Jan Ilavsky

Evolution of the void structure in plasma-sprayed YSZ deposits during heating☆

Abstract The evolution of the anisotropic void microstructure of plasma-sprayed yttria-stabilized zirconia (YSZ) deposits has been observed as a function of temperature by small-angle neutron scattering. Scattering experiments were carried out in-situ, in a furnace between 600 and 1400°C. The terminal slope (Porod scattering) of the scattering spectra was used to derive the specific surface area of the voids. For samples with sufficient scattering anisotropy, the two major void systems — intersplat (inter-lamellar) pores and intrasplat cracks — could be characterized separately. The pore and crack specific surface areas were found to depend on temperature differently. The specific surface area of the intrasplat cracks decreased markedly at temperatures below 1000°C, whereas the specific surface area of the intersplat pores began to decrease above 1000°C. This indicates important differences in the sintering of these two void systems probably related to their size and shape. Changes in the void surface were observed at temperatures as low as 800°C, a temperature comparable to, or less than, the usual operational temperature for this material.

Elastic modulus measurements in plasma sprayed deposits

A commercial hardness indenter has been modified to record load displacement as a spherical ball is elastically loaded onto the surface of the material to be measured. The resulting data are used to calculate the elastic modulus. This technique has been used to characterize the elastic modulus of zirconium oxide-8% yttrium oxide plasma sprayed deposits. Moduli were measured both on the cross section and on the plan section, and the differences were correlated with the microstructure. Since relatively small areas of the material were sampled by the indenter, local mapping of elastic modulus variations on the size scale of the microstructure was possible. A periodic variation in modulus with position in the cross section was found on a length scale that corresponded to the average plasma spray pass thickness. Elastic modulus variations also have been found on a macro scale through the thickness of freestanding plasma sprayed deposits. These large scale variations were probably a result of self annealing during the production of these thick samples. Finally, significant increases in elastic modulus have been found in samples annealed for a total of 2.5 h at 1100 °C. These changes have been correlated with small angle neutron scattering measurements of void surface area.

Thermal spray yttria-stabilized zirconia phase changes during annealing

Phase stability of thermal barrier deposits made from yttria partially stabilized zirconia (Y-PSZ) is a requirement for extended service lifetime. The response of Y-PSZ plasma-sprayed deposits to annealing at 1000, 1200, and 1400 °C with times from 1 to 1000 h has been evaluated using the Rietveld analysis of neutron diffraction data. Results show that the yttria concentration of the as-sprayed tetragonal zirconia component generally decreased with increasing annealing temperature and time. As the yttria content in the tetragonal phase approached a limiting concentration, about 3.5 mol.% of YO1,5, the tetragonal phase transformed into monoclinic phase on cooling. An increase in monoclinic phase content was clearly observed after annealing for 24 h at 1400 °C and was nearly 35% after 100 h at 1400 °C. A similar trend was observed at 1200 °C for longer annealing times, with monoclinic phase formation beginning after 400 h. At 1000 °C, experimental times were not sufficient for monoclinic phase to form, although a decrease in the yttria concentration in the tetragonal phase was observed.

Advanced Microstructural Characterization of Plasma-Sprayed Zirconia Coatings Over Extended Length Scales

Achieving control of the microstructure of plasma-sprayed thermal barrier coating (TBC) systems offers an opportunity to tailor coating properties to demanding applications. Accomplishing this requires a fundamental understanding of the correlations among processing, microstructure development, and related TBC properties. This article describes the quantitative characterization of the microstructure of plasma-sprayed partially stabilized zirconia (PSZ) coatings by means of x-ray and neutron-scattering imaging techniques. Small-angle neutron scattering, ultra-small-angle x-ray scattering, and x-ray microtomography were used to characterize and visualize the nature and structure of the features in these material systems. In addition, the influence of processing parameters on microstructure development is discussed along with thermal cycling effects on the pore morphology, and their resultant influence of the porosity on the thermal conductivity and elastic modulus of plasma-sprayed PSZ TBCs.

Microstructural characterization studies to relate the properties of thermal-spray coatings to feedstock and spray conditions☆

This paper reports how microstructural information, obtainable for thick free-standing thermal-spray (plasma-sprayed) ceramic deposits by small-angle scattering, can be used to explore the relationships among the feedstock or spray process conditions, the anisotropic void and crack microstructures of the deposits, and technologically important deposit properties such as thermal conductivity. The application of near-surface small-angle scattering to characterize the microstructure of sub-millimeter-thick coatings on the substrate is also reported for the first time.

