2019 年 66 巻 9 号 p. 442-448
The millimeter wave pressure sintering featuring power output (Wm) control of amorphous B4C powders is proposed without the need for any additive, in order to reveal definitely divided densification far below a glass transition. With the constant-rate heating technique, amorphous B4C containing nanocrystalline rhombohedra can be consolidated without sub-μm sized pore at 50 MPa. The heating rate used is found to be a dominating variable; with an increase from 10 to 20 K·min-1, it leads to a decrease from 510 to 444 K in full-density temperature. Its second-stage densification is expressed by the constitutive equation of consolidating strain rate ε˙n via non-Newtonian flow: ε˙n=ε˙osinh(Cσeff/RT)exp(-Q/RT) with a decreasing apparent activation energy Q from 42.5 to 35.4 kJ·mol-1 under an increased Wm. The constant-power output technique with a maximum Wm of 2 kW enables one to obtain a considerable initial abrupt shrinkage and subsequently full consolidation at the lowest temperature of 386 K under 98 MPa. The process control of the first-stage densification is then characterized by a neck formation law based on the first order chemical reaction d(Af-A)/dt = -kp(Af-A) where A is the fraction of a contacted area between particles, having an increasing kp with Wm.