Ceramic & Metal Matrix Composites & Nanocomposites
Dr. Xujin Bao Email:
[email protected] Room: S2.21 Tel (office) 01509 223150
Topics / Contents Introduction of composite materials Ceramic Matrix Composites (CMCs)
Introduction, Reinforcement materials, Matrix materials Fabrication Techniques of CMCs Toughening mechanisms of CMCs Interface and interphase Applications of CMCs (Carbon-carbon composites)
Metal Matrix Composites (MMCs) Introduction, Matrix materials, Reinforcement materials Fabrication Techniques Physical and Mechanical Properties of MMCs Applications of MMCs
Nanocomposites (CMNCs, MMNCs & PMNCs)
Reference Books F.L. Matthews and R.D. Rawlings “Composite Materials: Engineering and Science”, Woodhead publishing ltd, 1999. D. Hall and T.W. Clyne “An introduction to Composite Materials”, Cambridge University Press, 1996 B. Harris “Engineering Composite Materials”, Cambridge University Press, 1999 D.Gay, S.V. Hao and S.W. Tsai “Composite Materials Design and Applications”, CRC Press, 2003 R. Warren “Ceramic Matrix Composites”, Blackie, 1992. J.N. Fridlyander “Metal Matrix Composites”, Chapman & Hall, 1995 W. Krenkel “Ceramic Matrix Composites” Wiley-VCH, 2008 K.K. Chawla “Compsite Materials Science and Engineering” , Springer, 2012
Assessment This module consists of 20% coursework (a literature review on composite materials) and 80% examination. Dr Richard Heath: PMCs: week 1-7 Dr Hongtao Zhang: Mechanics of Composites week 8-10 Gerry Boyce (EPL) : Case studies, week 11 Dr Xujin Bao: CMCs, MMCs & Nanocompsoites: week 2-8
1. Introduction 1.1 Conventional materials and their limitations Plastics low density; good short-term chemical resistance; poor mechanical properties, but are easily fabricated and joined. Ceramics low density; great thermal stability; resistance to most forms of attack (abrasion, wear, corrosion). very rigid and poor processibility Metals medium to high density except magnesium, aluminium; good thermal stability and may be made corrosion-resistant by alloying; useful mechanical properties and high toughness, and easy to shape and join; the preferred engineering materials.
Why composites Improved performance,
Less weight, more strength and lower cost, Currently used materials frequently reach the limit of their usefulness. Produce either improved traditional materials (e.g. engineering ceramics, polymers and metal alloys) or completely new materials Composites are an example of ‘new’ materials developed with the intention of satisfying the above criteria.
1.2. Composite – A definition Composites have two (or more) chemically distinct phases on a microscopic scale, separated by a distinct interface. The constituent that is continuous and is often but not always, present in the greater quantity in the composite is termed the matrix. A composite may have a ceramic, metallic or polymeric matrix. The second constituent is referred to as the reinforcing phase, or reinforcement, as it enhances or reinforces the mechanical properties of the matrix. In most cases the reinforcement is harder, stronger and stiffer than the matrix, although there are some exceptions; for example, ductile metal reinforcement in a ceramic matrix and rubber-like reinforcement in a brittle polymer matrix.
Composite – A definition (2) Traditional composites Composite is usually produced by intimately mixing and combining the constituents by various means. Matrix, reinforcement and interface Both constituents have to be present in reasonable proportions, greater than 5% (improving mechanical properties) New developing composites
Nano-composites Molecular Composites Functional composites ………
Development of composites The development in the UK of carbon fibres, and in the US of boron fibres in the early 1960s. The development of high strength SiC and Al2O3 fibres which maintain their properties at elevated temperatures has initiated much of the current interest in composites based on metals and ceramics. The key factor is their very high strength to weight and stiffness to weight ratios because of their low densities. This is of great importance in moving components, especially in all forms of transport where reduction in weight results in greater efficiency leading to energy and hence cost savings.
Terminology/Classification • Matrix phase:
-- Purposes are to: - transfer stress to dispersed phase - protect dispersed phase from environment
-- Types:
woven fibers
MMC, CMC, PMC
metal
ceramic
polymer
• Dispersed phase:
0.5 mm
cross section view
-- Purpose:
MMC: increase σy, TS, creep resist. CMC: increase KIc PMC: increase E, σy, TS, creep resist.
-- Types: particle, fiber, structural
0.5 mm
Composite development
Classification of Composite The mechanical properties of composites are a function of the shape and dimensions of the reinforcement. The reinforcement is usually either fibrous or particulate. In single-layer composites long fibres with high aspect ratio are called continuous fibre reinforced composites, whereas discontinuous fibres composites are fabricated using short fibres of low aspect ratio. Multi-layered composites are classified as either laminates or hybrids. Laminates are sheet constructions which made by stacking layers in a specific sequence. Hybrids are usually multi-layered composites with mixed fibres, which may be mixed in a ply or layer by layer and these composites are designed to benefit from the different properties of the fibres employed.
Classification of Composites
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)
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 or remesh - increases strength - even if cement matrix is cracked Prestressed concrete - Rebar/remesh 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 Posttensioning – tighten nuts to place concrete under compression threaded rod nut
Classification: Particle-Reinforced (iii) Particle-reinforced
Fiber-reinforced
Structural
• Elastic modulus, Ec, of composites:
-- two “rule of mixture” extremes: upper limit: Ec = Vm Em + Vp Ep E(GPa) 350 Data: Cu matrix 30 0 w/tungsten 250 particles 20 0 150 0
lower limit: 1 Vm Vp = + Ec Em Ep 20 4 0 6 0 8 0
(Cu) • Application to other properties:
10 0 vol% tungsten
(W)
-- Electrical conductivity, σe: Replace E’s in equations with σe’s. -- Thermal conductivity, k: Replace E’s in equations with k’s.
Classification: Fiber-Reinforced (i) Particle-reinforced •
Fiber-reinforced
Structural
Fibers very strong in tension – Provide significant strength improvement to the composite – 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
Classification: Fiber-Reinforced (ii) Particle-reinforced •
Fiber-reinforced
Structural
Fiber Types
– Whiskers - thin single crystals - large length to diameter ratios • graphite, silicon nitride, silicon carbide • high crystal perfection – extremely strong, strongest known • very expensive and difficult to disperse – Fibers • polycrystalline or amorphous • generally polymers or ceramics • Ex: alumina, aramid, E-glass, boron, UHMWPE – Wires • metals – steel, molybdenum, tungsten
Longitudinal direction
Fiber Alignment
Transverse direction
aligned continuous
aligned random discontinuous
Classification: Fiber-Reinforced (iii) Particle-reinforced Fiber-reinforced • Aligned Continuous fibers • Examples: -- Metal: γ'(Ni3Al)-α(Mo)
Structural
-- Ceramic: Glass w/SiC fibers
by eutectic solidification.
formed by glass slurry Eglass = 76 GPa; ESiC = 400 GPa.
matrix: α (Mo) (ductile)
(a)
fracture surface
2 µm
fibers: γ ’ (Ni3Al) (brittle)
(b)
20
Classification: Fiber-Reinforced (iv) Particle-reinforced Fiber-reinforced • Discontinuous fibers, random in 2 dimensions • Example: Carbon-Carbon -- fabrication process: - carbon fibers embedded in polymer resin matrix, - polymer resin pyrolyzed at up to 2500ºC. -- uses: disk brakes, gas turbine exhaust flaps, missile nose cones.
