Lecture 6 - CMCs and MMCs Flashcards
1
Q
If you have a new material, which industries is the best to focus on?
A
Military, medical, sports since they are willing to pay.
2
Q
Why is it hard to introduce a new composite to the automotive industri?
A
Because of the need of mass production.
3
Q
Why do the transportation industri want composites?
A
Less weight which gives more fuel savings and improved accelerations.
4
Q
Why do the wind energy want composites?
A
The lower the weight the more power can the turbine produce.
5
Q
What properties does PMC, MMC and CMC have that the material by it self cannot attain alone?
A
PMC: Increased modulus, yield and tensile strength, creep resistance
MMC: Increased yield and tensile strength, creep resistance.
CMC: Increased fracture strength.
6
Q
Which 3 industries has the largest growth of composites?
A
Automotive/transportation, Wind energy and Aerospace.
7
Q
Comparing MMC and PMC, what are the main disadvantages and advantages?
A
MMC are more expensive than PMC and the conventional materials they are replacing.
MMC have advantageous properties over PMCs (Operates in a wider range of temperature, do not absorb moisture, have better electrical and thermal conductivity and are resistant to radiation damage) but they are also difficult to fabricate.
MMC can be used up to 1000°C only CMC can be used above that.
8
Q
what does inert mean and why is that important in composite manufacturing?
A
Inert means that the materials are chemically compatible and this is important to prevent severe reactions between the components.
9
Q
What is CTE?
A
Coefficient of thermal expansion
10
Q
For CMC CTE can be a problem, why?
A
The CTE is low, compared to its reinforcement. which can lead to cracks.
11
Q
which are the typical MMC matrix materials?
A
Lighter metals: Aluminium, Magnesium, Titanium,
High Temperature applications: Nickel, Cobolt
Övrigt: Intermetallics, Superalloys
12
Q
Which are the most common lightweight metals used?
A
Aluminium: is the most common metal matrix material, due to its low density (2.7 g/cm3), its high processability (associated with the low melting temperature of 660°C), and its high ductility (associated with its fcc crystal structure).
Magnesium: is even lower in density (1.7 g/cm3) than aluminium and also has a low melting temperature (650°C), but it suffers from its relatively low ductility (consequence of the hcp crystal structure and the fewer slip systems).
Titanium: has a relatively high density (4.5 g/cm3) and is relatively brittle (due to its hcp structure), but it is still attractive due to its high temperature capabilities (melting temperature: 1668°C).
13
Q
Titanium is very reactive, what is important to do?
A
It is important to lead away high temperatures, since the material is more reactive at high temperatures.
14
Q
Magnesium has HCP crystal structure and fewer slip systems, what does that mean?
A
It is hard shape
15
Q
What is thermal conductivity depending on and what does a low value mean?
A
It is depending on electrons and photons. If the value is low it means that there is a lot of obstacles ( tex in an alloy with different atoms or impurities.)
16
Q
How much reinforcement is it typically in MMCs?
A
10-60 vol.%
17
Q
what is typical for a metallic matrix?
A
soft and flexible
18
Q
What is important for the reinforcement in a MMC?
A
It must have high strength and stiffness
19
Q
what is needed when it comes to the bond between reinforcement-matrix in MMCs?
A
The bond must be strong so the load can be transfered from the matrix to the reinforcement
20
Q
What kind of reinforcement is used in MMCs?
A
• Two types of particulates
– dispersion strengthened alloys
– large particulate composites (e.g. cermets)
• Fiber reinforcements
– continuous/discontinuous fibers of different materials
21
Q
Howw do we classify composites?
A
• Composites – Particle reinforced ~ Large particle ~ Dispersion strengthening – Fiber reinforced ~ Continuous (aligned) ~ Discontinuous (short) = Aligned = Randomly oriented – Structural ~ Laminates ~ Sandwich panels
22
Q
what is important to remember about the structure when it comes to discontinuous reinforcements?
A
It will be an anisotropic material due to the alignment of the fibers and this affects the strength
23
Q
what is it called when the particulate is smaller than 1μm in diameter and what could it be capable of ?
A
It is called dispersoid and can provide Orowan strengthening.
