Search

Thermochemical interactions between calcium–magnesium–aluminosilicate (CMAS) glass and an environmental barrier coating of ytterbium disilicate (Yb2Si2O7) and ytterbium monosilicate (Yb2SiO5) were investigated. Top coats were deposited by plasma spray-physical vapor deposition onto silicon carbide substrates. CMAS powder was prepared as a glass and cast into a tape to yield a CMAS loading of ~29 mg/cm2. Samples were heat treated with CMAS at 1300 °C for 1–10 h or at 1400 °C for 1 h in air. Polished specimen cross-sections were characterized using scanning electron microscopy, X-ray diffraction, X-ray energy-dispersive spectroscopy, and transmission electron microscopy to evaluate resulting microstructures, phases, and compositions at CMAS/Yb2Si2O7 interfaces. Coatings exposed at 1300 °C—10 h and 1400 °C—1 h were fully infiltrated and compromised by CMAS. Dissolution of ytterbium silicate into molten CMAS followed by precipitation of cyclosilicate, silicocarnotite, and Yb2Si2O7 at 1300 °C and Yb2Si2O7 at 1400 °C enabled CMAS to effectively infiltrate top coats, rendering the predominantly Yb2Si2O7 coating ineffective at arresting molten CMAS degradation.

Thermochemical interactions between a calcium-magnesium aluminosilicate (CMAS) glass and ytterbium disilicate (Yb2Si2O7), a candidate environmental barrier coating (EBC) material, have been investigated. Pellets of Yb2Si2O7 and CMAS glass powder were heat treated at 1200, 1300, 1400 and 1500 °C for 1, 10 and 50 h in air. Powder X-ray diffraction was employed to identify the resulting phases. In a second series of experiments, Yb2Si2O7 substrates were prepared by hot pressing, and cylindrical wells were drilled into the material surface. CMAS glass powder was added to the wells to achieve a loading of ~35 mg/cm2 and subsequently heat treated at 1200, 1300, 1400 and 1500 °C for durations of 1, 10 and 50 h in air. Sample cross-sections were characterized using scanning electron microscopy, X-ray energy-dispersive spectroscopy, X-ray diffraction, electron microprobe analysis and transmission electron microscopy to evaluate the resulting microstructure, phases and compositions at the CMAS/Yb2Si2O7 interface. Dissolution of ytterbium silicate into molten CMAS followed by precipitation of Yb2Si2O7 coupled with CMAS grain boundary penetration at elevated temperatures were the dominant mechanisms by which CMAS effectively infiltrated and altered the Yb2Si2O7 substrate microstructure.

Thermochemical interactions between Ca2Y8(SiO4)6O2 apatite, a potential environmental barrier coating (EBC) material, and a synthetic CMAS having the composition 23.3 CaO - 6.4 MgO - 3.1 Al2O3 - 62.5 SiO2 - 4.1 Na2O - 0.5 K2O - 0.04 Fe2O3 mole % were investigated. Pellets of apatite + CMAS powder and hot-pressed apatite disc-CMAS couples were annealed at 1200–1500 °C for 1–50 hours in air. Powder X-ray diffraction (XRD) was used to identify the phases present. Polished cross-sections of the heat treated pellets and diffusion couples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). Ca3Y2(Si3O9)2 cyclosilicate, apatite, and amorphous phases were present in the samples heat treated at 1200 and 1300 °C, whereas no cyclosilicate was detected in samples annealed at 1400 and 1500 °C. A distinct cyclosilicate layer was observed at the apatite-CMAS interface in the diffusion couples heat treated at 1200 and 1300 °C. However, at 1400 and 1500 °C, due to its much lower viscosity, CMAS quickly infiltrated the apatite substrate through pores and along the grain boundaries and no cyclosilicate was observed; the apatite grains dissolved in molten CMAS followed by re-precipitation of apatite needles within an amorphous phase on cooling.

The thermochemical behavior of EBC candidate materials yttrium disilicate (Y2Si2O7) and ytterbium disilicate (Yb2Si2O7) was evaluated with three calcium-magnesium-aluminosilicate (CMAS) glasses possessing CaO:SiO2 ratios relevant to gas turbine systems. Pellet mixtures of 50 mol% EBC powder to 50 mol% CMAS glass powder were heat treated at 1200°C, 1300°C, and 1400°C. The products of these interactions were evaluated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Above glass melting temperatures, exposure of the disilicates primarily resulted in dissolution into the molten glass followed by precipitation of a Ca2RE8(SiO4)6O2 (RE = Yb3+, Y3+) apatite-type silicate and/or rare earth disilicate. In glasses with high CaO concentrations, apatite readily forms while the disilicate material is consumed by the reaction. As CaO content decreases, the disilicate phase becomes the main reaction product. Overall, reactions with yttrium disilicate favored more apatite crystallization than ytterbium disilicate. The viability of using these disilicates in various operating environments is discussed.

