When the word “Ceramic” comes to our mind, we usually associate them with plates, saucers, cups and mugs. But, the word “Ceramic” encompasses more than just the word “plates” or “saucers”. Indeed, ceramic materials are hard and inherently brittle, but this is just the tip of the iceberg. They have multifarious properties and have acquired a status of high technical importance in the field of scientific research. Ceramics are the soul of the modern day’s structural applications owing to their high mechanical and thermal stability under different challenging conditions. They exhibit remarkable properties such as high hardness, high wear resistance, high corrosion resistance, high elastic modulus, high melting point and the ability to retain high strength at elevated temperatures. Alumina (Al2O3) is one such remarkable ceramic material known for its unique optical, mechanical and electrical properties. But the brittle nature of Al2O3 limits its use in certain engineering applications. Therefore, the strength of Al2O3 and Al2O3- based ceramics can be enhanced by tailoring the microstructural design through the application of strategic techniques that may involve secondary phase particle inclusion (such as Zirconia, ZrO2)

1. Fundamentals, synthesis and
applications of Al2O3-ZrO2
composites
Seminar and Technical writing (Autumn 2021)
Presented by
Tandra Rani Mohanta
519CR6005
Department of Ceramic Engineering
National Institute of Technology Rourkela, Odisha
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2. Presentation contents
1. Classification of materials
2. Ceramic-definition
3. Classification of ceramic materials
4. Alumina
5. Zirconia
6. Toughening mechanisms in Al2O3-ZrO2 ceramics
7. Synthesis of Al2O3-ZrO2 ceramics
8. Applications
9. References
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3. Materials
Metals Polymers Ceramics Composites
Classification of materials
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4. Ceramic
Definition:
• The term ceramic comes from the Greek word “keramikos”,
which means “burnt stuff or drinking vessel”, indicating that
desirable properties of these materials are normally
achieved through high temperature heat treatment process
called Firing.
• A ceramic is a material that is neither metallic nor organic.
It may be crystalline, glassy or both crystalline and glassy.
Ceramics are typically hard and chemically non-reactive and
can be formed or densified with heat.
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5. Classification of ceramic materials
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Source: subtech.com

6. Oxide
ceramics
Aluminum
oxide Al2O3
Magnesium
oxide
Zirconium
oxide
Aluminum
titanate
Al2O3.TiO2
Lead zirconate
titanate
Pb[ZrxTi1-x]O3
Silicate
ceramics
Porcelain
Magnesium
silicates
MgO.SiO2
Mullite
3Al2O3.2SiO2
Carbide
ceramics
Aluminum
carbide
Silicon
carbide
Molybdenum
carbide
Tungsten
carbide
Boron
carbide
Calcium
carbide
Nitrides
ceramics
Silicon
nitride
Silicon
aluminum
oxynitride
Aluminum
nitride
Composition based classification of ceramic
materials Silicide
ceramics
Calcium
silicide
Magnesium
silicide
Molybdenum
di-silicide
Tungsten
silicide
Niobium
silicide
Boride
ceramics
Aluminum
diboride
aluminum
dodecaboride
Calcium
hexaboride
Magnesium
boride
Nickel boride
Niobium
diboride
Niobium
monoboride
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7. • Al2O3 is a remarkable ceramic material known for its interesting
properties.
• Exists in different forms depending upon the crystal structure.
• α-Al2O3 is most stable form of oxide. Examples: Corundum and
Sapphire.
• Crystal Structure of α-Al2O3.
• O2- >>  140 pm
• Al3+ >>  53 pm
• Lattice parameters: a  476 pm
c  1300 pm
ρ  3.99 g /cc
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Alumina (Al2O3)

8. Properties of Al2O3
•High Melting point (2054 oC).
•High hardness (10-30 GPa) depending upon the
content of Al2O3, sintered density and final grain
size.
•High stiffness (Esapphire  335-460 GPa; Epolycrystal
 400 GPa).
•Dense polycrystalline Al2O3 ( >99 % density) has
a bending strength, σbending  150-600 GPa;
depending upon the final avg. grain size.
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9. Zirconia (ZrO2)
• Exists in 3 crystalline forms: m, t and c.
• Range of stability:
monoclinic
1170oC
tetragonal
2370oC
cubic
2680oC
liquid
• t m transformation>> 3-5 % vol expansion.
• Properties: ρ  5.68 g/cc; M.p.  2700 oC; VH  12 GPa; E  200
GPa.
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950 oC

10. Al2O3-ZrO2 ceramics- An Overview
• Al2O3 is inherently brittle so it limits its use in certain engineering applications.
• Introduction of secondary inclusions leads to reduced Avg. grain Size and
enhancement of Mech. Properties.
• For highly dense, finely grained microstructures>> homogeneous distribution of
ZrO2 >> desirable factor>> Careful control of processing steps.
• Transformation toughening
I. Stress-induced transformation.
II. t-ZrO2 > sufficient size.
III. Impending crack >> t→m>> Volume
expansion of 3-5 %.
IV. Counterforce>> crack arrest.
Image source: http://glidewelldental.com/education/chairside-dental-magazine/volume-11-issue-3/bruxzir-full-strength-
vs-anterior
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11. PROPERTY UNITS
General Chemical formula n/a Al2O3 ZrO2 Zirconia toughned
alumina
Density g/cm3 3.98 5.85 4.6
Mechanical Compressive
strength
MPa >2100 1600 2500
Flexural strength MPa 315 100-400 675
Young’s modulus GPa 380 200 330
Thermal Thermal
conductivity
W/mK 29.3 1.7 17
Coefficient of linear
thermal expansion
10-8/oC 6.9 9.7 8
Electrical Volume resistance Ωcm >1014 >1013 >1014
Dielectric constant - 9.6 10
Dielectric strength kV/mm 31 23 25
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12. Toughening mechanisms in Al2O3-ZrO2 ceramics
Two forms of toughening mechanisms associated with Al2O3- ZrO2 ceramics
1. Microcrack toughening
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 the spontaneous t → m transformation in Al2O3-ZrO2 induces a large
tensile stress caused by the volume expansion of ZrO2 particles as a
result of which small cracks (microcracks) are formed at the immediate
vicinity of the transformed grain. These microcracks increases the energy
required for the impending crack for its propagation. At the same time,
the size of the dispersed ZrO2 must be large enough to transform on
cooling, and yet provide only limited development of the microcracks
References
1. Riley, F.L., Structural ceramics: fundamentals and case studies. 2009: Cambridge University Press
2. Claussen, N., J. Steeb, and R.F. Pabst, Effect on induced microcracking on the fracture toughness of
ceramics. American Ceramic Society Bulletin, 1977. 56(6): p. 559-62

