Various alumina-based materials are applied to achieve different electrical insulation properties based on the variation of the material specific relative permittivity. Thermally sprayed mullite (Al 2 O 3 · SiO 2 ) can form an amorphous phase due to the high cooling rates of the process. The formation of amorphous phases causes a change in the capacitive behaviour of the coatings. The tendency to form amorphous areas can be influenced by the composition of the feedstock material or coating parameters. On the one hand, mullite coatings based on two different Al 2 O 3 to SiO 2 ratios are investigated. On the other hand, a parameter variation is used to achieve various particle temperatures during the process. The coatings are investigated via X-ray diffraction and DSC for phase formation, electron microscopy for coating structure and impedance spectroscopy for measuring the AC-resistance. The conducted variation of the feedstock material as well as the parameters causes differences in the XRD and DSC measurements correlating with a difference in the amounts of amorphous phases. For the capacitive behaviour, coatings applied with hydrogen as process gas showed decreased AC-resistance values. The chemical composition of the feedstock material indicates that the AC-resistance decreases with increasing amounts of SiO 2 . In summary, mullite has promising insulation properties which can be modified by the feedstock material composition as well as the coating parameters. For future application, mullite is a promising candidate for increasing the electrical insulation properties in conditions under high electrical and mechanical demands.

Plasma sprayed ceramic coatings are widely used for thermal barrier coating applications. Commercially available mullite powder particles and a mixture of mechanically alloyed alumina and silica powder particles were used to deposit mullite ceramic coatings by plasma spraying. Microstructure and morphology of both powder particles as well as coatings were investigated by using scanning electron microscopy (SEM). Phase formation and degree of crystallization of coatings were analyzed and estimated by using X-ray diffraction technique. Differential thermal analysis (DTA) method was used to study the phase transformation of coatings. Results indicated that the porosity level in the coating deposited using mullite as initial powder particles was lower than that deposited using the mixed powder particles. The crystallization degree of the coating deposited using the mixed powder particles are higher than that deposited using mullite powder particles. DTA curves of coatings deposited using the mixed powders have showed some phase transformation due to the crystallization of the retained amorphous phases such as mullite and alumina in the coatings. The degree of crystallization of both as sprayed coatings was significantly increased after post deposition heat treatments.

Compositionally graded mullite/ZrO 2 coatings, have been tested as environmental barrier coatings (EBCs) for protection against water vapor corrosion of Si-based ceramic components intended for application in turbine engines. Four and five layered systems were engineered by plasma spraying over SiC substrates consisting of a Si bond coat layer, 2 or 3 mullite/ZrO 2 composite graded layers as middle layers and a nanostructured YSZ topcoat. These coatings were heat treated at 1300 °C in both stationary and thermal cycling conditions in a controlled water vapor environment. The effect of these ageing conditions on the coatings was comparatively investigated. Crystallization of the composite coatings and sintering of the YSZ topcoat was perceived. A reduction of SiO 2 content was detected in the composite layers before aging. The porosity of the coating did not change appreciably with the ageing tests and only the evolution of the pre-existing cracks and the growing of a thermally grown oxide layer can be highlighted as the major effect of the ageing tests.

The ongoing development of environmental barrier coatings (EBCs) offers the prospect to implement the full potential of silicon-based ceramics for high temperature structural applications. The current state-of-the-art EBC system comprises a Si bond coat, a mullite (3Al 2 O 3 ·2SiO 2 ) interlayer and a (1-x)BaO·xSrO·Al 2 O 3 ·2SiO 2 , 0 ≤ x ≤ 1 (BSAS) crack-resistant and water vapour attack resistant top coat. In this study, the influence of water vapour corrosion on the structural and mechanical properties of plasma-sprayed Si/Mullite/BSAS architectures was assessed by furnace thermal cycle testing (e.g., 100 cycles, 2h/cycle at 1300°C). Commercially available mullite and BSAS powders were used to produce crystalline coatings by air plasma spraying. Fully crystalline mullite and celsian BSAS coatings were engineered under controlled conditions on silicon coated, sintered α-SiC Hexoloy substrates. The overall performance at high-temperature of these functionally graded EBCs is discussed and correlated to their microstructural characteristics.

