treated the same way as in naming alkanes.

Answer 106

Combustion reactions of alkanes with oxygen are vigorous. Carbon dioxide, water and heat are produced when alkanes burn in oxygen. These reactions are the basis for the use of hydrocarbons for heat and power.

Answer 107

Substitution Reactions occur when one or more of the hydrogen atoms are replaced by other atoms. Alkanes are halogenated by exposure to halogen in sunlight or at high temperatures. In this substitution reaction, a hydrogen atom on the alkane is replaced by a chlorine atom to form methyl chloride.

Answer 108

A dehydrogenation reaction is one where hydrogen atoms are removed and the hydrocarbon becomes unsaturated.

Answer 109

Cracking is a process used in petroleum refining to break high molecular mass hydrocarbons into light hydrocarbon molecules. Cracking uses the application of heat and pressure (with or without catalysts) to create a variety of small hydrocarbons. Cracking is the principal way for converting crude oil (large hydrocarbon molecules) into smaller molecules such as found in fuels or into the starting stock for pharmaceuticals and polymers.

Answer 110

thermal cracking, catalytic cracking and hydrocracking

Answer 111

Ceramics, Polymers, Composites

Answer 112

They have developed an increasing role in engineered materials due to their corrosion resistance and low cost of processing, and are available in a variety of forms including fibers, thin films, sheets, foams and bulk. Polymers use hydrocarbons as the primary building blocks, but may also contain other elements such as oxygen and nitrogen. Polymers usually consist of very large molecules that are built by joining a series of small molecules together. The term polymer consists of poly, which is Greek for many, and mer, which means unit (i.e., building blocks). Therefore, polymer means “many units”.

Answer 113

Polymerization is the joining together of many monomers through chemical reactions.

Answer 114

The two main forms of polymerization are chain growth (or addition polymerization), and step growth (or condensation polymerization).

Answer 115

1.Chain growth (addition polymerization) is the rapid growth of long linear chains of molecules as shown in Figure 3 (steps ‘1’ through ‘n’; where n is the chain length). These are the thermoplastic polymers.

Answer 116

The reaction starts with an initiator or free radical (OH- group). The initiation reaction breaks the double bond, leaving a single bond between the carbon atoms and one unpaired valence electron, which are free to react with the nearest monomer (Step ‘1’). This allows the chain to grow. Growth continues at the end of the chain as the carbon atoms in the monomers continue to add on (Steps ‘2’ & ‘3’). Finally another free radical is added once the desired chain length is achieved (step ‘n’). This acts as a terminator, which results in a stable polymer

Answer 117

Step growth (condensation polymerization) is the second type of polymerization which consists of individual chemical reactions between reactive monomers. Each monomer has multiple reaction sites and all monomers react simultaneously to form a single molecule. These are the thermosetting polymers.

Answer 118

thermoplastic and thermosetting polymers.

Answer 119

Thermoplastic polymers are created through chain growth polymerization. The carbon forms strong primary covalent bonds down the length of the chain, but the bonds between adjacent chains tend to be weak (due to polarity variations between different atoms).

Answer 120

The weak bonds between the chains give these polymers a low melting range (100 to 200° C), which makes them very soft and deformable upon heating (hence the name thermoplastic). They lose their strength and stiffness as temperatures increase because it is easy for molecules to slide past each other. In a sense, thermoplastics tend to creep in a similar fashion as metals; except this occurs at much lower temperatures since it happens throughout the thermoplastic while it only occurs at the grain boundaries in metals. Polymers can be easily deformed at 100ºC, where as some steels do not exhibit significant creep until 800ºC. This ease of forming gives thermoplastics a huge advantage for low cost, high volume production of parts that will be used near room temperature. All polymers have low impact toughness (a few joules) due to their low strength (small area under the stress-strain curve). Thermoplastics also exhibit a drop in impact toughness at low temperatures, which is similar to the transition temperature in BCC metals. In polymers this is called the glass transition temperature (Tg). Above Tg the polymer chains can easily slide past each other, making the thermoplastic soft and ductile, behaving like a viscous rubber. Below Tg the polymer chains will not slide as easily, making the polymer more brittle, but also stronger and stiffer, more like a glass. Hard plastics like polystyrene or polyvinylchloride (PVC) are used below their glass transition temperature for higher strength applications. Soft plastics or rubbers like polyethylene or polyisoprene are used above their Tg, making them soft and flexible. Metals form solid solution alloys when mixed while polymers can form mixtures of different monomers within the long chains. Acrylonitrile Butadiene Styrene (ABS) is such a compound and these are called copolymers. Figure 5 (below) illustrates a copolymer consisting of a mixture of ethylene and vinyl chloride. The resulting properties of the copolymer are significantly different from either of the base monomers.

