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

In this study, the optimal parameters for aerosol deposition (AD) of mullite coating and the microstructure change of mullite coating after heat exposure in an air were investigated. Mullite, which is one of the component materials for environmental barrier coatings was deposited on glass, Al2O3 and Si by AD method. In order to produce a homogeneous mullite coating, the angle of the gas flow direction from the nozzle to the substrate plane should be 60°. Deposition rate increased with increasing 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 heterogeneous coating. The mullite coating formed by the optimized parameters was almost dense and crystalline. The chemical composition of the mullite coating was almost the same as the composition of the mullite raw powder used for the deposition. The coating was composed of mullite single phase. Delamination was not observed at the interface between the Si substrate and the mullite coating. Since the interface showed undulation, it was considered that the substrate and the coating were bonded due to the anchor effect. Heat exposure was carried out at 1573 K in a specimen in which the mullite coating was deposited on the Si substrate. When the specimen was heat exposed for 10 h, 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 (Al2O3 + mullite) and (SiO2 + mullite) two-phase state, respectively. Further heat exposure formed an altered layer near the interface. The layer was composed of (SiO2 + mullite) two-phase state containing more than 80 mol％ of SiO2. The thickness of the layer increased with increasing heat exposure time. Formation of the altered 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.