1. CERAMIC PHASE DIAGRAMS
These diagrams are especially useful in assessing the high temperature
performance of ceramic materials.

2. Phase diagrams have been experimentally determined for a large number of ceramic systems. For binary or two-component phase diagrams, it is frequently the case that the two components are compounds that share a common element, often oxygen.
3. Remember them?
4. The Al2O3 Cr2O3 System
consists of single liquid and single solid phase regions separated by a two-phase solidliquid region having the shape of a blade.
A substitutionalsolid solutiononein which Al3+substitutes for Cr3+and vice versa.
5. The Al2O3 Cr2O3 System
Fig. 12.24
6. The MgOAl2O3 System
There exists an intermediate phase, or better, a compound called spinel, which has the chemical formula MgAl2O4 (or MgO Al2O3 ).
SPINEL
Spinel is a distinct compound
50 mol% Al2O350 mol% MgO
7. The MgOAl2O3 System
Fig. 12.25
8. The ZrO2CaO System
The horizontal axis extends to only about 31 wt% CaO (50 mol% CaO), at which composition the compound CaZrO3 forms.
one eutecticand two eutectoid reactions are found for this system.
9. It may also be observed that ZrO2 phases having three different crystal structures exist in this systemnamely, tetragonal, monoclinic, and cubic.
A relatively large volume change accompanies this transformation,
resulting in the formation of cracks
10. This problem is overcome by stabilizing the zirconia.
adding between about 3 and 7 wt% CaO.
Partially stabilized zirconia, or PSZ
Yttrium oxide (Y2O3 ) and magnesium oxide are also used as stabilizing agents.
11. The ZrO2CaO System
Fig. 12.26
12. The SiO2Al2O3 System
Commercially, the silicaalumina system is an important one since the principal constituents
of many ceramic refractories are these two materials.
13. cristobalite
polymorphic form of silica that is stable
mullite
3Al2O32SiO2
a raresilicate mineral
14. The SiO2Al2O3 System
Fig. 12.27
15. Mechanical Properties
limited in applicability by their mechanical properties.
inferior to those of metals.
The principal drawback is a disposition
to catastrophic fracture in a brittle manner with very little energy absorption.
16. BRITTLE FRACTURE OF CERAMICS
At room temperature, virtually all ceramics are brittle.
17. Microcracks
Its presence results in amplification of applied tensile stresses and accounts for relatively low fracture strengths (flexural strength).
18. At room temperature, both crystalline and noncrystalline ceramics almost always fracture before any plastic deformation can occur in response to an applied tensile load.
Crack growth in crystalline ceramics may be either transgranularor intergranular
19. The measured fracture strengths of ceramic materials are substantially lower than predicted by theory from interatomic bonding forces.
The measure of a ceramic materials ability to resist fracture when a crack is
present is specified in terms of fracture toughness.
20. very small and omnipresent flaws in the material
stress raisers
may be minute surface or interior cracks (microcracks), internal pores, and grain corners, which are virtually impossible to eliminate or control.
21. static fatigue, or delayed fracture
fracture of ceramic materials will occur by the slow propagation of cracks, when stresses are static in nature
this type of fracture is especially sensitive to environmental conditions, specifically when moisture is present in the atmosphere.
22. FRACTOGRAPHY OF CERAMICS
A fractographic study involves examining the path of crack propagation as well as microscopic features of the fracture surface.
23. Fig. 12.29
24. during propagation, a crack accelerates until a critical velocity is achieved;
for glass, this critical value is approximately one-half of the speed of sound.
Upon reaching this critical velocity, a crack may branch, a process that may be successively repeated until a family of cracks is produced.
25. 26. mirror region - The crack surface that formed during the initial acceleration stage of propagation
mist region- a faint annular region just outside the mirror
hackle region- a set of striations or lines that radiate away from the crack source in the direction of crack propagation
27. STRESS-STRAIN BEHAVIOR
The stressstrain behaviorof brittle ceramics is not usually ascertained by a tensile test.
28.

First, it is difficult to prepare and test specimens having the required geometry.

29. Second, it is difficult to grip brittle materials without fracturing them; 30. Third, ceramics fail after only about 0.1% strain, which necessitates that tensile specimens be perfectly aligned to avoid the presence of bending stresses, which are not easily calculated.