Gradient structure high emissivity MoSi2-SiO2-SiOC coating for thermal protective application

Abstract

This study presents a one-pot preparation of gradient porous high emissivity coatings on ceramic fibrous insulations. MoSi2-SiO2-SiOC coatings have been prepared on mullite fibrous substrates with polysiloxane-derived SiOC glass as low-temperature adhesive, SiO2 as high-temperature ceramic aggregate, MoSi2 as emission agent and polydimethylsiloxane (PDMS) as pore former by slurry-spraying, drying and sintering. Scanning electron microscopy shows the MoSi2-SiO2-SiOC coating gradually changes from a porous structure into a dense structure with obviously decreasing porosity from substrate to surface. The pore size and porosity of the bonding-layer increase with the PDMS content and the density of the surface-layer increases with the sintering temperature suggesting the gradient porous structure is formed by two-steps of “low-temperature decomposition pore-forming” and “high-temperature sintering pore-sealing” during sintering. Firstly, the pores come into formation after decomposition of PDMS and the organic-to-inorganic transformation of the SiOC glass at lower temperature (<1000 °C). Secondly, the formed pores are partly sealed accompanied by carbothermal reduction of the SiOC glass and carburization reaction of the MoSi2 at higher temperature (>1400 °C). The gradient MoSi2-SiO2-SiOC coating increases the bonding strength between the coating and substrate up to 0.6 MPa, which is nearly twice as high as that of dense MoSi2-SiO2 coating. The MoSi2-SiO2-SiOC coating exhibits high spectral emissivity (0.90 at 700 °C), excellent thermal shock resistance (90 cycles without cracking at 25–1500 °C) and oxidation resistance up to 1500 °C, all of which can meet requirements of high temperature application.