Understanding the oxidation behavior of a ZrB2–MoSi2 composite at ultra-high temperatures

Abstract

This basic research investigates the microstructure evolution of a composite based on ZrB 2-MoSi 2 from the as-sintered features to the changes occurring upon oxidation at ultra-high temperatures, 1650 and 1800°C, in a bottom-up loading furnace for 15 minutes. Scanning and transmission electron microscopy evidenced the formation of a matrix typified by ZrB 2-cores surrounded by (Zr,Mo)B 2-rims with dispersed MoSi 2 particles and SiO 2 glass trapped at the triple junctions. The oxidation at 1650°C induced the migration of silica to the surface, which formed a continuous and protective scale. Below this scale, the matrix evolved into ZrO 2 grains encasing MoB nano-inclusions, as a result of the oxygen and boron oxide partial pressures established in the subscales. Underneath, a MoSi 2-depleted boride region, but substituted by SiO 2 and MoB was found. The same phases were observed upon oxidation at 1800°C, but a thicker and more turbulent oxidized layer formed as a consequence of the rapid evolution of MoO 3 , SiO and B 2 O 3 gases from the scales beneath the outermost silica-layer. According to the observed phases and the calculated phase stability diagrams, the partial pressures gradient within the oxide layer were defined and the effect of Mo-doping in boride matrices on the oxidation behavior was compared to that of other transition metals to establish a criterion design for the realization of ceramics with improved oxidation resistance.

Ablation behaviour of ultra-high temperature ceramic matrix composites: Role of MeSi2 addition

“…For the silicide-containing composites, independent of the MeSi 2 used, the original ZrB 2 grains of the matrix were surrounded by a solid solution with general formula (Zr,Me)B 2 , in agreement with previous studies [25,27,28] and as a consequence of the densification process involving a transient liquid phase formed by partial dissociation of the silicide [25]. This feature is particularly visible in ZBH, Figure 2c, owing to a higher amount of Me, in this case Hf, being dissolved in the ZrB 2 lattice.…”

“…For the materials containing MeSi 2 , the (Zr,Me)B 2 solid solutions of the matrices oxidized following the typical oxidation behavior recently outlined in dedicated studies [27,28]…”

“…In particular, it has recently been determined that MeSi 2 has a key impact on the oxidation behavior of monolithic ZrB 2 ceramics via the modification of the chemistry and nature of the glassy, vapor and crystalline phases formed. This can result in relatively effective sublayers depending on the temperature ranges encountered [26,27,28].…”

“…The selection of these silicides was based on their compatibility with high-temperature environments, i.e. they offer melting points above 1600°C, high stiffness and, owing to their beneficial effects on densification, high temperature strength and potential oxidation resistance improvement for borides [23,25,26,27,28]. Additionally, a comparative material without MeSi 2 but containing the same total amount of SH additives, i.e.…”

Aligned continuous cylindrical pores derived from electrospun polymer fibers in titanium diboride

“…The pursuit of commercial hypersonic flights and space travel for the mission to Mars relies on innovative reusable materials with improved thermal protection systems (TPS) . Candidate materials must survive harsh conditions, especially those found upon atmospheric reentry with dissociative oxidation at surfaces, extreme heat fluxes, temperatures soaring over 3000°C and vibrational and stagnation pressure loads, in key components of the hypersonic vehicles and aircrafts . These key components are traditionally leading edges, combustors and insulation components .…”