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TITLE
DEDICATION
CERTIFICATE
DECLARATION
CONTENTS
ACKNOWLEDGEMENT
LIST OF FIGURES
LIST OF TABLES
1. INTRODUCTION
1.1 CERAMICS
Fig 1.1 Classification of ceramics
1.2 SOL GEL PROCESS
1.1 Chronology of events leading to the evolution of sol gel science
1.2.1 THE FUNDAMENTAL CONCEPTS
a Sols
b. Gels
c. Aqueous based process
d Alcohol based process
e Outline of the process
Fig 1.2 Schematic outline of the sol gel process and comparison of colloidal and polymeric routes
f Advantages and limitations of the process
1.2 Applications of sol gel process
1.2.2 SOL GEL ALUMINA CERAMICS
a Chemistry of Boehmite preparation
b. Phase transformations in sol gel Boehmite
Fig 1.3 Structure of complexes formed during the precipitation of aluminium hydroxide from metal salt solution.
Fig 1.4 Sequence of α-alumina formation with temperature from different alumina precursors.
1.2.3 SEEDING CONCEPTS IN SOL GEL SYSTEMS
1.3 Seeding effects in sol gel systems
a Epitaxial Nucleation
Fig 1.5 Epitaxial nucleation of stable α-phase on substrate α in contact with unstable θ phase
b. Conditions for epitaxial nucleation benefits
1.3 COMPOSITES
1.4 NANOCOMPOSITES
1.4 Classification of nanocomposites
1.5 STRUCTURAL CERAMIC NANOCOMPOSITES
1.5.1 PROCESSING
1.5.2 MECHANICAL PROPERTIES
a Densification
b. Strength and toughness
c. Wear resistance
d Creep resistance
1.5.3 NANOCOMPOSITE EFFECT AND ASSOCIATED MECHANISMS
a Microstructural refinement
a.1 Zener grain boundary pinning
a.2 Sub grain formation
a.3 Suppression of intergranular fracture
a.4 Residual stress relaxation behaviour
b. Toughening Mechanisms
b.1 Crack deflection
b.2 Crack tip bridging and R-Curve effects
b.3 Micro cracking
1.6 PARTICULATE COMPOSITES FROM PRECOATED DISPERSOIDS
1.7 CONSTRAINED SINTERING
Fig 1.6 Geometry of the Problem
1.8 DESCRIPTION OF THE PROBLEM
2. EXPERIMENTAL
2.1 RAW MATERIALS USED
2.1 Raw materials used in the present study
2.2 EXPERIMENTAL PROCEDURE
2.2.1 ACTIVATION OF SiC
2.2.2 COATING TECHNIQUE
2.2 Experimental details for obtaining alumina coated SiC particles
2.2.3 PREPARATION OF COMPOSITE
a Preparation of boehmite sol
b. Preparation of alumina seed suspension
c. Sol gel processing of composites
Fig 2.1 Flowchart showing the preparation of sol gel alumina-SiC nanocomposites
2.2.4 DEPENDENCE OF CALCINATION CONDITIONS
Fig 2.2 Sintering schedule followed for Alumina-SiC nanocomposites.
2.2.5 EFFECTS OF NUCLEATING SEEDS
2.2.6 COMPARISON WITH CONVENTIONAL ROUTES
2.2.7 INFLUENCE OF MgO
2.3 CHARACTERISATION
a Thermal analysis
b. Dilatometric studies
c. Fourier transform infrared spectroscopy
d Zeta potential analysis
e Surface area analysis
f XRD
g Microscopy (SEM and SPM)
h Transmission electron microscopy
2.4 MECHANICAL TESTING
a Density
b. Flexural strength
Fig 2.3 Schematic illustration of a four point bending fixture
c. Fracture toughness
d Hardness
2.5 GAS PRESSURE SINTERING
Fig 2.4 A typical gas pressure sintering cycle
3. RESULTS
3.1 COATING OF SiC
3.1.1 ACTIVATION TREATMENT
Fig 3.1 Thermal Analysis data of as received SiC powders
Fig 3.2 FTIR pattern of as received SiC powders
Fig 3.3 FTIR pattern of SiC powders heat treated at 650°C/30min.