(b)
(a)
Structural C fibers: very stiff very strong
C matrix: less stiff view onto plane less strong 500 µm
fibers lie in plane
• Other possibilities: -- Discontinuous, random 3D -- Discontinuous, aligned 21
Classification: Fiber-Reinforced (v) Particle-reinforced Fiber-reinforced Structural • Critical fiber length for effective stiffening & strengthening: fiber ultimate tensile strength
σf d fiber length > 2τc
fiber diameter shear strength of fiber-matrix interface
• Ex: For fiberglass, common fiber length > 15 mm needed • For longer fibers, stress transference from matrix is more efficient Short, thick fibers:
σd fiber length < f 2τc
Low fiber efficiency
Long, thin fibers:
σd fiber length > f 2τc
High fiber efficiency
22
Fibre reinforced composites
Composite Stiffness Particle-reinforced
Fiber-reinforced
• Estimate of Ecd for discontinuous fibers:
-- valid when fiber length < 15 σ f d
τc
-- Elastic modulus in fiber direction:
Ecd = EmVm + KEfVf efficiency factor: -- aligned: -- aligned: -- random 2D: -- random 3D:
K = 1 (aligned parallel) K = 0 (aligned perpendicular) K = 3/8 (2D isotropy) K = 1/5 (3D isotropy)
Structural
Composite Stiffness: Longitudinal Loading Continuous fibers - Estimate fiber-reinforced composite modulus of elasticity for continuous fibers • Longitudinal deformation
σc = σmVm + σfVf volume fraction
∴
Ecl = EmVm + Ef Vf
c = composite f = fiber m = matrix
and
εc = εm = εf isostrain
Ecl = longitudinal modulus
Composite Stiffness: Transverse Loading • In transverse loading the fibers carry less of the load εc= εmVm + εfVf
and
σc = σm = σf = σ isostress
∴
1 Vm Vf = + Ect E m Ef Ect = transverse modulus
EmEf Ect = VmEf + Vf Em c = composite f = fiber m = matrix
Classification: Structural Particle-reinforced
Fiber-reinforced
Structural
• 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
27
Composite Benefits • CMCs:
Increased toughness
Force
• PMCs: 10
particle-reinf
E(GPa) 10
Increased E/ρ ceramics
3 2
PMCs
10
fiber-reinf
metal/ metal alloys
1
un-reinf
0.1
polymers
0.01 0.1 0.3
Bend displacement
• MMCs:
Increased creep resistance
3
6061 Al
10 -6
10 -8
10 30
Density, ρ [mg/m3]
10 -4
εss (s-1)
1
6061 Al w/SiC whiskers
10 -10 20 30 50
σ(MPa) 100 200
The properties of composites – Modulus density
Advantages of composite materials High modulus and strength per unit weight compared to traditional materials Fabrication flexibility allowing to produce the components with complex structures Large components can be produced without welding or joints.
1.3. Reinforcement Materials Reinforcement: enhances or reinforces the properties of the matrix. In most cases, the reinforcement is harder, stronger and stiffer than the matrix and ideally has a lower density. One of the dimensions of the reinforcement is usually small, a few microns. The geometry of the reinforcement is one of the major . factors in determining its effectiveness. The mechanical properties of composites are a function of the type, shape and dimensions of the reinforcement.
Particulate reinforcements Particulate reinforcements have dimensions that are approximately equal in all directions. The shape of the reinforcing particles may be spherical, cubic, platelet or any regular or irregular geometry. The arrangement of the particulate reinforcement may be random or with a preferred orientation. .
In the majority of particulate reinforced composites the orientation of the particles is considered, for practical purpose, to be random.
Size, shape & dispersion
Reinforcements (Strong fibres) One of the important features of engineering materials is that they are never as strong as the strengths of the chemical bonds which holds them together. The reason for this is that all materials contain various kinds of defects which can never be entirely eliminated in practical manufacturing operations. The strength of any sample of a glass or ceramic is actually determined by the size of the largest defect, or crack. Griffith’s equation
σmax = √ 2E ץF / πa where σmax is the strength of the material, E is its elastic stiffness (Young’s modulus), ץF is the work required to fracture the sample and a is the flaw size.
Glass fibres, for example, are manufactured by drawing molten glass very rapidly down to form fine filaments of the order of only ten microns (10 μm) in diameter. Careful measurements of the strengths of freshly drawn glass fibres shows them to be up to 5GPa by comparison with the very modest 100 MPa or so of bulk glass.
Why use fibre as the reinforcement The smaller the size, the lower the probability of having imperfections in the material. (Carbon fibre as an example). A high aspect ratio (length/diameter, l/d) that allows a very large fraction of applied load to be transferred via the matrix to the stiff and strong fibre. The smaller the diameter, the more flexible of the fibres, which permits a variety of techniques to be employed for making composites with these fibres
Comparison of the mechanical properties of fibres and their monolithic counterparts
Material Young’s modulus /GPa 300 Alumina - Fibre (Saffil RF) Monolithic 382 Carbon - Fibre (IM) 290 Monolithic 10 Glass - Fibre 76 Monolithic 76 406 Silicon carbide - Fibre (MF) Monolithic
410
Strength /MPa …… 332 3100 20 ……… 100 3920
………
1.4 The Matrix Materials The composite matrix is required to fulfil several functions: The matrix binds the fibres together and loads applied to the composite are then transferred into the fibres, The matrix must also isolate the fibres from each other so that they can act as separate entities and cracks are unable to pass unimpeded through sequences fibres in contact, which would result in completely brittle composites. The matrix should protect the reinforcing fibres from mechanical damage (e.g. abrasion) and from environmental attack. Through the quality of its “grip” on the fibres (the interfacial bond strength), the matrix can also be an important means of increasing the toughness of the composite.
Properties of MMCs
Properties of CMCs
Alumina (99% purity) Al2O3 + 25%SiC whiskers LAS glass-ceramic LASa + 50%SiC CFb Mullite Mullite + 25%SiC whiskers
Young’s modulus (GPa)
Strength (MPa)
Toughness KIC (MPa m1/2)
340
300
4.5
390
8.0
86 135
160
1.1
-
244 452
2.8
a: LAS is lithiumaluminosilicate; b: CF is continuous fibres
Properties of PMCs
2. Ceramic Matrix Composites (CMCs) Introduction Matrix materials Monolithic ceramics Monolithic glasses Glass ceramics
Reinforcement materials SiC, C, ZrO2, TiC Fabrication techniques
Conventional mixing and processing Continuous fibre composites Liquid state processing Vapour deposition Direct metal oxidation In-situ techniques
Toughening mechanisms of CMCs Interface and interphase Carbon – carbon composites
Why CMCs The fundamental limitation of ceramics is their brittleness, which, despite the fact that they have high strength and stiffness, restricts their use as engineering materials. The main advantage of a ceramic matrix composite is the possibility of improving the toughness, with possible increases in strength and stiffness.
