Powder Spray Research Articles

Numerical modeling of the powder materials spraying

On the basis of the numerical solution of polydisperse gas suspension dynamics system of equations, the process of powder spraying on a plane surface in an electric field is described. The model includes motion equations of the carrier medium and disperse phase fractions under aerodynamic friction and Coulomb forces action taking into account interfacial exchange of pulse and energy. The system solves by an explicit predictor-corrector method with splitting on the spatial directions and the non-linear correction scheme. The numerical model is applied to gas suspension speed and density fields receiving in interelectrode space and on target electrode surface. The system of equations was written in the generalized curvilinear coordinates: the physical region of a current in variables (x, y) was written in a canonical computational region in variables (ξ,η) and was solved by an explicit second-order Mac-Cormack method. The Poisson equation for the electric field potential was written in the generalized coordinates and was solved by finite difference method with the use of the iterative Seidel scheme on a gas-dynamic computational mesh.

Mechanical performance of plasma-sprayed bulk-like NiCrMo coating with a novel shell-core-structured NiCr-Mo particle

In this study, novel heat-treated shell-core-structured (SCS) NiCr-23Mo powder is employed to realize the deposition of a bulk-like metallic coating with excellent mechanical performance by plasma spraying. The SCS NiCr-23Mo powder was fabricated by cladding spherical Ni20Cr particles with refractory Mo as an alloying element. This ensures the generation of ultra-high temperature Ni16Cr23Mo droplets during the in-flight process of plasma spraying. The interface bonding between the Ni16Cr23Mo single splats and steel substrate, as well as the microstructure of the Ni16Cr23Mo coating were characterized. Mechanical behavior of the coating was examined by erosion test. Results showed that the Ni16Cr23Mo coating exhibited a dense microstructure with significantly improved adhesive strength. Moreover, effective bonding was formed at 78% of interfaces between the Ni16Cr23Mo splat and steel substrate, which was much higher than the 36% of the conventional NiCr splat. Erosion test revealed that the Ni16Cr23Mo coating presented excellent erosion resistance, being comparable to that of the Ni-based bulk alloy. Moreover, the Ni16Cr23Mo coating presents ductile mechanical behavior during erosion test rather than the brittle behavior of the conventional Ni-based alloy coating. Thus, a mechanically bulk-like coating with excellent interface bonding and excellent erosion resistance can be achieved by plasma spraying of the novel SCS NiCr-23Mo powder.

Experimental and numerical study for Cu metal coatings at room temperature via powder spray process

The effect of Cu particle size on deposition behavior of Cu coating layer was investigated by experimental and numerical studies to understand coating behavior of metal films during aerosol deposition (AD) process. Experimental results showed that Cu film fabricated using 2 μm Cu powder had high deposition rate, low resistivity, and high internal micro-strain. Moreover, Cu films fabricated using 2 μm Cu powder had dense microstructure and Cu oxide phases due to elevated temperature caused by kinetic energy of impacted particles. However, Cu films prepared with 5 μm Cu powder formed filmy deposition layers with poor structural and electrical properties. Numerical results revealed that the generated strain and heat at the impact interface between the particle and the substrate were significantly decreased with increasing particle size. In contrast, the maximum shock pressure of particles was increased with increasing particle size. These results demonstrate that metal Cu particle with size of about 2 μm is suitable for forming good quality coating layers and that their bonding is associated with high compressive strain and thermal energy.

Experiments, Statistical Analysis, and Modeling to Evaluate the Porosity Influence in SPS Coatings

Suspension plasma spray (SPS) is far more complicated than conventional plasma spray and requires a deep knowledge about the influence of process parameters and their correlations. In this study, YSZ coatings were manufactured by SPS with six different process parameters such as plasma power, suspension mass load, original powder size, substrate surface topology, spray distance, and spray step. Afterward, the porosity of as-prepared coatings was investigated by image method and x-ray transmission technique. A multivariate analysis on the collected experimental data was carried out by employing mathematical statistics methods. The results showed that: (1) coating porosity has a negative correlation with plasma power and suspension mass load and a positive correlation with the original powder size, spray distance, spray step, and substrate roughness; (2) spraying distance is the main factor affecting to coating porosity, followed by suspension mass load and substrate surface roughness, respectively. A linear model for porosity prediction was developed and was verified by experiments. The mechanism by which process parameters influence coating porosity is also discussed.

