Download - Kuliah_9_KXEX 1110
-
8/14/2019 Kuliah_9_KXEX 1110
1/56
-
8/14/2019 Kuliah_9_KXEX 1110
2/56
-
8/14/2019 Kuliah_9_KXEX 1110
3/56
-
8/14/2019 Kuliah_9_KXEX 1110
4/56
-
8/14/2019 Kuliah_9_KXEX 1110
5/56
-
8/14/2019 Kuliah_9_KXEX 1110
6/56
-
8/14/2019 Kuliah_9_KXEX 1110
7/56
-
8/14/2019 Kuliah_9_KXEX 1110
8/56
-
8/14/2019 Kuliah_9_KXEX 1110
9/56
-
8/14/2019 Kuliah_9_KXEX 1110
10/56
-
8/14/2019 Kuliah_9_KXEX 1110
11/56
-
8/14/2019 Kuliah_9_KXEX 1110
12/56
-
8/14/2019 Kuliah_9_KXEX 1110
13/56
-
8/14/2019 Kuliah_9_KXEX 1110
14/56
-
8/14/2019 Kuliah_9_KXEX 1110
15/56
-
8/14/2019 Kuliah_9_KXEX 1110
16/56
-
8/14/2019 Kuliah_9_KXEX 1110
17/56
-
8/14/2019 Kuliah_9_KXEX 1110
18/56
-
8/14/2019 Kuliah_9_KXEX 1110
19/56
-
8/14/2019 Kuliah_9_KXEX 1110
20/56
-
8/14/2019 Kuliah_9_KXEX 1110
21/56
-
8/14/2019 Kuliah_9_KXEX 1110
22/56
-
8/14/2019 Kuliah_9_KXEX 1110
23/56
-
8/14/2019 Kuliah_9_KXEX 1110
24/56
-
8/14/2019 Kuliah_9_KXEX 1110
25/56
-
8/14/2019 Kuliah_9_KXEX 1110
26/56
-
8/14/2019 Kuliah_9_KXEX 1110
27/56
-
8/14/2019 Kuliah_9_KXEX 1110
28/56
-
8/14/2019 Kuliah_9_KXEX 1110
29/56
-
8/14/2019 Kuliah_9_KXEX 1110
30/56
-
8/14/2019 Kuliah_9_KXEX 1110
31/56
-
8/14/2019 Kuliah_9_KXEX 1110
32/56
-
8/14/2019 Kuliah_9_KXEX 1110
33/56
-
8/14/2019 Kuliah_9_KXEX 1110
34/56
-
8/14/2019 Kuliah_9_KXEX 1110
35/56
-
8/14/2019 Kuliah_9_KXEX 1110
36/56
-
8/14/2019 Kuliah_9_KXEX 1110
37/56
Imperfections in Ceramics
Atomic Point Defect Schottky, Frenkel
Impurities Interstitial, Substitutional
-
8/14/2019 Kuliah_9_KXEX 1110
38/56
Atomic Point Defect
Frenkel Defect--a cation is out of place.
Shottky Defect--a paired set of cation and anion vacancies.
Equilibrium concentration of defects kT / QDe~
Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 is from W.G.Moffatt, G.W. Pearsall, and J.Wulff, The Structure and Properties of Materials , Vol. 1,Structure , John Wiley and Sons,Inc., p. 78.)
Shottky
Defect:
Frenkel
Defect
-
8/14/2019 Kuliah_9_KXEX 1110
39/56
Impurities
Impurities must also satisfy charge balance = Electroneutrality
Ex: NaCl
Substitutional cation impurity
Na + Cl-
initial geometry Ca 2+ impurity resulting geometry
Ca 2+
Na+
Na +Ca 2+
cationvacancy
Substitutional anion impurity
initial geometry O 2- impurity
O 2-
Cl -
anion vacancy
Cl -
resulting geometry
-
8/14/2019 Kuliah_9_KXEX 1110
40/56
-
8/14/2019 Kuliah_9_KXEX 1110
41/56
Mechanical Properties
Strength of ceramics vary greatly but they are generallybrittle . Tensile strength is lower than compressive strength.
-
8/14/2019 Kuliah_9_KXEX 1110
42/56
Mechanism of Deformation
Covalently bonded ceramics : Exhibit brittle fracture due to
separation of electron-pair bonds without their subsequentreformation. Ionically bonded ceramics: Single crystal show considerable plastic
deformation. Polycrystalline ceramics are brittle. Example: NaCl crystal
Slip in {100} familyof planes is rarely
observed as same
charges come into contact.
