Ceramic Matrix Composites (CMCs). Major Advantages: High temperature operation Reduced weight Lower...
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Transcript of Ceramic Matrix Composites (CMCs). Major Advantages: High temperature operation Reduced weight Lower...
Ceramic Matrix Composites (CMCs)
Ceramic Matrix Composites (CMCs) Major Advantages:
High temperature operation Reduced weight Lower life cycle cost
Major Drawbacks: Inherent brittleness of ceramics Severe sensitivity to flaws and point contact stresses Low strain intolerance Unpredictable catastrophic failure
A potential solution to the above reliability problem in ceramics is the technology of ceramic matrix composites, consisting of particles, whiskers, continuous and short fibre reinforcements.
Important Matrix Materials
Crystalline ceramics SiC: -SiC (hexagonal) and -SiC (cubic) forms. Very hard and
abrasive material. Excellent resistance to erosion and chemical attack.
Si3N4 : - and - forms (both are hexagonal). Easily be oxidized. Can be processed by sintering, hot press, reaction bonding, hot deposition isostatic press and CVD.
Al2O3 : - Al2O3 (hexagonal) is stable form. - Al2O3 toughened with zirconia is popular.
3 Al2O3 –2SiO2 (mullite): a solid solution of alumina and silica with 71-75% of alumina. Excellent strength and creep resistance, low CTE.
BN: -BN (hexagonal, layered structure with a low density and lubrication properties), -BN (cubic, diamond-like structure and very hard) and -BN (hexagonal)
B4C (boron carbide): low density, high melting point, high hardness. Can be sintered to a dense material from powder.
Amorphous Ceramics (Glass) Supercooled viscous liquids containing tetrahedron SiO4
Behaves like solid, with disordered structure Can contain various compositions with different properties
(Fused quartz, soda-lime (window) glass, borosilicate (Pyrex) glass, E-glass, C-glass, S-glass)
Glass Ceramics Consisting of a fine ceramic crystallites and up to 95-98vol% in
a glassy matrix. Glass ceramics are regarded as composites of glass and crystalline ceramics.
Nucleating agents, TiO2 or ZrO2 are introduced during melting operation to facilitate controlled crystallisation
Fabrication of Ceramic Matrix Composites (CMCs) Currently, all successful techniques for the manufacture of CMC
require processing at temp. of the order of 1000°C and upwards. Chemical and thermal expansion compatibility between fibres and
matrix are thus of paramount importance. Residual stresses are induced by differential CTE when the composite cools down from the processing temperature which can cause the composite to crack and fragment even without the external loading.
The fibres and matrix materials which can be combined successfully are limited. A fibre with the same or a higher CTE than the matrix is preferred.
High temperature chemical reactions between fibre and matrix during fabrication can have significant effects on composite properties.
The fibre properties may be deteriorated or the fibre-matrix interface bond becomes too strong resulting in a brittle, low strength composite. To control the bond strength and improve toughness an interlayer is applied as a thermal barrier coating.
Hot pressing and slurry infiltration Simultaneous application of press and high
temperature Slurry infiltration
Incorporation of fibres into a matrix, followed by matrix consolidation by hot pressing
Best suited for glass or glass-ceramic matrix composites
Uniform fibre distribution, low porosity, high strength
Restricted to low m.p. material
Processing of CMCs
Liquid infiltration of fibre preformSimilar to molten polymer or metal infiltrationProduces homogeneous, pore-free, high
density CMCsHigh m.p. causing reaction between the melt
and reinforcement
Sol-gel (Infiltration and sintering)The (polymer) powder sol is converted to
gel, which is subjected to controlled heating to produce desired end products: glass, glass-ceramic
Very good matrix composition controlEasy to infiltrate fibre towsVery large shrinkage on sintering
Mechanical Behaviour of CMCs Failure strain of fibre (*f = 1-
1.5%) is much greater than that of matrix (*m = 0.1-0.2%), resulting in premature matrix cracking in tension
Linear stress-strain curve until microcracks appear in the matrix at the ‘microcrack yield stress’
Non-linear strain continues as the matrix shows multiple microcracking associated with reduced stiffness
Fibre debonding and fibre bridging occur with advancing matrix cracks
At ultimate stress, massive fibre fracture occurs
Toughening Mechanisms in CMCs Microcracking ahead of the main crack
Crack branches out Particle Toughening
Crack bowing between particles Crack deflection at the particle Crack bridging Pinning of the crack front Metallic particles which undergo plastic deformation
Fibre Pullout and Crack Bridging The bridged fibres counteract the crack opening and
thus reduce the stress intensity factor Frictional sliding between the fibre and matrix
requires additional energy to consume for crack propagation
Transformation Toughening Phase transformation of second phase
particles at the crack tip, reducing the tensile stress concentration at the crack tip
Alumina containing partially stabilized zirconia (ZrO2+ Y2O3): volume expansion (~4%) due to phase transformation in zirconia particle: ZrO2 (tetragonal) -> ZrO2 (Monoclinic)
The dilation in the transformed zone around a crack is opposed by the surrounding untransformed material, introducing compressive stresses which close the crack