Creating a Protective Shell for Reactive MoSi₂ Particles in High‐Temperature Ceramics

Abstract

Alumina encapsulated molybdenum silicide (MoSi2) intermetallic particles were synthesized using a simple precipitation method followed by calcining at temperatures of 800°C–1000°C, to prevent the premature oxidation of MoSi2 at high temperatures. The shell composition and the influence of the calcining temperature on microcapsule integrity were investigated by means of X‐ray photoelectron spectroscopy, X‐ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The results demonstrate that the composition and the mechanical stability of the alumina shell can be tuned by the annealing temperature. After calcining at 800°C and 850°C the alumina shell remains intact. Calcining at higher temperature promotes the formation of mullite, which leads to cracking of the shell. However, when annealed at 1000°C for 24 h these cracks were filled with mullite and preserved the molybdenum silicide particles. Furthermore, the mechanical stability of the shell was improved by applying an intermediate calcining treatment at 450°C prior to the annealing process at 1000°C.

Influence of embedded MoSi2 particles on the high temperature thermal conductivity of SPS produced yttria-stabilised zirconia model thermal barrier coatings

“…It has been demonstrated that B-containing molybdenum disilicide (MoSi 2 ) could act as a suitable healing agent for YSZ-based TBCs. Upon oxidation initially silica (SiO 2 ) is formed as a reaction product, which fills the crack and subsequently turns into mechanically stable zircon (ZrSiO 4 ) due to a secondary solid state chemical reaction with the YSZ matrix [7,8]. To prevent premature oxidation of the MoSi 2 in the absence of a crack, the MoSi 2 particles need to be protected by a very thin yet dense layer of alumina [8], such that the self-heling mechanism would only be initiated by a crack intersecting the particle and opening the shell, allowing MoSi 2 oxidation.…”

A micromechanical fracture analysis to investigate the effect of healing particles on the overall mechanical response of a self‐healing particulate composite

“…In some practical cases, encapsulation of healing particles are done leading to a thin third phase (interphase) layer between the particle and the interface. 14,43 In such cases, the results obtained from the current study should be interpreted in the context that the failure of the interface would mean the failure of the encapsulating interphase layer.…”

Self‐Healing Phenomena in Metals

“…This concept closely resembles the original White concept and the many derivatives thereof for other materials such as asphalt and high‐temperature ceramics, albeit that in this case the system is non‐autonomous and requires thermal activation. When the original approach from White and co‐workers is followed, there would be the additional technical complexity that a ceramic shell surrounding the solder droplet is required, analogous to the alumina coating around the Mo 2 Si healing particles used in novel self‐healing thermal barrier coating systems . Recently, Kim and co‐workers designed a (Al, Cu, Si) 100− x (Sn, Bi) x alloy system with a microstructure consisting of spherical particles of a Sn–Bi phase with a low melting point (137 °C) embedded without a shell in a Al–Cu–Si matrix phase with a high melting point (517 °C) .…”