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Title of Thesis

Saif Ullah Khan
Institute/University/Department Details
University of the Punjab, Lahore
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
silicon nitride, ceramic, sintering, sem, snal, snbn

In high temperature engineering applications, the role of silicon nitride cannot be ignored and it has been regarded as a promising engineering material of the future. In view of the increasing requirements of the engineering applications of silicon nitride, the present study was undertaken.

Fabrication of silicon nitride composites by the addition of different sintering additives in different wt.% composition with silicon nitride by reaction sintering technique. The sintering was carried out in nitrogen environment under the atmospheric pressure at a temperature of 1500oC ( pressureless sintering) for four hours. The sintering aids i.e., ZrO2, AIN, BN & SiC were added in 10wt.% , 20wt.% and 30wt.% composition with 90wt.%, 80wt.% and 70wt.% silicon nitride respectively, using 2wt.% composition of Al2O3 or MgO to each batch as binders.

Various properties such as density, hardness, flexural strength and creep resistance were studied. It was observed that higher the wt.% composition of the sintering additive has degraded all the mechanical properties of the composites. It was due to the incomplete reaction and the formation of glassy and amorphous phases during the sintering reaction, which resulted porosity in the materials. The nitride sintering additives i.e, AIN and BN have significantly degraded the mechanical properties of the composites but, in the case of Zr O2 as a additive with Si3N4, the new crystalline phase Si2N2O has enhanced the densification of the material. Therefore the mechanical properties, such as hardness, flexural strength and creep resistance of such the composite material has been improved. The sintering additive SiC remained unreacted with the starting material Si3 N, but the phase Si2N2O was appeared during the sintering process. The transformation of α-Si3 N4to β-Si3 N4 was also very small in this composite material.

The fabricated materials were characterized by X-ray diffractometer using JCPDS card for the identification of the phase and they were further characterized by coloured XRD spectra, linked with computer software programme to observe the transformation of α-Si3 N4to β-Si3 N4 and further to confirm the identified phase, which were produced in the composites during the sintering process. The α to Si3 N4 transformation has also been observed in all of the composite materials. The crystalline phase Si2 N2O was identified in all the composites except SNAL. The unreacted particles of AIN were observed in the composite SNAL. Some amorphous phase were also observed in the composites SNAL ;and SNBN. The SiC particles remained unreacted with the material and were observed in the composite SNSIC.

To see the microstructure of the composites and to observe the phase, they were further characterized by SEM. The cavities, process, flaws, the glassy phase, the amorphous, the crystalline phases the shape (elongated, spherical, prismatic, etc.) and size of the phase or grains and α and β-Si3 N4phase were observed from the micrographs of the each composite material.

The chemical analysis at different points (pockets, particles, phases etc.) of the composites have been carried out by EDX, to observed the wt.% composition of the elements present at these points of the composition.

In order to study the creep mechanism by evaluating the stress exponent €œn€™ and the activation energy of the composites. The creep tests were carried out under two different conditions.

Under the stress of 100 MPa for the first thirty hours and then by changing the stress to 150 MPa for the next 30 hours, while keeping the temperature of the composites constant at 1200oC.

At a temperature of 1200oC for the first 30 hours and then by changing the temperature to 1300 oC. for the next 30 hours, while keeping the composites under the constant stress of 100MPa.

The average values of stress exponents and activation energies of all the samples were determined as 1.7 and 185 KJ/mole, respectively. It was the mixed mode of creep deformation i.e., the cavitation, nucleation and diffusional process. The grain boundary sliding accommodated by void deformation at the triple point was occurred during the creep phenomenon.

From the creep phenomenon, it has been observed that the increase in wt.% composition of the additives have degraded the creep property of the composites, which was most probably due to the incomplete reaction during the sintering process. The creep rate of sintered Si3 N4 with nitride additives such as BN and AIN was higher as compared to the material with additives ZrO2 and SiC. It was due to the evaluation of nitrogen gas during the sintering process and production of amorphous phase in the material during the sintering process. It has been noted that creep rate of all the composites was higher in the initial few hours; a study state creep rate was then observed. From the creep curves one thing was more prominent that the temperature has significant effect on the creep rate rather than that of the stress. No. failure of the composites after 60 hours of creep was observed.

The crept samples again characterized by SEM to see the change in microstructure of the composites. From the SEM micrographs of the composites after the creep deformation it was observed that the diffusion of the viscous flow has been occurred during the creep phenomenon. Also sub-critical crack growths have been observed in some of the micrographs. The depletion of multigrain has resulted in various geometries. Different shaped cavities were found in the micrographs.

Download Full Thesis
5217.09 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
161.68 KB
2 1 Introduction
284.22 KB
  1.1 Introduction 1
  1.2 Silicon Nitride Ceramic 2
  1.3 Fabrication Materials 3
  1.4 Physical properties of Si 3 N 4 8
  1.5 Applications of Si 3 N 4 ceramics 13
  1.6 Atomic structure of Si 3 N 4 17
  1.7 Creep Phenomenon 19
3 2 Literature review 26
496.88 KB
  2.1 Mechanical properties of silicon Nitride Materials 26
  2.2 The Effect of the sintering Aids/ Additives 41
  2.3 Characterization of the Materials 49
  2.4 The creep behaviour of the materials 52
  2.5 State of Art component fabrication and applications 56
4 3 Experimental procedures & Technique 59
389.41 KB
  3.1 The choice of additives/ sintering aids and binders in the present study 59
  3.2 Fabrication of the materials for characterization and mechanical properties 60
  3.3 Preparation of the samples for various studies 65
  3.4 Hardness measurement 67
  3.5 Flexural strength measurement 70
  3.6 Density 71
  3.7 Microscopy 73
  3.8 X-ray diffractography 75
  3.9 Creep machine for the measurement of creep rate of the samples 82
5 4 Instrument Design for experimental studies 87
226.96 KB
  4.1 Die preparation for cold sintering of the samples 82
  4.2 Design and preparation of the furnace for the sintering of the material and for the creep test 95
  4.3 Temperature controller unit 99
6 5 Results and discussion 104
3425.35 KB
  5.1 Density measurement of the samples 104
  5.2 Hardness measurement of the samples 108
  5.3 Flexural strength measurement of the samples 113
  5.4 The microstructure examination of the samples before the creep test 117
  5.5 The X-ray diffraction of the samples 131
  5.6 The chemical analysis of the samples using energy dispersive X- ray( EDX) 185
  5.7 The creep study of the composites 203
  5.8 The Microstrual examination of the samples after creep phenomenon 230
7 6 Conclusion 235
106.76 KB
  6.1 Assessment of the results, general conclusion and proposal for the further research work 242
8 7 References 242
86.6 KB