Fabrication of dense ZrB2/B4C composites using pulsed electric current pressure sintering and evaluation of their high-temperature bending strength

Abstract

ZrB2/x·vol%B4C (x = 30–90) composites were fabricated from ZrB2 and amorphous B/C powders using pulsed electric current pressure sintering (PECPS) from 1600 °C to 1900 °C for 6.0 × 102 s (10 min) under 50 MPa in a vacuum, accompanied by self-propagating high-temperature synthesis (SHS). Since the B4C phase was formed at 1600 °C, the relative density (Dr) was evaluated; the composites sintered at 1900 °C attained the highest Dr. Their Dr values increased gradually from 99.35% to 99.99% with increasing B4C contents up to 60 vol% and showed a constant value above 60 vol%. At room temperature, the mechanical properties of Vickers hardness (Hv), fracture toughness (KIC) and three-point bending strength (σb) were measured. Hv exhibited a monotonous increase from 20.3 to 32.7 GPa. On the other hand, KIC and σb revealed the same behavior for each of the compositions; both exhibited the highest values, i.e., 10.2 MPa m1/2 for KIC and 870 MPa for σb, in the 60 vol%B4C sample, and then the KIC decreased gradually to 9.73 MPa m1/2, and σb dropped suddenly from 850 MPa (70 vol%) to 340 MPa (80 vol%) and stayed as low σb in the 90 vol% B4C sample. Next, the high-temperature σb values of the composites (40–70 vol%) were measured in Ar. The composites (40–60 vol%) revealed high σb (≥640 MPa) from R.T.~1600 °C; the maximum value of 803.5 MPa was observed for the 60 vol%B4C composites at 1600 °C, and then the σb of all composites dropped to around 340 MPa at 1800 °C. From their stress-strain curves, elastic and plastic deformations were observed at 1600 °C and 1800 °C, respectively.