Composite Process 3

Ceramic Matrix Composites (CMCs)

The reasons for reinforcing ceramics are fundamentally different

from polymer and metals.from polymer and metals.

Polymer and metals are reinforced to increase strength and

modulus While ceramics are reinforced for toughening andmodulus While ceramics are reinforced for toughening and

damage tolerance.

There two types of CMCs: one incorporates continuous fiber and

the other a discontinuous reinforcement (i.e. Whiskers).

Both of these approaches enhance fracture resistance.

Whiskers incorporated in a short fiber Ceramic Matrix

Ceramic Matrix Composites (CMCs) Whiskers incorporated in a short-fiber Ceramic Matrix

Composite improve its toughness resisting to cracks propagation.

However a character of failure of short-fiber reinforced materials

is catastrophic.

Failure of long-fiber Ceramic Matrix Composites is not

catastrophic.

The best strengthening effect is provided by dispersed phase in

form of continuous monofilament fibers which are fabricated byform of continuous monofilament fibers, which are fabricated by

chemical vapor deposition (CVD) of silicon carbide on a

b t t d f t t (W) b (C) fibsubstrate made of tungsten (W) or carbon (C) fibers.


Properties of long-fiber Ceramic Matrix Composites:High mechanical strength even at high temperatures;

High thermal shock resistance;g ;

High stiffness;

High toughness;

High thermal stability;g y

Low density;

i h i i hi hHigh corrosion resistance even at high temperatures

C i t i


Ceramic matrix1) Silicon Carbide:

Hi h t t f d iti ( 2500 oC) High temperature of decomposition (approx. 2500 oC). Displays good thermal shock resistance and maintains its

flexural strength at elevated temperatures.g p Chemically inert and possesses high thermal conductivity

(about four times that of steel).2) Silicon nitride: Exhibits excellent strength and creep resistance at elevated

operating temperaturesoperating temperatures But may be limited by its vulnerability to oxidation at

temperatures exceeding 1000oC

C i t i


Ceramic matrix3) Alumina:

V t bl d hi hl i t t t h i l tt k d b th• Very stable and highly resistant to chemical attack under bothoxidizing and reducing conditions.

• Alumina also possesses the advantage of being fairlyp g g yinexpensive to manufacture.

4) Other types:a) Zirconia (ZrO2)b) Aluminum Titanate (Al2TiO5)) Al i Nit idc) Aluminum Nitride

Note:The mostly used material as ceramic matrix is SiC as toughness isThe mostly used material as ceramic matrix is SiC as toughness is maintained at elevated temperatures

Ceramic Matrix Composite (CMC) Fabrication

Green body

Schematic overview of the approaches employed in the fabrication of ceramic matrix composites

Fabrication of Ceramic Matrix Composites by Liquid phase InfiltrationLiquid phase Infiltration

Reactive Melt Infiltration Process (RMI)

Reactive Melt Infiltration Process (RMI) is used primarily for Reactive Melt Infiltration Process (RMI) is used primarily for

fabrication of silicon carbide (SiC) matrix composites.

The process involves infiltration of carbon (C) containing

preform with molten silicon (Si). Infiltration is usually

capillary forced.

Carbon of the impregnated preform reacts with liquid silicon Carbon of the impregnated preform reacts with liquid silicon,

forming silicon carbide (SiC). Resulting matrix consists of

ili bid d id l ilisilicon carbide and some residual silicon.


Reactive Melt Infiltration Process (RMI)

When liquid aluminum (Al) is used for infiltration of a preform

in oxidizing atmosphere alumina aluminum (Al O Al)in oxidizing atmosphere, alumina-aluminum (Al2O3 – Al)

matrix is formed (Fabrication of Ceramic Matrix Composites

by Direct Oxidation Process).

Reactive Melt Infiltration method is fast and relatively cost

effective. Materials fabricated by RMI method possess low

porosity and high thermal conductivity and electricalp y g y

conductivity.


