Anisotropy of the bending strength of LaB6–W2B5 reactive hot-pressed ceramics

Abstract

Refractory composite ceramic material in the LaB6–W2B5 system with a component ratio of 50 : 50 vol.% was obtained  by reactive hot pressing in a graphite mold. A heterophase powder containing lanthanum hexaboride, metallic tungsten, and  amorphous boron preliminarily ball-milled for 20 h with tungsten balls was used as the initial reaction mixture. The average particle size of the milled mixture was 2.9 μm. A relative density of 92 % was achieved at a temperature of 1800 °C with isothermal holding  for 15 min at 30 MPa in an argon atmosphere. The structure and composition of the LaB6–W2B5 material were studied by X-ray  diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The composition of the ceramics contained  two phases – cubic LaB6 lanthanum hexaboride and hexagonal W2B5 tungsten pentaboride. The ceramic structure featured by  ordered lamellar W2B5 particles in a LaB6 polycrystalline matrix. During the reactive hot pressing of the LaB6–W–B mixture, the  predominant growth of W2B5 crystals along (101) atomic planes was observed. Resulting lamellar W2B5 particles were oriented in  the LaB6 matrix perpendicular to the pressing load. Images obtained with electron microscopy were used for the three-dimensional  visualization of the LaB6–W2B5 structure. Three-point bending tests were conducted on 3×3×30 mm samples. The dependence  of bending strength on the direction of applied breaking load was established. When a breaking load was applied perpendicular to  the surface of the lamellar W2B5 particles, the ultimate strength was 420 MPa, while when loaded along the plane of the particles,  bending strength increases to 540 MPa. The anisotropy coefficient of ultimate strength was 0.78.