Thin Yttria-Stabilized Zirconia Coatings Deposited by Low-Energy Plasma Spraying Under Very Low Pressure Condition

In recent decades, very low pressure plasma spraying (VLPPS) technology (less than 10 mbar), as a next-generation coating process, has been extensively studied, because it can fully evaporate the materials to deposit dense, thin, and columnar grain coatings. This research aims at applying VLPPS with low-energy plasma source to melt or evaporate ceramic materials to develop high-quality thermal barrier coatings. Thin and homogeneous yttria-stabilized zirconia coatings were deposited successfully on a stainless steel substrate using low-power plasma spraying torch F100 (23 kW maximal) under very low pressure (1 mbar). The optical emission spectroscopy was used to analyze the properties of the plasma jet. The phase composition and the microstructure of the coatings were characterized by x-ray diffraction and scanning electron microscopy. The results showed that the YSZ powder was fully melted and partially evaporated, and the coatings had a hybrid microstructure that was combined with the condensation of the YSZ vapor and the melted particles. In addition, the porosity and microhardness of the coatings were evaluated.     

In Situ Synthesis of FeAl Dense Coatings by Very Low Pressure Reactive Plasma Spraying

With the purpose of elaborating high-quality FeAl coatings, a so-called very low pressure reactive plasma spray technique that combines very low pressure plasma spray and self-propagation high-temperature synthesis processes was used in the present study. A dense and homogeneous FeAl coating was thus successfully in situ synthesized by reactive plasma spraying of an Al/Fe₂O₃ composite powder under 1 mbar. The phase composition and microstructural features of the coating were characterized by XRD and SEM. Results indicated that the B2 ordered FeAl phase was synthesized, and the coating featured a dense and defect-free microstructure. The fracture mechanism of the coating remains mainly a brittle failure but the appearance of some dimples in local zones suggested some unexpected toughness.     

Quantification of particle morphologies in view of quality control of the thermal spray process

The relationships between processing parameters, deposit microstructures, and deposit properties are not well understood, mainly due, on the one hand, to the numerous parameters governing the thermal spray process and, on the other, to the random mode of deposition of the material (for example, individual impacts of molten particles). The interactions are mainly related to the impact, spreading, and solidification of the material droplets onto the substrate, or previously deposited layers. A methodology was implemented to quantify individual splat morphologies using size and shape factors referring to an equivalent diameter, an elongation factor, and a degree of splashing, and also three-dimensional profilometry and stereology. This approach is mainly based on the use of an image analysis system and statistical tools. A few examples of the potential of this method are given. Such a lowcost and low-time-consuming protocol could be easily implemented for an optimization purpose.     

Determination of residual stress distribution from in situ curvature measurements for thermally sprayed WC/Co coatings

An in situ monitoring of curvature of the specimen during spraying using a high speed video system was implemented to determine stresses in thermally sprayed WC/Co coatings. Influences of different spray-ing techniques (atmospheric plasma spraying and high-velocity oxygen fuel) and cooling levels were con-sidered using a mathematical model. Results show that temperature history of a part is of paramount importance in stress generation and distribution.     

Microstructure and Magnetic Properties of Fe-Ni Alloy Fabricated by Selective Laser Melting Fe/Ni Mixed Powders

Fe—Ni alloy,as a widely applied ferromagnetic material,is synthesized using selective laser melting(SLM).The chemical compositions and microstructure of the SLM Fe—Ni alloy are characterized by X-ray diffraction(XRD), energy dispersive X-ray spectroscopy and scanning electron microscopy.It was found that the samples exhibited fine grains with homogenous distribution when a low laser scanning velocity was used.Moreover,the magnetic properties of the samples with different laser parameters are also measured.It shows that the SLM Fe—30%Ni alloy possesses a low coercivity and high saturation magnetization.It also can be obtained that SLM is an alternative faster method to prepare soft magnetic material with complex shapes.Moreover,the magnetic properties can be influenced by the laser parameters.     

