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Notes: Yttrium silicates are promising materials for improved oxidation and erosion protection for carbon fiber-reinforced composites. A two-layer coating system of low-pressure plasma-sprayed yttrium silicate on chemical vapor deposition-SiC-precoated C/C–SiC was tested under atmospheric re-entry conditions simulated within a plasma wind tunnel test facility. The thermal expansion behavior of Y2SiO5 and Y2Si2O7 was investigated. The chemical compatibility with and without increasing oxygen partial pressure at the interface of the two-layer system was calculated by the CALPHAD method. The calculations were compared with experimental results. Furthermore, a thermodynamic explanation is presented to understand and predict the observed coating failure mechanism, identified as blister formation.

Notes: Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO2-coated gamma-Al2O3 particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO2-coated alpha-Al2O3 particle microcomposites (AS-alphaA) and to other Al2O3-SiO2 reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites.

Notes: The heat capacity at constant pressure (Cp) of mullite (single crystalline and dense and powdered polycrystalline) was determined between −125° and 1400°C. Measurements were performed in three laboratories using two different disk-type differential scanning calorimeters (DLR and Netzsch) and one cylinder-type (Setaram). On the basis of these experimental data a new master curve Cp (T) is presented. Up to about 1100°C the Cp curve agrees well with those from previously published results: The Cp master curve displays a parabolic shape with Cp values of about 0.415 J/(g·K) at −125°C, 0.78 J/(g·K) at 25°C, and 1.25 J/(g·K) at 1000°C. At temperatures above 1100°C a yet unknown, steplike, weak Cp increase is observed with ΔC°p step values ranging between 0.03 and 0.10 J/(g·K). This Cp anomaly of mullite is reproducible under various heating conditions and is reversible both on heating and on cooling. The extrapolated onset temperature of the anomaly (Ton.) is controlled by the crystalline state of the samples (single crystal, Ton. above 1100°C; polycrystal, Ton. above 1250°C) but does not depend on the composition of mullite. Sillimanite, which is structurally closely related to mullite, displays no Cp anomaly. The absence of latent heat (ΔH= 0) and the shape of the Cp anomaly of mullite are comparable to the behavior of glass-forming materials at the glass transition. The observations can best be interpreted in terms of an onset of ion-jumping between adjacent structural sites T and T* and O(C) and O(C)*. Other interpretations, i.e., clockwise and counterclockwise rotations of structural units, or minor decomposition of mullite to α-Al2O3 plus SiO2 glass observed at the surface of the single crystals, are less probable.

Notes: Hardness indentations with a Vickers pyramid under a load as low as 0.1 N applied at room temperature induce amorphization in single-crystal mullite when applied to the (001) crystallographic face. Different zones of damage were identified via TEM. Directly under the indenter, in the region of high-compressive stress, mullite becomes amorphous. Further out toward the matrix material, there exists a region of high plastic deformation in the form of dislocation networks, radial microcracks, and bend contours. The Vickers-induced damage is comparable to that produced by dynamic shock or intense ball milling of mullite.

Notes: Nine elastic stiffness coefficients, cij, of a mullite single crystal (2Al2O3·SiO2) are measured using acoustic resonance spectroscopy. The obtained values are similar to those of the structurally related aluminosilicate phase sillimanite (Al2O3·SiO2). Characteristic elastic properties of the two minerals are interpreted with the help of their crystal structures and atomic force constants for sillimanite. The high longitudinal stiffness coefficients, c33, of mullite (∼352 GPa) and sillimanite (∼388 GPa) are caused by continuous “stiff” load-bearing tetrahedral chains parallel to c-axis, while the “soft” octahedral chains have minor direct influence. They stabilize the tetrahedal chains against tilting. The lower c33 value of mullite in comparison to the sillimanite value may be caused by a weakening of the load-bearing tetrahedral chains which are parallel to c-axis because of partial replacement of silicon by the weaker-bonded aluminum. The longitudinal stiffness coefficients perpendicular to c-axis are significantly lower, because of sequences of alternating “soft” octahedral and “stiff” tetrahedral units. Within the plane (001), the compliant octahedra exhibit stiffness-controlling influence with coefficients parallel to b-axis (c22∼ 233 GPa) being somewhat lower than parallel to a-axis (c11∼ 291 GPa). This is explained with the occurrence of compliant octahedral Al(1)–O(D) bonds, which are more effective parallel to b-axis rather than to a-axis. Because octahedra are unaffected by the aluminum to silicon substitution, c11 and c22 coefficients of mullite and sillimanite are very similar. Shear stiffness coefficients of mullite increase from c55 (∼77 GPa) to c66 (∼80 GPa) to c44 (∼110 GPa), indicating increasing resistance against shear deformation within the planes (010), (001), and (100). The lattice plane of the highest shear stiffness (100) is built up of an oxygen-oxygen network, diagonally braced along 〈011〉 (“Jägerzaun”). This network with short oxygen–oxygen distances can be sheared by compression and elongation along oxygen–oxygen interaction lines only which is rather unlikely. Because of the lack of such networks in the planes (010) and (001), bending and deformation of structural units become easier, and consequently c55 and c66 are 〈c44. All three shear stiffness coefficients of mullite are slightly lower than those of sillimanite because of the reduction of the mean tetrahedral bond strength in mullite caused by partial substitution of silicon by aluminum.