Answer 121

1. Increasing chain length (number of atoms or molecular weight) 2. Branching (or adding side chains during polymerization; Figure 6) increases the stiffness and strength as the individual molecules become more mechanically intertwined. More force is required to get the polymer molecules to slide past each other. Think of a stack of poles; it is easy to pull out an individual pole. It is very hard to pull out a tree from a stack of trees due to the tangling. 3. Crosslinking polymers greatly enhances their strength because the long chains are chemically attached to each other with strong covalent bonds. Normally the linear chains in thermoplastics are relatively free to slide past each other. When they are cross-linked, individual polymer molecules are linked to other polymer molecules by atomic bridges (covalent bonds), usually after the part is formed. Cross-linked polyethylene (commonly called PEX) makes the polymer more rigid, reduces ductility (formability) and increases the melting point. PEX burst strength is improved, performance under heat (thermal stability) is improved, liquid permeability decreases, abrasion resistance increases, and notch sensitivity decreases. Vulcanization is a form of crosslinking that is used to give rubber similar improvement in properties.

Answer 122

High-density polyethylene (HDPE) has much higher strength then low-density polyethylene (LDPE), however, due to the shorter chain length LDPE has much more formability for producing things like thin films for plastic bags. FYI: Common LDPE is less dense then HDPE but better process control can produce LLDPE (Linear Low Density Polyethylene) which is about the same density as HDPE (but still weaker due to the shorter chain length). Ultra high molecular weight polyethylene (UHMWPE) gives even better strength and wear resistance for products such as bushings. During rotation the long molecules will break into shorter chains (i.e. grease). 4. Chain stiffening works by adding large pendant groups to the polymer backbone, which in turn prevents flexing and increases stiffness. Similar to branching, this also prevents the polymers molecules from sliding as easily, thereby increasing strength. Examples of pendant groups include the benzene ring in polystyrene (Figure 7) and the methyl group in polypropylene (less effective). 5. Polar groups increase the electrostatic charge between the chains. All polymers have weak bonding between chains which gives them low strength and high ductility. Addition of electronegative side groups (e.g. Cl (Vinyl), CN (polyacrylonitrile), O (polyester), etc.,) will be attracted to the slightly electropositive hydrogen. This increases the bond strength between chains which increases the overall strength. Polar groups also cause the polymer to bond easily to water, making the polymer hydroscopic (it likes water). Because polyethylene has no such groups, it is hydrophobic and does not absorb water or wet easily. For the same reasons, skin does not interact with polyethylene strongly, making it feel slippery. This also makes it difficult to form a knot. The addition of polar groups will have the opposite effect. 6. Add fillers to create a composite. Materials such as powdered limestone, wood flour, glass fiber, etc. will reduce the cost and increase the hardness. Metal powders may even be added to create electrical conductivity. 7. Increase crystallinity by stretching the polymer, which lines up the molecules in the stretching direction. This will increase strength in the longitudinal direction; however the material will be more easily split in the other directions. Polymer fibers (such as those used to make fabric) take advantage of this when the fiber is produced.

Answer 123

Peroxide - Peroxides are heat-activated chemicals that generate free radicals for crosslinking. This is called the Engel Process (PEX-A). Moisture-cured Vinyl Silane - A reactive silane molecule is grafted to the backbone of the polyethylene in the Silane Process (PEX-B). The silane becomes reactive by adding water, then crosslinking takes place across silicon and oxygen molecules to form the bridge. Beta Irradiation - This method involves subjecting PE to high-energy electrons (beta particles produced from radioactive decay) which knock off individual hydrogen atoms. This allows the underlying carbon to bond to the neighboring chain. This is called the Radiation Process (PEX-C).

Answer 124

The polymerization of nylon occurs when an acid (e.g., nylon 6/6 uses Adipic Acid; Formula HOOC (CH2)4 COOH) reacts with an amine (e.g. H2N (CH2)6 NH2)). This is called a “condensation copolymer”. The strength and toughness of nylon is among the highest of the polymers. The strong covalent bonds within the molecules run down the full length of the strand. This polymer can absorb moisture, which reduces its strength and causes swelling (up to 2.5%).

