Curso Introducción a la Ciencia de los Materiales (Peñoles

Tipos de materiales
• Compuestos
Más de un tipo de material combinado
Busca la mejor combinación de características de cada elemento
Ejemplo: fibra de vidrio (resistente) en material polimérico (flexible).
Compósito
A composite material is basically a combination of two or more
materials that are mechanically bonded together.
The resulting material has characteristics that are different than
the components in isolation.
The concept of composite materials is ancient. An example is
adding straw to mud for building stronger mud walls. Most
commonly, composite materials have a bulk phase or matrix and a
dispersed, non-continuous, phase called the reinforcement.
Some other examples of basic composites include concrete
(cement mixed with sand and aggregate), reinforced concrete
(steel rebar in concrete), and fiberglass (glass strands in a resin
matrix).
Tecnología antigua
Composite technology with T-800 Carbon Fiber
4
Contenido
Composites types are designated by:
the matrix material (Ceramic Matrix Composite, Metal MC, Polymer MC)
the reinforcement (particles, fibers, structural)
Composite property benefits:
MMC: improved E, σy, creep performance, Tensile Strength
CMC: improved KIc
PMC: improved E, σy, TS, creep resistance
Particulate-reinforced:
Types: large-particle and dispersion-strengthened
Properties are isotropic
Fiber-reinforced:
Types: continuous (aligned) and discontinuous (aligned or random)
Properties can be isotropic or anisotropic
Structural:
Laminates and sandwich panels
Clasificación de compósitos
Composites
Particle-reinforced
Largeparticle
Dispersionstrengthened
Fiber-reinforced
Continuous
(aligned)
Structural
Discontinuous
(short)
Aligned
Laminates
Sandwich
panels
Randomly
oriented
Composite materials, a mix of fibers and resins designed to provide
great strength yet remain very light weight, have been synonymous with
all aerospace applications from airplanes to NASA spacecraft and have
advanced into lightweight, strong materials for helmets, tennis rackets
and other sporting goods.
Porque usar compósitos?
Composite Phases
Phase types:
-- Matrix phase is continuous
-- Dispersed phase is discontinuous; surrounded by a matrix
Dispersed phase can have various shapes and
arrangements.
Longitudinal
direction
Fiber Alignment
Transverse
direction
aligned
continuous
aligned
random
discontinuous
What is a prepreg?
Prepregs
When
selecting prepregs the maximum service
temperature is one of the key selection criteria for
choosing the prepreg matrix.
The cure can be simply represented by pre-polymers
whose reactive sites join together forming chains and
cross linking. Once this process has taken place the
polymer is fully cured.
The thermoset cure essentially joins the reactive sites
together with the help of added components (filler,
accelerator, hardener, thermoplastic resins).
What are the properties of different thermoset matrices ?
There are three main
matrix types:
epoxy
phenolic
bismaleimide
The table indicates
the advantages of
each type and typical
applications.
Classification: Structural
Particle-reinforced
Fiber-reinforced
Structural
Classification: Particle-Reinforced (i)
Particle-reinforced
• Examples:
- Spheroidite matrix:
ferrite (α)
steel
Fiber-reinforced
(ductile)
60 µm
- WC/Co
cemented
carbide
matrix:
cobalt
(ductile,
tough)
:
Structural
particles:
cementite
(Fe C)
3
(brittle)
particles:
WC
(brittle,
hard)
600 µm
- Automobile matrix:
tire rubber rubber
(compliant)
0.75 µm
particles:
carbon
black
(stiff)
Particulate Composites
Rule of mixtures - The statement that the properties of a composite
material are a function of the volume fraction of each material in the
composite.
Cemented carbides - Particulate composites containing hard ceramic
particles bonded with a soft metallic matrix.
Electrical Contacts - Materials used for electrical contacts in switches
and relays must have a good combination of wear resistance and
electrical conductivity.
Polymers - Many engineering polymers that contain fillers and
extenders are particulate composites.
Classification: Particle-Reinforced (ii)
Particle-reinforced
Fiber-reinforced
Structural
Concrete – gravel + sand + cement + water
- Why sand and gravel?