24
Q
What is a precipitation?
A
a solute [löst ämne] dissolved in a metal while both are molten, precipitates as small particles when cooled.
25
What is a dispersion?
Disperse small, strong particles into a liquid metal, trapping the particles when it is cast in to shape.
26
What happens if you try to shear a precipitate?
- Large shear stress needed to move dislocation towards precipitate and shear it (works for small precipitates!).
- Dislocation “advances” but precipitates act as “pinning” sites with spacing S.
27
Is Dispersion strengthened alloys a composite?
- Dispersion strengthened alloys can be considered as composites because there is little or no interaction between the two components and the reinforcement is not soluble in the metal matrix.
- The dispersoids are usually 10-250 nm diameter oxide particles and are introduced by physical means rather than chemical precipitation. They are located within the grains and at grain boundaries but are not coherent with the matrix as in precipitation hardening
- The dispersed particles are sufficiently small in size to hinder dislocation movement and thus improve yield strength as well as stiffness.
- Dispersion strengthened alloys are somewhat weaker than precipitation hardened alloys at room temperature but since overaging, tempering, grain growth or particle coarsening do not occur upon heating, they are stronger and more creep resistant at high temperatures.
28
What is Cermets?
(actually cemented carbides) are composite materials made of ceramic (cer) and metallic (met) materials that combine the properties of both a ceramic(high temperature resistance and hardness) and those of a metal(such as the ability to undergo plastic deformation.)
One or more carbide compounds bonded in a metallic matrix.
29
What is the metal used for in cermets?
The metal (mostly Ni, Mo and Co) is used as a binder for an oxide, boride, carbide, or alumina.
30
Is cermets a MMC or a CMC?
Depending on the physical structure of the material, cermets are considered as metal matrix composites. But cermets are usually less than 20% metal by volume.
31
What are Cermets used for?
* Tungsten carbide cermets (Co binder) - cutting tools are most common; other: wire drawing dies, rock drilling bits and other mining tools, dies for powder metallurgy, indenters for hardness testers
* Titanium carbide cermets (Ni binder) - high temperature applications such as gas-turbine nozzle vanes, valve seats, thermocouple protection tubes, torch tips, cutting tools for steels
* Chromium carbides cermets (Ni binder) - gage blocks, valve liners, spray nozzles, bearing seal rings
32
WC-Co,
What do you use them for?
How do you produce it?
What properties does WC and Co have separately?
* Carbides such as WC are used for cutting tool inserts. However, WC is very brittle so it cracks or chips under impact loads. Hence, Co is used as a matrix.
* WC-Co cermets are produced by pressing Co and WC powders into compacts, which are heated above the melting point of Co (liquid phase sintering).
* Upon cooling, the carbide particles become embedded in the solidified Co, which acts as a tough matrix for the WC particles.
* In addition to its strength and toughness, Co is also selected because it wets the carbide particles to give a strong bond.
* Bonding can be enhanced by slight solubility between phases at elevated temperatures used in processing.
33
What are the 3 different processing techniques for MMCs?
- Liquid state processes
- Solid-state processes
- Deposition processes
34
What is the DuralcanTM Process and what could be possible problems?
(Liquid state processing)
(Low cost process!)
Al ingot and ceramic particles (usually SiC or Al2O3 of 8−12 μm in size) are mixed and melted. The melt is stirred at a temperature slightly above the liquidus temperature of the alloy to distribute the ceramic particles evenly. After casting, the solidified ingot may also undergo secondary processing by extrusion, forging or rolling.
Possible problems: Reaction of Al and SiC and the formation of brittle compound Al4C3 in the interfacial reaction layer. Al4C3 can be extremely detrimental to the mechanical properties of the MMC.
35
What is Pressureless) Preform Infiltration ?
| Liquid state processing
Infiltration of a preform of fibers or particles with a liquid metal.
Difficult process because of wetting of the ceramic reinforcement with molten metal. When the infiltration of a fiber preform occurs easily (such as with metallic fibers), reactions between the fiber and the molten metal may take place which significantly degrade the properties of the fiber.