13. 2. Transformation toughening
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 Transformation of the metastable t-ZrO2 phase as a result of stress in the
surrounding of the impending/approaching crack. From the figure below, if the
tetragonal ZrO2 (t-phase) particles are sufficiently small, then during the process of
cooling from elevated, the surrounding matrix phase can provide some resistance
or restriction to the transformation, and hence, a metastable t-ZrO2 phase can be
retained
Image Source: Barsoum, M. and M. Barsoum, Fundamentals of ceramics. 2002: CRC
press.

14. Synthesis of Al2O3-ZrO2 ceramics
• Powder processing route
 depends upon the choice of the
raw materials
 particle size and its distribution
 degree of agglomeration
 high purity materials prevent the
formation of secondary phases
during the process of sintering
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Image source: www.pm-
review.com

15. • Sol-gel synthesis
 Helps produce near-net complex
shapes with controlled microstructural
homogeneity under low processing
temperatures
 Compounds with metal-organic origin
or inorganic salts are often used for
sol preparation, followed by hydrolysis
and condensation to produce a gel
(called gelation of the sol). The gel is
then dried to expel the extra liquid
phase
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Image source:
http://dx.doi.org/10.4236/msa.2012.39095

16. • Co-precipitation technique
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Synthesis of Al2O3-ZrO2 composite powders by co-
precipitation depends upon the concentration, pH, and
temperature of the solution along with proper choice of
precipitant and drying method which makes it a rather
complex co-precipitation method.
The drying methods, namely, spray drying, vacuum drying
and freeze drying are used.

17. • Spray pyrolysis
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Image source: Matmatch
 An aerosol precursor solution is first
prepared which is sprayed into the
high temperature chamber wall. The
solvent evaporates and solute
precipitation occurs which upon
thermal decomposition, yields
ceramic powder
 powder stoichiometry can be
effectively controlled
 the amount of particle agglomeration
is rather limited
 Disadvantage:- powder morphology
cannot be effectively controlled and
may result in porous particles
 Expensive process.

18. • Combustion technique
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Ref: Xie, Shian & Guo, Lijun & Zhang, Manbo & Qin, Jiangke & Hu, Ruixiang. (2021). Durable hydrophobic
ceramics of Al2O3–ZrO2 modified by hydrophilic silane with high oil/water separation efficiency. Journal of
Porous Materials. 28. 1-13. 10.1007/s10934-021-01055-7.
 The powders produced this
technique had fine crystallite
size with a narrow distribution
as compared to those
prepared by conventional
heating.
 The technique yielded fine
ZrO2 homogeneously dispersed
in the Al2O3 matrix

19. Applications of Al2O3-ZrO2 ceramics
Al2O3-ZrO2
ceramics
Valve
seals
Pump
parts
Electrosu
rgical
insulators
Oxygen
sensors
Medical
applicatio
ns
Dies
Cutting
tools and
inserts
Body
armour
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20. References
1. Evans, K. The manufacture of alumina and its use in ceramics and related applications. in Key
Engineering Materials. 1996. Trans Tech Publ.
2. Shirai, T., et al., Structural properties and surface characteristics on aluminum oxide powders. Ann
Rep Ceram Res Lab Nagoya Inst Technol, 2009. 9: p. 23-31.
3. Kim, Y. and T. Hsu, A reflection electron microscopic (REM) study of α-Al2O3 (0001) surfaces. Surface
science, 1991. 258(1): p. 131-146.
4. Wachtman, J., et al., Elastic Constants of Synthetic Single‐Crystal Corundum at Room Temperature.
Journal of the American Ceramic Society, 1960. 43(6): p. 334-334.
5. Ryshkewitch, E. and D.W. Richerson, Oxide ceramics. 1985.
6. Wachtman Jr, J., et al., Exponential temperature dependence of Young's modulus for several oxides.
Physical review, 1961. 122(6): p. 1754.
7. Chung, D. and G. Simmons, Pressure and temperature dependences of the isotropic elastic moduli of
polycrystalline alumina. Journal of Applied Physics, 1968. 39(11): p. 5316-5326.
8. Knudsen, F., Effect of porosity on Young's modulus of alumina. Journal of the American Ceramic
Society, 1962. 45(2): p. 94-95.
9. Wiederhorn, S., B. Hockey, and D. Roberts, Effect of temperature on the fracture of sapphire.
Philosophical Magazine, 1973. 28(4): p. 783-796.
10. Riley, F.L., Structural ceramics: fundamentals and case studies. 2009: Cambridge University Press.
11. Ma, Q. and D.R. Clarke, Piezospectroscopic determination of residual stresses in polycrystalline
alumina. Journal of the American Ceramic Society, 1994. 77(2): p. 298-302.
12. JH, W. and J. PJ, Indentation creep of solids. Transactions of the Metallurgical Society of AIME,
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