Sand fills voids between gravel particles
Reinforced concrete – Reinforce with steel rebar
- increases strength - even if cement matrix is cracked
Prestressed concrete
- Rebar placed under tension during setting of concrete
- Release of tension after setting places concrete in a state of compression
- To fracture concrete, applied tensile stress must exceed this
compressive stress
Post-tensioning – tighten nuts to place concrete under compression
threaded
rod
nut
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.6 The effect of clay on the properties of polyethylene.
Classification: Fiber-Reinforced
Particle-reinforced
Fiber-reinforced
Structural
• Fibers very strong in tension
– Provide significant strength improvement
– Ex: fiber-glass - continuous glass filaments in a polymer
matrix
• Glass fibers
– strength and stiffness
• Polymer matrix
– holds fibers in place
– protects fiber surfaces
– transfers load to fibers
Particle-reinforced
Fiber-reinforced
Structural
• Reinforcement Types
– Whiskers - thin single crystals - large length to diameter ratios
• graphite, silicon nitride, silicon carbide
• high crystal perfection – extremely strong
nanowires
• very expensive and difficult to disperse
– Fibers
• polycrystalline or amorphous
• generally polymers or ceramics
• Ex: alumina, aramid, E-glass, boron
– Wires
• metals – steel, molybdenum, tungsten
boron
SiC whiskers
Fiber-Reinforced Composites
The Rule of Mixtures in Fiber-Reinforced Composites
Strength of Composites - The tensile strength of a fiber-reinforced
composite (TSc) depends on the bonding between the fibers and the
matrix.
Reinforcement: fibers and particulates
Glass
Carbon
Kevlar
Silicon Carbide
Boron
Ceramic
Metallic
Aggregate
Thermoplastic weaves
Carbon
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.9 The influence of volume percent boron-coated SiC
(Borsic) fibers on the properties of Borsic-reinforced
aluminum parallel to the fibers (for Example 16.7).
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.10 Increasing the length of chopped E-glass fibers in
an epoxy matrix increases the strength of the composite. In this
example, the volume fraction of glass fibers is about 0.5.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.11 Effect of
fiber orientation on the
tensile strength of E-glass
fiber-reinforced epoxy
composites.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.12 (a) Tapes containing aligned fibers can be joined
to produce a multi-layered different orientations to produce a
quasi-isotropic composite. In this case, a 0°/+45°/90° composite
is formed.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.13 A three-dimensional weave for fiberreinforced composites.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.14
Comparison of the
specific strength and
specific modulus of
fibers versus metals
and polymers.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.15 The structure of KevlarTM. The fibers are joined
by secondary bonds between oxygen and hydrogen atoms on
adjoining chains.
Composite Production Methods (i)
Pultrusion
Continuous fibers pulled through resin tank, then to
preforming and curing dies
Composite Production Methods (ii)
Filament Winding
Ex: pressure tanks
Continuous filaments wound onto mandrel
Orientation
The fiber directions can be arranged to meet specific
mechanical performance requirements of the composite
by varying the orientation.
Classification: Structural
Particle-reinforced
Fiber-reinforced
• Laminates -- stacked and bonded fiber-reinforced sheets
- stacking sequence: e.g., 0º/90º
- benefit: balanced in-plane stiffness
• Sandwich panels
-- honeycomb core between two facing sheets
- benefits: low density, large bending stiffness
face sheet
adhesive layer
honeycomb
Structural
Sandwich Structures
Sandwich - A composite material constructed of a lightweight, lowdensity material surrounded by dense, solid layers. The sandwich
combines overall light weight with excellent stiffness.
Honeycomb - A lightweight but stiff assembly of aluminum strip joined
and expanded to form the core of a sandwich structure.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.32 (a) A hexagonal cell honeycomb core, (b) can be joined to two face sheets by means of
adhesive sheets, (c) producing an exceptionally lightweight yet stiff, strong honeycomb sandwich
structure.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.33 In the corrugation method for producing a
honeycomb core, the material (such as aluminum) is corrugated
between two rolls. The corrugated sheets are joined together with
adhesive and then cut to the desired thickness.
