Jay S. Wallace

Microstructural characterization of yttria-stabilized zirconia plasma-sprayed deposits using multiple small-angle neutron scattering

Density, electron microscopy, elastic modulus, and small-angle neutron scattering studies are used to characterize the microstructures of yttria-stabilized zirconia plasma-sprayed deposits as a function of both feedstock morphology and annealing. In particular, anisotropic multiple small-angle neutron scattering data are combined with anisotropic Porod scattering results to quantify each of the three main porous components in these thermal barrier coating materials: intrasplat cracks, intersplat lamellar pores and globular pores. An inverse correlation between the volume of porosity and its surface area is confirmed for the as-sprayed deposits, as is a preferential annealing of intrasplat cracks at elevated temperatures. The average elastic modulus is correlated with the total void surface area while the elastic anisotropy is related more closely to the intersplat porosity. However, depending on the feedstock morphology, globular pores are also shown to play a surprisingly significant role in post-anneal deposit microstructures and properties.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Micro-indentation and inverse analysis to characterize elastic /plastic graded materials

Properties of actual graded materials were characterized with a new procedure based on inverse analysis. This procedure utilizes indirect experimental records obtained from instrumented micro-indentation and extracts key properties of indented specimen through the Kalman filter technique. The graded material is composed of mixture of Yittria Partially Stabilized Zirconia (PSZ) and NiCrAlY and it possesses varying elastic � /plastic properties through its thickness. This procedure enables determination of the compositional profile and the effective mechanical property without resorting to complex experimental measurements. It relies solely on the load � /displacement records of instrumented spherical indentation and the inverse analysis during the post-processing. The graded specimens were fabricated by plasma spray deposition process under controlled feeding of PSZ and NiCrAlY powders. Prior to testing of the graded materials, single phased coatings were made with each component and analyzed. This process allows consistent material constants to be used in the graded material analysis. Here PSZ and NiCrAlY were assumed to be elastic and elastic � /plastic, respectively. The elastic moduli of both materials were estimated with a common indentation method while the plastic properties of NiCrAlY were determined by modifying the inverse method proposed for the graded materials. The latter procedure represents a new indentation method for characterization of homogeneous elastic � /plastic materials. Once the properties of constituents were identified, the properties of graded material were estimated with the Kalman filter technique. The indented load � /displacement relations simulated from finite element analysis with the estimated properties and that of measured record showed excellent agreement, which assures a high degree of accuracy in the current measurement procedure. # 2002 Elsevier Science B.V. All rights reserved.

The elastic constants of single crystal β-Si3N4

Using Brillouin scattering, we have determined the complete set of elastic constants of the technologically important β-Si3N4. Availability of single crystals has allowed us to measure all five independent cij, which (in units of GPa) are: c11=433(3), c33=574(3), c13=127(5), c44=108(2), c66=119(4), and c12=c11−2c66=195(8), giving a bulk modulus of 259. Our values are compared with existing calculations and cij determined from nano-indentation results combined with empirical relations and trends in related materials.

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.

Superconducting thin films of Bi‐Sr‐Ca‐Cu‐O obtained by laser ablation processing

Thin films of Bi‐Sr‐Ca‐Cu‐O, deposited on (100) cubic zirconia by laser ablation from a bulk superconducting target of nominal composition BiSrCaCu2 Ox , have been investigated by dc resistance and magnetically modulated microwave absorption measurements. The latter technique reveals important features regarding the phase purity of superconducting samples that are masked in the dc resistance measurements. The superconducting behavior of the films, as a function of the substrate temperature during deposition and the post‐deposition annealing conditions, is discussed.

Formation of alumina-chromia-chromium composites by a partial reduction reaction

Abstract Metal-ceramic matrix composites were formed by the partial reduction of Al 2 O 3 -Cr 2 O 3 solid solutions to form Cr metal particles in an Al 2 O 3 -Cr 2 O 3 matrix which had a lower Cr 2 O 3 concentration. In Al 2 O 3 -Cr 2 O 3 solid solutions containing 10 and 25 wt.% Cr 2 O 3 , the microstructures produced by reduction are different from that expected from simple models of internal reduction or oxide scale formation. Grain boundary diffusion of individual grains. Small Cr particles were then observed to coarsen along some, but not all, grain boundaries by concomitant process known as ‘discontinuous coarsening’. The mechanisms controlling microstructural evolution in this system were examined.

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.

Role of crack formation in the electric fatigue behavior of ferroelectric PZT ceramics

The electric fatigue behavior of commercial lead zirconate titanate (PZT) was investigated by optical microscopy and instrumented Hertzian microindentation. Macroscopic delamination cracks were found near the electrodes after large numbers of electrical cycles. Additionally, macroscopics edge cracks were found to orientate from the boundary between electroded and unelectroded material. Instrumented Hertzian microindentation measurements on specimen cross sections show larger indentation depths just beneath the electrodes than in the center of the sample. This behavior may result form a higher microcrack density near the electrodes. The role of micro- and macrocracks in the electrical fatigue behavior of ferroelectric ceramics is discussed and compared to macroscopic material parameters and acoustic emissions.

Polarized ultraviolet Raman spectroscopy of β-Si3N4

We have studied single crystals of β-Si3N4 with polarization-resolved Raman spectroscopy in order to unambiguously obtain the symmetry character of zone center optical phonon modes. By applying ultraviolet excitation radiation, approaching resonance condition, we have overcome the usual difficulties with overwhelming fluorescence obscuring Raman spectra of β-Si3N4 in the visible. Superior extinction ratios have been achieved and the application of group theory allows us to assign ten out of eleven Raman-active modes in excellent correlation with recent theoretical results. The probable nature of the eleventh mode is discussed.