Appunti completi Materiali cementizi e ceramici

Toughening Mechanism and R-Curve for Toughened Ceramics

The toughening mechanism is to change the relationship between crack opening and mechanical resistance. The material is modified to make crack opening more difficult, resulting in improved mechanical resistance without a drop in toughness. This is shown in the plot on the right, where the toughness (K) increases with the growing defect.

On slide 12, the R-curve for toughened ceramics is shown in detail. The plot illustrates the relationship between toughness and defect length. As the defect length increases, the toughness also increases. This is in contrast to traditional ceramics, where toughness decreases with larger defects.

On slide 13, we explore the two phases of a composite material. The presence of reinforcement acts as crack deflection and deviation, hindering crack propagation. If the reinforcement is harder than the surrounding material, the crack deviates inside the material. The crack shielding is related to energy, creating an energy field around the crack tip that obstructs the crack from deviating.

propagation.absorbed.SLIDE 14

We consider in detail the interaction with the crack tip. We have the first mechanism that is the crack curving, that occurs if the particles of reinforcement are harder than matrix.

The crack when faces the particle front curves around it, proceeding around particles until all the curved crack fronts belong to one single front, and the become coalescent.

At the beginning, the crack fronts are all parallel, they are oriented in a favourable way to the applied stress, generally perpendicularly; once we curve the fronts, the crack fronts are oriented in different ways, always opposite with respect to the applied stress. So we have changed the mode of crack opening (fracture mechanics, mode I, II, III).

Type I is the most dangerous with crack propagation perpendicular to stress, so with crack curving we change it into Type II with shear stress. In type II and III the material is less impacted by the stress, so they are less dangerous because the crack is less

favoured.

SLIDE 15

The second type of crack interaction is the crack deflection. The deviation is obtained by controlling the microstructure (grain boundaries) and through reinforcements (particles).

This second type of crack interaction is easier to achieve with particles. The particles must have different elastic modulus or expansion coefficient compared to matrix.

SLIDE 16

We now consider the case of different expansion coefficient. We consider in α.

During cooling, the matrix and particles will shrink in different ways, and the matrix shrinks more and will tend to compress the particles.

During heating, the matrix expands and tends to produce tensile tangential stress on particles.

Inequalities only mean where I have tensile or compressive stress, I can have a coefficient of expansion greater in both particles and the matrix:

In the first case, we have a higher coefficient in the particles, I will have tensile stress in the matrix.

Towards the particle and compressive within the particle at the boundary with matrix. In this case, the crack does not exceed the particle because it is compressed, it is unfavorable, it will pass only around the particles.

In the second case we have a higher coefficient in the matrix, so I will have compression in the matrix and tensile stress on the particle at the boundaries. The crack will propagate in a favorable path where we have tensile stress (inside particles), but it will keep changing directions, and switching between mode I or II or III.

Not all particles are good to deviate cracks, in detail the higher the reinforcement volume fraction, the highest the toughness. But the toughness increases only until 20%, in fact at this point we reach a plateau and the toughness can't increase anymore.

The shape of particles is affecting the toughened effect, in particular round particles are less efficient than cylindrical. Furthermore, also the dimension affects the toughened effect, if

we consider R the aspect ratio, we see that from the sphere (R=1) to cylinder (R > 1, height bigger than width) the toughness increases; so for higher ratios the particles are more efficient.
We have to notice that the toughness never increases more than 4.

SLIDE 18
The engineering of grain boundaries occurs when we create fibrous monolithic ceramics. We sinter together fibres aligned, but we want different materials. So we coat the fibers with another material that will act as a matrix. Most of the cross section is due to fibres, the coating will create only a small amount of the matrix.

The boundary must have weak bond between the fibers and the coating because the matrix must open so that the crack passes though. The crack in practice does zig zag around fibres without crossing them. We must design the reinforcement also according to the service loading and its direction.