A novel ceramic processing method, called Horizontal Dip Spin Casting (HDSC), enabled fabrication of tubular ceramic parts with an aligned chopped fiber phase. HDSC was demonstrated using highly loaded aqueous alumina suspensions with >50 vol.% solids loading and ≤5 vol.% water-soluble polymer employed as a rheological modifier. Chopped carbon fibers were added to the suspensions to attain maximum loadings of 30 vol.%. During forming, cylindrical foam molds were dipped into the suspension while being rotated radially about the long axis. Simultaneously, a doctor blade was placed at a specified distance from the foam surface to facilitate the flow of the suspension to align the fiber and control the thickness of the material that accrued on the mold. Rheological study of alumina-PVP suspensions with and without chopped carbon fiber showed that the suspensions exhibited a yield-pseudoplastic flow behavior. The degree of alignment of the carbon fiber phase in the green bodies was characterized for various suspension formulations, carbon fiber contents and forming speeds. Stereological characterization of green body specimens confirmed the effectiveness of HDSC to attain the desired tubular geometry with considerable fiber alignment for a suspension composition containing ≤20 vol.% chopped fibers.

The properties of a volcanic ash glass obtained from the Eyjafjallaj&ouml, kull eruption of 2010 were studied. Crystallization experiments were carried out on bulk and powdered glass samples at temperatures between 900 and 1300 &deg, C. Iron oxides, Fe3O4 and Fe2O3, and a silicate plagioclase, (Na,Ca)(Si,Al)4O8, were observed. Bulk samples remained mostly amorphous after up to 40 h at temperature. Powdered glass samples showed increased crystallinity after heat treatment compared to bulk samples. The average coefficient of thermal expansion of the glass was 7.00 &times, 10&minus, 6 K&minus, 1 over 25&ndash, 720 &deg, C. The Vickers hardness of the glass was 6&ndash, 7 GPa and the indentation fracture toughness, 1&ndash, 2 MPa &radic, m Values for density, elastic modulus, and Poisson&rsquo, s ratio were 2.52 g/cm3, 75 GPa, and 0.24, respectively. The viscosity of the glass was determined experimentally and compared to three common models from the literature. The implications for the deposition of volcanic ash on hot section components of aircraft turbine engines are discussed.

Viscosity of a calcium-magnesium-aluminosilicate (CMAS) glass, melted from a synthetic sand with composition replicating that of air-breathing turbine engine deposits, was experimentally measured between 1215 °C and 1520 °C using a rotating spindle viscometer. Chemical composition of the CMAS glass before and after viscosity measurements was nominally 23.3CaO-6.4MgO-3.1Al 2 O 3 -62.5SiO 2 -4.1Na 2 O-0.5K 2 O-0.04Fe 2 O 3 (mol.%) as determined using inductively coupled plasma atomic emission spectroscopy. Experimental viscosity values were compared with those estimated from composition-based calculators of Giordano et al., Fluegel and FactSage software. Although none of these models exactly predicted viscosity values, those determined by Fluegel and FactSage models were found to more closely match experimental viscosity of the CMAS glass.

Aqueous suspensions of zirconium diboride (ZrB2), boron carbide (B4C) and tungsten carbide (WC) with dispersant and water-soluble polyvinylpyrrolidone (PVP) were investigated for processing by room-temperature injection molding, a novel, environmentally benign ceramic processing method. B4C and WC were used as sintering aids, and the as-received powders were attrition milled to reduce particle size to promote full densification of ZrB2 specimens by pressureless sintering. Zeta potential measurements of individual ZrB2, B4C and WC powders and of powder mixtures revealed that maximum stability was achieved in aqueous solutions of attrition milled powder mixtures dispersed using an ammonium polyacrylate dispersant. A maximum powder loading of 49 vol% with ≤5 vol% PVP was attained for ZrB2/B4C/WC suspensions with dispersant. Although exhibiting a time-dependent rheological response determined by parallel-plate rheometry, suspensions containing 49 vol% powders and ≤3 vol% PVP, as well as suspensions of 46 vol% powders and ≤4 vol% PVP, were flowable under the conditions of the process. ZrB2 rings prepared by room-temperature injection molding were machinable prior to binder removal and exhibited maximum brown densities of 56% true density (TD). Sintered densities were >98%TD with ~20% linear shrinkage. Scanning electron microscopy revealed an average grain size of 7.3±2.8 µm, and chemical analysis confirmed that no undesirable oxide phases remained in the sintered ZrB2 specimens. Aqueous ZrB2-based suspensions containing B4C and WC sintering aids and PVP were effectively processed via room-temperature injection molding to yield dense ZrB2 rings after binder burnout and pressureless sintering.