Si-based ceramics (e.g., SiC and Si 3 N 4 ) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the thickness recess that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H 2 O vapour), appropriate refractory oxides are engineered as environmental barrier coatings (EBCs). Presently, the state-of-the art EBCs comprise multilayers of silicon (Si) bond coat, mullite (Al 6 Si 2 O 13 ) intermediate layer and BaO-SrO-Al 2 O 3 -SiO 2 (BSAS) top coat. Evaluating and understanding their mechanical properties, such as, the elastic modulus (E) and the strain-stress relationship is essential for their practical application and reliable employment. It was investigated via depth-sensing indentation the role of high-temperature treatment (1300°C), performed in H 2 O vapour environment (for time intervals up to 500 h), on the mechanical behaviour of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates. Laser-ultrasonics was employed to evaluate the E values of as-sprayed coatings and to validate the indentation results. The fully crystalline, crack-free and near crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The (i) absence of phase transformation and (ii) stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers even after harsh environmental exposures provides a plausible explanation for the almost crack-free coatings observed. The measured mechanical properties of the EBCs and their microstructural behaviour during the high-temperature exposure are discussed and correlated.

Mullite and mullite/ZrO 2 bi-layer systems are being considered as environment barrier coatings (EBCs) for protection of Si-based (Si 3 N 4 , SiC) substrates against water vapor corrosion for application in forthcoming turbine engines. An approach to reduce the thermal expansion mismatch between mullite and ZrO 2 layers in those coatings would be to tailor intermediate mullite/Y-ZrO 2 composite layers. The feasibility of these composite layers is studied in a comparative manner by plasma spraying both single mullite and bi-layer coatings of mullite and of mullite/ Y-ZrO 2 (75/25 vol %.) over Hexoloy SiC substrates. All feedstock materials are equally prepared using spray drying methods as the mix powders are not commercially available. Singular spraying conditions are used to assure enhanced crystallization of the mullite phase. Coatings are aged for 100 h at 1300 °C in a controlled water vapor environment. The effect of water corrosion on the exposed coatings is comparatively investigated, determining changes in crystalline phase by X-ray diffraction (XRD), the crystallization of amorphous phases is highlighted by the use of differential thermal analysis (DTA) tools and the microstructure of the polished coatings is analyzed by scanning electron microscopy (SEM).

Mullite (Al 6 Si 2 O 13 ) is the basis of efficient environmental barrier coatings (EBCs) for protecting Si-based ceramic matrix composites (CMCs) selected to replace specific hot-section metallic components in advanced gas turbines. Furthermore, YSZ-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat were envisioned as solutions to ease their coefficient of thermal expansion (CTE) mismatch induced stress. Consequently, a proper understanding of the mechanical properties such as the elastic modulus, hardness or plastic/elastic recovery work serve for an efficient design of such refractory oxide multilayers. In this work, three different mullite powder morphologies (fused and crushed, spray-dried and freeze-granulated) were employed. Using depth-sensing indentation with loads in the range 100 – 500 mN, the role of the microstructure and morphology of the powder feedstock on the mechanical behaviour of air plasma sprayed mullite bond coats deposited on SiC Hexoloy substrates was investigated. Fully crystalline as-sprayed mullite coatings were engineered under controlled deposition conditions. Mechanical properties were measured for the as-sprayed coatings as well as for coatings heat-treated at 1300°C, in water vapour environment, for periods up to 500 h. Both E and H values of the coatings are found to be highly dependent on the morphology of the starting powders.

