Transcript CHAPTER 8 CERAMIC/METAL NANOCOMPOSITES

Noraiham Mohamad, PhD
Department of Engineering Materials,
Faculty of Manufacturing Engineering,
Universiti Teknikal Malaysia Melaka
 Introduction
 Ceramic
Matrix Nanocomposites
• Type of CMN
• Processing
 Metal
Matrix Nanocomposites
• Type of MMN
• Processing
 Nanocomposites-
solid structures with
nanometer-scale dimensional repeat
distances between the different phases that
constitute the structure
 Typically consist of:
• Inorganic(host) solid containing an organic
component or vice versa
• OR consist of 2 or more inorganic/organic phases in
some combinatorial form. Constraint- at least one of
the phases or features be in nano size
• Extreme example: can be porous media, colloids,
gels and copolymers
 Core concept:
• Combination of nano-dimensional phases with
distinct differences in structure, chemistry and
properties.
 Nanocomposites-
can demonstrate
different mechanical, electrical, optical,
electrochemical, catalytic, and structural
properties than individual component
 Multifunctional behavior often more than
the sum of individual components.
 Efforts
to develop high-performance
ceramics for engineering applications:
• Gas turbines
• Aerospace materials
• Automobiles and etc.
 Conventional
processed ceramics pose
many unsolved problems:
• Relatively low fracture toughness & strength
• Degradation mechanical properties at high T
• Poor resistance to creep, fatigue and thermal shock
 Attempts
phases
to solve  incorporating second
 Either:
• Particulates
• Platelets
• Whiskers
• Fibers
 Disappointment
in the use of micron-size
range fillers
 To solve the problem  nano-size fillers
• Passive control of microstructures by incorporating
nanometer-size second-phase dispersions into
ceramic matrices.
 How
to produce?
 Incorporation of a very small amount of
additive into ceramic matrix
 2 mechanisms:
• Additive segregates at the grain boundary with a
gradient concentration OR
• Precipitates as molecular or cluster sized
particles within the grains or at the grain
boundaries
 Crucial: process
optimization
2 major type:
 Intragranular

• Aim to:
 generate and fix dislocations during the processing,
annealing, cooling, and/or
 the in-situ control of size and shape of matrix grains
• Role of nanodispersoids esp in nano scale, important in
oxide ceramics, some of which become ductile at high T.

Intergranular nanodispersoids
• Must play important roles in control of the grain boundary
structure of oxide (Al2O3, MgO) and nonoxide (Si3N4,
SiC) ceramics which improves their high T mechanical
properties
Microsize particle,
platelet, whisker or
fiber
Microcomposite
Toughness increase, strength
decrease, High Tem mech
properties decrease
• High strength & reliability,
• excellent high temp mech
properties
New Functions
• Machinibility
•Superplasticity
 Eg. AL2O3/W, Mo, Ni, Cu, Co, Fe; ZrO2/Ni,
Mo; MgO/Fe,Co,Ni
 Processing method:
• Conventional powder metallurgy
• Solution chemical processes (eg. Sol-gel and co-
precipitation method)
 Powders-
sintered in reductive
atmosphere to give homogenous
dispersions of metallic particles within
ceramic matrices.

Dispersing metallic 2nd phases into ceramics
improves mechanical properties (eg. Fracture
toughness, strength and/or hardness)
• Due to- microstructural refinement by the nanodispersions and
their plasticity.
• Eg. Incorporating a small amount of ceramic or metallic
nanoparticles into BaTiO3, ZnO or cubic ZrO2.
 Improve mechanical strength, hardness and toughness ( important in
creating highly reliable electric devices operating in severe
environment conditions)
• Dispersion of soft materials: adding hexagonal boron nitride to
silicone nitride ceramic can enhance its fracture strength not
only at Troom but also at high T up to 1500C.
 Some exhibit superior thermal shock resistance and machinibility
because of the characteristic plasticity of one of the phases and the
interface regions between soft materials and hard ceramic matrices.