Thermal stability of a synthetic sand composition, which was developed as a simulant for calcium–magnesium aluminosilicate (CMAS) turbine deposits, was characterized using thermogravimetric and differential thermal analysis (TG/DTA). The sand was melted into CMAS glass, which had a composition of 23.3CaO–6.4MgO–3.1Al 2 O 3 –62.5SiO 2 –4.1Na 2 O–0.5K 2 O–0.04Fe 2 O 3 (mol.%), determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Bulk density of the glass was measured to be 2.63 g/cm 3 , and the Young's and shear moduli were 84.3 GPa and 33.6 GPa, respectively, along with a Poisson's ratio of 0.26. Vickers microhardness and indentation fracture toughness were determined to be 6.14 ± 0.1 GPa and 0.70 ± 0.05 MPa m 1/2 , respectively. Glass transition temperature, softening point and coefficient of thermal expansion of the glass were measured by dilatometry. Glass viscosities were estimated over a temperature range of 600–1500 °C using dilatometric reference points of the glass and from composition-based methods. Times required for infiltration of molten CMAS glass into thermal or environmental barrier coatings were also estimated.

The crystallization kinetics of a calcium-magnesium aluminosilicate (CMAS) glass with composition relevant for aerospace applications, like air-breathing engines, were evaluated using differential thermal analysis (DTA) in powder and bulk forms. Activation energy and frequency factor values for crystallization of the glass were evaluated. X-ray diffraction (XRD) was used to investigate the onset of crystallization and the phases that developed after heat treating bulk glass at temperatures ranging from 690 to 960 deg for various times. Samples annealed at temperatures below 900 deg remained amorphous, while specimens heat treated at and above 900 deg exhibited crystallinity originating at the surface. The crystalline phases were identified as wollastonite (CaSiO3) and aluminum diopside (Ca(Mg,Al) (Si,Al)2O6). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were employed to examine the microstructure and chemical compositions of crystalline phases formed after heat treatment.

Room-temperature injection molding, a novel, environmentally benign ceramic processing method, produced dense, near-net shape alumina rings by utilizing unique flow properties of aqueous, highly loaded (>50 vol.%) ceramic suspensions with ≤5 vol.% polyvinylpyrrolidone (PVP) dispersed using Darvan 821A. The rheological behavior of suspensions along with microstructural and mechanical properties of resulting specimens were evaluated by varying PVP content to determine the optimal composition for forming. Parallel-plate rheometry revealed that suspensions containing ≤5 vol.% PVP were yield pseudoplastic at room temperature, which facilitated processing without heating or complex chemical reactions. Alumina rings with high green densities (>60% true density (TD)) were machined before binder removal, and increasing PVP content was observed to enhance green machinability. After binder burnout and sintering, bulk densities were ∼98%TD with

The control of the rheological behavior of highly loaded ceramic/polymer suspensions affords the development of near-net shape forming techniques. In this study, suspensions containing sub-micrometer diameter alumina (up to 56 vol%) were fabricated using an anionic dispersant (≈4 vol%) and water-soluble polyvinylpyrrolidone (PVP). The amount and ratio of molecular weights of PVP in the suspension were varied to influence flow behavior. The final pH of the system, ≈9.5, was higher than the isoelectric point (IEP) of alumina implying that the alumina powder possesses a negative surface charge. In the case of alumina at this pH, PVP does not adsorb onto the surface of the powder. The flow behavior of the PVP-containing suspensions displayed yield-pseudoplastic characteristics that closely agreed with the Herschel–Bulkley fluid model. The addition of PVP significantly changed the rheology of the system, increasing the shear yield stress and altering flow behavior. Interparticle interaction approximations of the suspensions were modeled to correlate with experimental observations.