Adhesive-corrosion resistance is an important issue for the application life of solid lubricant coatings. With abundant metal droplets deposited on the surface of a solid self-lubricating coating, the coefficient of friction of the coating changes and results in adhesive wear. In this paper, a new method for evaluating the adhesive resistance for solid self-lubricating Ni-WSe2-BaF2·CaF2-Y-Ag-hBN coatings was reported. The microstructures and anti-adhesive characteristics under different angles of aluminum metal droplets obtained with an arc supersonic nozzle were investigated for high temperature solid self-lubricating coatings produced by plasma spray. The results demonstrate that the friction coefficient of Ni-WSe2-BaF2·CaF2-Y-Ag-hBN solid self-lubricant coatings is distributed between 0.086 and 0.299 at 25-800℃. The effect of molten metal drops on the coating adhesive-corrosion rate increases with the deposition angle. At 90°, the deposition rate of metal droplets on the coating and substrate surface is maximized, and the hexagonal boron nitride (hBN) self-lubricating coating deposition rate is only 58 mg/(cm2·s). This work demonstrates that hBN can effectively decrease the adhesive layer of the coating by the rate of the polishing and stripping.

In recent studies, the suspension plasma spray technology (SPS) was shown as a promising method for deposition of photocatalytic TiO 2 coatings on glass substrates. However, only little information about the effects of SPS process parameters on the resulting photocatalytic activity is available. In this study, several suspensions were prepared from different powders and various dispersion mediums successively sprayed. Four titania powders with a proven photocatalytic track record, Evonik P25, Kronoclean 7050, Hombicat UV100 and Sigma Aldrich were used to study the influence of starting powder type. Furthermore, compositions of the dispersing medium were varied in order to investigate their influence on coating structure, adhesion to substrate and photocatalytic activity. Degradation of methylene blue was tested as a marker for the photocatalytic activity. At chosen torch parameters, alcohol and water in a ratio of 1:3 was identified as the best composition. Suspensions sprayed with higher alcohol content suffered from the reduction in anatase phase; higher water content reduced the coating adhesion to the substrate. Regarding the initial powders, films made of Evonik P25 and Sigma Aldrich TiO 2 had higher photocatalytic activity than others. Moreover, the photocatalytic activity of doped TiO 2 coatings illuminated by both an artificial light and sunlight were investigated.

In this study, YSZ coatings were deposited on different substrate materials (stainless steel and aluminum) using suspension plasma spray (SPS) technique. The effects of substrate properties (material, surface topology, temperature, and thickness) on the formation of coatings were investigated. The results showed that, with the identical spray parameters, the porosity is higher for the coatings deposited on aluminum than that on stainless steel due to the high thermal transfer ability of the former substrate material. The SEM results revealed that the microstructure of as-prepared coatings could be tailored from the vertical cracked structure to the columnar structure by increasing the substrate surface roughness and their formation mechanisms were discussed. The substrate preheating temperature has an influence on the microstructure of the coatings, especially in the interfacial region and increasing the substrate temperature is an effective means for reducing the interface defects in the coatings. With the increase of the substrate thickness, the quantity of the vertical cracks in the coatings is reduced and their width becomes narrower.

The in-flight particle temperature and velocity are very important factors that determine the deposition characteristics and coating qualities in plasma spray. Therefore, predicting the in-flight particle temperature and velocity with spray conditions is benefit to control the prepared coating qualities. In the present study, a steady-state 3D model was used to calculate the gas flow field of a plasma jet. The plasma gas temperature and velocity profiles applied to the plasma jet as the boundary conditions were obtained with our previous studies of plasma arc behaviors inside plasma torch. After obtained the gas temperature and velocity of the plasma jet, the in-flight particle trajectories, temperature, and velocity were investigated with numerical calculation method. Also the inflight particle temperature and velocity were verified by experimental measurements.

In most plasma spraying SMEs a One-Cathode-One-Anode-Plasma-Generator (OCOAPG) is used due to its cost-effectiveness. To achieve high deposition rates, the highest possible fraction of the injected powder has to be melted and accelerated towards the substrate. Adequate to the amount and size of the particles, a sufficiently long and reproducible residence time in plasma is therefore needed. This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. In OCOAPG an arc is operated between a cathode and a central tubular anode, which causes different instabilities in the effluent plasma jet. Due to the instable interaction between the plasma jet and the carrier gas jet continuously incorporating the powder, fluctuations occur resulting in reduced coating quality. Coating systems with a higher amount of electrodes (and hence using several arcs) show higher stability and therefore can provide higher coating quality. However, due to their complexity and cost intensity, the investment hurdle for SMEs usually turns out to be too high. Recently, research steps to improve the plasma spraying process with OCOAPG have been undertaken by using a controllable current source to create a uniform particle gas jet interaction. As the movement of the anodic arc attachment point can be actively controlled by current pulses, the plasma jet can be lengthened and shortened at periodic intervals. Furthermore, by pulsing the particle delivery synchronously to the power modulation an improved particle penetration and consistent residence times can be achieved. First diagnostic results, including coatings, are presented and discussed within the paper.