Answer 125

PTFE (Polytetrafluoroethylene or Teflon (TM DuPont)) is very non-reactive. The most electronegative element, fluorine, replaces the hydrogen atoms in the ethylene molecule (Figure 8). Fluorine forms an extremely strong bond, which makes it excellent for corrosion resistance (both acid and bases) and strong at high temperatures. Above 260ºC, its properties degrade. PTFE has the lowest coefficient of friction of any known solid material. Applications include seals, tubing, small corrosion resistant vessels, coatings (non-stick) and cookware.

Answer 126

n general, thermosetting polymers are stronger, more rigid (i.e., higher modulus of elasticity), less ductile and have lower impact toughness properties than thermoplastic polymers. They are often characterized by crosslinking (covalent bonds forming between adjacent molecules), and are often referred to as networks.

Answer 127

Elastomers (rubber)

Answer 128

twisted or coiled polymer molecules, and cross-links. 1)Twisted or coiled chains are important to give the elastomer the ability to stretch. As strain is applied, those twisted chains will straighten without breaking. This gives large amounts of deformation prior to failure. Most elastomers are made using dienes, monomers with two double bonds. One of the double bonds is broken during polymerization, while the other stays within the polymer molecule. Double bonds are not straight since the carbon atom is attached to three neighbouring atoms instead of four, so each creates a kink in the long chain. Enough kinks add up to create that coiled chain. 2)Cross-linking is important to make the elastomer’s deformation elastic instead of plastic. After that twisted chain is stretched, it would stay stretched as permanent deformation. The cross-links pull the molecule back into its coiled up position, making the elastomer act as a spring. Most elastomers are cross-linked by vulcanization, which consists of mixing sulfur (1 to 5%) with the elastomer and then heating it within a mould. The sulfur breaks a few of the double bonds in the polymer molecules to create a bridge between the chains (Figure 9). Increasing the sulfur content creates more covalent crosslink bonds between adjacent chains, which in turn increases strength, rigidity (stiffness) and reduces solubility in organic solvents and acids. It also, however, reduces resilience. The crosslink density is low compared to those in thermosetting polymers.

Answer 129

Isoprene (or polyisoprene) is a common synonym for the chemical compound 2-methylbuta-1, 3-diene. Dienes are hydrocarbons which contain two double bonds to give the “spring.” Isoprene is the simplest diene that can be used to make an elastomer.

Answer 130

Good chemical resistance (solvents, oxidation, acids, bases, etc.). Low compression set (able to maintain elastic properties after prolonged compressive stress). High operating temperature range. The working temperature range for Viton® is -30 to 200 ºC, but it will take temperatures up to 310 ºC for short periods of time.

Answer 131

Silicones (or polysiloxanes) are inorganic polymers that consist of a silicon-oxygen backbone . Side groups can be used to crosslink two or more of these backbones together. By changing the chain lengths, side groups and crosslinking, silicones can vary in consistency from liquid to gel to rubber to hard plastic. The most common type is linear polydimethylsiloxane or PDMS ([SiO (CH3)2] n). Silicones are odorless, colorless, water resistant, chemical resistant, oxidation resistant and stable at high temperatures. They also have weak forces of attraction, low surface tension and low freezing points, and also do not conduct electricity. They have many uses, including lubricants, adhesives, sealants, gaskets, breast implants, pressure compensating diaphragms for drip irrigation emitters, dishware, Silly Putty™, and many other products. Due to their thermal stability and relatively high melting and boiling points, silicones are often used where organic polymers are not applicable. Their very low reactivity generally makes them nontoxic for human implants.

Answer 132

Low fabrication costs (single step processing). High strength to weight ratio in thin films. Cheaper assembly (low stiffness) - able to use snap fits, friction welds and self-tapping fasteners. Good electrical resistance. Good corrosion resistance. Able to recycle the material. Transparent. Low finishing costs since painting is not required. Resistant to acids.

Answer 133

Strength. Strength-to-weight-ratio in large sections. Higher rigidity (Young’s Modulus). Can be used over a wider temperature range (high temperature oxidation resistance and toughness at lower temperatures). Immune to degradation to some acids and organic solvents where polymers dissolve. Immune to UV degradation (bonds break in polymers). Opaque. Resistant to organic solvents.

Answer 134

Unfortunately, recycling thermoplastics has proven difficult. The biggest problem with recycling polymers is that it is difficult to automate the sorting of polymer waste, and therefore it is labour intensive. Typically, workers sort the thermoplastic by looking at the resin identification code, though common containers like soda bottles can be sorted from memory. Other recyclable materials, such as metals, are easier to process mechanically. Another recycling problem with polymers is that they’re primarily use for food packaging, which requires that only virgin material must be used (i.e., no recycled plastic may be used to produce a food container).