Principles for the safety evaluation of cosmetic powders

Consumer exposure to cosmetic (personal care) products is mostly by dermal contact, however additional considerations with regards to potential inhalation exposure from some cosmetics, such as sprays and powders, may be needed for a robust and reliable safety assessment.To get a deeper understanding of the exposure to airborne particles and droplets during product application, a team of international experts was founded under the umbrella of the European Association of the Cosmetic Industry “Cosmetics Europe” (CE) in Brussels. This expert team has worked out a pragmatic strategy how small and medium sized enterprises (SMEs), but also relevant authorities, could handle the safety evaluation of cosmetic powder products. Sufficient information on the aerodynamic diameter of sprayed droplets and here specifically of airborne particles is essential in addition to knowing the exposure after typical product application. The current article is focused on the determination of inhalation exposure to solids, and the derivation of safe exposure levels for cosmetic powder products found in the market. The principles described herein are very similar to spray products as published earlier and should be applied in a similar way (Steiling et al., 2014).Prediction models for the best estimate of inhalation exposure, developed with data from computer simulation programs, individual real-time measurements or finally by experience from the market were introduced and applied. Safety assessment approaches for exposure from powder spray products were developed and have been already considered in regulatory guidelines like the EC Cosmetics Regulation.

Modification of cryomilling process to tailor geometrical characteristics of nanostructured Al powder for cold spraying

Cryomilling is the most commonly used method to fabricate ultra-fine grained (UFG) or nanostructured metallic powder. However, it has relatively low control over powder's geometric characteristics, particularly for the powders having FCC crystal structure. This shortcoming makes traditional cryomilling process unattractive to produce UFG/nanostructured feedstock for cold spraying process. In the present work, a modified pot vibration milling process was successfully employed to produce nanostructured powder of pure aluminum at 100 K. The geometrical characteristics of Al powder was controlled by using various combinations of the milling media (i.e. steel balls) of different sizes. The results indicate that with increase in milling time, the initial grain size of Al (~1–5 μm) was progressively refined to ~30–50 nm. Moreover, lamellar powder morphology was obtained with the addition of process control agent (PCA) while equiaxed polygonal structure was evolved without using PCA. It was also revealed that, in the absence of PCA, the particle size distribution was mainly controlled by the interstice size distribution of the milling media.

Investigation of tillage elements of agricultural machinery

The efficiency of agricultural activity is significantly influenced by the condition of power machines used in the agricultural production. Among others, the formation and maintenance of suitable soil-conditions play a very important role in obtaining the desired harvest yield. Many types of wear have been recognized such as abrasive, erosive, corrosion, oxidation etc. The abrasive wear in agriculture equipments is the most common problem. The tillage elements are exposed to an extraordinary high load and a significant wear, therefore it is necessary to investigate the relationship between their active layer and durability. Abrasive wear is probably the most significant cause of mechanical damage of equipment components coming in contact with abrasive bodies. The purpose of our research work is to change the old, worn cultivator tines to tines having a longer durability; hot metal powder spray fusing has been used in order to realize our aim. The products made by us will be used in the agricultural production in the future therefore the type of layer build-up technology to be used during the production can be determined. As a cultivator is equipped with a lot of cultivator tines, the expenses can significantly be decreased by the use of cultivator tines made by a well-chosen layer build-up technology and a higher yield/agricultural area can be obtained during the cultivation.

Numerical simulation of vortex structures in the spraying of electrically charged gas suspension on a plate

Based on the numerical solution of polydisperse gas suspension dynamics system of equations, the process of powder spraying on a plane surface in an electric field is described. The model includes motion equations of the carrier medium and disperse phase fractions under aerodynamic friction and Coulomb forces action taking into account interfacial exchange of pulse and energy. The system solves by an explicit predictor-corrector method with splitting on the spatial directions and the non-linear correction scheme. The numerical model is applied to gas suspension speed and density fields receiving in interelectrode space and on target electrode surface. The system of equations was written in the generalized curvilinear coordinates: the physical region of a current in variables (x, y) was written in a canonical computational region in variables (ξ,η) and was solved by an explicit second-order Mac-Cormack method. The Poisson equation for the electric field potential was written in the generalized coordinates and was solved by finite difference method with the use of the iterative Seidel scheme on a gas-dynamic computational mesh.