Wide variety of properties (magnetic, electric
and optical properties can be tailored)
• Due to- size effect of nanosized metal dispersions.
• Eg. Dispersing conducting metallic nanoparticles or
nanowires can enhance the electrical properties
Ferromagnetism- value added for transition
metal particle dispersed oxide ceramic
composites
 Good magnetic response to apply stress- found
in ceramic/ferromagnetic metal nanocomposites

• Allow the possibility to remote sensing of initiation
fractures or deformations.
Silicon nitride (Si3N4) and silicon carbide/silicon
nitride (SiC/Si3N4) composites
 SiC/Si3N4 composites

• Crystallites of microcrystals of Si3N4 and nanocrystals of
SiC.
• Produced from amorphus silicon carbonitride (obtained
from pyrolysis of compacted polyhydridomethylsilazane
[CH3SiH-NH]m[(CH3)2Si-NH]n at about 1000C.
• Can withstand high temperature without oxidation or
degradation (it is arises from the formation of a thin (few
microns) silicon oxide layer.

Application: motor engines, gas turbines,
catalytic heat exchangers and combustion
system (aircraft and spacecraft applications)
• Originally invented to form
small-particle (oxide, carbide,
etc.) dispersion-strengthened
metallic alloys
• High energy ball milling
process (eg. Planetary mill)
• Advantages:
• Simplicity & possibility of
scaled-up manufacturing.
• Disadv:
• Low purity and less
homogeneity of the
structures
Planetary mill
Welding - solid state reaction
Cracking - crystal refinement
MgB2-Magnesium diboride
(superconductor)

Alloying occurs as a result of repeated breaking up
and joining (welding)
• Can prepare highly metastable structures such as amorphus
alloys and nanocomposites structures with high flexibility.

In addition of erosion and agglomeration milling
provoke chemical reactions- induced by the transfer
of mechanical energy
• Can influence the milling process and product properties
• Idea is used to prepare magnetic oxide-metal nanocomposites
via mechanically induced displacement reactions between a
metal oxide and a more reactive metal.

Induce chemical changes in nonmetallurgical
systems, including silicates, minerals, ferrites,
ceramics and organic compounds.



Also called “Replacement reactions”
Chemical reaction in which a less reactive element is
replaced in a compound by a more reactive one.
For example, the addition of powdered zinc to a solution of
copper(II) sulphate displaces copper metal, which can be
detected by its characteristic colour:

Zn(s) + CuSO4(aq)  ZnSO4(aq) + Cu(s)

The copper is taken out of the solution and is deposited as a
solid (s).

May initiate self-propagating combustive
reaction by mechanical stresses developed
during high impact hits. The nature depends on:
• Thermodynamic parameters (esp. in highly exothermic
systems melting the reaction mixture & destroying
ultrafine (nanocrystalline) microstructure)
• Microstructure of the reaction mixture
• The way microstructure develops during the milling
process

Milling mixture of ceramic and metal powderscan induce mechanochemical reaction (an
efficient way to produce nanocermet).


reaction mixture
(chemistry) A product of the act or process or an
instance of mixing and reacting ( COMBINING)
two or more substances together causing
reaction(s); mixture causing chemical
transformation or change; the interaction of
entities :
• the state resulting from such a reaction
• a process involving change in atomic nuclei

A product formed by the combination of two or
more elements, compounds or substances
together causing reactions and transformation of
original items or substances.
Thermodynamic of the metal/metal oxide
systems
 Kinetics of the exchange (displacement)
reactions during processing.
 Eg. Reduction of metal oxides with aluminium 
nanocomposites of Al2O3 and metallic alloys (Fe,
Ni, Cr; particularly binary alloy systems)