Answer 135

1. Mixing, melting and plasticizing the raw materials, usually done by introducing the mix through a hopper into an externally heated cylinder with a screw, which mixes and pushes the mixture to the heated zone to be plasticized. 2. Shaping the melt by pushing it through a die or into a mold. 3. Fabrication by drawing, blowing, injecting or extruding. 4. Finishing and solidification of the melt.

Answer 136

Oil-based coatings. These coatings involve pigments in oil (e.g., oil-based paint) that react and polymerize with oxygen from air and provide thin, strong, resistant film. These materials have good workability and wetting characteristics to surfaces. Water-based or emulsion-type coatings are usually based on vinyl or acrylic dispersed in water as an emulsion. Many of these react and polymerize with oxygen from air. These coatings are easy workable. Lacquer-based coatings are based on synthetic resins, which are dissolved in solvents and dry rapidly. These coatings include vinyl, chlorinated rubber and nitrocellulose. Co-reactive coatings include epoxies and urethanes. The two thermosetting chemicals are mixed during application. During curing, they react forming a single molecule film. The surface temperature must be sufficiently high to allow curing. The chemical and abrasion resistance of these coatings make them widely used as machine coatings (enamel). Powder coat paints typically apply polyester powders to the part. It then reacts during heating cycle of form a single molecule at the surface. Solid coatings, such as asphalt or polyethylene waxes, may be applied by dipping the part in hot polymer melt. Polyethylene powder may also be applied onto a hot surface where it will melt. Special-purpose coatings include fluorocarbons and silicones. Fluorocarbons can be sprayed or dip coated, such as PTFE or PTFE-polyimide. They provide chemical resistance and lubricating characteristics. Silicones have high temperature resistance.

Answer 137

Ceramics are inorganic compounds where the primary bonds are ionic and/or covalent in a 3-dimensional structure. Ionically bonded ceramics are typically complex compounds of a metal with a non-metal (e.g., alumina A12O3, silica SiO2, magnesium oxide MgO). Covalent ceramics are different in that they are compounds of two non-metals (e.g., SiO2) or just pure elements (e.g., diamonds). Whether the ceramic is ionic or covalent, they form hard and brittle three dimensional networks.

Answer 138

Hard (good wear resistance). High compressive strengths (but poor tensile strength). Brittle (low toughness is the main cause of failure). The ability to withstand high temperatures (creep and oxidation resistance). Good electrical insulators. Good corrosion resistance (already in oxidized state).

Answer 139

1. Glasses - silica (SiO2) based with other compound additions to decrease the melting range, or give other special properties. 2. Vitreous ceramics (e.g., clay products such as dishes, tiles, brick, porcelain) 3. High performance ceramics (e.g., cutting tools, dies, and wear resistant parts).

Answer 140

Glasses are usually based upon silica (SiO2) with the addition of oxides (soda (Na2O), PbO, B2O3, etc.) to make it amorphous. The amorphous structure allows glasses to be easily molded; however it produces very low creep resistance. High.temperatures and stress will cause it to flow. While these ceramics may be formed into intricate shapes, crystalline ceramics (e.g., aluminum oxide) are more suitable for high temperature applications (> 300°C).

Answer 141

Vitreous Ceramics are made from clays which are formed in the wet plastic state and then dried and fired to reach their final properties. Clays are composed of alumina and silica; the most common clay mineral is Kaolinite clay with a glass binder. Once fired they consist of crystalline phases (mostly silicates) held together by a glassy phase based on silica (SiO2). The glassy phase melts when the clay is fired and spreads around the surface of the inert, but strong crystalline phases, bonding them together.

Answer 142

1) Oxides: Used as insulators (thermal and electrical), abrasives and wear parts, bio-ceramics, ceramic knives, and for ballistic vests or cockpits (lightweight). This class includes alumina (Al2O3), zirconia (ZrO2), silica (SiO2), chromium oxide (Cr2O3), and magnesium oxide (MgO). 2) Carbides: Very high hardness. Used in abrasives, cutting tools and wear-resistant parts. This class includes the carbides of tungsten, chromium, silicon, boron, titanium, vanadium and tantalum. 3) Nitrides: Used for wear-resistant parts and specialized electronic devices. Extremely thermal shock resistant (low thermal expansion coefficient), so also used for gas turbines and rocket engines. This class includes TiN, AlN, Si3N4. 4) Intermetallics: Includes nickel aluminide (NiAl), which is used in wear coatings. Two metals together results in a different crystal structure from the constituents which gives special properties. The most abrasion resistant material is diamonds embedded in NiAl matrix. 5) Diamond: Highest naturally occurring hardness. Used in cutting tools, dies and abrasives.

