Development of boron suboxide composites with improved toughness

Abstract

The research efforts by material scientists in the area of boron suboxide materials have been directed towards improving the fracture toughness by microstructural tailoring and adopting various effective toughening mechanisms. In this perspective, the current work reports the development of boron suboxide composites with improved toughness. The relationships between densification and microstructure and microstructure and mechanical properties of boron suboxide (B6O) composites have been studied in detail using hot pressing technique. Various suitable sintering additives were selected based on thermodynamic calculations. Laboratory prepared B6O powders were initially milled using steel balls in an attrition mill to drive the particle sizes down to submicron range. Contaminations from steel balls were acid washed in HCl. The amounts of impurities remaining in the powders were characterised using Inductively Couple Plasma (ICP) technique. Submicron B6O powders were then admixed with Al2O3-Y2O3, Al2O3-Y2O3-SiO2, Al2O3-SiO2, Al2O3, TiC, TiB2 and Pd additive systems in a planetary ball mill. Al2O3-Y2O3 systems, with various molar ratios were used to study the influence of grain boundary composition on densification, microstructure and mechanical properties. Hot pressing experiments were conducted in a temperature range of 1600 – 1900oC under a pressure of 50 MPa for 20 minutes in argon atmosphere. The microstructure and phase composition of the hot pressed composites were characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). Densities of the samples were measured to determine the extent of densification. Vickers hardness and indentation toughness were used to characterise the mechanical properties of the hot pressed materials. Careful analysis of the indentation data has been made and such analysis has provided an estimate of the toughness properties of the B6O composites. Abstract ~ iv ~ ‘Pure’ B6O hot pressed at 1900oC had Vickers hardness of 35 GPa (500g load) but was found to be brittle. The experimental results revealed that B6O composites containing Al2O3-Y2O3 and Al2O3-Y2O3-SiO2 systems processed at 1800oC exhibit excellent combination of hardness (27 - 34 GPa using 500g load) and fracture toughness (3 - 6 MPam1/2). Densities higher than 97% of theoretical densities were achieved. Hot pressing these composites at higher temperature (1850oC) is observed to slightly lower the hardness values. Amorphous grain boundary phases were formed. Composites containing Al2O3, TiB2, and TiC, respectively were hot pressed at 1900oC. Vickers hardness in the range of 27 – 31 GPa were obtained using 500g load whilst fracture toughness values were in the range of 4 – 7 MPam1/2. B6O composite produced by addition of Pd had a hardness of 22 GPa using 5kg load but a high fracture toughness of 13 MPam1/2. Fracture toughness values obtained in this work are the highest recorded so far for B6O composites. The mechanisms leading to the achievement of superior toughness and the possible reasons for toughness variation with the different additives as well as with hot pressing temperature are critically analysed. It has been shown that B6O can be sintered using a wide range of oxide additives which could result in improvement of mechanical properties, especially fracture toughness. Microstructural tailoring of these composites could potentially see widespread application of B6O composites as structural material.