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L. Xie
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Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 249-252, May 10–12, 2016,
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This study assesses the microstructure and mechanical properties of tungsten boride (WB) powder and cemented carbide coatings with WB additions. HVOF-sprayed layers produced from 60WC-30WB-10Co composite powders are compared with conventional 88WC-22Co and 86WC-10Co-4Cr coatings based on phase composition, hardness, wear resistance, and wear surface structure. The results indicate that Co reacts with WB during spraying, forming ternary phases (WCoB, W 2 CoB 2 ) that increase hardness as well as sliding wear resistance.
Proceedings Papers
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 405-410, May 14–16, 2007,
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Thermal barrier coatings were produced using both Ar and N 2 as the primary plasma gas. Various aspects of the process and the coatings were investigated. It was found that higher in-flight particle temperatures could be produced using N 2 , but particle velocities were lower. Deposition efficiencies could be increased by a factor of two by using N 2 as compared to Ar. Coatings having similar values of porosity, hardness, Young’s modulus and thermal diffusivity could be produced using the two primary gases. The coatings exhibited similar changes (increased hardness, stiffness and thermal diffusivity) when heat-treated at 1400°C. The results point to the potential advantage, in terms of reduced powder consumption and increased production rate, of using N 2 as compared to Ar as the primary plasma gas for TBC deposition.
Proceedings Papers
Effect of Heat Treatment on Pore Architecture and Associated Property Changes in Plasma Sprayed TBCs
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 411-416, May 14–16, 2007,
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Plasma sprayed Thermal Barrier Coatings (TBCs) exhibit many interlamellar pores, voids and microcracks. These microstructural features are primarily responsible for the low global stiffnesses and the low thermal conductivities commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400°C, for times of up to 10 hours) on the pore architecture in detached YSZ top coats has been characterised by Mercury Intrusion Porosimetry (MIP) and gas-sorption techniques. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (inter-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high treatment temperatures (~1400°C). These changes are thought to be associated with intra-splat microcrack healing, improved inter-splat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Measured thermal conductivity data are compared with predictions from a recently-developed analytical model, using the deduced inter-splat contact area results as input parameters. Good agreement is obtained, suggesting that the model captures the main geometrical effects and the pore size distribution measurements reflect the most significant microstructural changes.
Proceedings Papers
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 423-427, May 14–16, 2007,
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It was found that the content of impurity oxides in 7YSZ, such as SiO 2 and Al 2 O 3 , has a significant effect on the coating sintering resistance and phase stability of 7YSZ thermal barrier coatings (TBCs). The reduction of the impurity content will significantly improve the sintering resistance and phase stability of 7YSZ TBCs and thus allow the 7YSZ TBCs to be used at higher temperatures.
Proceedings Papers
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 495-500, May 14–16, 2007,
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Abradable seals have been used in jet engines since the late 1960's. Today abradable seals are seeing applications not only in low pressure and high pressure sections of the compressor but also in the high pressure turbine module of jet engines and are gaining ever more attention in industrial gas turbines. Thermal spraying is a relative simple and cost effective means to apply abradable seals. These work by minimizing gaps between rotating and stationary components by allowing the rotating parts to cut into the stationary ones. The types of coatings employed are zirconia based abradable material systems with polymer and, in some cases, hexagonal boron nitride additions. The coatings are designed to work at service temperatures of up to 1200 °C. The objective of this paper is to review various types of commercial and experimental ceramic abradable systems and to assess their performance profiles. The paper will review yttria stabilized zirconia based systems with modified polymer additions and with variable particle sizes of the ceramic phase. Alternative stabilizers and their influence on key coating properties such as thermal shock resistance and abradability will be studied. The paper will also review the influence of plasma spray parameters on coating properties and study the general influence of coating porosity on coating erosion properties and abradability.
Proceedings Papers
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 501-506, May 14–16, 2007,
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The monoclinic and tetragonal phase compositions and distribution in air plasma sprayed (APS) yttria-partially stabilized zirconia (YPSZ) thermal barrier coatings were studied. The coatings were produced from powders with varying phase concentrations, chemical purity and powder production processes. Both the powder and coatings were characterized using X-ray diffraction (XRD) and Raman spectroscopy. The use of environmental scanning electron microscopy (ESEM) and X-ray energy dispersive analysis (EDS) added morphological and elemental information to the study. XRD and Raman spectroscopy were shown to be powerful combined tools and shows an overall decrease in the monoclinic phase within the coatings produced from the different powders. The distribution of both the monoclinic and the tetragonal phases could be highlighted both in the coatings and the individual powder particles. This indicated changes in monoclinic concentration in the less dense areas of some of the coatings and a varying distribution across particles in some of the powders. Raman mapping over small areas also showed how phase surface distribution, on the coatings surfaces, could be assessed.
Proceedings Papers
ITSC 2006, Thermal Spray 2006: Proceedings from the International Thermal Spray Conference, 477-482, May 15–18, 2006,
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Factors affecting the appearance of 7~8 wt.% yttria stabilized zirconia thermal barrier coatings were investigated. The coating appearance can be separated into two aspects, darkening and coloring. Coating darkening is related to the retaining of off-stoichiometric zirconium oxide (ZrO 2 -x) or yttrium oxide (Y 2 O 3 -x). These off-stoichiometric oxides are stable at temperatures between 2300 °C and 2700 °C. Rapid cooling of these oxides leads to the retaining of them in the as-sprayed coatings. A heat treatment in air at temperatures higher than 500 °C transforms these off-stoichiometric oxides to stoichiometric oxides and therefore eliminates the coating darkening effect. Coloring of TBCs is attributed to the existence of impurity oxides. Coatings made from the ultra high purity powder, Z204NS, do not exhibit coloring phenomenon before or after heat treatment. As-sprayed coatings made from premium purity (M204NS) or regular purity (A204NS-1) powders exhibit metastable coloring effect. A 1-hour heat treatment at 500 °C can eliminate the metastable colors and turn the coating color back to the color of the starting powder.