Knudsen Effect on the Estimation of the Effective Thermal Conductivity of Thermal Barrier Coatings

A numerical model based on the use of cross-sectional micrographies and a 3D image of thermal barrier coatings for the estimation of the material effective thermal conductivity is presented. The case of a YSZ thermal spray coating consisting of a 2 phase network, namely, the coating material and pores, is considered. The variation of the thermal conductivity of pores caused by their small size was considered by taking the Knudsen effect into account. The quantification of this effect on the effective thermal conductivity of the coating was achieved with the help of image analysis combined with an in-house program coded in C language. Finite-difference (FD) and finite-element (FE) models were applied using both 2D images and a 3D image. Despite the differences in the computed values obtained with these two numerical methods, the decrease of the computed thermal conductivity caused by the Knudsen effect was found to remain quite moderate for both methods (i.e., about 3-5% for the 3D results).     

Effect of Gun Current on Electrical Properties of Atmospheric Plasma-Sprayed Lanthanum Silicate Coatings

Apatite-type lanthanum silicate (ATLS) electrolyte coatings for use in intermediate-temperature solid oxide fuel cells were deposited by atmospheric plasma spraying (APS). Plasma-sprayed coatings with typical composition La₁₀(SiO₄)₆O₃ exhibiting good densification and high oxide ionic conductivity were obtained by properly adjusting the spraying parameters, particularly the gun current. The highest obtained ionic conductivity value of 3.3 mS/cm at 1,173 K in air is comparable to other ATLS conductors. This work demonstrated empirically that utilization of the APS technique is feasible to synthesize dense La₁₀(SiO₄)₆O₃ electrolyte coatings using gun currents within an unusually broad range.     

Deposition behavior of thermally softened copper particles in cold spraying

Finite-element analysis (FEA) combined with experimental observation was conducted on preheated Cu particles deposited on Cu substrate to clarify the deposition behavior of thermally softened particles in cold spraying. An explicit FEA code, ABAQUS, was used to predict the deformation features of the thermally softened particles. The experiment was performed by a home-made cold-spray system with a powder preheating device. Considering the possible serious oxidation of the cold-sprayed particles under high-temperature conditions, the preheating temperature was limited to 300 °C for each test. Based on the numerical and experimental results, a new concept called the thermal softening zone within which thermal softening occurs is proposed in the present work. It is found that thermally softened particles deform more intensively compared to non-preheated particles, and a more prominent metal jet can be achieved at the rim of the deformed particles with higher initial temperature. Moreover, the results also reveal that increasing the particle preheating temperature can stimulate the occurrence of thermal softening. For non-preheating or low-temperature preheating particles, thermal softening mainly occurs at the interfacial region. If the preheating temperature is sufficiently high, the whole particle can experience thermal softening. In addition, it is also found that preheated particles are more likely to deposit on the substrate surface than non-preheated particles. In addition, particle preheating is also found to facilitate the coating formation process, enabling the coating to be very thick. The coating microhardness decreases with increasing particle preheating temperature due to the elimination of work hardening by thermal softening.     

Microstructure and mechanical properties of flame-sprayed PEEK coating remelted by laser process

Poly-ether-ether-ketone (PEEK) is one of the high-performance thermoplastics. It is being increasingly used for many industrial applications due to its excellent properties. In this paper, a flame spraying technique is used to deposit PEEK coating on 304L stainless substrates. CO₂ and Nd:YAG laser treatments are chosen to remelt the as-sprayed polymer coating to get a dense coating. The microstructures of the as-sprayed and remelted coatings are characterized by SEM and XRD. The results show that both CO₂ and Nd:YAG lasers are suitable for densifying the PEEK coating on stainless substrate. However, the remelted coatings present different crystalline structure due to their laser processing parameters. Hardness measurements, tribological and scratch tests are conducted to characterize the mechanical properties of remelted coating. The coatings’ mechanical properties are correlated with their structures.