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$ceramics, or polymers Composite properties Ceramic matrix composites Why is toughening important for CMC fracture$

1 FOLLOW THE RECIPE Sub-topics 1 Classes and types of composites Why are composites used instead of metals, ceramics, or polymers Composite properties Ceramic matrix composites Why is toughening important for CMC fracture resistance

2 COMPOSITES Composites:Multiphase material with significant proportions of each phase. Combine materials with the objective of getting a more desirable combination of properties Principle of combined action Mixture gives averaged properties 2

3 BASIC(1) Constituent phases are : -Chemically dissimilar; -Separated by distinct interface 3

4 BASIC(2) 4

5 COMPOSITES CLASSIFICATION The properties of composites are a function of -the properties of the constituent phases; -their relative amounts; -the geometry of the dispersed phase 5 Shape of the particles; size; distribution; orientation

6 GEOMETRICAL AND SPATIAL CHARACTERISTICS OF PARTICLES OF THE DISPERSED PHASE concentration size shape distribution orientation 6

7 PARTICLE-REINFORCED COMPOSITES 7

8 PARTICULATE COMPOSITES consist of a matrix reinforced with a dispersed phase in form of particles. Effect of the dispersed particles on the composite properties depends on the particles dimensions Very small particles (less than 0.25 micron in diameter) finely distributed in the matrix Impede movement of dislocations and deformation of the material. Such strengthening effect is similar to the precipitation hardening. In contrast to the precipitation hardening, which disappears at elevated temperatures when the precipitated particles dissolve in the matrix, dispersed phase of particulate composites (ceramic particles) is usually stable at high temperatures, so the strengthening effect is retained. Many of composite materials are designed to work in high temperature applications. 8

9 CERMETS CERAMIC-METAL COMPOSITES Composed of extremely hard carbide particles embedded in a matrix of a metal Tungsten carbide based Chromium carbide based Titanium carbide based 9

10 LARGE-PARTICULATE COMPOSITES Largedispersed phase particles have low strengthening effect but they are capable to share load applied to the material, resulting in increase of stiffness and decrease of ductility. The degree of improvement of mechanical behavior depends on Bonding at the matrix particle interface. Cement - matrix 10

11 CONCRETE Concreteis a common large-particle composite in which both matrix and dispersed phases are ceramic materials. Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material. Outer view of the Roman Pantheon, still the largest unreinforced solid concrete dome to this day 11

12 RULESOFAVERAGING-E Isostrain and isostress 12 E and V denote the elastic modulus and volume fraction

13 DISPERSION-STRENGTHENED COMPOSITES Metals and metal alloys may be strengthened and hardened by the uniform dispersion of several volume percent of fine particles of a very hard and inert material. The dispersed phase may be metallic or nonmetallic; oxide materials are often used. The strengthening mechanism involves interactions between the particles and dislocations within the matrix, as with precipitation hardening. The dispersion strengthening effect is not as pronounced as with precipitation hardening; the strengthening is retained at elevated temperatures and for extended time periods because the dispersed particles are chosen to be non-reactive with the matrix phase. 13

14 FIBER REINFORCED COMPOSITES 14

15 FIBERS 15

16 FIBERS ARRANGEMENT 16

17 LOAD TRANSFER TO THE REINFORCEMENT Mechanical properties depend not only on the properties of fiber, but also on the degreeto which an applied load is transmitted to the fibers by the matrix phase Fiber and matrix will experience locally different axial displacements because of different E 17

18 LOAD TRANSFER AS A FUNCTION OF FIBER LENGTH 18

19 CRITICAL FIBER LENGTH Some critical fiber length is necessary for effective strengthening and stiffening of the composite material. This critical length lcis dependent on the fiber diameter d and its ultimate (or tensile) strength, and on the fiber matrix bond strength(or the shear yield strength of the matrix) τ c For a number of glass and carbon fiber matrix combinations, this critical length is on the order of 1 mm, which ranges between 20 and 150 times the fiber diameter. 19

20 INFLUENCE OF FIBERS LENGTH continuous To affect a significant improvement in strength, the fibers must be continuous

21 FIBERS LENGTH

22 ALIGNED FIBER COMPOSITES

23 ELASTIC BEHAVIOR LONGITUDINAL LOADING The total load sustained by the composite Fcis equal to the loads carried by the matrix phase Fm and the fiber phase Ff or or or 23