Mercury intrusion porosimetry of plasma-sprayed ceramic

Limitations and corrections for the application of mercury intrusion porosimetry (MIP) in measuring plasma-sprayed ceramic (alumina-titania) deposits are studied. The data reduction procedures of the MIP technique are discussed; the importance of which changes between different machines and samples. Thus, it is proposed that each published result should be accompanied by the specific data reduction procedures and assumptions used so that data may be compared. Preparation of plasma-sprayed ceramic samples has a significant influence on the MIP result. Sample fragmentation into irregular pieces (below about 1.2 mm effective diameter) prior to the MIP measurement resulted in an increase of surface effects such that the surface roughness dominated the MIP data and the measured porosity volume increased. Variation in sample thickness between 0.8 and 4.7 mm did not change the measured porosity. Orientation (parallel or perpendicular to the substrate) of the flat surface did not have a measurable effect on the MIP results.

The Ultra-Small-Angle X-Ray Scattering Instrument on UNICAT at the APS

A new ultra-small-angle X-ray scattering (USAXS) instrument has been commissioned as part of the UNICAT facility on the 33-ID line at the Advanced Photon Source. The instrument offers continuously-tunable optics for anomalous USAXS, 1000 times the throughput of earlier USAXS instruments1,2, high sensitivity and high resolution at low scattering vector, and a scattering vector range from below 0.00015 A−1 to above 0.5 A−1. Early results include USAXS from colloidal silica suspensions, and anomalous USAXS from rare-earth oxides in the presence of similarly-sized cavities in silicon nitride. The addition of side-reflection optics, in an optional configuration of this instrument, enables USAXS measurements of anisotropic as well as isotropic materials.

Characterization of the closed porosity in plasma-sprayed alumina

The porous phase of plasma-sprayed alumina was characterized using mercury intrusion porosimetry, water immersion methods, and small-angle neutron scattering. A comparison of the results shows that the intrusion techniques did not yield a full porous phase characterization. It has been found that while the amount of closed porosity was 1.4%(±0.5%) of the sample volume, this volume accounted for only 60% of the total internal surface area. The smallest void diameter was found to be 10 nm.

Versatile USAXS (Bonse‐Hart) Facility for Advanced Materials Research

The USAXS facility at UNICAT Sector 33 at the Advanced Photon Source (APS) is a world‐class resource for advanced materials research emphasizing full‐range characterization of nanometer‐scale to micrometer‐scale microstructures. Receiving photons from an APS Undulator A X‐ray source, the instrument delivers ≈ 1013 ph s−1 incident in a 0.4 mm × 2.5 mm area at the sample position for 10 keV photons, has an incident photon energy range from 7 keV to 19 keV, a single‐scan Q range (where Q = 4π/λ sin θ, λ is the photon wavelength, and 2θ is the angle of scatter) from 0.0001 A−1 to 1 A−1, over 10 decades of detector intensity range, absolute intensity calibration by primary methods, fluorescence rejection in the scattered beam, excellent energy resolution (Δλ/λ) for anomalous USAXS, and a maximum unfocused beam size of 0.4 mm × 2.5 mm, with useful beams as small as 20 μm × 20 μm. The facility offers semi‐automated data reduction, rapid and rigorous data analysis using state‐of‐the‐art structure factors and mode...

Small-angle neutron scattering study of the role of feedstock particle size on the microstructural behavior of plasma-sprayed yttria-stabilized zirconia deposits

(SANS) and multiple small-angle neutron scattering (MSANS) experiments. Three main void components were identified in the deposits: intrasplat cracks, interlamellar planar pores, and globular pores. The SANS and MSANS measurements were analyzed using the traditional theory for Porod scattering and a recently developed three-component model for MSANS evaluation. The average size, volume fraction, internal surface area, and orientation distribution for each void component within the deposits were determined. This study focused on gaining a better understanding of the effects of initial feedstock particle size and annealing temperature so n the microstructure of deposits sprayed under equivalent particle-impact conditions. Quantitative results are presented for each of four deposit samples: one prepared using the as-received feedstock particle wide-size distribution and three prepared from feedstock powder of different and relatively narrow particle size ranges with average sizes of 32, 47, and 88 m. Except for the coarse (88 m) feedstock powder, only mild monotonic variations were found in the microstructural anisotropies, the porosities (13 ± 1%), and the internal surface areas in the as-sprayed deposits. The internal surface area was independent of the feedstock particle size, even with the coarse feedstock. When the deposits were annealed at high temperatures (1100 and 1400 °C), the microstructures were altered with a reduction of the total internal surface area and a mild coarsening of the voids. These changes in the microstructural evolution were well-captured and described by the three-component model. The results were compared and related with those obtained from scanning electron microscopy images and elastic moduli measurements.