Slurry Infiltration Process (SIP) and Hot Pressing

Slurry Infiltration Process (SIP) involves the following operations:

Passing fibers (tape) through a slurry containing particles of the Passing fibers (tape) through a slurry containing particles of the

ceramic matrix;

Winding the fibers infiltrated by the slurry onto a drum and

drying;

Stack of the slurry impregnated fibers in a desired shape;

Consolidation of the matrix by hot pressing in Graphite die at Consolidation of the matrix by hot pressing in Graphite die at

high temperature

Slurry Infiltration Process (SIP) and Hot PressingSlurry Infiltration Process (SIP) and Hot Pressing

Slurry Infiltration Process (SIP)


Hot Pressing

Consolidation of the matrix by hot pressing in Graphite die at

high temperature or Hot Isostatic Pressing (HIP)high temperature or Hot Isostatic Pressing (HIP)

Consolidation can be achieved at low temperature (800-1000oC)

and pressure. This mean that the detrimental fiber/ matrix

reaction will be minimized. The ability of the matrix to flow

under low pressure minimized the fiber mechanical damage.


Slurry Infiltration Process (SIP) and Hot Pressing

Hot Isostatic PressingHot Isostatic Pressing of MMCs – sintering under a pressureapplied from multiple directions through a liquid or gaseousmedium surrounding the compacted part and at elevatedmedium surrounding the compacted part and at elevatedtemperature.

Chemical Vapor Infiltration (CVI)Chemical Vapor Infiltration method of Ceramic Matrix Composites fabrication isp pa process, in which reactant gases diffuse into an isothermal porous preform made oflong continuous fibers and form a deposition. Deposited material is a result ofchemical reaction occurring on the fibers surface.

Chemical Vapor Infiltration (CVI)

The infiltration of the gaseous precursor into the reinforcing

ceramic preform is driven by either diffusion process or an

imposed external pressure.

Chemical Vapor Infiltration (CVI) is similar to Chemical Vapor

Deposition (CVD), in which deposition forms when the reactant

gases react on the outer substrate surface.

CVI is widely used for fabrication of silicon carbide matrix CVI is widely used for fabrication of silicon carbide matrix

composites reinforced by silicon carbide continuous fibers.


Commonly the vapor reagent is supplied to the preform in a

stream of a carrier gas (H Ar He) Silicon carbide (SiC) matrixstream 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. MTS is decomposed

according to the reaction: CH3Cl3Si → SiC + 3HCl

The gaseous hydrogen chloride (HCl) is removed from the

preform by the carrier stream. Carbon matrix is formed from a

methane precursor (CH4).

Chemical Vapor Infiltration (CVI)Chemical Vapor Infiltration (CVI)

Advantages : L fib d d t l ti l l i filt ti Low fiber damage due to relatively low infiltration

temperatures;Matrices of high purity may be fabricated;g p y y ; Low infiltration temperatures produce low residual mechanical

stresses; Enhanced mechanical properties (strength, elongation,

toughness); Good thermal shock resistance; Good thermal shock resistance;Matrices of various compositions may be fabricated (SiC, C,

Si3N4, BN, B4C, ZrC, etc.).


Disadvantages : Slow process rate (may continue

up to several weeks); High residual porosity (10-15%); Hi h i l d d i High capital and production

costs.

a) A SiC monofilament with 35 micron vapour deposited layer Ti alloy.b) A Ti-alloy/ 80 vol.% SiC composite produced by HIPing

Direct Metal Oxidation (DIMOX)DDirect 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 moltenq gmetal with an oxidizing gas.

Preform of dispersed phase (fibers, particles) is placed on the surface ofparent molten metal in an atmosphere of oxidizing agent (Oxygen).p p g g ( yg )

Direct Metal Oxidation (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 phaseforming a thin layer of ceramic with some dispersed phase

incorporated in it.

C ill ff f h l h h h 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.

Direct Metal Oxidation (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• Good mechanical properties at high temperatures (e.g. creep

strength) due to the absence of impurities or sintering aids;• Low residual porosity.p y

The disadvantages of DIMOX process:• Low productivity – growth rate is about 0.04”/hour (1mm/hour).

The fabrication time is too long: 2-3 days.• Residual (non reacted) aluminum may be present in the oxide• Residual (non-reacted) aluminum may be present in the oxide

matrix.

Powder Based RoutesProduction of ceramics by powder based routes is very well established inindustrial practice.Cold compaction of the fine powder often with binder, and these are sintered orhot pressed. In some cases a liquid is present during consolidation.These process has many problems such as the introduction of fibers in suchoperations.S i ki b d h fib i h i lSevere matrix cracking observed as the fiber resist the matrix volumecontraction

Composite Process 3

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