Thick deposits were produced from pure Al powder of three different sieve sizes using cold spraying at the same process parameters. The in-plane mechanical and fracture properties of the deposits were investigated using bending of small specimens in four specimen orientations. It was shown that increasing the Al particle size by approximately 50% and 100% leads to small, but statistically significant differences of yield strength. Further, the increase in the powder particle size led to higher fracture toughness K IC but lower fatigue crack growth threshold ΔK thr . This can be attributed to two different fracture mechanisms in the cold sprayed deposits. A trans-particular fracture in the near-threshold fatigue regime is controlled by the microstructure and work hardening of the particles. At higher cyclic loads and in quasi-static regime, the particle decohesion and the resulting crack path determine the fracture behavior instead. However, the observed effect of particle size was rather small, much smaller than the effect of spray process parameters observed in the previous research.

The composition of the cored wires is inhomogeneous and contains solid velum as well as a powder filling which strongly influences the particle formation, in-flight particle behavior, the coating microstructure, and consequently the behavior of the desired coating. To study the effect of particle size distribution of the filling powder in cored wires the parameters of the twin wire arc spraying process such as current, voltage, and atomization air pressure are changed for different intervals of particle size distributions (-45µm+25µm, and -95µm+63µm). Fluctuations in arc voltage and current are measured and found to be higher at smaller particle sizes. The characteristics of inflight particles showed a higher particle velocity in case of smaller particle sizes. The particle temperature is higher in case of bigger particle sizes. The splats tend to form a pancake shape in case of smaller particle sizes. Therefore, the lamellas are more homogenous and the porosity is low. This investigation is important for deep understanding of twin wire arc spraying with cored wires.

Three different types of polyethylene powders were flame sprayed onto pre-heated steel substrate previously coated by electrostatic spray system with a thin epoxy primer layer. Properties of the polyethylene (PE) powders, including powder density, particle size and melt flow rate (MFR) were measured in order to study their influence on the mechanical properties of the coating. The spray experiments started with optimization of spraying parameters. The main variables were pre-heating temperature of the substrate, temperature increase during spraying (influenced by the spraying distance), and thickness of the PE coatings. The laboratory tests performed for the coatings were coating characterization by microscopy and mechanical testing. Porosity and thickness of the coatings were determined by optical and stereo microscopy studies from polished cross-sectional samples. Hardness, impact strength, peel strength, and adhesive strength of the coatings were also investigated. Also some hot water sinking and heat cycling tests were performed. As a result from the present studies it can be concluded that powder properties have great influence on the mechanical properties of the final coating.

The thermal spray industry identified the need for repeatable and reproducible feedstock powder characterization methods, especially particle size distribution (PSD), for cost effective manufacturing of thermal barrier coatings. The PSD measurement by a laser light scattering method was identified as the technique most widely used in the industry. This technique offers high resolution, rapid measurements and ease of use. A round robin study by nine laboratories using different models of a commercial light scattering instrument has been completed as the first step towards the development of a Standard Reference Material (SRM) for the calibration of light scattering instrument. Other measurement techniques were also employed for additional comparison. The PSD measurements employing light scattering techniques evidenced some method dependence, despite the use of identical sample preparation procedures. The round robin results will serve as reference values for the development of the SRM.

In our laboratory, we have developed a method to simultaneously inject different powders from the central axis direction and radial direction of the cold spray nozzle and are producing a composite coating by this method. In the previous research of our laboratory, an Al-12Si alloy coating with excellent wear resistance was produced by micro-forging assisted cold spray using the simultaneous nozzle injection method of powder in the axial and radial directions. Here, Al- 12Si alloy, which has excellent wear resistance, was used for the coating-formed particles, and stainless steel was used for the micro-forging particles. However, because the micro-forging particles were hollow, they remained in the coating. In this paper, we evaluated the influence of increasing the mixing ratio of micro-forging particles instead of solid (no holes) micro-forging particles on the coating structure. At the same time, the behaviors of particles by computational fluid dynamics are also investigated.

To improve the mechanical properties of aluminum coatings, ceramic reinforcement may be added resulting in an aluminum matrix composite. Two processing routes were investigated to manufacture aluminum matrix composite powders for thermal spraying: ball milling and mixing. Three sizes of SiC reinforcement particles were used: 2, 15, and 25 µm. For the ball-milled powders, morphology and microstructure were investigated as a function of SiC grain size and milling time. It is shown that the hardness of the composite and the efficiency of the spray process depend on the size of the hard particles as well as the preparation method. Friction tests were also carried out and the results are shown to correlate with coating microstructure.

In this investigation, chromium alloy powders of varying size are used to deposit coatings by HVOF spraying in order to study the effect of powder size on particle oxidation and oxygen content in the coatings. The results show that oxide content in both the layer and sprayed particles is extremely dependent on particle size and that little additional oxidation is likely to occur after particle impact. Paper includes a German-language abstract.