Schematic force displacement curves for a monolithic ceramic and CMCs showing the greater energy of fracture for CMCs.
Development of CMCs CMCs development has lagged behind that of metallic and polymeric composites for a number of reasons: processing routes involve high temperatures be used with high temperature reinforcement The development of CMCs was only made possible when fibres and whiskers of high temperature ceramics, such as SiC, became available. There is little point reinforcing a high temperature ceramic with a ductile metal which can lose its strength at temperatures much lower than the possible operating temperature of the ceramic. The high processing temperatures also can lead to thermal stresses being present during cooling due to the different thermal expansion coefficients of the matrix and the reinforcement material.
Application of CMCs The chief application of CMCs is for cutting tools, such as those made by Greenleaf, and are they widely replacing metallic carbide cutting tools due to their enhanced properties. Cutting rates of 450 m per minute are achievable, which is a significant improvement over more conventional materials they are used extensively in the machining of Ni-based superalloys. Alumina is also used extensively in pipework carrying solids suspended in liquids or gases - this requires good erosion resistance, which is significantly enhanced by making a composite with SiC.
Potential problems in fabrication of CMCs The high processing temperatures can lead to thermal stresses being present during cooling due to the different thermal expansion coefficients of the matrix and the reinforcement material. In MMCs the thermal stresses can be relieved by plastic deformation of the matrix, but not possible in CMCs and cracking of the matrix can result. For fibrous reinforcements, if ar>am the axial tensile stresses induced in the fibre produce an overall net residual compressive stress in the matrix and, as the fibres contract, there is a tendency for them to pull away from the matrix. The stress situation is reversed when am>ar and then cracking of the matrix may occur due to the axial tensile stresses. The coefficients of expansion of the fibre and reinforcement must therefore be matched to prevent cracking occurring.
2.1. Ceramic Matrix Materials Traditional / conventional ceramics: These are developed earlier and used primarily for nonmechanical properties such as thermal insulation, electrical resistance/ conductivity. These include bricks, pottery, tiles, and a variety of art objects. Advanced / high performance ceramics: A new and improved class of ceramic materials which are of interest in ceramic matrix composites. Examples of these high performance ceramics include oxides, nitrides, and carbides of silicon, aluminium, titanium and zirconium.
Structure of monolithic ceramics Ceramics can be defined as chemically very stable, inorganic, crystalline, non-metallic compounds or mixtures of such compounds. The chemical bonding in ceramics is generally a hybrid of ionic and covalent, the proportions of which vary between different compounds. The covalent bond is stronger than the ionic bond, and is also directional. The characteristic properties which are the result of such bonding are for example, high melting point, high chemical stability, high elastic moduli and low dislocation and atomic mobility, the latter leading to high hardness and creep resistance but also to brittleness.
Structure of monolithic ceramics
Material
Bonding
Melting Point /oC
SiC
Covalent
2500
Si3N4
Covalent
1900
NaCl
Ionic
801
MgO
Ionic
2620
SiO2
Covalent-ionic
1713
Al2O3
Covalent-ionic
2045
Material Density Melting (gcm-3) point (oC)
E (GPa)
n
CP (J/g K)
Thermal Coeff. cond. thermal (W/m K) expan. (K-1)
Elect. Resist. (W m)
Al2O3 SiC
3.99 3.2
2050 ~2500a
390 440
0.23 0.15
1.25 1.25
6.0 40
8.0 4.5
>1015 1
Si3N4 B4C BN (cubic)
3.2 2.5 3.5
1900a 2450 ~3000a
300 440 -
0.22 0.18 -
1.25 2.1 2
15 15 -
3 5.5 -
0.5 -
BN (hex) BN (hex) AlN TiB2 TiC TiN MoSi2 ZrO2 (tetr.) ZrO2 (mono)
2.3
-
45
-
-
21
7.5
1011
2.3
-
70
-
-
14
0.8
-
3.26 4.5 4.9 5.4 6.25 6.1
2300a 2980 3070 3090 2100 2400a
320 570 450 400 -
0.25 0.11 0.18 0.17 -
1.1 1.23 0.85 0.85 0.56 0.7
50 25 30 ~30 20 2
6 5.5 8.5 8.5 8.5 12
2x1012 10-5 10-4 5x10-5 2x10-5 -
5.55
-
240
0.3
-
-
15
-
2.8
1850
150
0.24
1
5
5.5
-
Mullite
Structure of monolithic glasses The atoms in a crystalline material are arranged in an ordered structure over long ranges. Glasses have a more disordered, but not completely random arrangement with short range order. Silica, SiO2, can exist in either state. If cooled slowly it will crystallise at a specific temperature, TM, whereas if cooled more quickly it can form a glass at Tg, the glass transition temperature. The open network of the glass is that it can accommodate atoms of a different species with relative ease - network formers, e.g. B and network modifiers, e.g. Na.
Structure of monolithic glasses
Crystalline SiO2
Network of glassy SiO2
Soda-silica glass
Structure of monolithic glass-ceramics A glass is called a vitreous solid, has a higher free energy than the crystalline state and is therefore metastable. Under certain conditions it is possible to crystallise, or devitrify, a glass to form a glass-ceramic. Glass-ceramics are polycrystalline materials which are formed when glasses of a suitable composition are heat treated and hence undergo controlled crystallisation - typically 50-98% of the volume may be crystalline. The properties are clearly dependent on the relative proportions of the glassy and crystalline phases. A glass-ceramic has superior mechanical properties to its parent glass and may exhibit unusual properties such as an extremely small coefficient of thermal expansion. One of the attractions of glass-ceramics is that the glass can be shaped using a number of inexpensive glass forming techniques, and then heat treated to crystallise it.
Formation of glass-ceramics
2.2 Reinforcement Materials - Ceramic fibres High strengths and moduli, with high temperature capability with freedom from environmental attack. Three types of ceramic fibre fabrication methods: chemical vapour deposition, polymer pyrolysis and sol-gel techniques. The great breakthrough in the ceramic fibre area has been the conception of pyrolysing, under controlled conditions, polymers containing silicon, carbon and nitrogen to produce high-temperature ceramic fibres. The most widely used are SiC and alumina, however other fibres based on Si3N4, B4C and BN are used.
Boron fibres Boron fibre is in itself a composite fibre, being made with a W core because of the high temperatures involved in its manufacture. It is made by chemical vapour deposition (CVD) from the reactants boron trichloride and hydrogen onto a tungsten substrate. The reaction involved is: BCl3(gas) + H2 (gas) = 2B (solid) + 6HCl (gas) The W wire is about 10 μm in diameter and is passed through a reaction chamber and a layer of approximately 50 μm thick is deposited onto the wire.
Boron fibres (2) The density of B is 2.34 gcm-3, which is 15% less than that of Al, and the density of the B fibres with W cores is 2.6 gcm-3 for a 100 μm diameter fibre with melting point of 20400C. Boron fibre composites are used in the US F15 Eagle fighter plane and in the US space shuttle, and have recently found more mundane applications in sports.