The problem is related to degradation of boron nitride at high temperatures, it has in fact a limited service.temperature zirconia is in the tetragonal phase, but as we increase thetemperature it transforms into the monoclinic phase. This phase transformationleads to a volume expansion and the creation of microcracks, which act asbarriers to crack propagation and contribute to toughening.SLIDE 23-24The bridging zone is achieved with the addition of fibers or particles. Thesereinforcements act as obstacles to crack propagation, creating a bridging effectand increasing the energy required for crack growth. The size, shape, anddistribution of the reinforcements play a crucial role in determining the tougheningefficiency.SLIDE 25-26The mixed mechanism combines both the process zone and bridging zone. Itinvolves the interaction between the crack tip and the reinforcements, leading toa combination of crack deflection, crack branching, and crack pinning. Thismechanism can further enhance the toughness of the material.SLIDE 27In conclusion, the toughening mechanisms in ceramic materials involve theprocess zone, bridging zone, and mixed mechanism. By optimizing the size,shape, and distribution of reinforcements, we can enhance the mechanicalresistance and toughness of ceramics, making them suitable for a wide range ofapplications.temperature we can never obtain the tetragonal form, but only monoclinic or cubic. The tetragonal particles are metastable at room temperature!

SLIDE 23
We illustrate the situation of zirconia cubic matrix, or alumina matrix, reinforced with tetragonal zirconia particles. The tetragonal particles are metastable, so they maintain their structure until the crack propagates.

When the crack starts propagating and builds process zone, a tensile stress is built on crack tip and this creates new free volume. The particles see the new free volume available and they transform into a monoclinic structure (stable at room temperature) with a volume increase.

The effect of increase of volume is compressive state at edges of crack, they tend to close the crack hindering propagation. The transformation from T to M structure needs energy that is absorbed from the crack.

SLIDE 24
The more the defect propagates, the more particles transform and expand, the more compressive stress is created and the more will be

The toughened effect. At the end we have obtained an increasing curve of K .IC

With this expansion, we may create some new microcracks inside the matrix: but they don’t help the big crack propagation, actually they consume energy for their formation (due to surfaces formation) and stop the crack propagation.

Microcracks phenomenon occurs only with zirconia particles of 1 micron.

SLIDE 25

Bridging zone is obtained with fibers composite or particle composite.

SLIDE 26

When we talk about particle composite we can have:

Composites with ductile reinforcements, - in particular a metal, nickel. We get reinforcement by plastic deformation of the particles and when they break they emit a sound.

Composites with brittle reinforcements, - we reach energy consumption through disbonding between fibres and matrix. We also break and pullout of reinforcement.

SIDE 27-28

The particles have low yield stress, so when the crack propagates, they don’t detach from the matrix, but absorb energy through plastic

deformation.Particles remain correctly bonded to matrix and slightly disbond to permitThis is the most energy consuming mechanism and mostplastic deformation.difficult to achieve.We see that for the value 15 we have a deflection and curving of crack, it isefficient but less than shielding.

SLIDE 29

Metallic CMC can be:

• Reinforced with PARTICLES
• WIDIA: both particle and matrix undergo deformation
• Reinforced with CONTINUOUS FIBRES (0.6%)

High TOUGHNESS, close to 20o142 Expensive production: CVD or CVI techniqueso C – C compositeso- Reinforced with SHORT FIBRES:Whiskerso

SLIDE 30

Considering composites with brittle reinforcement we have that when the crackpropagates, the first phenomenon is disbonding, then we have the breakage offibers, not on the crack line but inside the matrix, and at last the pull out.The pull out consists of the removal of fiber from the matrix due to pulling ofthe opening crack, the fiber remains inside the matrix on one side, and on theother side it is pulled out,

In correspondence of the cracking point. Pull out dissipates a lot of energy due to friction. Why fibers don't break at the crack line but inside the matrix? We want very hard fibers in order to not be immediately broken, so the crack tries to go around the fiber by disbonding matrix-fiber. Moreover, we cause, during production, a stress state that induces fibers to break in another point. SLIDE 31 Now we consider: - The plot on the left: at the first defect of the matrix we would have failure in a single material component, but in a composite, the matrix and the fibers are keeping it together. The materials resist until fibers break up, but at this point, they still have energy consumption, without failure, due to pullout. - The plot on the right: wrong material, the bond between fibers and matrix is too strong and so they remain bonded and we don't have the pullout. We only have breakage of fibers due to crack propagation and failure, with lower energy consumption. SLIDE 32 With fibers, we obtain the

best result but they are very expensive, moreover the sintering around fibers is the most difficult process. Metallic dispersion has a limited service temperature and durability due to the oxidation in metals. Zirconia reaches 20 of toughness, similar to metals, and it is easy to achieve, but has a maximum service temperature of 900°C. This is a problem because we have to work far from 1100°C, because above 1170°C tetragonal structure becomes stable and we cannot exploit its phase transformation in metastable state.

SLIDE 33-34

We have added advanced composites. We see the huge inc