Fabrication of dense B4C/CNF composites having extraordinary high strength and toughness at elevated temperatures

Fully dense B4C/CNF composites have been fabricated from the mixture of amorphous fine powders of boron and carbon, and carbon nano-fiber (CNF) using pulsed electric-current pressure sintering (PECPS) at 2173 K (1900 °C) for 10 min under 30 MPa in a vacuum. They showed a moderate mechanical properties at room temperature: bending strength σb more than 450 MPa, Vickers hardness Hv of 29-36 GPa, and fracture toughness KIC ranging from 4.1 to 5.7 MPa m1/2. However, they revealed extraordinary high strength more than 600 MPa at 1673-1873 K (1400-1600 °C) in an inert gas atmosphere, especially, σb of 830 MPa at 1773 K (1500 °C) (12.5 vol% CNF added composite) and σb of 800 MPa at 1873 K (1600 °C) (10.0 vol% CNF added composite). In addition, these B4C/CNF composites also showed much improved high-temperature fracture toughness by a factor of around 20, in comparison with monolithic B4C ceramics at 1773 K (1500 °C). These extraordinary both high strength and toughness might be explained in terms of the suppression of grain growth by CNF at elevated temperatures, an inverse strength temperature-dependence for carbides, and anchor effect induced by CNF.