Leon L. Shaw

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.

Advances and challenges of sodium ion batteries as post lithium ion batteries

Energy and climate concerns have made the need for research towards electrical energy storage. In this context, sodium ion batteries (SIBs) have attracted significant attention lately. Sodium is an abundant resource that is low cost and safe which makes it an attractive alternative to lithium. Its chemical properties are similar to that of Li which makes the transition into using Na chemistry for ion battery systems feasible. This review focuses on the latest progress in both cathode and anode materials for SIBs. It also details research in binders and additives and their effects on the SIB system. It further highlights the optimization of organic electrolytes and ionic liquid based electrolytes for utilization in SIBs. The mechanisms of sodium ion storage, transport, and solid electrolyte interphase formation are also discussed to better understand the behavior of ions and battery materials during de/intercalation. Finally, personal perspectives on outlook and major challenges ahead for SIBs are offered. These comprehensive and in-depth discussions along with proposed directions can enlighten ideas and offer avenues in the rational design of durable and high performance SIBs in the near future.

Thermal stability of nanostructured Al93Fe3Cr2Ti2 alloys prepared via mechanical alloying

Abstract Thermal stability of nanostructured Al 93 Fe 3 Ti 2 Cr 2 alloys prepared via mechanical alloying (MA) starting from elemental powders was investigated using a variety of analytical techniques including modulated differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, transmission electron microscopy coupled with energy dispersive spectrometry and microdiffraction. The results showed that the MA-processed Al 93 Fe 3 Ti 2 Cr 2 alloy in the as-milled condition was composed of an Al-based supersaturated solid solution with high internal strains. Release of internal strains, intermetallic precipitation and grain growth occurred upon heating of the MA-processed Al alloy. Nevertheless, grain growth in the MA-processed Al alloy was very limited and fcc-Al grains with sizes in the range of 20 nm were still present in the alloys after exposure to 450 °C (0.77 T m ).

Synthesis of nanocrystalline SiC at ambient temperature through high energy reaction milling

Abstract This study investigated the in-situ synthesis of nanosized crystalline SiC powders at room temperature through high energy ball milling of elemental silicon and carbon mixtures. Milling conditions including the mill design, the milling speed, the milling time and the ball-to-powder weight ratio (i.e. the charge ratio) necessary for the in-situ synthesis were studied. It was found that uniform formation of nanosized crystalline SiC powders within the powder charge could be achieved with a correctly designed attritor and the contamination could be minimized with proper selections of milling conditions. The crystalline β-SiC powders synthesized were themselves in nanosize scale, quite different from many previous studies which have shown that it is the internal grain structure of milled powders that is the “nanocrystalline” component of the powders (typically 5–20 nm), while the powders are themselves typically 0.1 μm to > 1 μm in size. Furthermore, it was found that the product structures generated by high energy reaction milling depended strongly on the milling speed, the charge ratio and the milling time.

Nanocrystalline hydroxyapatite with simultaneous enhancements in hardness and toughness

Using a series of dense hydroxyapatite (HA) bodies with well controlled grain sizes ranging from sub-micrometers to nanometers, we show that simultaneous improvements in hardness and toughness can be attained for nanocrystalline (nc) HA. It is demonstrated that the hardness of HA follows the Hall-Petch relationship as the grain size decreases from sub-micrometers to nanometers. In the same grain size range, the toughness of HA increases by as much as 74% because of the enhanced crack deflection associated with a transition from transgranular to intergranular cracking, promoted by the reduced grain size in the nanoscale. The mechanisms for simultaneous enhancements in the hardness and toughness of nc HA are discussed. It is anticipated that the principle of simultaneous improvements in hardness and toughness discovered in this study is also applicable to other nc ceramics, particularly non-cubic ceramics, with anisotropic elastic and thermal expansion properties.

Polymorphic transformation and powder characteristics of TiO2 during high energy milling

Many studies have indicated that the reactivity of reactants can be enhanced greatly by mechanical activation through high energy ball milling. To understand this enhanced reactivity, the polymorphic transformation and the evolution of the powder characteristics of TiO2 and graphite mixtures during high energy ball milling was investigated using various analytical instruments. It was found that polymorphic transformation of anatase to srilankite and rutile took place during milling. Furthermore, amorphization of crystalline phases and crystallization of the amorphous phase occurred at the same time during milling. High energy milling also led to ultrafine crystallites, large specific surface areas, and substantial amounts of defects in the powder particles. Effects of the graphite addition and the milling temperature on the polymorphic transformation and the evolution of the powder characteristics were also investigated. It was proposed that the polymorphic transformation of TiO2 during milling could be explained in terms of the temperature-pressure phase diagram if the temperature rise and high pressure at the collision site were taken into consideration.