THE EFFECT OF MECHANICAL ACTIVATION ON THE COMPACTION AND STRUCTURIZATION OF HOT-PRESSED ZrB₂ CERAMICS

Abstract

The effect of preliminary mechanical activation on the structural and physical state of the starting reagents, phase formation, and structure of sintered 80 vol.% ZrB₂–15 vol.% SiC–5 vol.% B₄C ceramics was studied. The starting powders were mechanically activated in steel drums of an AIR high-energy planetary-ball mill at a rotation speed of 1440 rpm in a water-cooled argon atmosphere. The mechanical activation lasted 15, 30, 60, and 120 min. The samples were produced by hot pressing using an SPD-120 pilot plant with induction heating in graphite dies in a СО–СО₂ atmosphere. The particle-size distribution was determined by laser diffraction. Metallographic studies were conducted using a Superprobe 733 electron microscope (Japan). The mechanical activation of ultrahigh-temperature ceramics was found to influence the compaction kinetics of the samples. When mechanical activation time increased from 15 to 120 min, the sample density increased by 10% in the hot pressing process at 2150 °С (15 min). The samples had 5–10 µm particles. During hot pressing, the ZrB₂ matrix phase formed a skeletal structure: the SiC phase in an amount of 15 vol.% acted as reinforcement in partial dissociation and B₄C in an amount of 5 vol.% acted as a boron and carbon donor. The preliminary mechanical activation of the 80 vol.% ZrB₂–15 vol.% SiC–5 vol.% B₄C powder mixture thus activated sintering in the temperature range 2100–2150 °С, featuring the formation of secondary phases and recrystallization (at 2150 °С), and promoted high density and low porosity of the hot-pressed ceramics. The studies showed that the densest samples were sintered from the powder mixture mechanically activated for 120 min. The density of the samples hot-pressed at 2100 °С (5 min) and 2150 °С (15 min) increased.