Daniel Goberman

The dependency of microstructure and properties of nanostructured coatings on plasma spray conditions

Abstract In this paper, Al2O3-13 wt.% TiO2 coatings formed via a plasma spray approach using reconstituted nanosized Al2O3 and TiO2 powder feeds are described. Effects of various plasma spray conditions on the microstructure, grain size, phase content and microhardness of the coatings have been evaluated. It is found that phase transformation of nanosized Al2O3 and TiO2 during heat treating, sintering and thermal spraying is, in general, identical to that of micrometer-sized counterparts. Furthermore, the particle temperature during thermal spray could be divided into three regimes, i.e. low, intermediate and high temperature regimes, according to the characteristics of the coating produced from the nanopowder. The hardness and density of the coating increase with the spray temperature. The phase content and grain size of the coating also exhibits a strong dependency on the spray temperature. The coating sprayed using nanopowder feed displays a better wear resistance than the counterpart sprayed using commercial coarse-grained powder feed. The observed phenomena are discussed in terms of physics of thermal spraying, mechanisms of coating growth and phase transformation of the oxides.

Development and implementation of plasma sprayed nanostructured ceramic coatings

Abstract A broad overview of the science and technology leading to the development and implementation of the first plasma sprayed nanostructured coating is described in this paper. Nanostructured alumina and titania powders were blended and reconstituted to a sprayable size. Thermal spray process diagnostics, modeling and Taguchi design of experiments were used to define the optimum plasma spray conditions to produce nanostructured alumina–titania coatings. It was found that the microstructure and properties of these coatings could be related to a critical process spray parameter (CPSP), defined as the gun power divided by the primary gas flow rate. Optimum properties were determined at intermediate values of CPSP. These conditions produce limited melting of the powder and retained nanostructure in the coatings. A broad range of mechanical properties of the nanostructured alumina–titania coatings was evaluated and compared to the Metco 130 commercial baseline. It was found that the nanostructured alumina–titania coatings exhibited superior wear resistance, adhesion, toughness and spallation resistance. The technology for plasma spraying these nanostructured coatings was transferred to the US Navy and one of their approved coating suppliers. They confirmed the superior properties of the nanostructured alumina–titania coatings and qualified them for use in a number of shipboard and submarine applications.

Fabrication and evaluation of plasma sprayed nanostructured alumina-titania coatings with superior properties

Reconstituted nanostructured powders were plasma sprayed using various processing conditions to produce nanostructured alumina‐titania coatings. Properties of the nanostructured coatings were related to processing conditions through a critical plasma spray parameter (CPSP) that in turn, can be related to the amount of unmelted powder incorporated into the final coating. Those coatings that retain a significant amount of unmelted powder show optimum microstructure and properties. Selected physical and mechanical properties were evaluated by X-ray diffraction (XRD), optical and electron microscopy, quantitative image analysis and mechanical testing. Constituent phases and the microstructure of the reconstituted particles and plasma sprayed coatings were examined with the aid of quantitative image analysis as a function of processing conditions. Mechanical properties including hardness, indentation crack growth resistance, adhesion strength, spallation resistance during bend- and cup-tests, abrasive wear resistance and sliding wear resistance were also evaluated. These properties were compared with a commercial plasma sprayed alumina‐titania coating with similar composition. Superior properties were demonstrated for nanostructured alumina‐titania coatings plasma sprayed at optimum processing conditions.

Microstructure development of Al2O3-13wt.%TiO2 plasma sprayed coatings derived from nanocrystalline powders

The development of constituent phases and microstructure in plasma sprayed Al2O3–13wt.%TiO2 coatings and reconstituted nanocrystalline feed powder was investigated as a function of processing conditions. The microstructure of the coatings was found to consist of two distinct regions; one of the regions was completely melted and quenched as splats, and the other was incompletely melted with a particulate microstructure retained from the starting agglomerates. The melted region predominantly consisted of nanometer-sized γ-Al2O3 with dissolved Ti4+, whereas the partially melted region was primarily submicrometer-sized α-Al2O3 with small amounts of γ-Al2O3 with dissolved Ti4+. The ratio of the splat microstructure to the particulate microstructure and thus the ratio of the γ-Al2O3 to α-Al2O3 can be controlled by a plasma spray parameter, defined as the critical plasma spray parameter (CPSP). This bimodal distribution of microstructure and grain size is expected to have favorable impact on mechanical properties of nanostructured coatings, as has been observed before.