24 ELASTIC BEHAVIOR TRANSVERSE LOADING 24

25 COMPOSITE STRENGTH Fracture strength for fiber Elastic deformation of fiber and matrix Matrix yields and deforms plastically; proportion of the load that is borne by the fibers increases

26 STRENGTH OF DISCONTINUOUS, ALIGNED FIBER COMPOSITES

27 DEFORMATION AND FRACTURE MECHANISMS IN COMPOSITES Parallel to fiber orientation: Stage I: Strain is small, fibers and matrix both elongate (or deform) elastically Fibers carry the load Stage II: Incompatibility of the lateral matrix and fiber deformation strains Matrix deforms plastically; Fibers deform Elastically Stage III: Fiber deformed plastically before fracture (found in metallic fibers) Both matrix and fibers deform plastically The onset of composite failure begins as the fibers start to fracture. Failure is not catastrophic: -Not all fibers fracture at the same time; -After fiber failure, the matrix is still intact for some longer time

28 FRACTURE OF FIBER-REINFORCED COMPOSITES

29 STRUCTURAL COMPOSITES

30 COMPOSITES BENEFIT

31 COMPOSITES OF DUCTILE MATRIX In a ductile matrix, like most polymers and metals, a strong interfacial bond is important, since the fibers carry most of the load in such matrices. Fiberstend to fail first, usually by cohesive failure through the fiber cross-section. This is because the fibers cannot strain as much as the matrix (e.g. carbon in epoxy). Cracks are few, and tend to propagate slowly. When the cracks hit the interface, strong interfacial bonds stop them.

32 CERAMICS

33 SOME IMPORTANT CERAMIC MATERIALS Bonding: Mostly ionic and some covalent. Ionic transfer of electrons between atoms Covalent sharing of electrons between atoms in very specific directions Single oxides Alumina Zirconia Titania Mullite Carbides Silicon carbide Boron carbide Titanium carbide Nitrides Silicon nitride Intermetallics Al2O3 ZrO2 TiO2 3Al2O3 + 2SiO2 SiC B4C TiC Si3N4 NiAl; Ni3Al; TiAl; MoSi2 Very high Peierls- Nabarropotential High resistance to dislocations motion Low plasticity

34 COMPOSITES OF BRITTLE MATRIX In a brittle matrix, like ceramics, the matrix carries most of the load, which is usually compressive (like in teeth or bone), and fibers (or particles) are added to increase toughness. That is, to increase the time to catastrophic failure by holding the matrix together after cracking. Fibershere are more ductile than the matrix (e.g. glass in alumina) and the matrix fails first. As the cracks propagate and reach the interface, a weak interfacial bond is desired. This enhances debonding, and the cracks are not stopped, but deflected along the length of the fibers. This effectively delays the time it takes the cracks to propagate through the entire matrix, and thus increases toughness.

$CERAMIC MATRIX COMPOSITES The fracture toughness of ceramics have been improved significantly by the development of a new generation of CERAMIC MATRIX COMPOSITES particulates, fibers, or whiskers of$

35 CERAMIC MATRIX COMPOSITES The fracture toughness of ceramics have been improved significantly by the development of a new generation of CERAMIC MATRIX COMPOSITES particulates, fibers, or whiskers of one ceramic material that has been embedded into a matrix of another ceramic. Improvement in fracture properties results from interactions between advancing cracks and dispersed phase particles. Crack initiation normally occurs with the matrix phase, whereas crack propagation is impeded or hindered by the particles, fibers, or whiskers.

DISCONTINUOUS REINFORCED CERAMIC COMPOSITES The

high-temperature stability, high thermal shock

resistance, light weight, nonmagnetic and

36 DISCONTINUOUS REINFORCED CERAMIC COMPOSITES The desirable characteristics of CMCs include high-temperature stability, high thermal shock resistance, high hardness, high corrosion resistance, light weight, nonmagnetic and nonconductive properties, and versatility in providing unique engineering solutions.

37 FRACTURE TOUGHNESS Aim to improve fracture toughness of CMC fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications. It is denoted K Ic and has the units of. The subscript 'Ic' denotes mode I crack opening under a normal tensile stress perpendicular to the crack, since the material can be made thick enough to resist shear (mode II) or tear (mode III). Toughening mechanisms Compressive prestressing of the matrix Crack impeding Crack deflection Fiber pullout Phase transformation??

