ME328 Materials Engineering
Homework 6
At the following link, we find the article “Ceramics for turbine engines” by David W. Richerson.
http://www.memagazine.org/backissues/membersonly/september97/features/ceramic/ceramic.html
In this article, Richerson reviews the progress in the development of high temperature structural ceramics up to the mid 90’s.
This is the period in which the primary materials in the field were established. Since then, progress toward sufficiently
durable ceramics has continued more incrementally.
1.
Richerson describes the first promising ceramics as formed by reaction sintering (e.g. reaction bonded silicon nitride and
reaction sintered silicon carbide.) How is reaction bonded silicon nitride, RBSN made.
The so called Reaction-Bonded Silicon Nitride (RBSN) materials are usually produced through the
chemical reaction of silicon powder compacts in a nitriding atmosphere
2.
Hot pressed silicon nitride is stronger than RBSN.

What is hot pressing? Hot pressing is a process used to sinter metal powder to full density
and ceramic compacts with controlled microstructures. The process involves the
simultaneous application of pressure and heat to a 'green' component.

3.
Why is it stronger?The combination of heat and pressure leads to lower amounts of porosity.
Also,
in reaction bonded materials, a variety of microstructures with a variety of properties is possible
(e.g. pure silicon may be left unreacted).
Richerson discussed the development of testing.

Why is three or four point bend testing used for ceramics rather than tensile testing? Making dogbone tensile

specimens is harder than making the rectangular blocks use for bend testing. Specimen
alignment (to avoid bending moments is critical for brittle ceramics and is challenging.
Why is four point bend testing better than three point bend testing? A larger volume of material is sampled
at a particular stress, therefore you are more likely to sample the “weak links” and get more
useful data with fewer samples.
HIPS involves relatively uniform pressure from all
directions while pressing generally is done along a single axis.
4.
How does Hot Isostatic Pressing differ from hot pressing?
5.
Richerson discusses the change of Weibull Modulus from about 7 to about 20 as a “guage of progress.”

What does Weibull Modulus indicate about a ceramic? The relative scatter of the material’s strength

Why is a Weibull Modulus of 20 better than 7? It reflects a distribution with less relative scatter and thus better
predictability.
6.
Injection molding is mentioned as a possible manufacturing method for ceramics. Since injection molding is a process for
low melting temperature polymers, this seems like an unlikely method for high melting temperature ceramics. How does
injection molding work for ceramics? You must combine sufficient polymer with the ceramic powder to allow flow in the
injection process. The polymer is lost during the sintering process.
7.
The normal method for making sintered ceramics (pressing a powder perform into shape followed by sintering to diffuse
the powders) produces a part with some porosity. As porosity increases

Strength (MOR)
a) increases
b) decreases
c) stays the same


8.
Stiffness (E)
Fracture Toughness (KIC)
a) increases
a) increases
Typical structural ceramics include Si3N4, SiC, Al2O3, ZrO2

The toughest of the four ceramics is a) Si3N4

For thermal shock resistance we would select a) Si3N4

The lowest cost would be
a) Si3N4
b) decreases
b) decreases
b) SiC
b) SiC
b) SiC
c) stays the same
c) stays the same
c) Al2O3
c) Al2O3
c) Al2O3
d) ZrO2
d) ZrO2
d) ZrO2
9. Both alumina and zirconia are used in hip replacements, but not so much in gas turbine blades. This is primarily because:
Both have good properties at room temperature, but not so good at high temps.
10. What is the difference between glass and ceramic?glass is amorphous and ceramic is crystalline
11. Tempered glass has significant internal stresses. How do those internal stresses help the material to resist breakage when
compared to regular plate glass? The outside compressive layer is useful when the glass goes into bending. The part of the
outside surface that would normally carry tensile stresses will at first only decrease its compressive stress. The part of the
class that would normally see compressive stress now sees larger compressive stress. The high compressive stresses are
ok because glass has a large compressive strength. Since glass has a low tensile strength, the reduction in the tensile
stresses at the surface means that the tempered glass can carry higher load in bending. Note that the residual tension in
the center of the glass is less relevant in the case of bending since those stresses are near the neutral axis.