Michael J. Lance

As-deposited mixed zone in thermally grown oxide beneath a thermal barrier coating

Abstract Gas turbine designers are increasingly using electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBC) to meet the challenge of higher efficiency gas turbine engine requirements. A key feature for expanding the use of TBCs is increased spallation life and reduced spallation life variability. Such a coating system comprises a substrate (Ni-based single crystal alloy), a bond coat (diffusion aluminide or MCrAlY), a ceramic (7 wt.% yttria stabilized zirconia), and a thin thermally grown oxide (TGO) between the bond coat and the ceramic. The TGO is intended to be α-alumina, but evidence reported by other researchers suggests that in some cases the as-deposited TGO may not be entirely α-alumina. The thin nature of the TGO in as-deposited TBCs ( 2 O 3 between the mixed oxide zone and the bond coat; and (2) a mixed oxide zone between the continuous γ-Al 2 O 3 and the TBC layer. An explanation for the creation of the mixed oxide zone found in these TGO morphologies is presented.

The importance of amorphous intergranular films in self-reinforced Si3N4 ceramics

Abstract High-fracture-strength and high-toughness β-Si 3 N 4 ceramics can be obtained by tailoring the size and number of the elongated bridging grains. However, these bridging mechanisms rely on debonding of the reinforcing grains from the matrix to increase toughness. Interfacial debonding is shown to be influenced by sintering aids incorporated in the amorphous intergranular films. In one case, the interface strength between the intergranular glass and the reinforcing grains increases with the aluminum and oxygen content of an interfacial epitaxial β-SiAlON layer. In another, the incorporation of fluorine in the intergranular film allows the crack to circumvent the grains. Atomic cluster calculations reveal that these two debonding processes are related to (1) strong Si–O and Al–O bonding across the glass/crystalline interface with an epitaxial SiAlON layer and (2) a weakening of the amorphous network of the intergranular film when difluorine substitutes for bridging oxygen.

Characterization of commercial EB-PVD TBC systems with CVD (Ni,Pt)Al bond coatings

Abstract Failure of electron-beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) with aluminide bond coats is strongly influenced by bond coat oxidation behavior. This study investigated oxide (Al2O3) formation during EB-PVD processing of TBCs with (Ni,Pt)Al bond coats. The effects of substrate composition, coating impurities and bond-coat grit-blasting on the oxide phases, residual stress and microstructure were evaluated. As-deposited, high-purity commercial bond coats contained high concentrations of sulfur and other impurities at their surfaces. Numerous small voids formed at the oxide–metal interface when as-deposited bond coats were oxidized during EB-PVD processing. Grit-blasting of (Ni,Pt)Al had a significant impact, since the formation of α-Al2O3 during EB-PVD processing was significantly enhanced and voids did not form beneath the scale. Preliminary cyclic oxidation testing suggested an influence of superalloy sulfur content on TBC durability.

Evidence for nanoindentation-induced phase transformations in germanium

Nanoindentation experiments were performed using Berkovich and cube-corner indenters to investigate whether nanoindentation-induced phase transformations, such as those observed in silicon, also occur in germanium. Although the indentation load-displacement curves for germanium do not show the unloading pop-out or elbow phenomena observed in silicon, clear evidence for phase transformations was obtained by scanning electron microscopy (SEM) and micro-Raman spectroscopy. SEM showed that there is extruded material around the contact periphery of cube-corner hardness impressions that is metalliclike in its flow characteristics, just as in silicon. Micro-Raman spectroscopy revealed more direct evidence by identifying amorphous and what may be the crystalline BC8 (Ge-IV) phase. The fact that these phenomena are observed primarily and reproducibly only for the cube-corner indenter suggests that the contact geometry significantly affects the transformation behavior. Results are discussed in terms of possible def...

Characterization and Performance of LiFePO4 Thin-Film Cathodes Prepared with Radio-Frequency Magnetron-Sputter Deposition

LiFePO4 films with a thickness of 1{micro}m were deposited on a stainless steel substrate by radio-frequency magnetron sputtering of a LiFePO{sub 4}/carbon composite target. Raman spectra revealed the presence of carbon in the film, which increased the electronic conductivity of the film relative to reports of the carbon-free material. X-ray diffraction revealed that the films were well crystallized, free of second phases, and may have a texture with a (011) orientation. The LiFePO{sub 4} plus carbon film showed a good capacity and rate capability at 25 and -20 C when cycled as the cathode of a lithium battery, although the film was still inferior to the carbon-coated LiFePO{sub 4} powder electrodes with 0.5 {micro}m particle size.