PARTICLE TOUGHENING The mechanism of ductile particle bridging is a crack wake effect with an associated process zone so these composites would be expected to show resistance-curve (Rcurve) behaviour.

38 PARTICLE TOUGHENING The mechanism of ductile particle bridging is a crack wake effect with an associated process zone so these composites would be expected to show resistance-curve (Rcurve) behaviour. At steady state, the toughening increment, Gc, is given as Schematic diagram of ductile particles bridging an advancing crack. where V f is the area fraction of ductile particles intersected by the crack plane (usually taken to be equal to the volume fraction of ductile particles), σ(u) is the stress/stretch relationship for the metallic particle, and u * is the crack opening displacement when the metallic particle fails.

$PARTICLE TOUGHENING Particle changes the length of the crack. This leads to dispersion of the fracture energy and significantly change in fracture toughness.$

39 PARTICLE TOUGHENING Particle changes the length of the crack. This leads to dispersion of the fracture energy and significantly change in fracture toughness. Crack propagation in an Al2O3-Mo composite crack deflection and bridging by Mo particles. For example, in a composite of Al2O3-with 20 volume % of Mo for the case when the crack surrounds the metal particles the fracture toughness increases by about 22% as compared with the value of K IC in Al2O3 Crack bridging can happen while matrix incorporated by ductile particles

40 CRACK DEFLECTION SEM image (a) and TEM image (b) of Al 2 O 3 /10vol.% Fe 3 Al composite. Crack propagation in an Al2O3-Mo composite crack deflection by the particles.

$CRACK BRIDGING For small-scale bridging, the fracture toughness increment can be estimated where Eis the elastic modulus of the composites, σ 0 the yield stress, fthe volume fraction of bridging$

41 CRACK BRIDGING For small-scale bridging, the fracture toughness increment can be estimated where Eis the elastic modulus of the composites, σ 0 the yield stress, fthe volume fraction of bridging particles, t the characteristic dimension of the reinforcement, and χ a dimensionless function representing the work of rupture of the bridging phase. Taking values for Eof 370Gpafor composite with Fe 3 Alm, volume fraction fof 0.2, and σ 0 of 450MPa for Fe 3 Al particle the tof 1.3µm, an estimated value of χof 0.4 was used. According to Eq. the toughness increment by the grain bridging of Fe 3 Al is approximately 4.16MPam 1/2.

42 INFLUENCE OF THERMAL RESIDUAL STRESSES Thermal residual stress caused by the mismatch in CTE of the matrix and particle leads to change in fracture mode. TS can serve to toughen the materials if matrix is put under compressive stresses. Ductile particles of high CTE The fracture toughness increment arising from the thermal residual stress can be calculated by CTE where t is the average grain size of toughening phase, l the average interval between toughening particle, and f the volume fraction of toughening particles the radial pressed stress of matrix

43 IDEAL DUCTILE PARTICLE CMC The ideal ductile particle CMC comprises metallic particles within a ceramic matrix such that an advancing crack is attractedto a particle. That particle then debondspartially from the matrix, ideally to its polar regions, and deforms plastically, thus absorbing energy and bridging the crack, providing closure tractions, both of which will provide a toughening increment. However, putting this concept into practice poses several challenges and has resulted in a wide range of materials with some interesting properties. For an advancing crack to be attracted to an inclusion, rather than repelled by it, the elastic modulus of the inclusion must be lower than that of the matrix; clearly this is not a problem for most engineering ceramic/metal combinations The toughening increment should increase with the yield strength of the metal and with the size of the inclusion. However, if the inclusion becomes too large then the difference in the coefficients of thermal expansion of the metal and the ceramic matrix is likely to result in cracking, which may lead to an advancing crack being able to by-pass the particle.

EXAMPLES OF METALLIC PARTICLES IN ALUMINA MATRICES (a) scanning electron micrograph showing the necking of a nickel particle (courtesy of XudongSun), (b) scanning electron micrograph showing the

44 EXAMPLES OF METALLIC PARTICLES IN ALUMINA MATRICES (a) scanning electron micrograph showing the necking of a nickel particle (courtesy of XudongSun), (b) scanning electron micrograph showing the necking of an iron particle (courtesy of Matthew Aldridge), (c) extended focus confocalscanning laser micrograph of limited plastic deformation of an iron particle (courtesy of Paul Trusty). J.A. Yeomans/ Journal of the European Ceramic Society 28 (2008)