36
Describe Applied Pressure MMC Production Routes (Squeeze casting and gas pressure assisted infiltration) and pros and cons with the technique.
(Liquid state processing)
Pressure is continuously applied until solidification is complete. The pressure forces the molten alloy through the interconnected pores in the particulate or fibrous perform (pressure up to 100 MPa in direct squeeze casting).
Advantage: No problem with the use of a poor wettability reinforcement due to pressure; no/minimal reaction between the reinforcement and molten metal since the casting and solidification times are both short.
Alternative: Pressure supplied using gas (pressure vessel using inert gas).
37
Describe Spray Deposition
| Liquid state processing
A spray gun is used to produce an atomized stream of Al alloy, into which heated silicon carbide particles are injected. An optimum particle size is required for efficient transfer into the atomized melt stream.
• High melt feed rates can be used.
• Reaction products at the particle-matrix interface can be avoided since contact times in the liquid state are very short.
• The MMC billets are usually porous and the material requires consolidation extrusion or rolling) for obtaining a fully dense MMC product.
• The ceramic content of the MMC can be controlled by the feed rates of melt and ceramic, but the content of the reinforcement is limited to 25%.
38
Describe Physical Vapor Deposition (PVD)
| Liquid state processing
Evaporation of a relatively thick layer of matrix material onto the surface of a fiber.
Principle: Thermal vaporization of the matrix material in a high vacuum. The reinforcing fiber is then passed through a region having a high vapor pressure of the matrix metal to be deposited. Condensation of the matrix material then occurs on the fiber to produce a thick surface coating). Typical deposition rates for the fiber coating are 5-10 μm/min.
Afterwards, the coated fibers can be bundled and consolidated using hot pressing or HIPping. A very uniform distribution of fibers (here 80%) or monofilaments can be produced.
39
Describe Powder processing
| Solid state processing
Matrix and reinforcement powders are blended together using high energy mixing; followed by cold compaction, canning, degassing and a high-temp. consolidation process such as hot pressing, extrusion or hot-isostatic pressing (HIPping) to produce a fully dense composite.
Also possible with use of whiskers.
40
Describe Diffusion Bonding
| Solid state processing
The process is mainly used for producing monofilament reinforced Ti alloys. Diffusion bonding is a very suitable technique for Ti alloys since Ti dissolves its own surface oxide layer at elevated temperatures in controlled atmospheres.
Advantage: Ability to process a wide variety of metal matrices allied with good control of fiber orientation and volume fraction.
Disadvantage: Long processing times, high processing temperatures and pressures; high costs.
41
When is the maximum strength obtained? (Fibers, load)
Maximum strength is obtained when long fibers are oriented parallel to the applied load. A three dimensional weave is also possible.
42
What is a quasi-isotropic material?
By using combinations of different fiber orientation quasi-isotropic materials may be produced.
43
Give 3 examples of Secondary processing of MMCs and describe them and ev problems.
Rolling and forging can rapidly impose high deviatory strains on a material. These high strains and strain rates can lead to cavities, fiber fracture and macroscopic cracking of an MMC, particularly if the deformation temperature is low (risk for cracks).
Forging can be carried out on MMCs provided that elevated temperatures are maintained and low strain rates are employed. Careful control of temperature needs to be maintained to avoid matrix melting and hot tearing (crack formation) during the forging operation.
HIPping is used for consolidation of MMCs since it generates no deviatory strains (no generation of material defects).
44
what is the possibility to machine MMCs?
| What techniques are used?
Poor, The main problem is that the hard abrasive reinforcements in most MMCs cause rapid tool wear during machining (high machining costs). Non-conventional machining processes, such as Electro Discharge Machining (EDM), laser cutting and Abrasive Water Jet (AWJ), can also be used for machining of MMCs.
45
what are the 3 Abrasive wear modes?
a) 3 body abrasive wear: freely moving particles
b) 2 body abrasive wear: embedded hard particles
c) 2 body abrasive wear: hard rough surface
46
Conventional fusion welding is not suitable for all MMCs, which one and why?
Even when it is working, what are the major issues?