This study was undertaken to investigate the influence of the initial powder particle size on the microstructure development in a Cu-3wt% Ag alloy obtained by vacuum plasma spraying of a mixture of copper and silver powders. Various mixtures of powder particles were used where the copper powder particle size was kept as –80/+40 μm, while the silver powder particle size fraction was varied. Vacuum plasma spraying (VPS) in inert gas atmosphere was used to elaborate thick coatings (2 mm) with limited oxygen contamination. Significant variations in microstructure were obtained depending on the Ag powder size fraction that was used. These differences in behavior are discussed taking into account differences in the solid state precipitation mechanisms in the alloy.

HVOF processes represent the state of art for the spray deposition of wear and corrosion resistant coatings since their supersonic gas velocities in combination with moderate flame temperatures allow the deposition of optimal coatings with very high bond strengths, fine surface finishes and low oxide levels. However, new generation coating materials (fine powders), stringent quality requirements and the high productivity demanded by the industry, push the HVOF technologies to their limits. Recently, a novel air-oxygen controlled high velocity combustion process has been development by Tecnalia. The system operates within the supersonic regime using a broad range of fuel/oxidant ratios thanks to the use of air-oxygen mixtures and a carefully optimized gun design. Extremely low flame temperatures can be achieved while keeping a supersonic flow of combustion products, thus allowing the solid state deposition of almost all industrially relevant metal alloys with superior deposit qualities. In this work, a systematic investigation of the influence of the powder particle size and gun configuration on resulting coating microstructural features has been performed. For comparison, two fine structured commercially available WC10Co4Cr powders with different particle size distributions have been investigated. The coating structure has been characterized with by high resolution SEM cross-section imaging and X-ray diffraction analysis. Resulting coatings are characterized by highly dense structures, a high retention of the primary carbides, average microhardness of up 1885 HV0.3 and fracture toughness varying between 3 and 7 MPa.m -1/2 depending on the powder particle size distribution and the process conditions used.

Rare-earth complex oxide ceramics are promising candidate materials for next-generation thermal barrier coatings due to their low thermal conductivity and phase stability. During plasma spraying, the component with a higher vapor pressure may experience severe preferential vaporization, resulting in significant composition change from the starting powder. In this study, Gd 2 Zr 2 O 7 (GZO) powder with a hollow spherical structure is used as the feedstock material to assess the vaporization behavior of G d2 O 3 during atmospheric plasma spraying (APS). Isolated Gd 2 Zr 2 O 7 splats with regular disc shapes in different sizes were deposited on stainless steel substrates at a deposition temperature of 300 °C to study the effects of particle size on vaporization loss. The elemental composition of each splat was analyzed by EDS, and the ratio of Gd to Zr in different splats with different diameters was obtained. The results show that the vaporization loss of Gd increases markedly with decreasing particle size due to the preferential vaporization of G d2 O 3 . Using Gd 2 Zr 2 O 7 powders of a certain size can reduce Gd loss, although the effect is determined by molten droplet size rather than apparent powder particle size.

Ceramic deposition produced by cold spraying was studied for functional surface applications. Several oxide materials and metal matrix composite (MMC) powders were used to model the behavior of ceramic powder deposition on soft metallic substrate materials. The manufacturing method, density and size of ceramic powders and matrix material of MMC were found to affect the deposition on soft metal surfaces. The powder density influences the deposition greatly and it is also an important factor in finding an ideal powder particle size. Fusing and crushing the powder can be a good manufacturing method if the fusion does not cause phase transformations in the powder. In that case, spray drying with sintering can give better results. Spraying parameters, such as the process gas parameters and the effect of multiple sweeps of the torch were also studied to optimize the amount of deposition. Cold spraying was found to be a promising manufacturing method for functional surfaces.

A brief study was performed to examine the influence of starting powder composition, starting powder particle size and spraying environment on the chemistry, phase assemblage and porosity of Al-Cu-Fe plasma sprayed coatings involving a quasicrystaliine phase. It was found that a loss of Al during spraying results from the extremely low thermal conductivity of the quasicrystaliine phase in the starting powders. This loss changes the bulk composition of the deposited coating and partially controls the phases that develop. Smaller starting powder particles tended to lose more Al and, therefore, form less of the quasicrystaliine phase. Larger starting powder particles did not lose Al to the same degree, and produced coatings with more of the quasicrystaliine phase. However, these powders produced coatings also had a higher amount of porosity.

SiC coatings were prepared with pack powder in different particle sizes in a vacuum atmosphere by pack cementation technique to protect the C/SiC composites substrate from oxidation. The phase and microstructure of the coatings were investigated by XRD, SEM analyses. The relationship between powder granularity in the pack and microstructure of SiC coatings was studied. Cyclic oxidation test at 1573K in air atmosphere was performed and the effect of powder particle size in the pack on high-temperature oxidation resistance of SiC coatings was discussed in detail. It is observed that with powder granularity in the pack increasing thickness and density of SiC coatings increases, corresponding oxidation resistance of the coating is improved. Possible mechanisms related to oxidation were preliminarily discussed.

