Recently MoSi₂ sacrificial particles embedded in yttria partially stabilized zirconia (YPSZ)have been proposed as attractive healing agents to realize significant extension of the lifetime of the thermally loaded structures. Upon local fracture of the YPSZ, the embedded healing particles in the path and in the vicinity of the crack react with the oxygen atoms transported via the crack and first fill the crack with a viscous glassy silica phase (SiO₂). The subsequent reaction between this freshly formed SiO₂ and the existing tetragonal ZrO₂ of the YPSZ leads to the formation of rigid crystalline zircon (ZrSiO₄), which is key in the crack-healing mechanism of YPSZ based materials. The isothermal kinetics of the self-healing reaction and the mechanism of zircon formation from the decomposing MoSi₂ and the surrounding YPSZ were assessed via X-ray diffraction (XRD). The obtained results revealed that at 1100 °C the reaction between amorphous SiO₂ and YPSZ is completed after about 10 h. For a more accurate determination of the kinetics of the self-healing reaction, bilayer samples of YPSZ – MoSi₂ (with and without boron addition)were annealed in air over a temperature range of 1100–1300 °C. This led to the formation of a MoSi₂/amorphous (boro)silica/zircon/YPSZ multi-layer, which was investigated with scanning electron microscopy (SEM)and electron probe X-ray microanalysis (EPMA). Kinetic modeling of the growth of zircon and silica or borosilicate layers showed that zircon growth was dominated by the diffusion of Si⁴⁺ in zircon whereas the growth of the silica or borosilicate layer was controlled by oxygen diffusion. Moreover, a significant increase in the rate of ZrSiO₄ formation was observed due to the presence of B in the MoSi₂ particles.

The present paper focuses on the Spark Plasma Sintering (SPS) manufacturing of a new type of self-healing thermal barrier coating (TBC) and a study of its thermal cycling behaviour. The ceramic coating consists on an Yttria Partially Stabilized Zirconia (YPSZ) matrix into which healing agents made of MoSi₂-Al₂O₃ core-shell particles are dispersed prior to sintering. The protocol used to sinter self-healing TBCs on MCrAlY (M: Ni or NiCo) pre-coated Ni-based superalloys is described and the reaction between the particles and the MCrAlY bond coating as well as the preventive solutions are determined. Thermal cycling experiments are performed on this complete multilayer system to study the crack healing behaviour. Post-mortem observations highlighted local healing of cracks due to the formation of silica and the subsequent conversion to zircon at the rims of the cracks.

A successful sol-gel process to encapsulate molybdenum di-silicide MoSi₂ particles with a closed and thermally stable Al₂O₃ layer using aluminium tri-sec-butoxide as a precursor is presented. The processing conditions such as precursor selection and temperature were optimized through analysing the interaction of the MoSi₂ particles with the sol. The application of the sol-gel based coating was followed by calcining the coated particles at temperatures between 900 and 1200 °C in Ar. The shell composition and the mechanical stability of the microcapsule were analysed by means of X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. Upon calcining at 1200 °C in Ar, the MoSi₂ core remains intact as it is, covered by an α - alumina shell with a thickness of about 0.6 μm. The stability tests proved that the encapsulate particles are about five times more oxidation resistant than the uncoated MoSi₂ particles.

Yttria – partially stabilised zirconia (YPSZ) MoSi₂ composites have been designed to prolong the lifetime of the matrix by self – healing cracks during thermal cycling. The healing reaction at high temperatures is based on the decomposition of MoSi₂, leading to a volumetrically expanding reaction product, which seals the crack. In this work, coefficient of thermal expansion (CTE) and the fracture toughness of composites containing MoSi₂ particles, produced by spark plasma sintering (SPS) have been compared to conventional YPSZ. The CTE mismatch between YPSZ and MoSi₂ was found to be small, implying that thermally induced mismatch stresses will be small and the composites have a similar CTE to conventional YPSZ. Fracture toughness was found not to be affected by the particles and showed similar values to unreinforced YPSZ. Cracks introduced by indentation have been shown neither to prefer, or avoid, the particles suggesting that such a composite system is capable of autonomously activating the self – healing reaction.

To prolong the lifetime of thermal barrier coatings (TBCs) recently a new method of microcrack healing has been developed, which relies on damage initiated thermal decomposition of embedded molybdenum disilicide (MoSi₂) particles within the TBC matrix. While these MoSi₂ particles have a beneficial effect on the structural stability of the TBC, the high thermal conductivity of MoSi₂ may have an unfavourable but as yet unquantified impact on the thermal conductivity of the TBCs. In this work the thermal conductivity of spark plasma sintering (SPS) produced yttria-stabilised zirconia (YSZ) model thermal barrier coatings containing 10 or 20 vol.% of MoSi₂ healing particles was investigated using the laser flash method. Measurements were performed on free-standing composite material over a temperature range from room temperature up to 1000 °C. Microstructural analysis was carried out by SEM combined with image analysis to determine the size, distribution and area fraction of healing particles. The measurements were compared with the results from microstructure-based multi-physics finite element (FE) models and analytical models (the asymmetric Bruggeman model and the Nielsen model) in order to study the effects of the addition of MoSi₂ particles as well as the presence of micro-pores on the apparent thermal conductivity. The results show a strongly non-linear increase in the thermal conductivity of the composite material with the MoSi₂ volume fraction and a dependence on the aspect ratio of MoSi₂ particles. Interparticle connectivity is shown to play a big role too.