•The role of bond coat morphology in APS TBC system lifetime is explored.•Lifetime-relevant BC morphology is quantified via optical confocal microscopy.•Amplitude, slope, and summit density parameters are correlated with TBC lifetime.•Unique combinations of roughness amplitude, slope, summit density optimize lifetime.•TBC durability is correlated with BC–TBC interface morphological optimization.•Specific BC morphology upon which coatings are applied affects local TBC compliance.The role of bond coat (BC)–thermal barrier coating (TBC) interfacial morphology on the failure of yttria-stabilized zirconia (YSZ) air plasma sprayed (APS) thermal barrier coatings on a NiCoCrAlY bond coat has been investigated. Interfacial morphology plays a key role in the generation of compressive and tensile stresses normal to the global surface plane through oxide growth processes and microstructural evolution, but a functional correlation between BC morphology and coating lifetime for these systems has proved to be elusive. A compilation of quantitative surface morphology parameters – average roughness (Sa), areal root-mean square slope (Sdq), and areal summit density (Sds) – have been assembled to provide a lifetime-relevant description of interfacial morphology, relative to observed coating failure mechanisms. Current data suggest that a combination of the following BC–TBC morphology parameters have a positive effect on coating system lifetime: (1) an average roughness of 15 μm ± 3 μm, (2) a slope distribution with a summit near 66° ± 3°, and (3) a peak-to-peak summit spacing of approximately 120 μm ± 10 μm. The observed increases in coating lifetime are attributed to the increased effective toughness of the TBC microstructure as a direct result of the morphology of the BC onto which the TBC is sprayed.

Oxidation of MCrAlX (M = Ni and Co; X = Y or Re) bond coats was carried out at 1,125 °C in a range of N2–O2–H2O environments. A three-step process of (1) oxidation, (2) taper-polishing, and (3) re-oxidation was used to evaluate steady state development of thermally grown oxide (TGO). During initial oxidation, transient (Ni,Co)(Al,Cr)2O4 spinel formed above α-Al2O3. Following taper-polishing, no new spinel grew during 1–200 h of re-oxidation in any water vapor environment; spinel growth at the TGO surface by a steady state mechanism—owing to Al-depletion of the bond coat, as predicted elsewhere—was deemed unlikely. Observations of transient spinel volatilizing in wet environments were supported by measurements of nickel volatilizing from pre-fabricated NiAl2O4 spinel pellets as a function of humidity. In some cases, following volatilization, water promoted vapor phase-redeposition of spinel onto adjacent specimen surfaces. Spinel-related conclusions from past humid oxidation experiments for which volatilization phenomena were not considered—and especially for which Al-depletion of the bond coat is cited as the cause for spinel growth—should be reevaluated.

•We explore inter-diffusion in composite SOFC electrodes as a durability issue.•Composite SSC/LSGM cathodes were synthesized at varying constituent weight ratios.•System degradation was evaluated via electrochemical testing and TEM microanalysis.•Degradation was correlated with microstructure-dependent cation inter-diffusion.•TEM/EDS analysis is critical to understanding mechanisms and kinetics of evolution.Composites of Sm0.5Sr0.5CoO3−δ (SSC), a good electronic conductor, and La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM), a good ionic conductor, have been evaluated as cathode systems for La-gallate electrolyte based SOFC systems via microstructural analysis and electrochemical testing. SSC and LSGM composite cathode half-cells were fabricated with varying phase fraction ratios. X-ray diffraction and transmission electron microscopy studies show that these electrode and electrolyte materials will inter-diffuse with long-term exposure in service environments. Within an electrode, inter-diffusion occurs between SSC and LSGM particles of varying initial particle sizes, resulting in heterogeneity in the local cation ratios. Of the SSC/LSGM weight ratios analyzed (pure SSC, 80:20, 70:30, and 50:50), the 80:20 ratio was identified to possess the lowest resistance, according to AC impedance results. The microstructural basis of this finding, and implications for optimal materials design, are discussed.

The protein streptavidin exhibits unique properties advantageous for “bottom-up” nanofabrication applications. It self-assembles into various 2-D crystalline lattices onto which nanoparticles can be attached through both electrostatic and ligand−receptor mechanisms. We examine the electrostatic adsorption of gold nanoparticles onto non-close-packed streptavidin crystals and show that site-specific attachment preferentially occurs in between protein molecules. The resulting nanoparticle arrangement consequently displays a long-range structural coherence with the underlying protein lattice, although with a reduced degree of order relative to that of the biological template. Monte Carlo simulations reveal that this remittent ordering is due to (1) the random offset between the nanoparticles and their respective adsorption sites and (2) nonspecific binding to the surface of the protein molecules. Overall, our results indicate that streptavidin crystals are capable of templating ordered nanoparticle arrays.