Indentation fracture behavior of plasma-sprayed nanostructured Al2O3–13wt.%TiO2 coatings

Abstract Indentation crack growth resistance of nanostructured Al2O3–13wt.%TiO2 coatings plasma sprayed using nanosized powders was investigated. Comparisons were made between the nano-coatings and a commercial baseline coating of the same composition, Metco 130. In Metco 130 coatings that contain only the single-phase splat microstructure, long cracks initiate at the indent corners and propagate along splat boundaries. In contrast, the nano-coatings are composed of a bi-modal microstructure (a fully melted splat structure and a partially melted particulate structure), and the partially melted particulate region serves to trap and deflect the splat boundary cracks. The interface between the fully melted region and the partially melted region also provides additional crack arrest mechanisms. At optimized conditions, these toughening mechanisms can produce an approximately 100% improvement in the crack growth resistance. The optimized microstructure for the nano-coatings is the microstructure containing 15–20% of the partially melted particulate region, which can be systematically controlled by changing the plasma flame temperature.

Low temperature synthesis and characterization of MgO/ZnO composite nanowire arrays

Large scale dendritic MgO/ZnO composite nanowire arrays have been successfully synthesized on Si substrates using a two-step sequential hydrothermal synthesis at low temperature for the first time. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and x-ray photoelectron spectroscopy (XPS) were systematically carried out to confirm and elaborate the potentially localized Mg surface alloying process into the ZnO nanowire arrays. Both room temperature and low temperature (40 K) photoluminescence results revealed an enhanced and blue-shifted near-band-edge (NBE) ultraviolet (UV) emission for the MgO/ZnO nanowires compared to those of the pure ZnO nanowire arrays, indicating the success of Mg alloying into ZnO nanowires. This enhancement might be due to the 155 degrees C hydrothermal process and the amorphous MgO layer in the MgO/ZnO nanowires. The specific template of densely packed ZnO nanowire arrays was suggested to be instrumental in enabling this type of MgO/ZnO composite nanowire growth.

Valence states of nanocrystalline Ceria under combined effects of hydrogen reduction and particle size

This study reveals that the Ce4+ to Ce3+ ratio in CeO2 nanoparticles (<20 nm) increases with increasing temperature under a hydrogen atmosphere. Such anomalous behavior is due to the physical effect of nanoparticle sizes on increasing the oxidation state of Ce ions in CeO2, which is stronger than the chemical effect of hydrogen reduction. This discovery offers an effective way to independently control the specific surface area and Ce3+ concentration of CeO2 nanoparticles.

Polarization Controlled Kinetics and Composition of Trivalent Chromium Coatings on Aluminum

Combined in situ spectroscopic ellipsometry and electrochemistry have been employed to monitor, in real-time, the formation of trivalent Cr conversion coatings on polished Al substrates at applied sample potentials. It is found that the formation kinetics and chemical composition of the film can be controlled by adjusting the anodic and cathodic reactions. The growth kinetics are accelerated at more positive anodic potentials or more negative cathodic potentials. At more negative potentials, the percentage of chromium in the coating is found to increase, while the zirconium percentage decreases.

Deposition of multi-layered alumina–titania coatings by detonation waves

Multi-layered alumina–titania coatings are formed from conventional and nano-structured powders by detonation technique. Alternate layers of the developed coatings contain partially melted nano-structured material. The microhardness across the coating thickness exhibits high and low values for the conventional and the nano-structured layers respectively.

Polypeptide-catalyzed Biosilicification of Dentin Surfaces

In situ formation of mineral particles by biocatalysis would be advantageous for occluding dentin tubules to reduce permeability or for sealing of material-tooth interfaces. One approach would require that the peptide-catalyst remain functional on the dentin surface. Based on recent observations of retained activity on other surfaces, we hypothesized that poly(L-lysine) (PLL), an analog of the protein catalyst responsible for silica formation in primitive marine species, would remain functional on dentin. PLL was applied to dentin discs along with a pre-hydrolyzed silica precursor, tetramethyl orthosilicate (TMOS). Discs were analyzed microscopically (scanning electron microscopy, SEM) and chemically (x-ray photoelectron spectroscopy, XPS). The treated discs, but not the negative controls, exhibited partial distinct coating whose XPS survey was consistent with that of silica, demonstrating that the polypeptide was required and retained its mediating activity. Peptide-catalysts that mediate mineral formation can retain functionality on dentin, suggesting a wide range of preventive and treatment strategies.