In this study, we deposited alumina (Al 2 O 3 ) coatings from powder consisting of dense particles using aerosol deposition. The powders were ball milled with zirconia (ZrO 2 ) milling media for 0 to 9 hours to optimize the deposition performance. We investigated the impact of high-energy ball milling on the shape, size, and crystal structure of the Al 2 O 3 powders, as well as their deposition behaviors.

Material’s tensile strength can be improved by the presence of a body-centered cubic (BCC) phase, which is essential in highstrength applications and highly corrosive environments. Thus, synthesizing a BCC single-phase, equiatomic AlCoCrFeNi high-entropy alloy (HEA) feedstock particle using a highenergy mechanical alloying (HE-MA) method was investigated. The transient alloy particles were developed using a planetary mill at a constant rotational speed of 580 rpm employing milling times in the range of 4 to 24 hours. During the process, stearic acid of 3 wt.% of the precursor composition was used as a process-controlling agent (PCA). Two HE-MA manufacturing regimes were utilized: i) conventional (milling constituent elements simultaneously), and ii) sequential (progressive milling while adding elements in a certain order). In addition to the conventional method, a sequential regime was employed to develop FeNiCoCrAl, wherein individual elements were added every 4 hours to the starting/milled Fe + Ni mixture. Based on the results, the HE-MA FeNiCoCrAl showed a BCC single-phase formation after 24 hours, with no intermetallic or contamination traceability. Finally, a nanoindentation hardness measurement was carried out to support the observed phase transformation before and after the HE-MA process.

High-entropy alloys (HEAs) represent an innovative development approach for new alloy systems. These materials have been found to yield promising properties, such as high strength in combination with sufficient ductility as well as high wear and corrosion resistance. Especially for alloys with a body-centered cubic (bcc) structure, advantageous surface properties have been revealed. However, typical HEA systems contain high contents of expensive or scarce elements. Consequently, applying them as coatings where their use is limited to the surface represents an exciting pathway enabling economical exploitation of their superior properties. Nevertheless, processing conditions strongly influence the resulting microstructure and phase formation, which in turn has a considerable effect on the functional properties of HEAs. In the presented study, microstructural differences between high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) sprayed coatings of the alloy AlCrFeCoNi are investigated. A metastable bcc structure is formed in both coating processes. Precipitation reactions are suppressed by the rapid solidification during atomization and by the relatively low thermal input during spraying. The coating resistance to corrosive media was investigated in detail, and an improved passivation behavior was observed in the HVAF coatings.

Cation-deficient perovskite-type oxides have received considerable attention as new thermal barrier coating materials because of their extremely low thermal conductivities. In this study, sintered samples produced from RTa 3 O 9 (R: Y, La or Yb) powders are examined and the mechanisms behind their low thermal conductivity are investigated. Thermal conductivity was found to vary primarily with the ionic radius of the R element. As ionic radius decreases, nanodomains form via tilting of the TaO 6 octahedra. Phonon scattering at the domain boundaries is thus likely responsible for the low thermal conductivity of cation-deficient perovskite oxides.

The aim of this study is to understand how the characteristics of feedstock powders impact the morphology of thermal spray coatings. In the experiments, YSZ powders were prepared, characterized, and deposited on nickel-base superalloy substrates by plasma spray physical vapor deposition (PS-PVD) and the coatings were examined and tested. The results indicate that particle size distribution is an important factor in coating quality and that very fine-grained powder (< 12 µm) is not conducive to the formation of a columnar crystal structure. A powder with a rough, porous structure, on the other hand, readily absorbs heat and is thus easily vaporized, leading to a good columnar structure in the coating. In contrast, dense powder is difficult to vaporize, which promotes the formation of layered structures. Among the coatings with a columnar structure, one produced from powders with a particle size of 16.7 µm exhibited the lowest porosity and highest microhardness.

This study assesses the influence of particle size and spray parameters on the structural, mechanical, and electrical insulation properties of alumina coatings deposited by atmospheric plasma spraying. It has been found that the combination of a relatively fine feedstock powder and high velocity plasma spraying promotes the formation of denser coatings with high dielectric strength. Correlations between dielectric strength and deposition efficiency, coating hardness, crystal structure, and surface roughness are also assessed.

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.