• Ceramics with ductile metal inclusion produce
toughened materials with superior mechanical properties
• Have better thermomechanical properties (higher
thermal shock resistance due to better metal-ceramic
interfacial strength)
 Nanoparticles
of iron embedded in
insulating alumina matrix
(Nanocomposites with magnetic phases)
• Mixture of iron and alumina powder
• Process: direct milling using ball milling
• Ave particle size: 10 nm range
• Magnetization of iron particles: 25-40% less than
expected in bulk iron.
 Fe3O4
particles dispersed in Cu (smaller
magnetic particles embedded in
nonmagnetic matrix)
• Prepared by high-energy ball milling of Fe3O4 and
Cu powders OR mixture of CuO and metallic iron.
• Both results in: magnetic semi-hard nanocomposites
with a significant superparagamagnetic fraction (due
to very small particles sizes of the dispersed
magnetic phase)
 Hard
magnetic SmCoFe phases in soft
magnetic (Nanocomposite magnet)
• Prepared by mechanical milling and heat treatment
 Metastable
structures
nanocrystalline/amorphus
• Result from repeated deformation and fracture
events during collisions between powders and
balls.
• Plastic deformation in powders: formation of
shear bands reach high dislocation densities
shear bands degenerate into randomly oriented
subgrains.
• Large surface area of nanocrystalline grains help
in transformation of crystalline amorphus
A
shear band (or, more generally, a
'strain localization') is a narrow zone of
intense shearing strain, usually of plastic
nature, developing during severe
deformation of ductile materials.
two X-shaped shear bands are clearly
visible (see also the sketch on the right,
where initial vertical scratches on the
external surface help understanding the
shear deformation).
 Ideal
starting materials: Aerogels
 Aerogels:
• High-porosity structures (nanosize pores)
• Made by sol-gel polymerization of selected
silica, alumina or resorcinol-formaldehyde
monomers in solution
• Extremely light (~0.5-0.001 g/cc-1)
 Generation of a dispersion of colloidal particles suspended in Brownian
motion within a fluid matrix.
 Colloids are suspension of particles of linear dimensions between 1nm
and 1 m. The colloidal suspensions can subsequently convert to viscous
gels and then to solid materials
 Sol-gel preparation leads to the greatest possible homogeneous
distribution of reinforcing phases in the host matrix.
 Products have high purity and homogeneity, ease of processing and
composition control
Sol-gel synthesis involving following steps