In plasma spray-physical vapor deposition (PS-PVD), deposition takes place not only from liquid splats, but also from nanosized clusters as well as the vapor phase. As a result, thin, dense, and porous ceramic coatings can be produced for special applications using this method. In this study, columnar-structured YSZ coatings were deposited by PS-PVD on graphite and zirconia substrates and the effect of substrate temperature on coating microstructure was investigated. A deposition mechanism of heterogeneous nucleation is presented based on the observations and findings of the study.

The aim of this work is to fabricate nanostructured ceramic coatings using unsintered agglomerated powder and to characterize differences in microstructure, especially those at the nanoscale, due to spraying conditions. Feedstock powders were prepared from commercial YSZ nanoparticles that were reconstituted into solid spheres (70-100 μm) by spray drying. The surface morphology of sintered and unsintered agglomerates was examined by FE-SEM prior to deposition by atmospheric plasma spraying using two gun configurations, one with a lengthened barrel and one with water cooling. YSZ coating cross-sections were examined by optical and electron microscopy, revealing details at the micro and nano scale. The results show that the unsintered agglomerates, which were successfully deposited using both spray guns, are favorable for developing bimodal coating structures with fine grains and porous nano zones.

A plasma torch with a converging-diverging nozzle has been developed for low pressure plasma spraying. This study investigates the current-voltage characteristics of the plasma arc for argon and argon-hydrogen plasma gases and the effect of hydrogen volume percentage on the plasma jet. Emission spectroscopy is used to analyze the plasma spectra and electron temperatures in the center of the plasma jet are determined based on a Boltzmann distribution. The results show that an increase in input power considerably increases electron temperature and that gas composition has a significant effect on current-voltage characteristics. The impact of detection distance is also addressed.

In this study, high-emissivity NiCr oxide coatings, one with TiO 2 and one with TiB 2 compounds, were deposited on stainless steel substrates by spray granulation and atmospheric plasma spraying. The main phases in both coatings are NiTiO 3 , spinel (NiCr 2 O 4 ), and residual Cr 2 O 3 . The emissivity of the layer with TiO 2 was found to be higher in the 2.5-8 μm range than that of the layer with TiB 2 , which is attributed to higher surface roughness and larger particle size. These results indicate that the NiCr oxide coatings with TiO 2 compounds are more suitable for high-temperature IR applications.

In this study, (La 0.9 Ca 0.1 )(Cr 0.9 Mg 0.1 )O 3 ceramic powders prepared by solid-state synthesis were deposited on nickel-base superalloy substrates by atmospheric plasma spraying. Powder morphology and coating surfaces were examined by SEM, and composition and phase structure were evaluated by EDS and XRD. Coating porosity and bond strength were measured and emissivity and thermal shock tests were carried out. The results show that the powders maintained their perovskite structure during spraying and that no impurities were introduced in flight. The emissivity of the coatings was found to be 0.88 at 600 °C and 0.89 at 800 °C, which is attributed to lattice distortion stemming from differences between doping and original ions and the valence states of Mg 2+ and Cr 3+ . Coating crystal structure was stable over the thermal shock range from room temperature to 1100 °C and no spalling or fracture occurred after ten shock cycles.

In this study, zirconia coatings were fabricated by vacuum plasma spraying using hollow spherical and fused and crushed YSZ powders. Relationships between spray parameters and in-flight particle velocities and temperatures were investigated in real time and correlated with coating microstructure and density obtained under vacuum as well as atmospheric spraying conditions. The results indicate that plasma sprayed particles reach higher velocities under vacuum and slightly higher temperatures in atmospheric conditions. Powder morphology and structure play a major role in determining coating microstructure and porosity, especially in vacuum spraying. The fused and crushed powder yielded the densest coatings under the vacuum process conditions employed.

In this study, stainless steel splats were deposited on preheated stainless steel substrates with oxide scales of different thickness in inert low-pressure plasma spay (LPPS) conditions to examine the effect of in-situ oxidation of prior splats on the morphology and bonding of subsequently formed splats. Splat-substrate interface cross-sections were prepared by focus-ion-beam milling. Splat morphology and bonding state with the substrate were characterized by SEM. The results show that with oxide films up to 35 nm thick, disk-type splats are deposited that bond well to the substrate except in the periphery region. As oxide films become thicker (100 nm) and present a surface with micro-scale roughness, splats take on a finger-like shape with poor bonding at the interface.