Conventional fusion welding is not suitable for fibrous MMCs, since the distribution of fibers will be radically altered in the joint area and may be completely lost. In the case of particulate reinforced MMCs, there are still some major issues with the use of fusion welding processes:
• high melt viscosity in the weld pool
• segregation effects during re-solidification
• reactions at the interface between reinforcement and matrix
• gas evolution
47
What is the Classification of joining methods for MMCs ?
```
• Solid state processes
- Inertia friction welding
- Friction stir welding
- Ultrasonic welding
- Diffusion bonding
• Fusion processes
- Laser beam welding
- Electron beam welding
- Gas metal arc welding
- Gas tungsten arc welding
- Resistance spot welding
- Capacitor discharge welding
• Other processes
- Brazing
- Soldering
- Adhesive bonding
- Mechanical fastening
- Cast-insert joining
- Transient Liquid phase
- Rapid Infrared Joining
```
48
What are the three forms of interface between the two phases?
* Direct bonding with no intermediate layer; adhesion (”wetting”) is provided by either covalent bonding or van der Waals force
* Intermediate layer (inter-phase) is in form of solid solution of the matrix and dispersed phase constituents
* Intermediate layer is in form of a third bonding phase (adhesive)
49
What are the possible failure mechanisms (schematic) in fiber composite materials for loading vertical to the fiber orientation?
a) Poor adhesion: Fiber-matrix debonding
b) Medium adhesion: Fiber crack near the interface and on fiber by matrix
c) Good adhesion remains: Fracture within the matrix
d) Good adhesion: Splicing of the fiber
50
Why are cracks a good thing when it comes to failure?
They are energy consuming.
51
What is wetting depending on?
Time and temperature
52
Can MMCs be recycled?
Since ceramic materials usually occur in the form of particles, short fibers, or continuous fibers in MMCs, a material separation of the components with the goal being the reuse of the matrix alloy and the reinforcement is almost impossible. However, with conventional melting treatments in re-melting factories the matrix alloy can be recycled without problems.
53
Recycling is important how much energy do you save by recycling Al and steel?
energy committed to produce 1 kg recycled Al is 1/10 of the virgin material; for steel it is 1/3
54
What are the main advantages with ceramics?
* High melting temperature
* Low density
* Chemical inertness
* High hardness
* Potential for extending performance limit beyond that of metallic materials
55
What are the main problems with ceramics?
* Brittleness/low fracture toughness
| * Under tensile or impact loading, they fail catastrophically.
56
What are the main properties of CMCs?
high strength and modulus, low density, high-temperature use capability, and greater toughness in comparison to monolithic ceramics.
57
What applications are common for CMCs?
* cutting tools
* dental prostheses
* thermal barrier coatings
* wear resistant parts
* structural material for nuclear, energy, military, and aerospace.
58
What temperatures can thermal barrier coatings stand?
over 1000°C, while a superalloy is less than 700°C
59
What is TBC?
thermal barrier coatings
60
What are TBC used for and how do they work?
TBC are applied to metallic surfaces, such as stationary gas turbines or aero-engine parts, operating at elevated temperatures.
TBCs are insulating the metallic component and allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue.
61
What 2 TBCs are available?
Electron beam-physical vapor deposition (EB-PVD)
| Atmospheric Plasma Sprayed (APS)
62
compare APS vs EB-PVD
Electron beam-physical vapor deposition (EB-PVD) coatings can accommodate thermal stresses better than APS coatings due to their columnar structure, but the production cost is high. EB-PVD have longer life-time due to their microstructure, i.e. they are strain tolerant under cyclic thermal loads.
63
What is CMCs?
CMCs are a material consisting of a ceramic matrix combined with a ceramic (oxides, carbides) dispersed phase.
64
What are CMCs reinforced with?
CMCs are reinforced by continuous (long) fibers or discontinuous (short) fibers.
65
What is the main difference between ceramics and CMCs?
CMCs are designed to improve toughness of conventional ceramics (their main disadvantage is brittleness).
66
Which is the most common reinforcement in CMCs?
Most of the CMCs are reinforced by silicon carbide fibers due to their high strength and stiffness (modulus of elasticity).
67
Whiskers incorporated in short-fiber CMCs improve the materials toughness resisting to cracks propagation. But what happens in case of failure?