Mullite based compositions have interest for thermal barrier coatings because they have thermal expansion coefficients close to those of silicon ceramic substrates. In this work, mullite-zirconia coatings are obtained by flame spraying and characterized based on microstructure, crystal phases, hardness, elastic modulus, and thermal conductivity. Crystallinity is improved by in-situ heating with a flame torch, which is also shown to increase hardness and elastic modulus. Thermal diffusivity measurements show that the thermal properties of mullite-zirconia coatings are relatively stable over a wide temperature range and adequate for many thermal barrier applications.

This study investigates the influence of powder morphology and spray processes on the microstructure, crystallinity, hardness, and elastic modulus of mullite coatings. Coatings produced from mullite powders and suspensions are deposited by plasma spraying while measuring in-flight particle temperature and velocity. Powder morphology and spraying conditions are correlated with measured coating properties, creating a process map for engineering mullite coatings for specific applications. It is shown that coating crystallinity, microstructure, and mechanical properties vary widely depending on powder morphology, processing, and in-flight particle characteristics.

In this study, two processing routes are used to produce mullite powders for thermal spraying and the influence of each method on particle morphology and microstructure is investigated. Different thermal treatments are performed to improve grain cohesion and powder flow and their effect on the crystal structure of the powder is assessed as well. The powders are plasma sprayed, in-flight characteristics are measured, and splats are collected and analyzed. A correlation among powder morphology, in-flight particle properties, and splat morphology is established to better understand the influence of powder processing route on coating formation.

Zircon (ZrSiO 4 ) is a technologically important oxide ceramic material known for its high refractoriness and chemical stability. It shows excellent thermal shock resistance as a result of its very low thermal expansion coefficient and a low heat conductivity coefficient. Plasma spraying is a convenient method to produce large area coatings with high growth rates, necessary for many applications. ZrSiO 4 is among the least expensive spraying materials for refractory applications. In this study, a single-step process was used to prepare mullite/zirconia ceramic composites by plasma spraying zircon/alumina mixtures. Mixtures of ZrSiO 4 and Al 2 O 3 powders with Al 2 O 3 to SiO 2 molar ratios of 3:2 were milled for 2 h in a zirconia medium using a ball mill. The as–milled powders were dried in the furnace and sintered at 1300 and 1350 °C for 2h then crushed to a size less than 100 μm. The powders were sprayed by an atmospheric plasma spray gun (Metco 3MB) using C/C+SiC ceramic matrix composite substrates. Scanning electron microscopy (SEM) was used to analyze the microstructures of the powders and plasma coatings. The phase composition analysis of the powder showed the presence of alumina and zircon. After plasma coating, alumina, zircon, and zirconia phases were determined.

Mullite coatings (3Al 2 O 3 ·2SiO 2 ) were deposited by suspension thermal spraying of micron-sized (D50 = 1.8 µm) feedstock powders, using a high-velocity-oxy-fuel gun (HVOF) operated on propylene (DJ-2700) and hydrogen fuels (DJ-2600). The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to coatings with low porosity and fine and homogeneous porosity distribution, making this process potentially suitable for creating thin layers with low gas permeability. In-flight particle states were measured for a number of spray conditions of varying fuel-to-oxygen ratios and standoff distances and related to the resulting microstructure, stoichiometry, phase composition (EDS, SEM, XRD) and hardness (VHN 300gf) of the coatings. In an attempt to retain the crystalline phase in the coatings, HVOF operating conditions were varied to limit in-flight particle melting. However, fully crystalline coatings were only obtained by gradually heating the coating during deposition to temperatures above 400°C.

This paper examines the dielectric properties of silicate coatings including mullite (3Al 2 O 3 -2SiO 2 ), steatite (MgSiO3), spodumene (Li 2 O-Al 2 O 3 -4SiO 2 ), and olivine with near-forsterite (Mg 2 SiO 4 ) composition. The materials were sprayed using a water-stabilized plasma gun and the deposits were removed from the substrate, polished, and sputtered with aluminum on both sides. Electrical tests consisting of voltage, resistance, and capacitance measurements showed that the relative conductivity of plasma-sprayed silicates is stable between 200 Hz and 1 MHz, which is suitable for many insulation applications. Paper includes a German-language abstract.