In this study, NiCoCrAlYTa-10%Al 2 O 3 powder was deposited on heat-resistant steel substrates by supersonic atmospheric plasma spraying. Coating microstructure, adhesion strength, porosity, oxygen content, microhardness, and wear resistance were systematically evaluated. The results show that the coatings are dense and well adhered and exhibit excellent wear resistance.

The microstructure of plasma sprayed AlCoCrFeNiTi-NiCrBSi composite coatings was analyzed and microhardness and high-temperature wear resistance were measured. The results show that dense, hard coatings with good adhesion strength were achieved. In wear tests, the volume loss of the as-sprayed coating at room temperature was 21% of that of 316 stainless steel and only 5% at 900 °C. Abrasion wear was the main failure mechanism over the temperature range, with adhesion wear appearing at around 900°C. Peeling was also observed on some worn surfaces.

This study investigates the feasibility of spraying aluminum nitride-alumina-yttria mixtures in a nitrogen plasma ambient. Coatings consisting of h-AlN, c-AlN, Al 5 O 6 N, and γ-Al 2 O 3 with small amounts of α-Al 2 O 3 and aluminum-yttrium oxide phases were produced. Although using the Y 2 O 3 additives significantly affected the process and microstructure, it did not achieve the high thermal conductivity desired in as-sprayed coatings. However, a high thermal conductivity (>90 W/m·K) AlN coating was fabricated by increasing the AlN content and enhancing sintering during heat treatment.

This study investigates the influence of particle temperature and velocity during reactive plasma spraying and the effect of plasma gases on coating properties. Using hydrogen gas with low flow rate was found to be better for reactive plasma spraying of fine Al 2 O 3 -AlN mixtures. The H 2 gas increased in-flight particle temperature, affecting in-flight vaporization, AlN content, phase transformation, deposition efficiency, and coating thickness. N 2 gas, on the other hand, increased particle velocity, thereby reducing particle residence time in the plasma, which affects melting, nitride conversion, and phase transformation.

In this work, YSZ coatings were deposited on air-cooled substrates by atmospheric plasma spraying. High-resolution transmission electron microscopy (HR-TEM) was used to examine local microstructure near the interface of bonded splats. The coatings mainly consist of typical columnar grain microstructure with metastable tetragonal phase. At the bonded zones, the top surfaces of previously deposited splats act as heterogeneous nucleation sites for the next splat. The examinations also revealed large columnar grains perpendicular to the bonded interface along with high defect density.

Agglomerated and sintered Cr 3 C 2 -25%NiCr powders possess excellent flow ability and appearance that have been extensively applied to resist abrase and erosion in high temperature applications such as power boiler and turbine blade. Microstructure of Cr 3 C 2 -25%NiCr coatings were observed through scanning electronic microscope (SEM), and bond strength and microhardness of coatings were measured by tensile shearing test and Vickers hardness test. It is indicated that ultrafine Cr 3 C 2 -25%NiCr coatings have some outstanding properties to traditional Cr 3 C 2 - 25%NiCr coatings by plasma sprayed.

The adhesion of plasma-sprayed coating is to a large extent controlled by the cleanness and roughness of the surface on which the coating is deposited. So, most of the plasma spray procedures involve surface pretreatment by grit-blasting to adapt the roughness of the surface to the size of the impacting particles. This preparation process brings about compressive stresses that make it inappropriate for thin substrates. The present works aims to elaborate a ceramic coating on a thin metal substrate with a smooth surface. The coating system is intended for use in a generation–IV nuclear energy system. It must exhibit a good adhesion between the ceramic topcoat (about 0.5-mm thick) and the smooth metal substrate (1-mm thick) to meet the specifications of the application. Our approach has consisted in depositing the ceramic layer on a few micrometers thick ceramic layer made by suspension spraying. We have observed the interface between both ceramic layers by transmission electronic microscope and studied the adhesion of the nanostructured layer by the Vickers Indentation Cracking technique and that of the coating system by tensile test.

FeAl intermetallics matrix composites reinforced by ceramics particles such as titanium carbide have attracted much attention in recent years. In this study, shrouded plasma spraying with nitrogen as a protective gas was employed to deposit TiC particles dispersed FeAl/TiC composite coatings. Fe-35Al powder and Fe-35Al/TiC composite powders containing 35 vol.% and 45 vol.% TiC prepared by mechanical alloying were used as feedstock powders. The microstructure of the ball-milled powders and the as-sprayed coatings was characterized by scanning electron microscopy and X-ray diffraction. The mean coefficient of thermal expansion (CTE) of FeAl and FeAl/TiC was measured. The results showed that dense FeAl and FeAl/TiC coatings with low oxide inclusions were deposited by shrouded plasma spraying. The mean CTEs calculated based on the Reuss formula are reasonably consistent with those measured in the present study. As a result, the CTE of FeAl-based composite coating can be properly controlled by adjusting TiC content in the composite coating to match with that of the substrate.