Boron fibres (3)
Microstructure of boron fibres prepared via CVD
Alumina fibres Alumina exists in 4 crystalline forms - a, d, g and h: it is polycrystalline a Al2O3 which dominates in fibres and has a hexagonal close packed structure. Alumina fibres can also contain other oxides, mainly SiO2, in appreciable quantities. The fibres are used with a refractory binder in mats, boards and tiles for thermal insulation and also as reinforcements in MMCs especially with Al alloy matrices.
Alumina fibres The melting point of alumina is above 2000oC but has a low viscosity when molten so it is not possible to produce the fibres by melt spinning. Two main methods based on solutions of Al compounds are used Slurry Sol-gel
Alumina PF fibres via slurry method
The main commercial fibre produced by this method is Al2O3 - FP by Du Pont. It contains 99% alumina and is 10-20 μm in diameter with a grain size of about 0.5 μm.
Properties of PF fibres Diameter (µm)
Density (g cm-3)
20 + 5
3.95
Tensile strength (MPa) 1380
Young’s modulus (GPa) 379
Melting point (°C) 2045
Alumina fibres via sol-gel method The second method is sol-gel processing in which a viscous, highly concentrated solution of an aluminium containing compound is spun to produce either continuous or discontinuous fibres depending on the process parameters. Commercial fibres produced by this method are SAFFIL (short) and SAFIMAX (long ~0.5m) which are typically 3 μm in diameter and contain 4% SiO2.
Alumina fibres via sol-gel method
Properties of Alumina fibres
Silicon carbide fibres There are two main methods for the production of SiC fibres CVD onto W or C substrates decomposition of a polymeric precursor CVD involves decomposition of gaseous silane:
CH3SiCl3 (gas) = SiC (solid) + 3HCl (gas) Reaction vessel very similar to that for the production of B fibres. A W core is usually used, but also carbon core. Compos Diamete Density Tensile Young’s ition r (µm) (g cm-3) strength modulus (MPa) (GPa) β-SiC
140
3.3
3500
430
Avco Special fibres Special fibres have been developed by Avco (Avco SCS2 and SCS-6 for use with Al and Mg, and Ti matrices respectively) Have a composition gradient at the surface, resulting in a carbon rich surface which promotes bonding with the matrix.
Avco Special fibres
SiC fibres via polymers Various steps involved in the polymer route are: Identify a suitable starting polymer Devise an efficient polymer preparation method Characterise the polymer (e.g. yield, molecular weight and purity) Melt spin the polymer into a precursor fibre Cure the precursor fibre to crosslink the molecular chains, making it infusible during the subsequent pyrolysis. Pyrolyse to obtain the ceramic fibre.
SiC fibres via polymers
SiC fibres via polymers Reaction proceeds via decomposition of polydimethylsilane. Note that some oxygen, and excess Si and C are present resulting in some SiO2 being present in the fibres. Nicalon fibres (12 μm diameter) have a composition of 59 Si-31 C-10 O wt.% Tyranno fibres (12 μm diameter), made from polytitanocarbosilane also contain up to 5 wt.% Ti. Fibres produced by this method are much finer, and therefore more useful, and are also cheaper than those produced by CVD.
CH3
Cl Si
CH3
Cl
Dichlorodimethylsilane Dechlorination with Na CH3 Si CH3 Polydimethylsilane
SiC fibres via polymers
Polymerisation at 470oC CH3 H Si C H H Polycarbosilane Melt spinning at 350oC Polycarbosilane fibre Curing 190oC in air, or r.t. in ozone Polycarbosilane fibres with molecular cross linking by oxygen to avoid subsequent melting Pyrolysis - 1300oC in vacuum SiC fibre Amorphous or nanocrystalline β-SiC
Properties of SiC fibre
Fibre
Tensile strength (GPa)
Young’s modulus (GPa)
Nicalon 200
2.0
200
Hi-Nicalon S
2.5
400
Tyranno SA3
2.9
375
Other ceramic fibres Si3N4 fibres can be prepared by reactive CVD using volatile Si compounds such as SiCl4 and NH3 onto a W or C substrate. It can also be prepared by a polymer route involving organosilizanes (containing Si-NH-Si bonds) with methyl groups on the Si and N. After pyrolysis however this produces a mixture of SiC and Si3N4. BN has a similar density to C fibre, but has a greater oxidation resistance and dielectric properties. Boric oxide precursor fibres can be converted into BN by nitriding with NH3, followed by a stabilising high temperature heat treatment which removes any oxide traces. …….
Glass fibre manufacture
Carbon fibres Worldwide production capacity of carbon fibre exceeds 30 000 tons a year, despite the fact that carbon fibre is relatively expensive, costing more than the equivalent strength synthetic organic fibre. It is used widely in the aerospace industry and in sporting goods with polymeric and metallic matrices. The structure and properties of carbon fibre vary considerably depending on process route and new forms are always being investigated hollow fibres - improved impact toughness coiled fibres - extend significantly without loss of elasticity.
Manufacture of Carbon fibres There are three main stages in production: Stabilisation - in which the precursor is heated at low temperatures to prevent melting during subsequent processing at high temperatures Graphitisation - which increases the degree of order in the molecules and hence improves properties Graphitisation - which increases the degree of order in the molecules and hence improves properties
Manufacture of Carbon fibres
Properties of CF
Strength and elastic modulus of CF as a function of final heat treatment temperature
Carbon-carbon composites Carbon-carbon composites are formed from fibrous carbon in a carbonaceous matrix. The reinforcement can either be chopped fibre or continuous, usually in the form of mats. The matrix can be highly crystalline or glassy graphite depending on the method of production, and the degree of porosity can range from virtually none to up to 80%. The properties therefore vary considerably from composite to composite given the variety of possible matrices and reinforcements and processing parameters.
Carbon-carbon composites There are two main classes of carbon-carbon composites: ‘porous’ and ‘dense’. Three possible routes for production of carbon fibres: Cellulose or rayon fibres (thermosetting polymers) Petroleum and coal tar pitches Polyacrlyonitrile (PAN)
Ceramic particulate reinforcement Particulate reinforcements generally much cheaper. SiC is the most widely used - traditionally made by heating sand and coke at 2400oC in a furnace. This produces large particles of SiC which then need to be reduced in size. SiC can also be made as whiskers, small single crystals, of the order of 1 μm diameter and up to 100 μm long. These are commonly grown from the saturated gas phase or manufactured from Nicalon or Tyranno fibres.
Ceramic particulate reinforcement Recently produced commercially by pyrolysis of rice husks (containing 15-20% silica). The reaction is: 3C + SiO2 = SiC + 2CO 10% of the SiC is formed as whiskers, with the rest being particulate. Particulate SiC, in the form of platelets 10-100 μm, is a popular choice for reinforcement, particularly for ceramic matrix composites.