Synthesis and characterization of single-wall carbon nanotube-amorphous diamond thin-film composites

Thin-film single-wall carbon nanotube (SWNT) composites synthesized by pulsed laser deposition (PLD) are reported. Ultrahard, transparent, pure-carbon, electrically insulating, amorphous diamond thin films were deposited by PLD as scratch-resistant, encapsulating matrices for disperse, electrically conductive mats of SWNT bundles. In situ resistance measurements of the mats during PLD, as well as ex situ Raman spectroscopy, current–voltage measurements, spectroscopic ellipsometry, and field-emission scanning electron microscopy, are used to understand the interaction between the SWNT and the highly energetic (∼100 eV) carbon species responsible for the formation of the amorphous diamond thin film. The results indicate that a large fraction of SWNT within the bundles survive the energetic bombardment from the PLD plume, preserving the metallic behavior of the interconnected nanotube mat, although with higher resistance. Amorphous diamond film thicknesses of only 50 nm protect the SWNT against wear, providi...

NDE assessment of TBCs: an interim report of a photo-stimulated luminescence 'round-robin' test

Abstract Photo-stimulated luminescence spectroscopy (PSLS) can be used as a non-destructive tool to gain quantitative information about the stress and strain in the thermally grown oxide (TGO) formed beneath a thermal barrier coating (TBC) and possibly assess damage. In response to the increasing popularity of this technique, a ‘round-robin’ test has been initiated to compare methods of luminescence collection using different instruments as well as various spectral analysis methods. The interim results of this ‘round-robin’ are reported here with a series of PSLS measurements and analyses performed at four different laboratories on the same set of electron beam physical vapor deposited (EB-PVD) TBC samples, having undergone various oxidation treatments. In addition, a set of standard electronic spectra (collected from one group) were circulated for analysis in order to compare the curve-fitting methods used by each group. Apart from sample variations, there was very good agreement between the results obtained in the different laboratories, and there were no systematic differences in the data, indicating that present collection and analysis methods are comparable.

High-pressure phase transformation of silicon nitride

We provide evidence for a high-pressure phase transformation (HPPT) in the ceramic material silicon nitride. This HPPT is inferred by a high-pressure diamond anvil cell, Raman spectroscopy, scanning/transmission electron microscopy, and optical and acoustic microscope inspection. In the case of silicon nitride, the HPPT involves a ductile or metallike behavior that is observed in severe deformation processes, such as nanoindentation and micromachining. This pressure-induced plasticity is believed to be similar to that found in silicon and germanium with its origin in the high-pressure metallic β-Sn phase formation.

Residual Stress Measurements

Portions of the contribution have been prepared by UT-Battelle, LLC, Operator of Oak Ridge National Laboratory under Contract No. DE-AC05-00OR22725, with the U.S. Department of Energy.

Characterization of Field-Aged EGR Cooler Deposits

Exhaust gas recirculation (EGR) cooler fouling has become a significant issue for compliance with NOx emissions standards. In order to better understand fouling mechanisms, eleven field-aged EGR coolers provided by seven different engine manufacturers were characterized using a suite of techniques. Microstructures were characterized using scanning electron microscopy (SEM) and optical microscopy following mounting the samples in epoxy and polishing. Optical microscopy was able to discern the location of hydrocarbons in the polished cross-sections. Chemical compositions were measured using thermal gravimetric analysis (TGA), differential thermal analysis (DTA), gas chromatography-mass spectrometry (GC-MS), x-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Mass per unit area along the length of the coolers was also measured. Despite coming from different sources and applications, many common features were observed in the cooler deposits including mud-cracking, hydrocarbon condensation near the metal surface, and erosion of the deposit. Differences and commonalities between the coolers will be discussed in the context of better understanding cooler fouling and ways to prevent it.