Tipos de materiales
• Semiconductores
Monocristales con impurezas
Propiedades eléctricas intermedias entre conductores y aislantes.
Para fabricación de circuitos integrados: industria microelectrónica.
Semiconductores
Materiales como el silicio o el germanio, que son pobres conductores de la
electricidad, hasta que son “dopados” con pequeñas cantidades de otros
materiales como arsénico, fósforo o boro.
Los semiconductores se utilizan para construir dispositivos como diodos,
leds y transistores.
LEDs
Señales de tráfico
Materiales semiconductores (I)
Semiconductores elementales: Germanio (Ge) y Silicio (Si)
Compuestos IV: SiC y SiGe
Compuestos III-V:
Binarios: GaAs, GaP, GaSb, AlAs, AlP, AlSb, InAs, InP y InSb
Ternarios: GaAsP, AlGaAs
Cuaternarios: InGaAsP
Compuestos II-VI: ZnS, ZnSe, ZnTe, CdS, CdSe y CdTe
Son materiales de conductividad intermedia entre la de
los metales y la de los aislantes, que se modifica en
gran medida por la temperatura, la excitación óptica y
las impurezas.
Materiales semiconductores (II)
•Estructura atómica del Carbono (6 electrones)
1s2 2s2 2p2
•Estructura atómica del Silicio (14 electrones)
1s2 2s2 2p6
3s2 3p2
•Estructura atómica del Germanio (32 electrones)
1s2 2s2 2p6
3s2 3p6 3d10
4s2 4p2
4 electrones en la última capa
Diagramas de bandas (I)
Energía
Diagrama de bandas del Carbono: diamante
4 estados/átomo
Eg=6eV
- - - 4 electrones/átomo
Banda de conducción
Banda prohibida
Banda de valencia
Si un electrón de la banda de valencia alcanzara la energía necesaria
para saltar a la banda de conducción, podría moverse al estado vacío
de la banda de conducción de otro átomo vecino, generando corriente
eléctrica. A temperatura ambiente casi ningún electrón tiene esta
energía. Es un aislante.
Diagramas de bandas (II)
Diagrama de bandas del Carbono: grafito
Energía
4 estados/átomo
- - 4 electrones/átomo
Banda de
conducción
Banda de
valencia
No hay banda prohibida. Los electrones de la banda de
valencia tienen la misma energía que los estados vacíos de
la banda de conducción, por lo que pueden moverse
generando corriente eléctrica. A temperatura ambiente es
un buen conductor.
Diagramas de bandas (III)
Energía
Diagrama de bandas del Ge
4 estados/átomo
Eg=0,67eV
- - - 4 electrones/átomo
Banda de conducción
Banda prohibida
Banda de valencia
Si un electrón de la banda de valencia alcanza la energía necesaria para
saltar a la banda de conducción, puede moverse al estado vacío de la
banda de conducción de otro átomo vecino, generando corriente
eléctrica. A temperatura ambiente algunos electrones tienen esta
energía. Es un semiconductor.
ATE-UO Sem 07
Diagramas de bandas (IV)
Banda de
conducción
Eg
Banda de valencia
Aislante
Eg=5-10eV
Banda de
conducción
Banda de
conducción
Eg
Banda de valencia
Semiconductor
Eg=0,5-2eV
Banda de valencia
Conductor
No hay Eg
A 0 ºK, tanto los aislantes como los semiconductores no conducen, ya
que ningún electrón tiene energía suficiente para pasar de la banda de
valencia a la de conducción. A 300ºK, algunos electrones de los
semiconductores alcanzan este nivel. Al aumentar la temperatura
aumenta la conducción en los semiconductores (al contrario que en los
metales).
ATE-UO Sem 08
Superconductores
Los materiales superconductores son capaces de levitar en
presencia de un campo magnético intenso (imán).
• Tren de levitación magnética (MAGLEV)
Preguntas?