Ceramic whiskers Whiskers are monocrystalline, short fibres with extremely high strength because of absence of crystalline imperfections such as dislocations. Production process of SiC whiskers
Metallic fibres Metallic fibres have the advantage that they exhibit consistent strength values whereas ceramic fibres can vary considerably in strength due to inherent flaws and imperfections. Conventional wire drawing processes can produce metal wires down to 100 μm in diameter, whereas for wires of smaller diameters, 10 μm, the Taylor process has to be used. the metallic wire is encased in a sheath of sacrificial material (e.g. glass) heated - sheath goes soft and the core wire melts or softens. wire/sheath combination is then heavily drawn in the plastic state down to a very fine diameter the sheath is subsequently removed chemically
Metallic fibres
2.3. Processing of CMCs Objectives: To produce materials with a predetermined microstructural geometry with a minimum of harmful defects To optimise the strength of the fibre/matrix interface and hence mechanical properties For particulate and whisker reinforced composites - the processing techniques developed for monolithic ceramics. For long fibre composites new techniques have had to be developed because of the difficulty of maintaining continuous fibre geometries during processing with the minimum of fibre damage, and because the fibres can degrade at the high temperatures required for sintering.
Conventional mixing and pressing Difficulties for manufacture of CMCs: Difficult to obtain a homogeneous mixture of the two constituents Cannot add high proportions of the reinforcement Whiskers can form aggregates, which reduces packing efficiency Damage possible to whiskers during the mixing and pressing operations
Fabrication of ceramics and composites from Powders Powders
This route involves the production of the desired body from an assemblage of finely divided solids (i.e. powders) by the action of heat.
Additives: binders, Platicisers, reinforcements etc
Mixing
Pressing, Injection Moulding, casting etc
Forming
Figure: A flow-chart of processing of ceramics from powders
Drying & Sintering
Densified products 91
Processing of CMCs from powders Powder forming processing has two functions: To shape the powder as nearly as possible into the shape of the component To bring the powder particles as closely as possible to permit effective sintering. In cold forming processes the powder is formed into a green body prior to subsequent sintering. in hot forming processes forming and sintering occur simultaneously. The effectiveness of the sintering of the green body is favoured by high green density and uniformity of the distribution of the powder particles. The control and retention of the shape are also favoured by a high green density which implies low shrinkage and controllable variation of green body density within the compact.
Cold forming processes The choice of method is determined by the geometry, the complexity of shape and the production volume of the component.
Cold forming processes – Uniaxial pressing In uniaxial cold pressing, the powder is poured into a rigid die and then pressed with a closely fitting punch. Friction between the powder particles and between the powder and die/punches lead to an uneven distribution of stress throughout the compact This leads to non-uniformity of density. Uniaxial pressing becomes difficult if the fibre contains whiskers or short fibres since the random packing density of particles with fibrous shape is very poor. If the fibres are not to become crushed, the achievable green density is very poor. The shape and the size of the compact is limited by the friction effects and that it has to be ejected from the die. Thus, the method is suitable for small components with relatively simple geometry.
Cold forming processes – Cold isostatic pressing (CIP) In this process the powder is filled into a flexible mould generally prepared from rubber and then subjected to isostatic pressure via a fluid in a pressure chamber. Friction effects are much less in this process than in uniaxial pressing and the pressing of larger more complex shapes is possible. The rubber moulds are relatively easy to produce but the process cycle itself is slow and so the method is appropriate for small series of components.
Cold forming processes – Cold isostatic pressing (CIP)
Cold forming processes – Slip casting In slip casting, a slurry of the powder is poured into a porous mould, usually made of gypsum, which absorbs the liquid carrier causing the powder to be drawn to the mould walls. This method is mostly used for production of traditional ceramics and is ideal for preparation of large, thinwalled, hollow components. For optimum casting, the slurry or slip should be a stable suspension with as high solid contend as possible but with low viscosity.
Cold forming processes – Slip casting
Dispersants; pH, solid content, viscosity……
Cold forming processes – Tape casting A process related to slip casting is tape casting in which thin layers of slurry are cast onto a substrate of thin plastic film. Both slip casting and tape casting are suitable for shortfibre composites, tending to produce a twodimensionally random fibre orientation.
Cold forming processes – Tape casting
Casting processing control (1) Careful process control is necessary in the slip casting process. Some of the critical factors include: Constancy of properties Viscosity Settling rate Freedom from air bubbles Casting rate Drain properties Shrinkage Release properties Strength
Casting processing control (2) Constancy of properties refers to the reproducibility of the casting slip and its stability as a function of time. The slip must be easily reproduced and preferably should not be overly sensitive to slight variations in solids content and chemical composition or storage time. The viscosity must be low enough to allow complete fill of the mould, yet the solid content must be high enough to achieve a reasonable casting rate. Too-slow casting can result in thickness and density variations due to settling. Too-rapid casting can result in tapered wall (for a drain casting), lack of thickness control, or blockage of narrow passages in the mould.
Cold forming processes – Injection moulding Injection moulding involves mixing of the ceramic powder with a sufficient amount of polymer or other soft binder to produce a mouldable dough. This is then injected under pressure into a mould. Injection moulding has the potential for the fabrication of short-fibre reinforced composites. However, the powder content is limited to a critical volume fraction above which the viscosity of the mix increases sharply.
Cold forming processes – Injection moulding Particular interest is the possibility of controlling fibre orientation through control of the flow of the mix through the mould. The main problem is the limit to fibre fraction set by packing geometry (uneven shrinkage). Injection moulding is used mostly in the production of large series of components with complex shape.
Processing of whisker and short fibre reinforced composites Whisker and short fibre composites are difficult to produce using conventional cold forming techniques followed by pressureless sintering due to poor packing characteristics of particles with high aspect ratio. However, these composites can be prepared using conventional processing with relatively straightforward modifications provided pressure assisted sintering methods such as hot pressing. Processing of these composites includes following stages: whisker cleaning to remove impurities wet mixing of matrix powder and whiskers sintering (hot pressing); i.e. uniaxial hot pressing is the most common method in the commercial production of cutting tools.
Processing of long fibre CMCs These composites can be divided into two as follows: Composites prepared by impregnation of a continuous, multifilament yarn of fibres with matrix (most commonly in the form of a powder slurry). The impregnated yarn can be laid up into various geometries prior to consolidation. Composites prepared by infiltration of a fibre preform of predetermined shape and usually with a multiaxial fibre geometry. An underlying principle of techniques developed for long-fibre composites is that a preform of fibres with the required geometry is infiltrated with the matrix or a matrix precursor. The infiltrating matrix can for example take the form of a powder slurry(slurry infiltration), a liquid solution (liquid infiltration), or a mixture of gases or vapours that react to form the matrix.(chemical vapour infiltration, CVI)
Fabrication of Ceramic Matrix Composites by Liquid phase Infiltration The methods of fabrication of Ceramic Matrix Composites, utilising infiltration of a liquid into long continuous fibres, are as follows: Infiltration of molten ceramic Slurry Infiltration Process (SIP) Reactive Melt Infiltration (RMI) Polymer Infiltration and Pyrolysis (PIP)
Fabrication of Ceramic Matrix Composites by Liquid phase Infiltration Infiltration of molten ceramic Infiltration of molten ceramic into a fibre preform is limited by low viscosity of molten ceramics and by high temperature causing chemical interaction between the molten matrix and the dispersed phase (fibres). This process is sometimes used for fabrication glass matrix composites.