Answer 143

1. Blending. All components of the mixture in their proper proportions are blended together to get the final properties required. A binder such as paraffin is added to temporarily bind the particles together during forming. A small portion of glass is added to fuse the crystalline particles together during sintering. 2. Forming. Pressing or compaction methods include: i. Extrusion ii. Hot pressing iii. Isostatic pressing iv. Pill pressing 3. During presintering the part is a heated to over 800ºC. Pre-sintering removes the paraffin from the “green” compacts and the glass particles begin to soften and lightly bond with the crystalline particles. 3a. Where closer tolerances are required, the part can be machined to approximately 5% oversize now. Machining is easier while the material is still soft, and the 5% will allow for further shrinkage during sintering. 4. Sintering heats the ceramic to 1400 - 1500ºC. The glass component softens and acts as a solvent to allow crystalline phases to fuse. The glass is not providing strength, only allowing the ceramic particles to bond to each other. The final product is strong, but brittle. It has excellent corrosion, wear resistance and high temperature performance. Due to the high hardness, machining is very difficult after sintering.

Answer 144

Ceramics joining can be achieved by either mechanical joints such as fasteners or external shrinkage techniques or it can be achieved by using bonding methods such as applying adhesives or diffusion bonding.

Answer 145

In general, oxides are the least expensive and easiest to process among engineering ceramics. Alumina is good in thermal conductivity and thermal shock. Silicon nitride and silicon carbide provide high strength over a wide range of temperatures. Partially stabilized zirconia (PSZ) yields excellent strength that has a limited range of temperatures. If toughness at low temperatures is the main design criterion, then PSZ would be the best candidate.

Answer 146

Diamond-like carbon coatings. These are amorphous hard carbon coatings that provide wear resistance in a wide range of temperatures. Typical applications include punches and dies, cutting tools, forming tools and injection moulding tools. Some versions of these coatings provide good lubricating properties, which reduces friction in sliding systems. Titanium nitride coatings (~2 μm thick) are applied on high-speed steel or tool steel cutting tools, giving them a golden colour and improving their hardness and wear resistance. These coatings also act as thermal barriers, provide longer tool life and improve cutting process performance. Titanium carbonitride coatings provide higher hardness and better wear resistance than titanium nitride coatings. Other ceramic coatings, such as zirconium nitride and chromium nitride, are also used for wear and abrasion resistance.

Answer 147

Physical vapour deposition (PVD) is when the substrate is placed in a vacuum chamber and the coating material is evaporated from a heated container. All surfaces in the line of sight are coated with the vapour (deposition). Evaporation of ceramics requires high energy to cause evaporation, (e.g., electron beam gun). The adhesion of ceramic coatings produced by thermal evaporation PVD is poor, and this method is not generally used for ceramic coatings. DC (direct current) sputtering techniques are used for ceramic coatings. The part is placed in a vacuum chamber with a small amount of argon (10-3 torr). A high-voltage DC power supply creates plasma between the part to be coated (anode) and the coating material (cathode), which sputters off its atoms through the plasma to coat the part. A RF (radio frequency) supply is used for non-conductive targets. Ion plating combines the thermal evaporation PVD and sputtering techniques. The coating material is evaporated as in PVD and then ionized by the plasma using an electron beam gun. The ionized ceramic bombards the substrate with high velocity, which results in better adhesion between the part and the coating than PVD. In the case of titanium nitride coatings, pure titanium is sublimated and reacts with nitrogen in the chamber and then coats the part. Chemical vapour deposition (CVD) involves the deposition of coating material vapour on a preheated substrate. It has an advantage over PVD in that it does not require line of sight. Plasma-assisted CVD is used for diamond-like carbon coatings and silicon carbide coatings. For diamond-like carbon coatings, after reaching high vacuum in the coating chamber, argon and a carbon-containing gas (such as methane) are introduced at low pressure. When plasma is applied, the carbon dissociates and impinges onto the part at high velocity. This forms a carbon coating with good adhesion to the part. Plasma arc spray and high-velocity thermal spray processes melt and spray the ceramics onto a metal surface. Ion implantation is a surface modification process that involves ions of one material impinging and embedding into the substrate with sufficient energy, which results in modification and improvement of the surface properties. The process is performed in a vacuum.

