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, Al 2 O 3 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, (Al 2 O 3 +mullite) and (SiO 2 +mullite), respectively. Further heat exposure results in the formation of a reacted layer of two phases (SiO 2 +mullite) containing more than 80 mol% of SiO 2 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.

Optimal deposition parameters for the aerosol deposition of a ¢-SiAlON coating and the microstructure change of an EBC after heat exposure in air are investigated. Dense and crystalline SiAlON coating having developed texture, where the (0001) plane is declined approximately 10°from the coating plane is formed. The deposition rate increases with the gas flow rate when the rate is ranging from 12 to 16 L/min. Further increase of the gas flow rate decreases the deposition rate. Regarding the 15 µm thick mullite coating deposited on SiAlON substrate heat exposed at 1573 K over 30 h, delamination of the coating occurs due to the oxidation of SiAlON. 30 µm thick mullite coating prevents the oxidation. As for the EBC deposited on SiSiC substrate, delamination occurs at SiSiC/SiAlON interface by the oxidation of SiC during heat exposure at 1573 K. At the bonded region during heat exposure, SiAlON prevents the mullite coating to become (SiO 2 +mullite) two-phase state by supping Al to the mullite. Residual Si at the substrate moves to SiAlON and mullite coating under heat exposure at 1673 K. Structure of EBC is maintained by using SiC substrate in which the Si will not move to the coating during 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, Al 2 O 3 and Si by AD method. In order to produce a homogeneous mullite coating, the angle of the gas ow direction from the nozzle to the substrate plane should be 60 . Deposition rate increased with increasing gas ow rate, when the gas ow rate was in the range from 18 to 36 L/min. Further increase of the gas ow 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 Al 2 O 3 + mullite and SiO 2 + mullite two phase state, respectively. Further heat exposure formed an altered layer near the interface. The layer was composed of SiO 2 + mullite two phase state containing more than 80 mol of SiO 2 . 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.