Infiltration Fabrication of CMCs All Infiltration technique incorporate the following stages of fabrication: Fabrication of preform. A preform of the required shape is prepared by laying-up and moulding the fibres reinforcing phase. Deposition of interphases. The fibres are coated with interphases during either the filament production or after the preform fabrication. Infiltration. The fibrous preform is infiltrated with a pre-ceramic fluid. The fluid contains either ceramic matrix particles (slurry) or a substance, which may be converted into a ceramic as a result of chemical reaction. Thermal processing. Ceramic matrix forms in the space between the fibres when the pre-ceramic fluid incorporated into the reinforcing structure is heated.
Classification of infiltration methods of ceramic composites fabrication (1) Infiltration techniques employ different types of the fluids and the processes of conversion of the fluid into a ceramic: Polymer Infiltration and Pyrolysis (PIP). Ceramic matrix is formed from a low viscosity preceramic organo-metallic polymer infiltrated into a preform as a result of pyrolysis. Chemical vapour infiltration (CVI). A preceramic gaseous precursor (vapour) infiltrates into the fibre reinforcing preform and converts into ceramic as a result of chemical decomposition.
Classification of infiltration methods of ceramic composites fabrication (2) Reactive Melt Infiltration (RMI). The preform is infiltrated with a liquid metal, which produces ceramic matrix when reacting with a surrounding substance. Liquid Silicon Infiltration (LSI). Silicon carbide matrix forms during the reaction of molten silicon infiltrated into the preform with the porous carbon. Direct melt oxidation (DIMOX). Ceramic matrix is produced from a molten metal (commonly aluminium) oxidised by the surrounding air.
Classification of infiltration methods of ceramic composites fabrication (3) Slurry Infiltration. The matrix is formed from a slurry containing fine ceramic particle, which infiltrates into the preform and converts into ceramic after drying and hot pressing. Sol-Gel Infiltration. A sol preceramic precursor infiltrates into the preform, undergoes polymerisation (gelation) and then is converted into a ceramic at an elevated temperature.
Classification of infiltration methods of ceramic composites fabrication (4) Combined infiltration methods. Slurry Infiltration + Polymer Infiltration and Pyrolysis (PIP). Infiltration of the preform with a preceramic polymer blended with fine ceramic particles (slurry) followed by pyrolysis. Slurry Infiltration + Liquid Silicon Infiltration (LSI). Chemical Vapour Infiltration (CVI) + Liquid Silicon Infiltration (LSI). Chemical Vapour Infiltration (CVI) + Polymer Infiltration and Pyrolysis (PIP).
Fabrication of CMCs by Polymer Infiltration and Pyrolysis (PIP) Polymer Infiltration and Pyrolysis (PIP) is the method of fabrication of CMCs comprising an infiltration of a low viscosity polymer into the reinforcing ceramic structure (e.g. fabric) followed by pyrolysis. heating the polymer precursor in the absence of oxygen when it decomposes and converts into a ceramic.
Fabrication of CMCs by Polymer Infiltration and Pyrolysis (PIP)
Polymer Precursors Preceramic polymers (polymer precursors) are the polymers, which can be converted into ceramics by pyrolysis. Molecules of preceramic polymers are commonly contain carbon (C) and/or silicon (Si) but may also contain nitrogen (N),oxygen (O), boron (B), aluminium (Al), titanium (Ti). Polymer Infiltration and Pyrolysis (PIP) technique is used mainly for fabrication composites with silicon carbide (SiC) matrices from polycarbosilanes (silicon derived polymer precursors): polymethylsilane (PMS) and allhydridopolycarbosilane.
Polymer Precursors Polysilazane may be converted into SiCN or Si3N4 with ceramic. Carbon matrices composites are fabricated by pyrolysis of either thermosetting resins (phenolics, oxidized polystyrene, polyvinyl alcohol) or thermoplastic resins (pitches or coal tar). Different polymer precursors:
Process of CMCs by PIP (1) Fabrication of pre-impregnated material (prepreg). The the reinforcing fibres are impregnated with a resin and then dried or cured to B-stage (partial curing). In such condition the viscosity of the polymer is increased and the prepreg may be shaped (laid-up). Lay-up. The prepreg is shaped by a tooling (mould). Moulding. The laid-up prepreg is moulded. Various moulding methods may be used: bag moulding by either atmospheric pressure (vacuum bag mould) or increased air pressure (gas pressure bag mould).
Process of CMCs by PIP (2) Infiltration of a preceramic polymer. The pores of the reinforcing structure are filled with a low viscosity solution of a preceramic polymer when the preform is immersed into it. The infiltration process is driven by the capillary forces therefore it is commonly conducted at normal pressure, however it may also be vacuumor pressure-assisted. Pyrolysis. Pyrolytic decomposition of the preceramic polymer is performed in the atmosphere of Argon at a temperature in the range 800-1300°C.
Process of CMCs by PIP (3) Nitride matrices (e.g.silicon nitride) are fabricated in the atmosphere of Nitrogen (N2) or Ammonia) (NH3). Volatile products such as CO, Hydrogen (H2), CO2, CH2, H2O are released as a result of pyrolysis forming a porous structure of the resulting ceramic matrix. The value of the ceramic yield is determined by the weight loss (amount of the released volatiles). Multiple re-infiltration and pyrolysis…….
Advantages of PIP Advantages of Polymer Infiltration and Pyrolysis (PIP): Fibres damage is prevented due to the processing at a relatively low temperature; Good control of the matrix composition and the microstructure; Reinforcing phase of different types (particulate, fibrous) may be used; Net shape parts may be fabricated; Matrices of various compositions (silicon carbide, silicon nitride, silicon carbonitride) may be obtained; No residual silicon is present in the matrix.
Disadvantages of (PIP) The disadvantages of the Polymer Infiltration and Pyrolysis (PIP): The fabrication time is relatively long due to the multiple infiltration-pyrolysis cycle; There is a residual porosity decreasing the mechanical properties of the composite; Relatively high production cost (higher than in Liquid Silicon Infiltration method).
Fabrication of CMCs by Chemical Vapour Infiltration (CVI) Chemical Vapour Infiltration method of CMCs fabrication is a process, in which reactant gases diffuse into an isothermal porous preform made of long continuous fibres and form a deposition. Deposited material is a result of chemical reaction occurring on the fibres surface. The infiltration of the gaseous precursor into the reinforcing ceramic continuous fibre structure (preform) is driven by either diffusion process or an imposed external pressure. The deposition fills the space between the fibres, forming composite material in which matrix is the deposited material and dispersed phase is the fibres of the preform. Chemical Vapour Infiltration is widely used for fabrication of silicon carbide matrix composites reinforced by silicon carbide long (continuous) fibres.
