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, 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.

Microstructural Change during Heat Exposure in Air of Modeled Environmental Barrier Coating Processed by Aerosol Deposition Method

“…Although the shape of the particles was slightly angular, it was almost equiaxed. 28) The chemical composition of SiAlON powder was Si 29.8, Al 27.6, O 17.7, N 22.9, Fe 0.034, Ti 0.003, and Y 0.004 mass%. There was dispersion in particle size, and the median particle diameter was 1.04 µm (Fig.…”

“…Partial delamination of the coating was observed for the coating deposited at the highest gas flow rate. The relationship between the gas flow rate and the A d v a n c e V i e w deposition rate by the AD method has been reported for TiN 34) and Al 2 O 3 25) powder at a nozzle angle of 90°, and for mullite powder 28) at a nozzle angle of 60°. For both nozzle angles, the deposition rate increased with the gas flow rate up to a maximum value and then decreased.…”

“…As a result, a reacted layer is formed at the center of the coating and in the vicinity of the interface between the substrate and the coating, and the coating becomes a two-phase mixture of SiO 2 and mullite that contains about 80% of SiO 2 . 28) This phenomenon occurs when the surface of the mullite coating is exposed to high oxygen partial pressure and the vicinity of the interface between the Si substrate and the mullite coating is exposed to low oxygen partial pressure. Under such an oxygen partial pressure gradient, Al in the mullite diffuses from the low oxygen partial pressure side to the high oxygen partial pressure side, and the Al component is depleted in the mullite A d v a n c e V i e w coating on the bond coat layer side, so that the mullite layer decomposes.…”

Mullite-Based Ceramics from Mining Waste: A Review

“…Monolithic mullite ceramics find application in tableware [48], porcelain stoneware [49,50], refractory [51] or electronic devices [52]. Mullite coatings have been used successfully as environmental barrier coatings for the protection against oxidizing agents, reducing agents or aggressive chemicals at high temperatures [53,54]. Besides ceramic or metallic surfaces, mullite composites, including composites prepared with mullite matrix or mullite fiber, find important applications as components in turbine engines, high-performance furnaces or heat shields for space vehicles [55][56][57].…”

Formation mechanism of texture in α-alumina coatings produced by aerosol deposition

“…4) The AD method gained recognition through research on deposition by Akedo. 5), 6) Many powders such as ZrO 2 , 7)10) ¡-Al 2 O 3 , 5),6),11)18) Mullite, 19) ¢-SiAlON, 20) Pb(Zr 52 ,Ti 48 )O 3 , 6),21) NiZnFe 3 O 4 , 6) Li 4 Ti 5 O 12 , 22) AlN, 6) TiN, 23) MgB 2 , 6) Ni 24) and Cu 25) have been used to produce coatings by AD.…”