Xinqing Ma

Deposition of thermal barrier coatings using the solution precursor plasma spray process

The solution-precursor plasma-spray (SPPS) process is capable of producing highly durable thermal barrier coatings. In an effort to improve the understanding of the deposition mechanisms in this novel process, a series of specific experiments, where the substrate is held stationary and the plasma torch is programmed to scan a single pass across the substrate, were conducted and the resulting deposits were carefully characterized. In addition to the deposition mechanisms identified previously in the stationary torch experiments, the deposition mechanisms of two other types of deposits, thin film and fine spherical particles, were identified in this study. The melting of inflight formed 7YSZ particles and their rapid solidification to form ultra-fine splats on the substrate was found to be the dominant deposition mechanism. The characterization of actual SPPS coatings confirmed that the various coating-deposition mechanisms identified in the model experiments occur in concert during the actual coating process. Adherent deposits (ultra-fine splats, deposits from gel-like precursor and film formed via chemical vapor deposition), unmelted particles (spherical particles, deposits from non-decomposed precursor) and porosity were estimated to constitute ∼65, ∼19 and ∼16 vol%, of the coating, respectively.

A new high-velocity oxygen fuel process for making finely structured and highly bonded inconel alloy layers from liquid feedstock

High-velocity oxygen fuel (HVOF) thermal spray processes are used in applications requiring the highest density and adhesion strength, which are not achievable in most other thermal spray processes. Similar to other thermal spray processes, however, a normal HVOF process is unable to apply fine powders less than 10 µm via a powder feeder. The advantages of using smaller and even nanosized particles in a HVOF process include uniform microstructure, higher cohesion and adhesion, full density, lower internal stress, and higher deposition efficiency. In this work, a new process has been developed for HVOF forming of fine-grained Inconel 625 alloy layers using a liquid feedstock containing small alloy particles. Process investigations have shown the benefits of making single and duplex layered coatings with full density and high bond strength, which are attributed to the very high kinetic energy of particles striking on the substrates and the better melting of the small particles.

Intermediate temperature solid oxide fuel cell based on fully integrated plasma-sprayed components

This work addresses the fabrication of membrane-type solid oxide fuel cells (SOFCs) operating at medium temperatures, where all components are fabricated by plasma spray technology, and the evaluation of the performance of the SOFC single unit in a temperature range of 500 to 800 °C. Single cells composed of LaSrMgO3 cathodes, LaSrGaMgO3 (LSGM) electrolytes, and Ni/yttria-stabilized zirconia anodes were fabricated in successive atmospheric plasma-spraying processes. Plasma-spraying processes have been optimized and tailored to each layer to achieve highly porous cathode and anode layers as well as high-density electrolyte layers. A major effort has been devoted to the production of the LSGM electrolyte that has a high density and is free of cracks. Electrochemical impedance spectroscopy was used to investigate the conductivity of the electrode layers, and particularly the resistance of the electrolyte layer. It revealed that the heat treatment had a great influence on the specific conductivity of the sprayed electrolyte layers and that the specific conductivity of the heat-treated layers was dramatically increased to the same magnitude as is typical for sintered LSGM pellets. The experimental results have demonstrated that the plasma-spraying process has a great potential for the integrated fabrication of medium-temperature SOFC units.

Structural and magnetic properties of nanostructured Ni0.5Zn0.5Fe2O4 films fabricated by thermal spray

Nanostructured Ni0.5Zn0.5Fe2O4 thick films were fabricated by a high velocity oxygen fuel (HVOF) thermal spray approach with over 98% of the theoretical density and with no cracks, followed by heat treatment under an oxygen atmosphere. Crystallographic, microstructural, as well as static and dynamic magnetic properties of the films were studied by x-ray diffraction, high-resolution transmission electron microscopy, superconducting quantum interference device magnetometer, and high-frequency impedance analyzer. By controlling the nature of the flame, the crystal structure of the ferrite can be retained during thermal spraying while the grain size as small as 10–20 nm can be attained. By controlling the spray conditions and postannealing, the real part of initial complex permeability μ′ reaches 120 while the image part μ″ remains small in the frequency range up to 13 MHz. In comparison with a conventional Ni0.5Zn0.5Fe2O4 (μ′=800, cutoff frequency fc=1.5 MHz), a nanostructured sample possesses a much higher ...

The Solution Precursor Plasma Spray Process for Making Durable Thermal Barrier Coatings

The Solution Precursor Plasma Spray (SPPS) process involves the injection of atomized droplets of precursor into the plasma plume, instead of powder that is used in conventional plasma spray. The resultant thermal barrier coating (TBC) microstructure consists of (1) through-coating-thickness cracks, (2) ultra-fine splats, and (3) nanometer and micrometer-sized dispersed pores. These unique SPPS microstructural features provide highly durable TBCs. The SPPS TBCs in 1121°C /1 hour cyclic furnace tests exhibit a significantly improved spallation life compared to APS, DVC, and EB-PVD/Pt-Al TBCs. Extensive process diagnostic and modeling studies have been conducted to provide a foundation for understanding and control of the process. Process/microstructure/property relationships have been defined. Extension of the process for making thick coatings (> 3mm) and low thermal conductivity coatings are described.© 2005 ASME

Structure and magnetic properties of NiFe/SiO2 and Co/SiO2 nanocomposites consolidated by detonation compaction

In this article, the structural and magnetic properties of NiFe/SiO2 and Co/SiO2 nanocomposites fabricated via powder processing are presented. The NiFe/SiO2 and Co/SiO2 nanoparticles were both synthesized by a wet chemistry approach. The as-synthesized nanoparticles were characterized by scanning electron microscopy, x-ray diffraction, and superconducting quantum interference device magnetometer, yielding detailed information concerning the structure and size of NiFe or Co particles, coating, and composition purification. The nanoparticles were then consolidated into solid components via detonation compaction. Depending on the powder morphology and detonation conditions, the density of the consolidated sample can reach over 91% of the theoretical density of the conventional materials. X-ray diffraction experiments on the samples both before and after consolidation indicate that the crystal structure of the nanocomposites remains unchanged during detonation consolidations; however, a rather large increase...

Detonation Consolidation of NiFe/SiO2 and Co/SiO2 nanocomposites

Consolidation of nanostructure magnetic particles is required not only for manufacturing bulk component, it is actually a fundamental requirement for obtaining novel magnetic properties from the material. Consolidation (assembly) of nanoparticles to full density without deteriorating their nanostructure (size and morphology) is a big challenge. Here we present the consolidation experiments of NiFe/SiO 2 and Co/SiO 2 nanocomposites via detonation consolidation. This approach is based on the explosive pressure created when an acetylene and oxygen mixture gas fires in a sample containing tube, the very high hypersonic propulsion force makes nanoparticles deposit onto the target. Depending on the powder morphology and operation conditions, the density of the consolidated sample can reach over 91% of the theoretical density of the bulk materials. X-ray diffraction experiments on the samples before and after consolidation indicate that the denotation consolidations can be optimized such that it does not cause any phase transition. However, a particle size increase was observed. Static magnetic studies carried out on the samples before and after detonation operation shows that the saturation magnetization does not. This indicates that the operation does not cause an oxidation of the nanopowders. These experiments show that detonation approach is a good candidate for consolidating magnetic nanoparticles.