The erosion behavior of yttria stabilized zirconia thermal barrier coatings is investigated with respect to powder particle size. Solid particle erosion experiments were conducted at room temperature to determine the mechanism of erosion for ceramic thermal spray coatings. Testing was carried out on as-sprayed as well as thermally cycled specimens. Porosity and bend testing measurements indicate that a decrease in porosity and an increase in inter-lamellar strength leads to an increase in the erosion resistance of ceramic thermal spray coatings.

Plasma-sprayed alumina-waste glass composite coatings on ceramic substrates were produced. Two kinds of alumina powders, different alumina volume fractions, and two glass powders particle size distributions were tested. Post-process thermal treatments were performed. The coatings were characterized by SEM, XRD, Vickers microhardness, fracture toughness, abrasion resistance tests. Coatings superior to traditional tile glazes were obtained with as high as 50 vol.% of waste glass. Fine glass powders (<45µm) must be employed to achieve adequate toughness. A low-cost spray-dried alumina can be used instead of the expensive commercial powders. The thermal treatment enhances the coating properties. A FEM thermo-mechanical simulation was performed. Elastic modulus calculations show a definite coating anisotropy (higher mechanical properties in the longitudinal direction). Compressive residual stresses in the alumina and tensile ones in the glass are developed after the thermal treatment. Crack propagation studies based on Griffith model show cracks initiating from larger pores and propagating easily through the glass, thus explaining the coating toughening achieved through the employment of finer glass powders. Cracks are stopped by alumina; this effect is enhanced in the thermally treated coatings. The numerical and experimental (from indentation fracture toughness test) crack propagation patterns are in good agreement.

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.

Aluminum nitride (AlN) and iron nitride (Fe 4 N) coatings were fabricated by reactive plasma spraying using fine feedstock powders. Reactive plasma spraying, in which element particles react with surrounding active species in the plasma, enables to fabricate nitride ceramics which decompose without stable melting phase. However, it is difficult to fabricate the coatings which include higher concentration of nitride phase by reactive plasma spraying using conventional particle size of feedstock powders. Therefore, fine feedstock powders were used in order to enhance the nitriding reaction during spraying. Aluminum or iron particles were injected into Ar/N 2 plasma and were deposited onto graphite substrates. It was possible not only to increase the nitride phase content in the coatings but also to densify the microstructure in both materials. Thus, it became clear that using fine feedstock powders are useful for fabrication of nitride ceramic coatings by reactive plasma spraying.

Plasma spraying was used to deposit Li3PO4 coatings from sintered dense powders in three size ranges to study the effects of particle size and spraying distance. Coating microstructure, crystal structure, and composition were characterized using SEM, XRD, ICP-MS, and FTIR. It was found that sintered dense powders have a high temperature orthorhombic phase (γ-Li3PO4) that differs from the β-Li3PO4 phase associated with agglomerated Li3PO4. Plasma-sprayed coatings produced from these powders have similarly dense microstructures with fracture-surface morphology like that of sintered bulk. The effect of particle size and spraying distance on atomic ratio is also investigated in the study.

The widespread use of additive manufacturing and modern powder-based technologies (thermal spraying, hardfacing, sintering) encourages the search for alternative routes enhancing the development of metal and metal alloy powders. The state-of-the-art powder production processes, like gas, water or plasma atomization, are dedicated to mass production, which limits the availability of new powder compositions with desired characteristics. In this study, stainless steel powders were investigated. The powders were atomized by an in-house developed ultrasonic (UT) atomization set-up, called ULTRAMIZER. In this system, the atomization of melt is possible by using a high-power ultrasonic field. The atomized powders were characterized in terms of morphology and particle size distribution (PSD). The powder features were then correlated with operating parameters of: (i) UT atomization system, mainly frequency and root mean square power (RMS), and (ii) the orientation of the atomization plate against the melting system, by means of distance and tilting angle. The study shows that the ultrasonic atomization allows producing nearly spherical, defect-free powder particles, with a very narrow and controllable size distribution. These are important advantages over other metal powder production methods, especially when it comes to the development of new types of powder.

The plasma spray process is one of the oldest and most useful thermal spray processes used in industry today. The principles of plasma spray are complex. The many factors that affect plasma spray make understanding and manipulating the process a difficult task. Although it is impossible to isolate each of those factors, the understanding of the way they affect the quality of the coating can help the sprayer make an improved judgement for coating development and application. This paper is an attempt to identify the various factors, including plasma spray equipment configurations, plasma gases, feedstock and manipulation, and discuss their effect on a particular plasma spray coating application from a job shop perspective. Also, plasma spray coating applications are discussed.