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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