In case failure occurs, it is catastrophic.
68
Describe Long-fiber (continuous) composites (CMCs)
Long-fiber (continuous) composites are reinforced by long mono-filament or long multifilament fibers. Monofilament fibers produce stronger interfacial bonding with the matrix material improving its toughness. Failure of long-fiber CMCs is not catastrophic.
69
Name some Typical properties of long-fiber CMCs
* High mechanical strength even at high temperatures
* High thermal shock resistance
* High stiffness
* High toughness
* High thermal stability
* Low density
* High corrosion resistance even at high temperatures
70
What is a ceramic?
Ceramics are inorganic crystalline oxide materials that are solid and inert (often hybrids of ionic and covalent bonding).
Ceramic materials are brittle, hard, strong in compression, weak in shearing and tension.
Ceramics generally can withstand very high temperatures such as temperatures that range from 1000°C - 1600°C. Exceptions are inorganic materials that do not include oxygen such as silicon carbide.
71
Why do we want porosity when it comes to TBCs?
Because air is a bad thermal conductor (like windows)
72
Tell us a bit about Zirconium Oxide (ZrO_2) properties
```
• Use temperatures up to 2400°C
• High density
• Low thermal conductivity (20% that of alumina)
• Chemical inertness
• Resistance to molten metals
• Ionic electrical conduction
• Wear resistance
• High fracture toughness
• High hardness
Zirconia is an extremely refractory material (i.e. it retains its strength at high temperatures). It offers chemical and corrosion inertness at temperatures well above the melting point of alumina. The material has low thermal conductivity and it is electrically conductive above 600°C.
```
73
Pure zirconia exists in which three crystal phases and at what temperatures?
```
High temperatures (> 2370°C): cubic structure.
Intermediate temperatures (1170-2370°C): tetragonal structure.
Low temperatures (< 1170°C): monoclinic structure.
```
74
Describe the different transformations that can occur for pure Zirconia.
The transformation from tetragonal to monoclinic is rapid and is accompanied by a 3-5 % volume increase that causes extensive cracking in the material. This behaviour destroys the mechanical properties of fabricated components during cooling and makes pure zirconia useless for any structural or mechanical application.
Several oxides which dissolve in the zirconia crystal structure can slow down or eliminate these crystal structure changes, e.g. MgO, CaO, and Y2O3. With sufficient amounts added, the high temperature cubic structure can be maintained to room temperature. Cubic stabilized zirconia is a useful refractory and technical ceramic material because it does not go through destructive phase transitions during heating and cooling.
75
How are SiC-matrix composites manufactured?
SiC-matrix composites are fabricated by chemical vapor infiltration (a variant of CVD) or liquid phase infiltration methods of a matrix material into a preform prepared from silicon carbide fibers.
76
How are alumina ans alumina-silica composites manufactured?
- Sol gel method
| - Direct Oxidation Process
77
Hos does spark plasma sintering work?
The SPS process heats the powder compact directly by the pulse arc discharges, thus achieving very high thermal efficiency. As a result, material densification by SPS is generally very fast (within a few minutes) and can be achieved at temperatures 200 to 500°C lower than those used in conventional sintering. The sintering process is pressure-assisted, which helps the plastic flow of the material and accelerates the sintering process.
78
What are common failures for CMCs?
- Stress corrosion cracking due to environmental effects at the crack tip (combo of stress and corrosion)
- Thermal shock due to constraint of thermal expansion
- Thermal shock due to uneven rapid heating/cooling
- Intergranular fracture is most likely, when the grain boundaries are weak.
- In transgranular (cleavage) fracture, the crack passes through the grains. The fracture surface may be smooth or shows small steps.
- Brittle cleavage fracture due to local stress rises at the crack tip. If it exceeds the stress required to break inter-atomic bonds, they separate, leading to cleavage fracture.
79
What is the differences between a ductile and a brittle fracture visually?
Ductile: Occurrence of necking and cup and cone fracture surfaces with dimples.
Brittle: Flat fracture surface (grainfacets).
80
What value does fracture toughness have for ceramics and what does it mean?
