Factorial design to minimize residual oxygen in reaction hot‐pressed zirconium diboride

Abstract

Processing parameters to minimize the residual oxygen content of ZrB2 ceramics prepared by reactive hot‐pressing were selected using statistical analysis. Additions of carbon and excess boron were used to react with oxygen present in the starting ZrH2 and B powders as an impurity. A 32 full‐factorial experimental design was used to determine the carbon and excess boron contents that minimized residual oxygen content in the final ZrB2 ceramic while also minimizing formation of any impurity phases. Carbon additions were effective at reducing the oxygen content, but resulted in the formation of residual ZrC. Boron additions were also effective at removing oxygen, but to a lesser extent compared to carbon. In addition to being more effective at removing oxygen, carbon additions also improved the densification behavior, whereas boron additions inhibited densification. The stoichiometric reaction resulted in a ZrB2 ceramic with a relative density of 99%, but that contained 6.9 vol% of (Zr1−x,Mgx)O2−x as a residual phase following reactive hot‐pressing. The composition with a carbon addition to starting oxygen content having a molar ratio of 1, and a boron to zirconium molar ratio of 2.1, resulted in a 95% dense ceramic with only 0.1 vol% of residual (Zr1−x,Mgx)O2−x and trace amounts of ZrC.

Microstructure and mechanical properties of reaction‐hot‐pressed zirconium diboride based ceramics

“…A crystallographic density of 6.107 g/cm 3 was calculated for nominally pure ZrB 2 from the lattice parameters determined from XRD ( Figure 1). 29 Using the calculated theoretical density of 6.094 g/cm 3 results in a relative density of 97.9%, which is in close agreement with the observed porosity of 1.6 vol%. Porosity, ZrC x , and (Zr 1-x Mg x )O 2-x were observed in the cross-sections in the amounts of 1.60, 0.50, and 0.42 vol%, respectively.…”

“…Thus, minimizing oxygen content may improve densification. 29 Tungsten and zirconium carbide additions to ZrB 2 based ceramics have been examined by several research groups for their enhancements to oxidation resistance and/or mechanical properties. Synthesizing ZrB 2 from metallic Zr required hot-pressing at 2100°C to reach full density, while the material employing ZrH 2 was hot-pressed to full density at 1900°C.…”

“…[16][17][18][19][20][21]23,24 Chamberlain et al 16 and Lonergan et al 9 both reported approximately 2 vol% residual ZrO 2 in their dense ceramics. 29,31,41,42 Additionally, zirconium carbide has been shown to improve toughness by contributing superplasticity to ZrB 2 based ceramics at temperatures above 1600°C, due to the presence of active slip planes and carbon diffusion through the ZrC lattice. 12,[25][26][27][28] Neuman et al utilized boro-carbothermal reduction and a design of experiments methodology to minimize the residual oxide impurities.…”

Solute distributions in tantalum‐containing zirconium diboride ceramics

“…Commercial ZrB 2 powders typically contain impurities such as Hf, Cr, Fe, W, and Mg that either are present in the source minerals or are introduced during comminution of the powders . Impurities are incorporated into ZrB 2 differently depending on processing conditions and their solubility in ZrB 2 .…”

“…Some metallic impurities, such as Cr and Fe, tend to segregate to grain boundaries and form metal‐rich liquid phases that promote liquid‐phase sintering . Alkaline‐earth metals such as Ca and Mg and metalloids such as Si and Al form oxide solutions with ZrO 2 or amorphous oxides that reside at triple points and grain boundaries . Finally, transition metals (TMs) including Ta, Mo, Hf, Nb, and W are soluble in ZrB 2 and form (Zr,TM)B 2 solid solutions.…”