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.

Processing of CMCs 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

Liquid infiltration of fibre preform Similar to molten polymer or metal infiltration Produces homogeneous, pore-free, high density CMCs High 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 control Easy to infiltrate fibre tows Very 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