Ageing

Gelation

Drying

Densification



Freeze drying or lyophilization is a drying method where the solvent is
frozen prior to drying and is then sublimed, i.e., passed to the gas
phase directly from the solid phase, below the melting point of the
solvent. It is increasingly applied to dry foods, beyond its already
classical pharmaceutical or medical applications. It keeps biological
properties of proteins, and retains vitamins and bioactive compounds.
Pressure can be reduced by a high vacuum pump (though freeze
drying at atmospheric pressure is possible in dry air). If using a
vacuum pump, the vapor produced by sublimation is removed from
the system by converting it into ice in a condenser, operating at very
low temperatures, outside the freeze drying chamber.
Supercritical drying (superheated steam drying) involves steam
drying of products containing water. This process is feasible because
water in the product is boiled off, and joined with the drying medium,
increasing its flow. It is usually employed in closed circuit and allows a
proportion of latent heat to be recovered by recompression, a feature
which is not possible with conventional air drying, for instance. The
process has potential for use in foods if carried out at reduced
pressure, to lower the boiling point.
Natural air drying takes place when materials are dried with unheated
forced air, taking advantage of its natural drying potential. The process
is slow and weather-dependent, so a wise strategy "fan off-fan on"
must be devised considering the following conditions: Air
temperature, relative humidity and moisture content and temperature
of the material being dried. Grains are increasingly dried with this
technique, and the total time (including fan off and on periods) may
last from one week to various months, if a winter rest can be tolerated
in cold areas.
In a typical phase diagram, the
boundary between gas and liquid
runs from the triple point to the
critical point. Regular drying is the
green arrow, while supercritical
drying is the red arrow and freeze
drying is the blue.
 The
product consists of:
• A substrate (eg. Silica aerogel)
• One or more additional phases (of any
composition or scale)
 Most
commonly made- silica-based
nanocomposite system (can be used to
other aerogel (alumina, etc.) precursor
Microstructure of
aerogel-encapsulated
phase nanocomposite
A flower is on a piece of aerogel which is
suspended over a bunsen burner.
Aerogel has excellent insulating
properties, and the flower is protected
from the flame
 Depending
introduced:
on when second phase is
• Can be added during the sol-gel processing (before
supercritical drying)
• Or added through the vapor phase (after
supercritical drying)- vapor phase infiltration
• Or chemical modification of the aerogel backbone
(may be effected through reactive gas treatment)
• Non-silica material is added to the silica sol before
gelation
 Produce
varieties of composites
 Can be:
• Soluble organic
• Inorganic compound
• Insoluble powder
• Polymer
• Biomaterial etc.
 Must withstand the
subsequent processing
steps used to form aerogel (alcohol soaking,
supercritical drying)
 Prevent from settling before gelation (for
bulk insoluble materials)
2
criteria
• Added component MUST NOT interfere with the
gelation chemistry of aerogel precursor
• MUST NOT leaching out during the alcohol soak
or supercritical drying steps
 Big
problem- if a high loading 2nd phase
is desired
Better to use chemical binding agent (can bind
to the silica backbone & bond the metal
complex)
 Many use to prepare nanocomposites of silica
aerogels or xerogel
 Nanocomposites of aerogel with metal atoms or
ions uniformly (atomically) dispersed throughout
the material
 Thermal post-processing- creates nanosize metal
particles within aerogel matrix
 Eg. Application:

• Catalysts for gas-phase reactions
• Catalysts for catalyzed growth of nanostructures
 Through
open pore network of aerogels
 Create various forms of aerogel-base
nanocomposites
 Almost any compound can be deposited
uniformly throughout aerogel
 Absorbed material in silica aerogel  solid
phases by thermal or chemical
decomposition
 Eg. CNT have been deposited within pores
of zeolites to create superconductor

Silica aerogel/carbon composites
• Decomposition of hydrocarbon gases at T (200-450 C)
• Carbon loadings from 1-800% (at low loading-C desposited
uniformly, at high-loading-C begins to localize at the exterior surf
of composites)
• Have interesting electrical conductivity (> certain level) & higher
mech strength than aerogel

Silica aerogel/silicon composites
• Thermal decomposition of various organosilanes on silica gel
forms deposits of elemental silicon
• Rapid decomposition of the silane precursor leads to deposits
localized near the exterior surface of the aerogel substrate
• Nanocomposites with 20-30 nm diameter silicon particles exhibits
strong visible photoluminescence at 600nm
 Silica
aerogel/transition metal
composites
• organo/transition-metal complexes can be used
to thermally deposit metal compounds
intermediate composites
• Intermediate composites can be converted to
metal oxides, sulfides or halides
• Control loading- can be changed by repeated
deposition steps
• Crystals of desired metal (range; 5-100 nm
diameter)
Metal matrix composites (MMC)
 Materials in which rigid ceramic reinforcements are
embedded in a ductile metal or alloy matrix
 Combine metallic properties (ductility & toughness)
with ceramic characteristics (high strength and
modulus), greater shear strength, compression
strength, higher service temperature capabiilties.
 Interest in MMCs increased over the past 30 years
(aerospace and automotive industries)
 Resulted from:
• Availability of inexpensive reinforcements
• Development of various processing routes- results in
reproducible microstructure and properties
• Availability of standard or near-standard metal working
methods (can be utilized to fabricate the composites)
 Type
of reinforcements: particulate,
whiskers or short fiber
 Most glamour: particulate (ease of
fabrication, lower costs and isotropic
properties)
 Traditional processing routes: powder
metallurgy, spray deposition, mechanical
alloying (MA) and various casting
techniques.
 All process: based on addition of ceramic
reinforcements to the matrix (molten or
powder form).
Ex situ process- reinforcing phases are prepared
separately prior to composite fabrication
 Scale of the reinforcing phase limited to starting
powder size, normally ~µm (10µm – 1µm)
 Less interfacial reactions between
reinforcements and matrix
 Poor wettability between reinforcements-matrix
(due to surface structure & contamination)
 Properties highly controlled by size, volume
fraction of reinforcements and nature of
matrix/reinforcements interfaces

