Study of sintering and structure-property relationships in boron suboxide (B6O) - alkaline earthmetal oxide, cobalt, and nickel compound

Abstract

Recently some progress has been reported in improving the fracture toughness of boron suboxide (B6O). This involved the introduction of second phases in it at low levels of doping. This study extends this work through the addition of different additives and through the improvement of densification, by controlled sintering. Boron suboxide powders were sintered with and without additives. Pure B6O powders were sintered at a temperature of 1900oC under a load of 50 MPa for 20 minutes in an argon environment. B6O powders mixed with different additives, including alkalineearth metal oxide, nickel and nickel compounds, cobalt compounds and chromium boride. These powders were sintered at a temperature of 1850oC under loads from 50 -80 MPa, for 20 minutes in an argon environment. Microstructure, phase composition and properties were investigated. Boride secondary phases were formed when hot pressing B6O admixed with nickel and nickel compounds, cobalt compounds and chromium boride additives while the alkaline earth metal oxide additives resulted in the formation of boride and borate phases, depending on the oxide used. The mechanisms leading to the achievement of increased toughness and the possible reasons for toughness variation with the different additives are critically analyzed. Good combinations of mechanical properties were achieved for B6O-cobalt compound additive with hardness values from 32.6 -34.9 GPa and fracture toughness from 5.2 to 5.8 MPam o.5 for B60-Co2B and B60-Co0 materials respectively.