This study investigates the heat-shock properties of metal-oxide films synthesized from ethylenediamine tetraacetic acid (EDTA) complexes using conventional flame-spray equipment. An EDTA·Y·H powder was placed in the feed unit of the sprayer and transported by a flow of oxygen to the gun. The powder was sprayed using a mixture of H 2 and O 2 as the flame gas, producing a layer of yttrium oxide on a stainless steel substrate. XRD analysis was used to examine the crystal structure of the deposits and SEM imaging revealed the surface and cross-sectional microstructure. A cyclic thermal shock test was conducted and the deposited film was analyzed for the existence of cracks, deformation, and delamination. Although the number of cracks, crack lengths, and cracks per unit area increased due to heat shock, delaminations were not observed. The results show that the Y 2 O 3 films have high thermal-shock resistance and are suitable for use as thermal barrier coatings.

This study assesses the viability of three suspension spray processes for producing photocatalytic TiO 2 . In the experiments, flame, plasma, and HVOF torches were used to spray TiO 2 suspensions onto stainless steel substrates, varying process parameters in order to gauge their effect on phase composition, crystal size and, in turn, photoactivity. The TiO 2 samples were characterized by means of XRD, SEM, and UV-Vis analysis and photocatalytic hydrogen-production testing. Suspension flame spraying proved to be the most effective method, producing phase-controlled nanostructured titania 32% more photoactive than the SPS samples and up to five times more active than analogous coatings produced by CVD.

In this work, micro-plasma spraying is used to produce hydroxyapatite coatings on Ti-6Al-4V substrates. To understand coating formation mechanisms, in-flight particle velocity and surface temperature were monitored under different spraying conditions. XRD measurements show that the resulting coatings have a high degree of crystallinity with little amorphous or metastable phases. Some of the coatings were also found to have a uniformly distributed columnar structure, corresponding to a strong (002) texture and excellent stability in Hanks’ salt solution even after 14 days of immersion.

The effect of substrate template effect on the crystalline structure of plasma sprayed 8YSZ (8mol%Y 2 O 3 ) splats was investigated by high resolution transmission electron microscopy (HR-TEM) examination of FIB-processed splat samples. 8YSZ splats were deposited by the atmospheric plasma spraying (APS) on the polished sintered tetragonal structure substrate (3YSZ) and cubic structure substrate (8YSZ) at different preheating temperatures. The focused ion beam (FIB) was utilized to prepared TEM cross-sectional sample of splats. The crystalline structures of both the splat and the underlying substrate were examined by HRTEM. Results showed that the 8YSZ splats deposited on the polished sintered cubic structure 8YSZ substrate at a substrate surface temperature of 900°C exhibited cubic structure and the epitaxial grain growth was confirmed between the crystalline of splat grain and immediately underlying cubic crystalline substrate grain. Moreover, epitaxial grain growth was confirmed between the crystalline of splat grain and the tetragonal structure substrate when substrate surface temperature was increased to 1200°C. The present results suggest that the crystalline structure formation of 8YSZ splats produced by plasma spraying was affected by the substrate template effect.

Lead-free piezoelectric materials are nowadays drawing considerable attention as lead titanium zirconium oxide (PZT) is considered “as a substance of very high concern” by the European Chemicals Agency because of its toxicity. An interesting PZT replacement material for high temperature capable ultrasonic transducers is bismuth titanate (Bi 4 Ti 3 O 12 ), which could be used for pipe thickness and corrosion monitoring in the oil & gas and nuclear industries. In this study, solution precursor plasma spraying (SPPS) is used to deposit Bi 4 Ti 3 O 12 coatings onto stainless steel substrates by means of inductively-coupled thermal plasma. The crystal structure and the morphology of the deposited coatings is studied as a function of the SPPS operating parameters such as plasma gases, electrical power, chamber pressure and spraying distance. SPPS of piezoelectric materials is an interesting one step process alternative to the time consuming layered-based chemical spray pyrolysis/calcination of sol-gel precursors.

This study deals with the deposition of coating materials that can be difficult to process by plasma spraying, including lanthanum and gadolinium zirconate, two pyrochlores of interest for thermal barrier applications, and lanthanum strontium cobalt ferrite (LSCF), a perovskite of interest for gas separation membranes. In addition to conventional atmospheric plasma spraying (APS), the feedstock powders were applied by suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD). The spraying processes are described in detail along with the characteristics of the powders and coatings and the effects of various spray parameters on splat behavior and coating composition and structure.

This study investigates a new evaluation method that has the potential to differentiate between particle interfaces and grain boundaries in cold spray coatings. The method uses confidence index (CI) and image quality (IQ) values obtained from EBSD analysis to determine the location of grain boundaries as well as grain orientation and crystallinity of the deposit. In the case of a cold-sprayed copper deposit, the new method helps to explain the observed characteristics of the coating.

