Homeworks 3 and 4 Flashcards
You have developed a kneecap replacement made of zirconia using injection molding. You have to
finalize the processing to make the final component. Are there any steps necessary for the green body
to undergo before sintering? If so, please explain briefly which these steps are and what their purpose
is.
injection molding – plastic route
- drying - removing the solvent
- pyrolysis (burn-out) - remove the organic material
Are there any differences between calcination and pyrolysis in ceramic processing? If so, please
explain.
Similarities
- Both are burn-out steps.
- temperature/objectives are the same
Differences
- Calcination is used for low amounts of additives, typically in wet/colloidal routes.
- calcination happens in air
- Pyrolysis is used for high amounts of additives, typically for plastic routes
- pyrolysis happens in a controlled atmosphere
What are the competing mechanisms that can be found during the sintering of a ceramic part?
Coarsening vs. densification
we want densification to
Name three phenomena that can be associated with a successful sintering of a ceramic component.
- shrinkage
- strengthening
- densification
Name the main characterization techniques that you would need to determine if your green body has
been processed correctly by slip casting. Briefly describe why you have chosen those techniques.
- SEM - look at microstructure, particle packing, homogeneity, flaws, etc.
- XRD - purity, composition/phases
- Archimedes - density (green density)
- Caliper/Scan - dimensions, shape, etc.
Which characterization techniques would allow you to design a suitable calcination and sintering cycle for that slip cast piece? Briefly describe those techniques.
- Calcination - DTA-TG (wt% vs. T)
- Sintering - dilatometer (% shrinkage vs. T)
What unit would bulk density be in? How do we express it?
g/mL or kg/m^3
express in % of theoretical density
How would you find % total porosity?
100- density = %total porosity
Suppose you have a 100g sample of MgO. After placing in water and allowing the piece to saturate,
you remove it and find that the saturated weight is 120g. You then place it back in the water and
determine that the saturated suspended weight is 90g.
c) Where does that porosity come from?
d) How could that porosity be removed?
c) the porosity could come from
- bad particle packing/presence of voids
- particle densification
- flaws
d) porosity could be removed by
- improved processing
- drying/burn-out
- change sintering
- increasing temperature/t
- change the furnace
What does “unconstrained shrinkage” during drying of ceramics mean? How can it be achieved?
When the rate of the evaporation from the surface is less than the rate from which the liquid can come to the surface
Shrinkage is uniform and unconstrained so there’s no cracking
What does Archimede’s method measure?
density of the green body
What does XRD measure?
Composition of the raw powder
What does a dilatometer measure?
coefficient of thermal expansion and shrinkage
What does differential thermogravimetric analysis DTA measure?
weight loss as a function of temperature
What does SEM measure?
grain size in fractured surfaces
What does TEM measure?
grain boundary features
What does centrifugation measure?
particle size
What does 4-point bending measure?
tensile strength
What does pycnometry measure?
density of the raw powderWha
What does a rheometer measure?
none of the above
The design of a ceramic engine is considering using ceramic whiskers additions in order to increase
the fracture toughness.
i. Which reinforcement mechanism is expected to take place? Make a sketch representing
the reinforcement that whiskers can provide.
Crack Bridging or crack deflection
The design of a ceramic engine is considering using ceramic whiskers additions in order to increase
the fracture toughness.
ii. Which other toughening mechanisms can be used in ceramic materials?
Transformation toughening
The design of a ceramic engine is considering using ceramic whiskers additions in order to increase
the fracture toughness.
iii. What is the main strategy in the context of crack propagation behind toughening mechanisms in ceramic materials?
Increase the energy required for crack propagation
How would the Weibull modulus change if it is measured for a more reliable set of samples? How can the reliability of ceramics increase?
The Weibull modulus will have an increased slope.
Raw material selection (smaller particle size), improved processing (remove agglomeration and flaws), so increased homogeneity
Reinforcing, controlled densification
What are the main design scenarios against crack propagation that can be used in ceramics?
Max flaw size
min strength – max flaw size
Why is the probability of failure for large components higher than for smaller components in ceramics?
more volume, more likelihood of critical flaws
How do you measure mechanical properties in ceramics?
stress t – 3 or 4 point bending
stress c – compression testing
K sub ic (fracture toughness) - 3 or 4 point bending + notch
How are the yield strength values in ceramic materials compared to other material classes?
It has no yield strength, non-existent