Fig 3.4 FTIR pattern of SiC powders after thermal activation and HF leaching
3.1.2 COATING
a FTIR
Fig 3.5 FTIR pattern of SiC-25 wt% alumina powders (gel)
Fig 3.6 FTIR pattern of SiC-25 wt% alumina powders calcined at 500°C
b. BET surface area
Fig 3.7 Plot of surface area values with increasing alumina weight fraction
Fig 3.8a Adsorption isotherms of SiC powders coated with varying weight fraction of alumina (gel)
Fig 3.8b Adsorption isotherms of SiC powders coated with varying weight fraction of alumina (calcined at 500° ()
Fig 3.9a t-plot analysis of coated SiC powders before calcination
Fig 3.9b t-plot analysis of coated SiC powders after calcination at 500°C
c. Transmission electron microscopy
Fig 3.10 TEM pictures of a) Uncoated SiC
Fig 3.10 TEM pictures of b) SiC-5 wt% alumina calcined at 500°C
Fig 3.10 TEM pictures of c) SiC-5 wt% alumina calcined at 500°C
Fig 3.10 TEM pictures of d) SiC-25 wt% alumina calcined at 500°C
d Zeta potential measurements
Fig 3.11 TEM picture of SiC particles coated with 25 wt% alumina (gel)
Fig 3.12 Zeta Potential as a function of pH for SiC particles coated with alumina compared with that of uncoated SiC and pure alumina
3.2 PROCESSING OF COMPOSITES
3.2.1 THERMAL ANALYSIS
Fig 3.13 Thermal analysis pattern of as received boehmite powders
Fig 3.14 Thermal analysis of boehmite gel seeded with 2 wt% a-alumina seeds
Fig 3.15 Thermal analysis pattern of alumina-5vol% SiC composite precursor seeded with 2 wt°!o a-alumina seeds
3.2.2 PHASE FORMATION
Fig 3.16 XRD pattern of the phases formed on calcination of alumina-5 vol% SiC seeded precursor at different temperatures.
3.2.3 DEPENDENCE OF CALCINATION CONDITIONS
3.1 Formation of alwnina phases on calcination
Fig 3.17a Dependence of calcination conditions on green density
Fig 3.17b Dependence of calcination conditions on sintered density
3.2.4 DILATOMETER STUDIES
Fig 3.18a Shrinkage profile of sol gel composite precursor calcined at 1000°C
Fig 3.18b Shrinkage profile of sol gel composite precursor calcined at 900°C
Fig 3.18c Shrinkage profile of sol gel composite precursor calcined at 800°C
Fig 3.19 FTIR pattern of alumina-SiC composite precursor calcined at 1000°C
3.2.5 COMPARISON BEHAVIOUR
Fig 3.20 Effect of CIP Pressure on green and sintered densities
3.3 EFFECT OF SEEDING
3.3.1 ON PHASE TRANSFORMATION
3.3.2 ON DENSIFICATION
Fig 3.21 Variation of a-alumina formation temperatures with % amount of seeds
3.3.3 DENSIFICATION AND MICROSTRUCTURE DEVELOPMENT
Fig 3.22 Sintered density with temperature for seeded and unseeded samples
Fig 3.23 Densification data of monolithic alumina and nanocomposite
Fig 3.24 SEM picture of polished and thermally etched monolithic alumina
Fig 3.25a SEM picture of alumina-5vol% SiC sintered at 1550°Cllh
Fig 3.25b SEM picture of alumina-5vol% SiC sintered at 1550°Cllh (higher magnification)
Fig 3.26a SEM micrograph of alumina-5vol% SiC nanocomposite sintered at 1650°Cllh
Fig 3.26b SEM micrograph of alumina-5vol% SiC nanocomposite sintered at 1650°Cl l h (higher magnification)
Fig 3.27 AFM picture of alumina-5 vol% SIC nanocomposite sintered at 1650°C/lh a) lower magnification b) higher magnification
Fig 3.28a SEM picture of alumina-5 vol% SiC nanocomposite sintered at 1700°C/90 min.