Fabrication of CMCs by CVI (2)
Fabrication of CMCs by CVI (3) Commonly the vapour reagent is supplied to the preform in a stream of a carrier gas (H2, Ar, He). Silicon carbide (SiC) matrix is formed from a mixture of methyltrichlorosilane (MTS) as the precursor and Hydrogen as the carrier gas. The ceramic deposition is continuously growing as long as the diffusing vapour is reaching the reaction surface. The porosity of the material is decreasing being filled with the formed solid ceramic. However in the course of the CVI process the accessibility of the inner spaces of the preform is getting more difficult due to filling the vapour paths with the forming ceramic matrix. The matrix densification stops when the preform surface pores are closed. The final residual porosity of the ceramic composites fabricated by CVI method may reach 10-15%.
Chemical Vapour Infiltration This technology has been used to produce tough SiC/SiC engine elements for aerospace applications. CH4(g) C(s) + 2H2(g)
CH3CI3Si(g) SiC(s) + 3HCl(g)
Process of CMCs by CVI Fabrication of the fibre preform. Application of a debonding interphase. A thin (commonly 0.1-1 µm) layer of pyrolytic carbon (C) or hexagonal boron nitride (BN) is deposited on the fibre surface by Chemical Vapour Infiltration (CVI) method. Infiltration of the preform with a preceramic gaseous precursor, which decomposes and forms a ceramic deposit (matrix) on the fibre surface. The process continues until the open porosity on the preform surface is closed.
Process of CMCs by CVI Abrading/machining the preform surface in order to open the paths of the percolating network, which allow further densification of the matrix. Multiple re-infiltration-abrading cycles until maximum densification is achieved. Protection surface coating. The open porosity is sealed in order to prevent a penetration of the environmental gases into the composite during the service.
Advantages of CVI Advantages of fabrication of CMCs by CVI: Low fibre damage due to relatively low infiltration temperatures; Matrices of high purity may be fabricated; Low infiltration temperatures produce low residual mechanical stresses; Enhanced mechanical properties (strength, elongation, toughness); Good thermal shock resistance; Increased Creep and oxidation resistance; Matrices of various compositions may be fabricated (SiC, C, Si3N4, BN, B4C, ZrC, etc.); Interphases may be deposited in-situ.
Disadvantages of CVI Disadvantages of fabrication of CMCs by CVI: Slow process rate (may continue up to several weeks); High residual porosity (10-15%); High capital and production costs.
Fabrication of CMCs by Liquid Silicon Infiltration (LSI) Liquid Silicon Infiltration (LSI) process is a type of Reactive Melt Infiltration (RMI) technique, in which the ceramic matrix forms as a result of chemical interaction between the liquid metal infiltrated into a porous reinforcing preform and the substance (either solid or gaseous) surrounding the melt. Liquid Silicon Infiltration (LSI) is used for fabrication of silicon carbide (SiC) matrix composites. The process involves infiltration of carbon (C) microporous preform with molten silicon (Si) at a temperature exceeding its melting point 1414°C.
Fabrication of CMCs by Liquid Silicon Infiltration (LSI) The liquid silicon wets the surface of the carbon preform. The melt soaks into the porous structure driven by the capillary forces. The melt reacts with carbon forming silicon carbide according to the reaction: Si(liquid) + C(solid) → SiC(solid)
Fabrication of CMCs by LSI (1)
In contrast to the composites fabricated by Polymer Infiltration and Pyrolysis (PIP) and Chemical Vapour Infiltration (CVI) ceramic matrices formed by Liquid Silicon Infiltration are fully dense (have zero or low residual porosity).
Fabrication of CMCs by LSI (3) SiC produced in the reaction fills the preform pores and forms the ceramic matrix. Since the molar volume of SiC is less than the sum of the molar volumes of silicon and carbon by 23%, the soaking of liquid silicon continues in course of the formation of silicon carbide. The porous preform may be fabricated by either pyrolysis of a polymerized resin or by Chemical Vapour Infiltration (CVI). The preform microstructure is important for complete infiltration. Large pores helps to obtain a complete infiltration but may result in noncomplete chemical interaction and formation of a structure with high residual free silicon and unreacted carbon. Small preform pores results in more complete chemical reaction but in non-complete infiltration due to the blockage (chock-off) of the infiltration channels. The infiltrated at high temperature molten silicon is chemically active and may not only react with the carbon porous preform but also attack the reinforcing phase (SiC or C fibres, whiskers, or particles). A protective barrier coating (interphase) of SiC, C or Si3N4 prevents the damage of the fibres by the melt. The barrier coatings are applied over debonding coatings.
Process of CMCs by LSI Application of Interphases. A thin (commonly 0.1-1 µm) layer of a debonding phase (pyrolytic carbon (C) or hexagonal boron nitride (BN)) is deposited on the fibre surface by CVI method. In addition to this the fibres are protected from the highly reactive liquid silicon by a barrier coating (commonly SiC). The interphases are deposited by CVI. Fabrication of the prepreg. The reinforcing fibres (tow, tape, weave) are impregnated with a resin and then dried or cured to B-stage (partial curing). The resin contains carbon, which further will react with molten silicon.
Process of CMCs by LSI Lay-up. Moulding. Pyrolysis. Infiltration of the porous prepreg with Liquid Silicon. The prepreg is immersed into a furnace with molten silicon where its porous carbon structure is infiltrated with the melt. The infiltration process is driven by the capillary forces. Liquid silicon reacts with carbon forming in situ silicon carbide matrix.
Advantages of LSI Advantages of fabrication of CMCs by LSI: Low cost; Short production time; Very low residual porosity; High thermal conductivity; High electrical conductivity; Complex and near-net shapes may be fabricated.
Disadvantages of LSI Disadvantages of fabrication of CMCs by LSI: High temperature of molten silicon may cause a damage of the fibres; Residual silicon is present in the carbide matrix; Lower mechanical properties of the resulting composite: strength, modulus of elasticity.
Fabrication of CMCs by Direct Metal Oxidation Process (DIMOX) Direct metal oxidation process (Dimox) of Ceramic Matrix Composites fabrication is a type of Reactive Melt Infiltration (RMI) technique, involving a formation of the matrix in the reaction of a molten metal with an oxidizing gas. Preform of dispersed phase (fibres, particles) is placed on the surface of parent molten metal in an atmosphere of oxidizing agent (Oxygen).
Fabrication of CMCs by Direct Metal Oxidation Process (DIMOX) Two conditions are necessary for conducting Direct oxidation process: dispersed phase is wetted by the melt; dispersed phase does not oxidize in an atmosphere of oxygen. Liquid metal oxidizes when it is in contact with oxygen, forming a thin layer of ceramic with some dispersed phase incorporated in it. Capillary effect forces the melt to penetrate through the porous ceramic layer to the reaction front where the metal reacts with the gas resulting in growing the ceramic matrix layer.
Fabrication of CMCs by DIMOX (2)
Fabrication of CMCs by DIMOX (3) Some residual metal (about 5-15% of the material volume) remains in the inter-granular spaces of the ceramic matrix. The resulting materials have no pores and impurities, which are usually present in ceramics fabricated by sintering (binders, plasticizers, lubricants, deflocculants, water etc.). DIMOX technique is used for fabrication composites with the matrix from aluminium oxide (Al2O3). A reinforcing preform (SiC or Al2O3 in either particulate or fibrous form) is infiltrated with a molten aluminium alloy heated in a furnace to a temperature 900-1150°C.