Answer 148

Toughness to the composite. Support for the reinforcement and maintain structure integrity. Protection for the reinforcement from surface damage during manufacture and during service of the composite. Load transfer to the reinforcement. Major control over electrical properties, chemical behaviour and high-temperature use of the composite.

Answer 149

Acts as the load-carrying component. Provides the strength, stiffness and hardness to the composite.

Answer 150

The interface is the surface between the matrix and the reinforcement where a physical, mechanical or chemical discontinuity occurs. The adhesion between the matrix and reinforcing materials is critical to attaining high properties.

Answer 151

Fiber reinforced composites (“string and glue materials”) use continuous (plies) or discontinuous thin fibers. These fibers carry most of the force and are embedded in the matrix of another. The matrix supports and transmits the load to the fibers and provides ductility and toughness. Since the fibers are very thin, they are very flexible. Oxidation is the first stage (200°C and 300°C). The fibers attain their black colour through this process. Carbonizing is the next stage (1000°C and 3000°C). This process produces high-strength carbon fibers. Higher temperatures increase (graphitizing) the stiffness at the expense of strength. Oxidation is the first stage (200°C and 300°C). The fibers attain their black colour through this process. Carbonizing is the next stage (1000°C and 3000°C). This process produces high-strength carbon fibers. Higher temperatures increase (graphitizing) the stiffness at the expense of strength. Particulate or aggregate composites - consist of discrete particles of one material surrounded by a matrix of another material. Short fiber composites are typically viewed as particulates as long as the orientation of the particles is random. Particulate composites will have strengths generally uniform in all directions.

Answer 152

Concrete: cement is the binder and gravel is the reinforcement. Further addition of rebar changes the concrete into a three phase composite with the rebar giving directional strength. Grinding / cutting wheels: (silicon carbide and a binder; vitreous glass binder (for grinding) / plastic resin binder (for cutting)). Bakelite: phenol formaldehyde and fillers such as wood, stone flour, metal particles. Polycrystalline diamond: diamond powder bonded with a metal, resin, silicon carbide, or other type of binder. Cermets (ceramic-metal): combinations of hard, wear resistance ceramic particles bonded together with high temperature metals. The ceramic gives high wear resistance and the metal gives toughness. Cermets are often used for machining tool bits and other wear resist surfaces. “Carbide bits” used in machining are a type of cermet. The ceramic portion may be tungsten carbide, aluminum oxide, silicon carbide, etc. and these ceramic particles are typically bonded together with nickel or cobalt since they have high melting points, toughness and strength.

Answer 153

oCarburizing tungsten powder to form carbide particles. oBlending the carbide with a metal powder (Co or Ni binder). oHydrostatic pressing and machined to slightly oversize. oSintering (heat to bind the metal to the ceramic particles).

Answer 154

Corrosion resistance. The polymer matrix will not corrode (industrial tanks, piping, etc.). Most steel pipe has to be over designed with a minimum 20% corrosion allowance and even then there are many failures. A recent analysis of the 12,000 annual releases (leaks) in Alberta showed 58% were caused by internal corrosion with external corrosion caused an additional 12% (The remainder of causes were welds 4%, other 27%). High strength to weight ratio. Even though these very high strength materials are brittle, they may still be used since the fracture cannot easily progress. High stiffness to weight ratio (60% of all aircraft and aerospace applications use composite materials, marine craft, leisure (skis, tennis rackets), etc.). Since the surface is smoother than steel and doesn’t corrode, the fluid friction is reduced in pipes (~25%). This reduces operating costs (smaller compressors, pumps, etc.). There are thousands of kilometers of GRE buried in Alberta.

Answer 155

Material and manufacturing costs (slow production rates) tend to be high. Low toughness. Prone to delamination. Strength is in the direction parallel to the fibers. UV degradation of polymer matrix. Coatings can eliminate this problem. Creep in the polymer matrix. Most composites are limited to <100°C. Hydrolysis (absorbed moisture and other solvents) can degrade the polymer matrix. Vinyl ester and polyester will quickly soften in hot water (90°C); however, epoxy is immune up to 135°C. Requires specialized joining and repair methods.

Answer 156

Composite components can be joined together by either mechanical joints such as fasteners or by using bonding methods such as applying adhesives. Some combinations of mechanical joints alongside using adhesives also have been found practical when a strong bonding for the composite material is required.