Microstructure Observation and Nanoindentation Size Effect Characterization for Micron-/Nano-Grain TBCs

Microstructure observation and mechanical properties characterization for micron-/nano-grain thermal barrier coatings were investigated in this article. Scanning electron microscope images demonstrated that both micron-grain coating and nano-grain coating had micrometer-sized columnar grain structures; while the nano-grain coating had the initial nanostructures of the agglomerated powders reserved by the unmelted particles. The mechanical properties (hardness and modulus) of micron-/nano-grain coatings were characterized by using nanoindentation tests. The measurements indicated that the nano-grain coating possessed larger hardness and modulus than the micron-grain coating; which was related to the microstructure of coatings. Nanoindentation tests showed that the measured hardness increased strongly with the indent depth decreasing; which was frequently referred to as the size effect. The nanoindentation size effect of hardness for micron-/nano-grain coatings was effectively described by using the trans-scale mechanics theory. The modeling predictions were consistent with experimental measurements; keeping a reasonable selection of the material parameters.

Effect of Particle In-Flight Behavior on the Microstructure and Fracture Toughness of YSZ TBCs Prepared by Plasma Spraying

The present study aims to elaborate particle in-flight behavior during plasma spraying and its significance in determining the microstructure and mechanical properties of plasma sprayed yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The as-sprayed YSZ coatings were characterized in terms of defects (such as pores, unmelted particles and cracks) and fracture toughness. The results showed that, due to the higher temperature and velocity of in-flight particles in a supersonic atmospheric plasma spraying (SAPS) compared to that of atmospheric plasma spraying (APS), denser coatings were formed leading to a better fracture toughness. The percentage of defects of the microstructure was similar to the temperature and velocity of particles in-flight during plasma spraying. Furthermore, the structural defects had a strong effect on its mechanical behavior. The total defect percentage and fracture toughness in SAPS-TBCs spanned 6.9 ± 0.17%–13.26 ± 0.22% and 2.52 ± 0.06 MPa m1/2–1.78 ± 0.19 MPa m1/2; and 11.11 ± 0.36%–17.15 ± 0.67% and 2.13 ± 0.08 MPa m1/2–1.4 ± 0.12 MPa m1/2 in APS-TBCs.

MICROSTRUCTURES, MICROHARDNESS, AND CRYSTALLIZATION BEHAVIORS OF AMORPHOUS Al2O3-YSZ COATINGS PREPARED BY AIR PLASMA SPRAYING

Three types of amorphous Al 2 O 3-yttria-stabilized zirconia (YSZ) coatings were prepared using air plasma spraying (APS). The effects of the APS parameters on the microstructure, porosity, amorphous-phase content, and microhardness of the coatings were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results indicated that the microstructure of the coatings was dependent on the APS parameters used. The porosity of the coatings and the number of unmelted particles deposited increased with the flow rate of the primary plasma gas increasing. The amorphous-phase contents and microhardness values of the coatings exhibited a trend opposite to that of their porosity and the number of unmelted particles present in them. The crystallization behaviors and microhardness values of Al 2 O 3-YSZ coatings annealed at different temperatures were also investigated. The results indicated that the coatings with higher amorphous-phase contents possessed homogeneous nanocrystalline structure and exhibited higher microhardness values after the heat treatments.

Effect of Laser Fusing on Hot Corrosion Resistance of Nickel Based Self Fluxing Alloy Coating

The thermal sprayed coatings are widely used in waste incineration boilers and fossil fuel-fired boilers. However, the defects, such as porosity, cracks and unmelted particles, in these coatings are detrimental to corrosion performance. In this study, the nickel based self fluxing alloy coating was fused by YAG laser to improve hot corrosion resistance of the coating. Under appropriate laser parameters, the nonporous, crack-free coating was produced. Hot corrosion test, conducted in the presence of a mixed salt of Na2SO4/NaCl/KCl at 550Гshowed that the modified coating exhibited excellent corrosion resistance compared with the as sprayed and gas fused coatings.