Fig 3.28b SEM picture of alumina-5 vol% SiC nanocomposite sintered at 1700°C190 min. (higher magnification)
3.4 FRACTURE MODE
Fig 3.29 AFM picture of alumina-5 vol% SiC nanocomposite sintered at 1700°C190 min. a) lower magnification b) higher magnification
Fig 3.30a Fracture surface of sintered monolithic alumina
Fig 3.30b Fracture surface of sintered alumina-5 vol% SiC nanocomposite
3.5 MICROSTRUCTURE DEVELOPMENT IN UNSEEDED COMPOSITES
Fig 3.31 a SEM picture of an unseeded composite sample sintered at 1550°C/60 min.
Fig 3.31 b SEM picture of an unseeded composite sample sintered at 1650°C/60 min.
3.6 MECHANICAL PROPERTIES
Fig 3.31c SEM picture of an unseeded composite sample sintered at 1700°C/90 min.
Fig 3.32 Four point bend strength values of alumina-5 vol% SiC nanocomposites with sintering temperature
3.2 Comparison of mechanical data for alumina and nanocomposite
3.7 COMPARISON OF PROCESSING METHODS
Fig 3.33 XRD patterns of the various composite precursors
Fig 3.34a Shrinkage profile of a-alumina + SiC mixture
Fig 3.34b Shrinkage profile of sol gel composite precursor calcined at 1000°C
Fig 3.34c Shrinkage profile of transition alumina + SiC mixture
Fig 3.35 Variation in densities of composite precursors a) green b) on sintering at 1700°C/90 min
3.8 INFLUENCE OF MgO ADDITION
Fig 3.36 Densification behaviour of MgO doped and undoped nanocomposites
Fig 3.37a AFM picture of undoped alumina
3.3 Variation in grain sizes with sintering temperature for 1 wt% MgO doped and undoped nanocomposite
Fig 3.37b AFM picture of MgO doped alumina
Fig 3.38a SEM picture of chemically etched alumina-5 vol% SiC doped with 1 wt% MgO sintered at 1450°C/lh
Fig 3.38b SEM picture of chemically etched alumina-5 vol% SiC doped with 1 wt% MgO sintered at 1550°Cll h
Fig 3.38c SEM picture of chemically etched alumina-5 vol% SiC doped with 1 wt% MgO sintered at 1650°C/lh
4. DISCUSSION
4.1 COATING OF SiC
4.1.1 ACTIVATION TREATMENT
4.1.2 PRECIPITATION OF ALUMINA PRECURSOR PHASE
4.1.3 COATING MECHANISM
Fig 4.1 Schematic illustration of the coating process
Fig 4.2 Increase in isoelectric points (IEP) of alumina coated SiC particles
4.2 PROCESSING OF COMPOSITES
4.2.1 EFFECT OF SEEDING ON PHASE TRANSFORMATION
4.2.2 DENSIFICATION
4.2.3 MICROSTRUCTURE DEVELOPMENT
4.3 DENSIFICATION AND GRAIN BOUNDARY PINNING
4.4 FRACTURE MODE
Fig 4.3 AFM picture of a) sintered alumina b) nanocomposite
4.5 DEPENDENCE OF CALCINATION CONDITIONS
4.1 Phase formation and characteristics of alumina-5 vol% SiC composite precursors on calcination
4.6 INFLUENCE OF MgO ON PROCESSING OF COMPOSITES
4.7 COMPARISON OF PROCESSING METHODS
Fig 4.4a TEM picture of so] gel derived alumina-SiC nanocomposite
Fig 4.4b TEM picture of nanocomposite showing SiC particles within grain
Fig 4.5a SEM picture of sol gel derived alumina-SiC nanocomposite
4.8 COMPARISON OF PROPERTIES BETWEEN ALUMINA AND NANOCOMPOSITE
Fig 4.5b SEM picture of nanocomposite derived from tamei alumina + SiC mixture
Fig 4.6 Schematic illustration of the sot gel coated process compared with conventional powder mixing route a) sol gel coated b) powder mixing
Fig 4.7 SEM picture of a typical processing flaw in nanocomposite
5. CONCLUSION
REFERENCES