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On the Toughening Mechanisms Present in Boron Suboxide Materials

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On the Toughening Mechanisms Present in Boron Suboxide Materials On the Toughening Mechanisms Present in Boron Suboxide Materials with Sintering Aids Peter Robert Bush A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. May 2011 Declaration I declare that this dissertation is my own unaided work. It is being submitted to the degree of Master of Science in Engineering to the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination to any other University. ------------------------ Peter Robert Bush --------------- day of ------------------------ 2011 Extracts from this body of work have been presented at: • Sigalas, I., Johnson, O.T., Bush, P. and Freemantle, C. Boron suboxide composites: Thermal stability and tribological assessment. 12 th International Ceramics Conference (Cimtec, Montecatini Terme, Italy) 6- 11 June 2010 • Sigalas, I., Johnson, O.T., Bush, P. and Freemantle, C. Boron suboxide composites: Recent developments. 5 th International Workshop on Spinel Nitrides and Related Materials (Rüdersheim/Rhine, Germany). 29 August – 3 September 2010. i Abstract The fracture toughness of polycrystalline boron suboxide has been substantially improved through the addition of a low amount of various sintering aids Determining the reasons behind the small amount of secondary phase increasing the toughness of boron suboxide to such an extent was the focus of this project. Fracture toughness testing was conducted on pure boron suboxide and boron suboxide containing CaO, NiO, Y 2O3-Al 2O3 TiB 2-Y2O3-Al 2O3 additions. Determining the fracture toughness through the SEVNB, IFT and COD methods, it was shown that these additives increase the toughness of boron suboxide significantly. Additionally, hardness testing of pure boron suboxide revealed a significant indentation size effect. Analysis of crack interaction with the microstructure was performed. Insufficient mechanical toughening events were observed to explain the increase in fracture toughness. Although the mechanism behind the increase in fracture toughness was not discovered, it was concluded that the sintering aids must change the surface energy of the boron suboxide matrix. ii Acknowledgements I would like to acknowledge the following people for the valuable contributions which facilitated the completion of this research work. i. Professor Sigalas, for overseeing this body of research ii. Doctor Herrmann, for his contributions to this body of work iii. Element Six (PTY) Ltd. for their financial support iv. DST/NRF Centre of Excellence in Strong Materials for their financial support v. The staff at Element Six (PTY) Ltd. who assisted in this project, namely, Dr. T. Heiligers, Dr. F. Van Staden, Mr. R. Cruz and Mr. S. Masete vi. Mr. O.T. Johnson for his assistance in obtaining decent SEM images vii. The support staff in the School of Chemical and Metallurgical Engineering, especially Mr. A. Xoseka and Mr. B. Mothebeli, for their technical assistance viii. My family and friends for their patience and understanding during this period iii Contents Declaration ............................................................................................ i Abstract ................................................................................................ ii Acknowledgements ............................................................................. iii List of Figures .................................................................................... vii List of Tables ........................................................................................ x List of Symbols .................................................................................. xii 1.0 Introduction & Motivation ........................................................... 1 2.0 Literature Survey .......................................................................... 3 2.1 Super hard materials ...................................................................................... 3 2.2. Structure of boron suboxide ......................................................................... 5 2.3. Synthesis .................................................................................................... 10 2.4. Sintering ..................................................................................................... 17 2.4.1 Pure boron suboxide ............................................................................. 17 2.4.2. Composites .......................................................................................... 18 2.5 Thermodynamics of boron suboxide ........................................................... 25 2.5.1 Enthalpy and heat capacity................................................................... 25 2.5.2 Static interactions with CGI ................................................................. 27 2.5.3 Oxidation resistance ............................................................................. 29 2.6. Hardness ..................................................................................................... 31 2.6.1. Hardness testing .................................................................................. 31 2.6.2. Hardness of boron suboxide ................................................................ 33 2.7. Fracture Toughness .................................................................................... 35 2.8. Toughening of ceramics ............................................................................. 38 2.8.1 Crack deflection ................................................................................... 40 2.8.3 Zone shielding ...................................................................................... 42 2.8.4 Contact shielding .................................................................................. 44 3.0 Experimental Procedure ............................................................. 46 3.1 Introduction ................................................................................................. 46 iv 3.2 Hot press runs .............................................................................................. 46 3.2.1 Capsule assembly ................................................................................. 47 3.2.2 Densifying boron suboxide powder ..................................................... 48 3.3 Materials to be tested .................................................................................. 49 3.4 Production of boron suboxide discs by SPS/FAST technology .................. 50 3.5 LASER cutting ............................................................................................ 52 3.6 Lapping and grinding .................................................................................. 54 3.7 Grinding and polishing ................................................................................ 56 3.8 Density and Porosity ................................................................................... 57 3.9 Microscopy .................................................................................................. 60 3.10 XRD analysis ............................................................................................ 61 3.11 Hardness Testing ....................................................................................... 61 3.12 Indentation size effect ............................................................................... 63 3.13 Indentation fracture toughness .................................................................. 64 3.14 Distinguishing the crack regime ............................................................... 69 3.15 Crack opening displacement analysis ....................................................... 72 3.16 Single-edge-V-notch beam test ................................................................. 75 3.17 Introducing the V-notch ............................................................................ 78 4.0 Results .......................................................................................... 80 4.1 Introduction ................................................................................................. 80 4.2 Analysis of hot pressed boron suboxide materials ...................................... 80 4.2 Analysis of Spark Plasma Sintered samples ............................................... 82 4.2.1 Density ................................................................................................. 82 4.2.2 Phase analysis of samples .................................................................... 84 4.3 Indentation Size Effect ................................................................................ 89 4.3.1 Hardness Measurements....................................................................... 89 4.3.2 Meyer’s law. ......................................................................................... 90 4.4 Hardness of SPS densified materials .......................................................... 93 4.5 Crack regime ............................................................................................... 94 4.6 Indentation fracture toughness .................................................................... 95 4.7 SEVNB ........................................................................................................ 97 4.8 Mode of failure ...........................................................................................
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