Direct metal oxidation (4) The Lanxide technology for CMC is based on growth of an oxide scale into ceramic fibrous (or particulate - or mixed) preform, which sits on top of a pool of molten metal. Although the technology is relatively cheap, there is always a metallic residue in the composite, which can deteriorate its wear resistance and high-temperature performance. The following issues are involved: direct metal oxidation or nitridation combination of melt-infiltration and reaction forming of matrix An example – AIN Matrix Formation: Alloy Melting: Al (s) Al (l) Reaction/Growth: 2Al(l) + N2(g) 2AIN (s)
Direct metal oxidation (5) reaction rate dependent upon temperature (850 - 1300 C) and partial pressure of reacting gas requires additives (e.g. 3-5 wt% Si and Mg) for infiltration typical matrices: Al2O3, AIN, ZrN, TiN
Figure 2.17. DIMOXTM directed metal oxidation process (courtesy of Lanxide Corporation).
Process of CMCs by DIMOX Lay-up. At the lay-up stage the fibrous preform is shaped. Application of Interphases. Deposition of a gas permeable barrier on the preform surface. The surface through which the melt should wick into the preform is not coated.
Process of CMCs by DIMOX Direct Metal Oxidation. The preform is put in contact with liquid aluminium alloy. The melt wicks into the reinforcing structure through the non-coated surface. The oxidant (air) penetrates into the preform in the opposite direction through the gas permeable barrier. Aluminium and oxygen meet at the reaction front and form the growing layer of the oxide matrix. The process terminates when the reaction front reaches the barrier coating. Removal of excessive aluminium. The residual aluminium is removed from the part surface.
Advantages of DIMOX Advantages of DIMOX process: Low shrinkage. Near-net shape parts may be fabricated. Inexpensive and simple equipment; Inexpensive raw materials; Good mechanical properties at high temperatures (e.g. creep strength) due to the absence of impurities or sintering aids; Low residual porosity.
Disadvantages of DIMOX
The disadvantages of DIMOX process: Low productivity – growth rate is about 1mm/hour. The fabrication time is too long: 2-3 days. Residual (non-reacted) aluminium may be present in the oxide matrix.
Fabrication of Ceramic Matrix Composites by Slurry Infiltration Slurry is a dispersion of ceramic particles in a liquid carrier, which may also contain additives such as binders and wetting agents. Slurry Infiltration method of fabrication of Ceramic Matrix Composites utilizes a slurry percolating into a porous reinforcing preform. The infiltration process is driven by the capillary forces. After the infiltration process has completed, the preform is dried and hot pressed forming a ceramic matrix composite.
Ceramic matrices produced by Slurry Infiltration Slurry Infiltration is used for fabrication of ceramic, ceramic-glass and glass matrices: Alumina (AL2O3); Silica (SiO2); Glass; Mullite (3AL2O3*2SiO2); Silicon carbide (SiC); Silicon nitride (Si3N4).
Ceramic matrices produced by Slurry Infiltration The method of fabrication of ceramic matrix composites by Slurry Infiltration technique is similar to Sol-Gel Infiltration. However Slurry Infiltration produces denser structure with smaller shrinkage during processing due to higher content of solids. Pressure or vacuum assisted Slurry Infiltration allows further increase of the density of the resulting ceramic composite.
Slurry Infiltration process Slurry infiltration. The reinforcing fibres pass through a slurry, which penetrates into the porous structure of the reinforcing phase. The driving force of the infiltration is capillary effect but the process may be enhanced by vacuum or pressure. Lay-up. The prepreg (infiltrated fibres) is wound onto a mandrel. Then it is dried, cut and laid-up. After drying they are cut and laid-up on a tooling (mould). Hot pressing. Hot pressing (sintering, densification) is performed at high temperature and increased pressure, which enhance the diffusion of the ceramic material between the particles incorporated into the fibres structure. The particles consolidate resulting in a low porosity densified composite.
Slurry Infiltration process (2)
An optical micrograph of a cross-section of a unidirectional alumina fibre/glass matrix composite made by slurry infiltration.
The pressure and temperature schedule used during hot pressing of this composite.
Hot Pressing Slurry Infiltration
Advantages and disadvantages of Slurry Infiltration Advantages of Slurry Infiltration: Low porosity; Good mechanical properties. Disadvantages of Slurry Infiltration: The reinforcing fibres may be damaged by the high pressure applied in the hot pressing stage. Hot pressing operation requires relatively expensive equipment; Relatively small and simple parts may be fabricated.
Fabrication of Ceramic Matrix Composites by Sol-gel process Sol-gel process of a fabrication of Ceramic Matrix Composites involves preparation of the matrix from a liquid colloidal suspension of fine ceramic particles (sol), which soaks a preform and then transforms to solid (gel). Colloidal suspension is formed as a result of chemical reaction when very small particles with radii up to 100 nm (nanoparticles) precipitate within a liquid (water or organic solvent).
Fabrication of Ceramic Matrix Composites by Sol-gel process Liquid sols have a low viscosity therefore they easily infiltrate into the preform. At elevated temperatures sols containing organometallic compounds (e.g. alkoxides) undergo cross-linking (polymerisation) by either the polycondensation or hydrolysis mechanism. Polymerisation converts sol into gel – a polymer structure containing liquid. Gels may be transformed into ceramics at relatively low temperature, which reduces the probability of the reinforcing fibre damage.
Sol-gel Infiltration process Fabrication of the prepreg. The reinforcing fibrous material is immersed into the sol. The sol wicks into the porous structure of the reinforcing phase. Vacuum/pressure may be applied to assist the infiltration process. Lay-up. The prepreg is shaped by a tooling (mould). Gellation and drying. The sol is heated to 150°C. It is converted into gel, which is then dried at a temperature up to 400°C. Water, alcohol and organic volatile components are removed from the material. Repeated re-infiltration and gelation. The sol infiltration-gelation cycle is repeated several times until the desired densification is achieved. Sintering. The ceramic matrix is consolidated (sintered) at the firing temperature.
Advantages of Sol-gel Infiltration Advantages of Sol-Gel Infiltration: less reinforcing fibre damage due to low processing temperature; Controllable matrix composition; Low equipment cost; Low machining cost due to near-net-shape fabrication; Large and complex parts may be fabricated.
Disadvantages of Sol-gel Infiltration Disadvantages of Sol-Gel Infiltration: Possible matrix cracking because of large shrinkage; Multiple infiltration-gelation cycles are required in order to increase the ceramic yield; Low mechanical properties; High cost of sols.
Pultrusion
Another version of the liquid infiltration technique is the pultrusion of continuous fibres through molten glass.
Reaction bonding formation of CMCs Reaction bonding (RB): Conversion of a compacted powder (porous) preform composed of one element of a ceramic (i.e. metal or nonmetal) to the final ceramic compound, through reaction with the environment, usually in a liquid or gaseous form. SiC + Si (s) /N2 (g) SiC(s) + Si3N4
Advantages of reaction bonding formation Advantages little or no matrix shrinkage works best with high modulus C fibers and CVD SiC fibers can automate process single step consolidation short time processing (