Ceramics have low fracture toughness. That means they are only absorbing little energy during fracture. Hence, a crack has to be stopped in its movement in another way and the energy required for crack propagation has to be increased.
81
What are the Toughening mechanisms in short fiber CMCs?
(a) Bridging toughening effect: Cracks pass through the matrix leaving intact fiber bridging them. Loading of the composite can continue until fiber failure (gradual failure of composite). This normally occurs by successive fracture and (b) pull-out of individual fibers leading to pseudo-plastic behaviour giving relatively high fracture energies.
(c) Deflection toughening mechanism: the crack is led around second phase particles or fibers leading to a reduction in stress intensity at the crack tip → apparent toughness increase.
(d) Microcrack induced toughening can be induced by the incorporation of ZrO2 particles in a ceramic matrix.
82
How does toughening by fibers work for CMCs?
When the crack grows in the matrix, the reinforcement (fibers) remains intact and bridge the crack. This promotes multiple cracking – each contributing its own energy and thereby raising the overall dissipation. When the fibers do break, the breaks are statistically distributed, leaving ligaments of fibers buried in the matrix. The pullout, as the crack opens up, dissipates more energy by friction.
83
How is the classification of toughening mechanisms in ceramics?
- Crack deflection: Tilt and twist out of the crack plane around grains and 2nd-phase additions
- Crack bowing: Bowing in the crack plane between 2nd-phase crack-pinning points
- Crack branching: Crack may subdivide into 2 or more roughly parallel cracks
- Crack tip shielding by process zone activity: Microcracking, Transformation toughening, ductile yielding in process zone
- Crack tip shielding by crack bringing: 2nd-phase brittle fibers with partial debonding, frictional & ligamentary grain bridges, 2nd-phase ductile ligament bridging
84
Which factors contribute to the fracture toughness of ceramics?
- Volume fraction of reinforcement
- Young’s modulus of matrix and reinforcement: If a matrix is reinforced with high modulus, high strength fibers then more stress can be carried by the fibers.
- Strength of the matrix/ reinforcement interface: In fiber- reinforced composites a strong interface can lead to transfer of stress from the matrix to the fibers. A weak interface can lead to debonding and crack deflection.
85
Tell us more about the toughening mechanism involving phase transformation in zirconia (ZrO2)
Zirconia toughened aluminia (ZTA) contains 10-20 vol% ZrO2 particles. Zirconia has tetragonal structure at elevated temperatures (t) and a monoclinic structure (m) at low temperature.
When cooling ZTA from high temperatures, the t → m transition occurs in the particles. The transformation is rapid and it is accompanied with a volume increase of 3%. This volume change produces stresses in the alumina matrix around the transformed particles leading to microcracking. These microcracks increase the toughening of the ceramic by their ability to deflect and bifurcate the propagating crack.
Control of the extent of the microcracking determines the increase of toughness. (Optimum: particles are large enough to transform but small enough to only cause limited microcrack development → crack interaction).
86
Tell us more about the toughening mechanism involving stress induced transformation in zirconia (ZrO2)
If a crack extends under stress, large tensile stresses are generated around the crack, especially ahead of the crack tip. These stresses release the matrix constrain on the tetragonal ZrO2 particles, and if sufficiently large may lead to transformation to monoclinic structure. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.
The volume expansion (3%) and shear strain (1-7%) developed in the particle lead to compressive strain in the matrix. Since this occurs in the vicinity of the crack, extra work would be required to move the crack through the matrix accounting for the increase in toughness and hence strength.
87
What microstructural parameters that are relevant to toughening?
* Volume fraction of constituents
* Shape (diameter, length and aspect ratio) of second phase such as particulate, platelet or fibers
* Orientation of fibers with respect to the loading direction.
88
If you want to machine CMCs, what is important to remeber?
CMCs are hard and very abrasive. Hence, they show high cutting forces and high cost on tool consumption. Machining requires diamond coated inserts for both drilling and milling operations. Electrical discharge machining (EDM) is a possible method for shaping/machining.
89
If you want to recycle CMCs, how do you do that?
Recycling of ceramics: often by crushing due to the high melting point. The same should apply for CMCs.
