Three metal‐organic chemical vapor deposition (MOCVD) coatings (ZrO2, Y2O3·ZrO2, Y2O3) were studied in contact with model CMAS to investigate the microstructural evolution and phase formation. The MOCVD coatings were covered with CMAS powder deposits and annealed for 1 h at 1250°C, respectively. The ZrO2 coating was completely infiltrated by CMAS, whereas the yttria containing coatings show a higher resistance against CMAS infiltration. This is explained by the formation of a continuous oxyapatite layer in the reaction zone of the coating and the CMAS deposit. The Y2O3·ZrO2 coating shows the best infiltration resistance despite the fact that the Y2O3–CMAS sample is the only completely crystallized. As crystallization products, oxyapatite, melilite, anorthite as well as new garnets bearing all available cations were formed. The garnet phase was confirmed by XRD and TEM. EDS measurements were used to calculate structural formula A3B2T3O12 (A = Ca, Y, Zr; B = Mg, Al; T = Al, Si).
2/1‐mullite single crystal (001) plates with thicknesses between 0.9 and 1.9 mm were exposed for 1.5, 3, 6, and 12 h at 1670°C to a slowly flowing (100 mL/min) water‐rich gas mixture (O2/H2O 80/20). Under the given experimental conditions, 2/1‐mullite yielded significant amounts of structurally bound OH groups across the bulk and decomposition of the crystal surface on a micrometer scale. Decomposition products are (i) sodium‐containing silicon‐rich alumino silicate glass formed from melt and (ii) α‐alumina, which crystallizes within melt cavities. The crystal plates that are free of any OH absorption before the corrosion experiments show a steep increase in OH absorption intensity up to 3 h of corrosion and a flattening toward longer times of exposure. The evaluation of OH intensity profiles implies an effective diffusion coefficient DH in the range between 1.5 and 2.5 × 10−7 cm2/s.
Alumina (Al2O3)‐coatings were fabricated by a metal–organic chemical vapor deposition at a substrate temperature of 950°C using two different precursor evaporation systems. The application of a screw feeding system for precursor delivery leads to the formation of coatings with a nanolamellar microstructure while precursor evaporation directly from a crucible produces coatings without a specific microstructure. X‐ray diffraction measurements show that both kinds of coatings consist of transitional Al2O3. Closer investigations of the lamellar structured coating by means of transmission electron microscopy reveal that the lamellae result from periodical fluctuations of nanoporosity and grain size. Annealing experiments show that the laminated microstructure is stable at high temperatures. Even though the coatings transform completely into α‐Al2O3 after heat treatment at 1200°C and nanopores coalesce to larger pores, a through‐thickness lamellar coating morphology is maintained.
Oxide ceramic coatings in the system Y2O3-Al2O3-ZrO2 were fabricated in laboratory scale by using a MOCVD unit. A hot wall reactor was used along with different precursor feeding systems. Most experiments were carried out by using powder flash evaporation including a screw feeder for precursor powder delivery. For comparison, further samples were fabricated by using band flash evaporation and continuous evaporation from a crucible. Oxygen was used in all cases as reactant gas. Aluminium-tris-2,4-pentanedione (Al(acac)3), yttrium-tris-2,2,6,6-tetramethyl-3,5-heptanedione (Y(thd)3) and zirconium-tetrakis-2,2,6,6-tetramethyl-3,5-heptanedione (Zr(thd)4) were applied as metal-organic precursors because of their similar vaporization behaviour under the given conditions. The coating stoichiometry was varied from pure alumina to complex ternary compositions in the system Y2O3-Al2O3-ZrO2. Both kinds of ternary coatings fabricated by using flash evaporation methods show a nanolamellar microstructure in the as deposited state. Heat treating experiments at 1200 degrees C for up to 5 days enhance the lamellar character of the coating deposited by using powder flash evaporation. The lamellar microstructure is due to alternating YSZ enriched layers and YAG enriched layers in this state. However, the coating fabricated by using band flash evaporation shows a dense interpenetrating network of YSZ and YAG after heat treating instead of a lamellar microstructure observed in the as deposited state.
Two series of mullite single crystal plates of thicknesses between 0.9 and 1.9 mm were
exposed up to 12 h to a slowly flowing water rich gas mixture (O2/H2O = 80/20; 100 ml/min) at
temperatures of 1670°C. The heat treated samples show up to about 50 μm into the mullite a partial
surface decomposition. Decomposition products are sodium containing silicon-rich aluminosilicate
glass and α-alumina within melt cavities. Concentration profiles through the plate cross sections
show the effect of inward and outward diffusion of OH forming species and Na and Mg impurity
enrichment on ppm level in the bulk.
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