Optimum: when fine,
thermally stable ceramic
particulates dispersed
uniformly in metal matrix
 Reinforcements
are formed in a metal
matrix by exothermal chemical reactions
between elements or between element
and compound during fabrication of the
composite.
 Wide range of matrix materials
(aluminium, titanium, Cu, Ni, Fe)
 Second phase particles (borides,
carbides, nitrides, oxides and mixtures)
 Formation
of reinforcements that
thermodynamically stable in matrix
 Reinforcement/matrix interfaces that
clean strong interfacial bonding
 Formation of reinforcing particles with
finer size, more uniform distribution in
matrix  yields better mechanical
properties
 Variety
of processing due to great
potential of in situ MMC for widespread
applications:
 Solidification process (most attractive)
• Reinforcements formed in situ in molten metallic
phase prior to its solidification
 Controlled crystallization
• Crystallization of amorphus solid under proper
heat treatment conditions
 Mechanical
alloying
 Why
attractive? (simple, economy and
flexibility)
 Factors affecting structures & properties
• Selection of solidification processing techniques
(solidification conditions)
• Matrix alloy composition
• Dispersoids
 Microstructure
refinement- rapid
solidification processing (RSP)
Reduce solute segregation
Enhance dispersion hardening (by reducing size
of reinforcing phases & modify their distribution)
 Ability to produce alloy composition not
obtainable by conventional processing
 Materials have excellent compositional
homogeneity, small grain sizes, homogenously
distributed fine precipitates or dispersiods.
 Eg of RSP:


• Large undercooling & high cooling rate of Ti/B or Ti/Si
alloys in situ Ti-based nanocomposites containing large
volume fraction of particles
• In-situ TiC particulate-reinforced Al-based composites
Master Material Ingot
Al, Ti, graphite powder
(Tmelt, graphite lined
furnace, Ar)
Weaknesses
• presence of agglomerates TiC
(0.2±1.0 µm) accumulate at Al
subgrain or grain boundaries
Direct Chill Cast
Chill block melt spinning
(rapid solidify in ribbon form)
Milling
Ribbons mill into powder (100±250
µm)
Result
TiC particles of 40±80 nm distributed
uniformly in Al matrix with grain
sizes of 0.3±0.85 µm
Canned & Degassed
Extruded
(into rods)
Nanocrystalline materials either single or multiphase polycrystals with grain sizes in nm.
 Properties are different from and often superior to
conventional polycrystals or amorphus solid:

•
•
•
•
•
•
•


Increase strength or hardness
Improve ductility or toughness
Reduce elastic modulus
Enhance diffusivity
Higher specific heat
Enhance CTE
Superior soft magnetic properties
How to produce?
Crystallization of amorphus solid via heat treatment
Complete crystallization  nanocrystalline materials
Controlled crystallization of amorphus alloys
partially crystallized materials with nanosized
crystallites embedded in residual amophus matrix
(special nanocrystal/amorphus nanocomposite
structure)
 Excellent mechanical & magnetic properties
 Eg. Fe/Cu/Nb/Si/B alloys (BCC Fe solid solution and
10 nm diameter nanostructures embedded in
amorphus matrix):


•
•
•
•
Excellent soft magnetic properties
High saturation flux density
Low magnetostriction
App: magnetic devices such as choke coils & transformers