Notes: The thermal diffusivities Dth of mullite single crystals with about 2/1-composition parallel to the crystallographic a, b, and c axes (i.e., parallel to [100], [010], and [001]) and of monophase, dense mullite ceramics were measured between room temperature and 1200°C using the laser flash method. The semi-transparent mullite disks were covered by thin layers of argon-sputtered platinum and sprayed colloidal graphite in order to minimize heat radiation transfer effects. The phonon-produced thermal conductivity of mullite was calculated according to K=DthCpρ using experimentally measured values for thermal diffusivity, Dth(T), and specific heat, Cp(T), and thermal expansion coefficients, αi, for the determination of temperature-dependent density ρ(T) and sample thickness d(T). The anisotropy of thermal diffusivity and conductivity is evident with highest values parallel to the crystallographic c axis (i.e., [001]; e.g., at T=100°C K[001]=6.862 W/mK), corresponding to the direction of strongest bonds and highest elastic stiffness in the mullite crystal structure. Perpendicular to the c axis, thermal diffusivities and conductivities are smaller, with the values parallel to the a axis (i.e., [100]) being slightly higher than parallel to b (i.e., [010]; e.g., at T=100°C K[100]=4.563 W/mK and K[010]=4.400 W/mK), indicating lower bond strength and elastic stiffness in these crystallographic directions. Conductivity anisotropy factors are around 1.33 for K[001]/Kaverage, 0.90 for K[100]/Kaverage, and 0.85 for K[010]/Kaverage with little variation between room temperature and 1200°C. The thermal diffusivity and conductivity of dense, monophase mullite ceramics display similar values as the averaged single crystal data, thus providing a master curve for the temperature-dependent thermal conductivity.

Notes: The order–disorder of the tetrahedrally coordinated aluminum and silicon atoms in mullite has been investigated by means of 29Si nuclear magnetic resonance (NMR) spectroscopy. Sinter (3/2) and fused (2/1) mullites in the as-received state and reheated at 1750°C, and a reference sillimanite were used for this study. All mullites display similar 29Si NMR spectra: The strongest peak occurs at about −88 ppm, with two subpeaks close to −92 and −96 ppm. The −88 ppm signal is assigned to a sillimanite-type environment with three aluminum oxygen tetrahedra as next nearest neighbors of the silicon oxygen tetrahedra. The two 29Si NMR signals near −92 and −96 ppm are assigned to silicon oxygen tetrahedra surrounded by two aluminum oxygen and one silicon oxygen tetrahedra, and one aluminum oxygen and two silicon oxygen tetrahedra, respectively. 29Si NMR spectra with different short-range-order parameters were simulated by an array of 2 × 10 000 tetrahedral positions by means of an adapted random generator. The comparison between measured and simulated mullite and sillimanite 29Si NMR spectra yields a moderate degree of tetrahedral aluminum–silicon order, with no tendency toward cation demixing.

Notes: The microhardness (H) of single crystal, orthorhombic mullite was measured on (010) and (001) faces from room temperature up to 1400°C. The microhardness versus temperature curves display sigmoidal shapes. The mean microhardness at room temperature is ∼16 Gpa and it decreases with temperature to H is ∼ 13 GPa at 300°C. At 〉300°C, the microhardness is only slightly reduced with temperature to H is ∼10 GPa at 1000°C. Above this temperature limit, it markedly decreases again, to a mean value of ≈6 GPa at 1400°C. Up to ∼1000°C the (010) and (001) microhardnesses are very similar. Above ∼1000°C, however, the hardness gradually becomes anisotropic, with H(010) being twice as high as H(001) at 1400°C (H(010)∼ 8 GPa, H(001)∼ 4 GPa).

Notes: A plane-parallel, polished, 0.9 mm thick, single-crystal (001) plate of 2:1 mullite was treated for 6 h at 1600°C in an Ar/H2O (90/10) gas mixture at 100 kPa. Optical microscopy studies and infrared (IR) reflection spectroscopy studies of the lattice vibrations yielded no evidence for change with respect to the untreated reference crystal. However, IR absorption spectroscopy showed that structurally bound OH groups were formed by the heat treatment in the Ar/H2O gas mixture. IR absorption depth profile analysis showed a rather homogeneous OH distribution through the crystal. Five different hydroxyl groups were separated according to dipole orientations and peak positions: E‖a, ωa1= 3447 cm−1, ωa2= 3579 cm−1; E‖b, ωb1= 3456 cm−1, ωb2= 3544 cm−1; and E‖c, ωc1= 3498 cm−1. All IR peaks were strongly broadened (between 90 and 150 cm−1) because of a distribution in O-H binding distances caused by the real structure of mullite.