INTERFACE, FRACTURE PROPAGATION AND TOUGHNESS High-resolution image of the interface between Fe 3 Al and Al 2 O 3 Regardless of the processing method chosen, a major challenge is to control the

45 INTERFACE, FRACTURE PROPAGATION AND TOUGHNESS High-resolution image of the interface between Fe 3 Al and Al 2 O 3 Regardless of the processing method chosen, a major challenge is to control the strength of the metal ceramic bondand hence the degree of debonding. If the bond is too strong then there will be no debonding and the particle will be almost fully constrained, giving little opportunity for plastic stretching. If the bond is too weak then the particle will debondfrom the matrix completely and the crack will by-pass the particle and the only toughening that can be achieved would be that associated with crack deflection, which does not have the same potential for large increases as plastic deformation. One way to circumnavigate the problem of the strength of the interface is to produce metallic particles that are mechanically interlocked with the matrix

$NANOCOMPOSITES Scanning electron micrographs of (a)inter-granular fracture surface of alumina and a change in fracture mode from intergranular in monolithic alumina to$

46 NANOCOMPOSITES Scanning electron micrographs of (a)inter-granular fracture surface of alumina and a change in fracture mode from intergranular in monolithic alumina to transgranular in alumina metal nanocomposites (b)transgranular fracture surface of alumina 5vol% chromium nanocomposite J.A. Yeomans/ Journal of the European Ceramic Society 28 (2008)

47 COMPLETE BRIDGING BY FIBERS Cracks bypass the fibers and leave them bridging the cracks At a certain stress cracks appear in a matrix elastic Non-brittle in character (no catastrophic failure)

$ROLE OF INTERFACE The local response of the matrix/fiber interface is of great importance E1 E2 E1 Debonding Mechanisms:matrix fracture; debonding at the crack tip followed by crack deflection;$

48 ROLE OF INTERFACE The local response of the matrix/fiber interface is of great importance E1 E2 E1 Debonding Mechanisms:matrix fracture; debonding at the crack tip followed by crack deflection; frictional sliding between matrix and fiber; fiber failure; fiber pull out. E1 E2 => modulus mismatch causes shearing of the crack surfaces => a mixed-mode stress state in the vicinity of a crack tip => instead of Kicwe need more complex formalism of fracture mechanics. More suitable is strain energy release rate, G Ic

TRANSFORMATION TOUGHENING Mainly zirconia(zro2) as toughening phase Tetragonal monoclinic cubic Stress-induced tetragonal-tomonoclinic (martensitic) phase transformation which is characterized by a

49 TRANSFORMATION TOUGHENING Mainly zirconia(zro2) as toughening phase Tetragonal monoclinic cubic Stress-induced tetragonal-tomonoclinic (martensitic) phase transformation which is characterized by a large volume change (3-5%) and shear deformation (1-7%). Transformation strain( V/V = 3 5%) leads to compressive stresses in zone of transformation. This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia.

$high wear resistance, and a superior fracture$

50 EXAMPLES For example, the alumina-zirconiamaterial system would combine the remarkable physical-mechanical properties of alumina, i.e. high wear resistance, and a superior fracture toughness compared to the monolithic material. crack deflection transformation toughening BIOLOX delta

51 TRANSFORMATION TOUGHENING MECHANISM

52 TOUGHENING INCREMENT

53 MICROCRACK TOUGHENING

54 CARBON CARBON COMPOSITES Their desirable properties include high-tensile moduli and tensile strengths that are retained to temperatures in excess of 2000C, Resistance to creep, Relatively large fracture toughness, Low coefficients of thermal expansion and relatively high thermal conductivities; These characteristics, coupled with high strengths, give rise to a relatively low susceptibility to thermal shock. Their major drawback is a propensity to hightemperature oxidation. The carbon carbon composites are employed in rocket motors, as friction materials in aircraft and highperformance automobiles, for hot-pressing molds, in components for advanced turbine engines, and as ablative shields for re-entry vehicles. 54

Lecture 08 Fracture Toughness and Toughening Mechanisms Ref: Richerson, Modern Ceramic Engineering, Ch17, Marcel Dekker, 1992

MME 467 Ceramics for Advanced Applications Lecture 08 Fracture Toughness and Toughening Mechanisms Ref: Richerson, Modern Ceramic Engineering, Ch17, Marcel Dekker, 1992 Prof. A. K. M. Bazlur Rashid Department