This study investigates microscale features in plasma sprayed alumina and how they correlate with mechanical property differences at the macroscale. Thick alumina coatings were deposited on aluminum and titanium substrates by atmospheric and low-pressure plasma spraying using α-Al 2 O 3 powder as the feedstock. Coating surfaces and cross-sections were examined by FE-SEM, phase distributions were identified by means of EBSD, and porosity was measured via image analysis. Mechanical properties at the micro and macroscale were assessed by nanoindentation, Vickers hardness, and cavitation erosion testing. Relevant test results and observations are presented and discussed in the paper.

This study compares the effects of plasma nitriding and nitrocarburizing treatments on HVOF sprayed stainless steel coatings with different crystal structure. The treatments were conducted at 550 °C for 10 h in a gas mixture of N 2 and H 2 for nitriding and N 2 , H 2 , and C 2 H 2 for nitrocarburizing. Optical microscopy, SEM-EDS, and XRD show that the treatments produced thick nitride layers consisting of a compound layer and a nitrogen diffusion layer. The treatments increased not only the surface hardness, but also the load bearing capacity of the coatings due to the formation of CrN, Fe 3 N, and Fe 4 N phases. Plasma nitrocarburized 410 stainless steel had the highest microhardness and load bearing capacity because of the precipitation of Cr 23 C 6 on the surface.

In this study, dense yttria coatings are produced by conventional flame spraying using a yttrium ethylene diamine tetra acetic acid (Y-EDTA) complex as the feedstock powder. By adjusting spray parameters, the organic components in the EDTA are completely decomposed and only crystalline Y 2 O 3 phases are found in the deposits. Compared to atmospheric plasma sprayed Y 2 O 3 , Y-EDTA coatings produced by reactive flame spraying have better surface morphologies with fewer microcracks and are obtained at lower cost.

MoB/CoCr, a novel material for thermal spraying, with high durability is used to resist erosion by molten Al-12.07wt%Si alloy. The durability of the MoB/CoCr coating prepared by low pressure plasma spraying (LPPS) has been investigated using a molten-metal immersion tester. The test revealed that the MoB/CoCr coating has much higher durability without dissolution in the molten Al-12.07wt%Si alloy. Little change of crystal structure, mainly composed of ternary borides of Co 2 MoB 2 and CoMoB, is observed after the immersion test, suggesting that the ternary borides have much higher durability.

Solution precursor plasma spray (SPPS) is a thermal spray process in which deposits are formed by injecting solutions with the appropriate chemistry directly into the plasma. The deposits consist of grains or particles as small as ~20nm, and may be very porous or nearly dense, depending on the solution and deposition parameters. Recently, the potential of SPPS to deposit fine particle, porous coatings suitable for use as electrochemical electrodes for fuel cells and gas sensors has been demonstrated. This paper describes the efforts to deposit LiFePO 4 coatings which may be of interest for Li ion battery electrodes with SPPS. In this case, along with the porosity, surface area, and microstructure of the deposited coatings, crystal structure also plays an important role in determining the performance of the LiFePO 4 electrodes. Solution precursors with different solution chemistries containing lithium, iron and phosphorus ions are injected into hydrocarbon plasma issuing from a DC-arc torch. The effects of solution chemistries on coating morphologies and crystal structure were investigated. The results indicate that the porosity and crystal structure of the coatings can be tailored by selecting different additives.

Yttrium oxide (Y 2 O 3 ) coatings have been prepared with high power axial injection plasma spraying using fine powder slurries. It is clarified that the coatings have high hardness, low porosity and high erosion resistance against CF4 contained plasma in the previous study. This suggests that the plasma spraying of Y 2 O 3 with slurry injection techniques is applicable to fabricating equipments for semiconductor devices, such as dry etching. Surface morphologies of the slurry coatings with splats are almost similar to conventional plasma-sprayed Y 2 O 3 coatings, identified from microstructural analysis by field emission SEM in this study. However, no lamellar structure has been seen from cross sectional analysis, which is apparently different from the conventional coatings. It has also been found that crystal structure of the slurry Y 2 O 3 coatings mainly composed of metastable phase of monoclinic structure, whereas the powders and the conventional plasma spray coatings have stable phase of cubic structure. Mechanism of coating formation by plasma spraying with fine powder slurries will be discussed based on the findings.