Cracking occurs at grain boundaries.
Figure 10.44
-
8/14/2019 Kuliah_9_KXEX 1110
43/56
Factors Affecting Strength
Failure occurs mainly from surface defects. Pores gives rise to stress concentration and
cracks. Pores reduce effective cross-sectional area.
Flaw size is related to grain size . Finer size ceramics have smaller flaws and henceare stronger.
Composition , microstructure , surface condition ,
temperature and environment also determinestrength.
-
8/14/2019 Kuliah_9_KXEX 1110
44/56
-
8/14/2019 Kuliah_9_KXEX 1110
45/56
Elastic Modulus
Room T behavior is usually elastic, with brittle failure.
3-Point Bend Testing often used.--tensile tests are difficult for brittle materials.
Adapted from Fig. 12.32,Callister 7e.
F L/2 L/2
d = midpointdeflection
cross section
R
b
d
rect. circ.
Determine elastic modulus according to:F
x
linear-elastic behavior
F
slope =E =
F
L3
4bd 3 =F
L3
12 R 4
rect.cross
section
circ.cross
section
-
8/14/2019 Kuliah_9_KXEX 1110
46/56
Creep
time
Elevated Temperature Tensile Test ( T > 0.4 T m).
creep test
slope = ss = steady-state creep rate.
x
-
8/14/2019 Kuliah_9_KXEX 1110
47/56
Toughness of Ceramic
Ceramics have low strength .
Research has been conducted to improve toughness. Hot pressing with additives and reaction bonding improve
toughness.
K IC values obtained by four point bend test .
aY K f IC =
f = fracture stress (MPa)
a = half size of target internal flaw
Y = dimensionless constant
Figure 10.46
-
8/14/2019 Kuliah_9_KXEX 1110
48/56
Transformation Toughening of Partially Stabilized ZrO 2
Transformation of Zirconia combined withsome other refractory oxides (MgO) can
produce very high toughness ceramics.
ZrO 2 exists in 3 structures. Monoclinic Up to 1170 0C
Tetragonal 1170 2370 0C
Cubic above 2370 0C
Adding 10% mol of MgO stabilizes cubicform so that it can exist in metastable statein room condition.
-
8/14/2019 Kuliah_9_KXEX 1110
49/56
If a mixture of ZrO 2 9 mol% MgO is sintered at about
1800 0C and rapidly cooled, it will be in metastable state. If reheated to 1400 0C and
held for sufficient timetetragonal structure
precipitates. Under action of stress,
this tetragonal structuretransforms to monoclinic
increasing volume andhence retarding crack growth.
Figure 10.47a
-
8/14/2019 Kuliah_9_KXEX 1110
50/56
-
8/14/2019 Kuliah_9_KXEX 1110
51/56
Thermal Properties of Ceramics
Low thermal conductivity and high heatresistance.
Many compounds are used as industrialrefractories.
For insulating refractories, porosity is desirable. Dense refractories have low porosity and high
resistance to corrosion and errosion. Aluminum oxide and MgO are expensive and
difficult to form and hence not used asrefractories.
-
8/14/2019 Kuliah_9_KXEX 1110
52/56
Porosity
Exist between powder particles due toforming processes.
Heat treatment will eliminate porosity butsome residual porosity will remain.
Influence the elastic properties and strength. Porosity reduce strength because:
Pores reduce cross sectional area Act as stress concentrator
-
8/14/2019 Kuliah_9_KXEX 1110
53/56
-
8/14/2019 Kuliah_9_KXEX 1110
54/56
Hardness
High hardness Suitable for abrasive materials
-
8/14/2019 Kuliah_9_KXEX 1110
55/56
Electrical Properties
Basic properties of dielectric:
Dielectric constant :- Q = CV
Q = Charge
V = Voltage
C = Capacitance
C = 0A/d 0 = permeability of free space
= 8.854 x 10 -12 F/m When the medium is not free space C = K 0A/d Where K is dielectric constant of the
material between the plates
Figure 10.35
-
8/14/2019 Kuliah_9_KXEX 1110
56/56
Dielectric Strength and Loss Factor
Dielectric strength is measure of ability of material to hold
energy at high voltage.Defined as voltage gradient at which failure
occurs.
Measured in volts/mil.
Dielectric loss factor: Current leads voltage by 90 degreeswhen a loss free dielectric is between plates of capacitor.
When real dielectric is used, current leads voltage by 90 0 where is dielectric loss angle .
Dielectric loss factor = K tan measure of electric energy lost .