The Current State of Application of Ultra- and Nanostructured Powders in Technologies in Welding Technologies (Review)

This paper presents an overview of applying nano-and ultra-disperse powders as modifiers in various procedures of arc welding, as well as in electro-slag welding and plasma powder spraying. Consideration of some methods of embedding nano-and ultra-disperse powders into a liquid weld pool is provided.

Formation of Composite Coatings by Sub- and Supersonic Plasma Spraying under and on Special Coating Materials

A technology was developed for the formation of a multilayer composite coating obtained by the sub – and supersonic plasma spraying method based on Al, Ti, Ni, and Mo. The results of the investigation of a coating of Al-Ti-Ni-Mo powder are deposited by subsonic plasma deposition on a steel substrate followed by the application of an anticorrosion coating from a paint coating material (PCM) of the CINEP brand, which is protected by the supersonic spraying powder Al-Ti-Ni-Mo. This coating layer is designed to protect the anticorrosive layers of the main multicomponent coating of products, which can be subjected to external mechanical stress during operation. It is shown that the resulting composite coating is sufficiently strong and withstands strains in the zone of elastic deformation of the matrix. Carried out corrosion tests showed a high resistance of these coatings.

Current Progress in Solution Precursor Plasma Spraying of Cermets: A Review

Ceramic and metal composites, known also as cermets, may considerably improve many material properties with regards to that of initial components. Hence, cermets are frequently applied in many technological fields. Among many processes which can be employed for cermet manufacturing, thermal spraying is one of the most frequently used. Conventional plasma spraying of powders is a popular and cost-effective manufacturing process. One of its most recent innovations, called solution precursor plasma spraying (SPPS), is an emerging coating deposition method which uses homogeneously mixed solution precursors as a feedstock. The technique enables a single-step deposition avoiding the powder preparation procedures. The nanostructured coatings developed by SPPS increasingly find a place in the field of surface engineering. The present review shows the recent progress in the fabrication of cermets using SPPS. The influence of starting solution precursors, such as their chemistry, concentration, and solvents used, to the micro-structural characteristics of cermet coatings is discussed. The effect of the operational plasma spray process parameters such as solution injection mode to the deposition process and coatings’ microstructure is also presented. Moreover, the advantages of the SPPS process and its drawbacks compared to the conventional powder plasma spraying process are discussed. Finally, some applications of SPPS cermet coatings are presented to understand the potential of the process.

Synthesis mechanism and preparation of LaMgAl11O19 powder for plasma spraying

Lanthanide magnesium hexaaluminate (LaMgAl11O19) powders were successfully synthesized by the solid-state reaction method. The objective of this study was to investigate the synthesis mechanism of LaMgAl11O19 and prepare LaMgAl11O19 powders suitable for plasma spraying. The results show that LaAlO3 reacts with MgAl2O4 and Al2O3 to form LaMgAl11O19 at approximately 1300 °C. Single-phase LaMgAl11O19 powders were prepared successfully by solid-state reaction at a synthesis temperature of 1600 °C for 6 h. Unlike the particles in the synthesized powders, those of the centrifugally spray-dried powders have a spherical shape with uniform granularity and good flowability, density, and particle size distribution, making them suitable for plasma spraying. The synthesized powders and centrifugally spray-dried powders remained as a single phase after heat treatment at 1300 °C for 100 h, indicating that LaMgAl11O19 has excellent high-temperature stability.

Influence of Powder Morphology and Microstructure on the Cold Spray and Mechanical Properties of Ti6Al4V Coatings

The powder microstructure and morphology has significant influence on the cold sprayability of Ti6Al4V coatings. Here, we compare the cold sprayability and properties of coatings obtained from Ti6Al4V powders of spherical morphology (SM) manufactured using plasma gas atomization and irregular morphology (IM) manufactured using the Armstrong process. Coatings deposited using IM powders had negligible porosity and better properties compared to coatings deposited using SM powders due to higher particle impact velocities, porous surface morphology and more deformable microstructure. To evaluate the cohesive strength, multi-scale indentation was performed and hardness loss parameter was calculated. Coatings deposited using SM powders exhibited poor cohesive strength compared to coatings deposited using IM powders. Images of the residual indents showed de-bonding and sliding of adjacent splats in the coatings deposited using SM powders irrespective of the load. Coatings deposited using IM powders showed no evidence of de-bonding at low loads. At high loads, splat de-bonding was observed resulting in hardness loss despite negligible porosity. Thus, while the powders from Armstrong process lead to a significant improvement in sprayability and coating properties, further optimization of powder and cold spray process will be required as well as consideration of post-annealing treatments to obtain acceptable cohesive strength.

