Formation of Mullite Coating by Aerosol Deposition and Microstructural Change after Heat Exposure

Abstract

Optimal parameters for the aerosol deposition (AD) of a mullite coating and the microstructural change of the coating after heat exposure in air were investigated. Mullite, which is a component of environmental barrier coatings, was deposited on glass, Al₂O₃ and Si by the AD method. The angle of the gas flow direction from the nozzle to the substrate plane should be 60° to produce a homogeneous mullite coating. The deposition rate increased with the gas flow rate when the gas flow rate was in the range from 18 to 36 L/min. Further increase of the gas flow rate resulted in the formation of a heterogeneous coating. The mullite coating formed with the optimized parameters was almost fully dense and crystalline. The chemical composition of the mullite coating was almost the same as that of the raw mullite powder used for deposition. The coating was composed of a single mullite phase. No delamination was observed at the interface between the Si substrate and the mullite coating. The interface did have undulations; therefore, it was considered that the substrate and the coating were bonded due to the anchor effect. Heat treatment was performed at 1573 K for a mullite coating deposited on a Si substrate. When the specimen was exposed to heat for 10 h, the coating at the surface side and the coatings at the central part and near the interface between the substrate and the coating were composed of two phases, (Al₂O₃+mullite) and (SiO₂+mullite), respectively. Further heat exposure results in the formation of a reacted layer of two phases (SiO₂+mullite) containing more than 80 mol% of SiO₂ near the interface. The thickness of the layer increased with increasing heat exposure time. The formation of the reacted layer was due to the diffusion of Al present in the mullite coating to the coating surface and the diffusion of Si into the coating from the Si substrate.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 186–192.

Fig. 7 SEM micrographs and EDX map showing a cross-section of a mullite coating deposited on a Si substrate. (a) SEM micrograph of an as-deposited coating, (b) EDX maps of Si, Al and O, and (c) SEM micrograph in the vicinity of the interface between the Si substrate and the mullite coating. Measurements of chemical composition were conducted for the yellow framed areas shown in (b).