Photo-Decomposition Property of Thermal Sprayed TiO2 Coatings

TiO2 is a promising photocatalyst due to its excellent photocatalytic activity, physical and chemical stabilities. Plasma spraying technology is good way to prepare for TiO2 coating. The effect of spraying parameters on photocatalytic performance was studied. The results showed that both the amount of unmelted particles and the porosity of as-sprayed TiO2 coatings increased with the increasing of Ar gas flow rate. And the external bias could improve TiO2 coating’s photocatalytical performance. The improvement degree is closed related its grain size and porosity.

Microstructure and Properties of Laser Fused Nickel Based Self Fluxing Alloy Coating

The thermal sprayed coatings are widely used in waste incineration boilers and fossil fuel-fired boilers. However, the defects, such as porosity, cracks and unmelted particles, in these coatings are detrimental to corrosion performance. In this study, the nickel based self fluxing alloy coating was fused by YAG laser to improve performance of the coating. Under appropriate laser parameters, the nonporous, crack-free coating was produced. The rubber wheel type abrasion wear test and hot corrosion test conducted in the presence of a mixed salt of Na2SO4/NaCl/KCl at 650°C showed that the modified coating exhibited excellent wear and corrosion resistances compared with the as sprayed and gas fused coatings.

Porosity-Thermal Conductivity Relationships in Plasma Sprayed Zirconia Coatings

Thermal barrier coatings (TBCs) are used on heat engine parts to impart protection to components against failure under excessive heat loads, to increase inlet temperatures with consequent improvements in efficiency, and to reduce requirements for cooling. Control of thermal conductivity is addressed since low thermal conductivity depends not only on the nature of the yttria stabilized zirconia (YSZ) layer, but also on the morphology of pores and cracks, which are closely linked to process parameters. This paper will present the influence of feedstock characteristics (particle size distribution and powder morphology) and thermal cycling on porosity content and thermal conductivity of zirconia coatings. The results show increased porosity with particle size, due to an increase in the degree of particle fragmentation and unmelted particles, leading to lower thermal conductivity. Coatings sprayed with powders of different powder morphology yielded changes in porosity and interlamellar contact, thus, influenced thermal conductivity. Sintering effects during thermal cycling resulted in reduced porosity and increased thermal conductivity.

Corrosion Resistance of Thermal Spray Inconel 690 Coatings

Inconel thermal spray coatings are potential candidates for wide industrial applications. However, the coating structure resulting from thermal spray processes characterized by unmelted particles, oxides and porosity can severely hinder their corrosion resistance in agressive media. The aim of the present work was to optimize elaboration parameters in order to enhance their chemical stability. Electrochemical criteria were used to identify the ability of those coatings to form a passive film in a sulfuric acid solution and also to determine their local corrosion resistance in a chloride solution. Results show that the passivity state is strongly conditionned by the coating microstructure. Coatings free of unmelted particles exhibit a notably reduced passive current density while the localized corrosion resistance seems conditionned by the amount of interlamellar oxide.

Numerical Modeling and Simulation of Particle Behaviors in an HVOF Supersonic Flow

Dynamic and thermal behaviors of particles injected into an HVOF supersonic flow were modeled using the Lagrangian formulation coupled with a moving grid system which allows to treat melting and solidification problems in a particle. The particle behaviors of alumina, Tribaloy™-400 and MCrAlY were examined by numerical simulations. Velocities calculated by this model were compared with experimental measurements available in publications. For investigating the particle temperatures for which experimental data are hardly available, qualitative experiments were conducted with an MCrAlY powder. It was thus observed that unmelted particles predicted by the numerical simulation were also found in the microstructure of the sprayed MCrAlY coating. From this work, it was shown that the established model provides a reasonable prediction of dynamic and thermal particle behaviors in HVOF flows.