Powder and Powder-Free Intra-Oral Scanners: Digital Impression Accuracy.

The purpose of this study was to evaluate the accuracy of digital impressions taken with two different intra-oral scanners (with and without optical powder), by dentists with different level of experience. Two intra-oral scanning devices (Apollo DI and Cerec Omnicam, Sirona Dental GmbH, Germany) were used to obtain a digital cast from scanning different preparations on a maxillary typodont. Three operator groups scanned five crown preparations using both powder and powder-free scanning systems. One group consisted of fourth year dental students, a second group consisted of prosthodontic residents, and third group consisted of prosthodontic specialists. The precision of spray application and impression expertise was compared between the three groups. The most frequently encountered errors were non-linear powder application, inappropriate intra-oral scanner manipulation, and insufficient digital data for crown fabrication. Experience is needed in order to achieve high quality digital impressions. A prosthodontic specialist avoided incorrect manipulation of an intra-oral camera, uneven application of powder spray and achieved a better marginal fit.

Detonation spraying behaviour of refractory metals: Case studies for Mo and Ta-based powders

In thermal spraying of refractory metal powders, two major issues need to be solved: particles of materials having high melting temperatures should be heated to reach a semi-molten/molten state or temperatures close to the melting point, while oxidation of the metals should be prevented. It has long been believed that it is rather difficult, if not impossible, to produce high-quality refractory metal coatings by detonation spraying. In this work, we demonstrated the capability of the detonation spraying method to produce tantalum-based and molybdenum coatings of low porosity. Using a computer-controlled detonation spray (CCDS2000) facility, the detonation spraying behaviour of a molybdenum powder and a partially oxidized tantalum powder was studied. Spraying was conducted onto steel substrates using an acetylene-oxygen mixture with O2/C2H2 = 1.1. The spraying process was studied by means of analyzing the splat morphology and calculating the particle temperatures and velocities. According to the X-ray diffraction phase analysis, the metals did not experience oxidation during the deposition. Rather, partial reduction of the oxide phase contained in the Ta-based powder occurred during spraying.

Photocatalytic performance of copper-based coatings deposited by thermal spraying

Visible light-activated photocatalysts are gaining general importance in advanced methods for removing persistent organic compounds. In this study, two types of composite oxide-metal coatings are obtained on copper substrates through thermal spraying of an aluminum bronze powder with different time-of-flight distances between the spraying nozzle and the substrate (150 and 200 mm). The surfaces of the coatings consist of nanopatterned α-Fe2O3 and CuxO produced by oxidation of the metal phase during the thermal spraying process. The optical band gap energies of the coating assemblies are 2.573 and 2.131 eV, respectively, and the Brunauer–Emmett–Teller (BET) specific surface areas are 4.65 and 4.87 m2 g−1, respectively. The photocatalytic performance of the two types of thermally sprayed coatings in the visible region is studied using methylene blue as a model compound. The coatings present remarkable stability in aqueous environments, good hydrophilicity, high photodegradation rates (0.20–0.26 h−1), high organic dye mineralization efficiencies (75.69 and 92.36%) and unaltered methylene blue removal efficiencies during three successive 12 h photodegradation cycles (91–99.60%).

A Study of Plasma-Sprayed Nanocomposite Coatings Based on Magnesium-Substituted Tricalcium Phosphate

A technique for plasma spraying of a magnesium-substituted tricalcium phosphate powder has been developed; optimum modes of the plasma-jet arc current and the particle size of the sprayed powder have been determined. It has been found that the structure of the resulting coating contains microparticles and nanoparticles with a size of up to 90 μm and up to 100 nm, respectively. The adhesion of the substituted coatings is 13 MPa; on average, it is 20% higher than the adhesion of coatings based on unmodified powders.

Composition, structure and mechanical properties of metal oxide coatings produced on titanium using plasma spraying and modified by micro-arc oxidation

Metal oxide coatings on VT6 titanium alloy were formed by plasma spraying of aluminum oxide powder and subsequent microarc oxidation at a current density from 1 to 3 kA/m2. As a result of combined treatment, metal oxide coatings consisting of a mixture of aluminum and titanium oxides were obtained on the surface of titanium samples. The most pronounced changes in the morphology of the plasma-sprayed coating were observed at the highest current density of 3 kA/m2. The open porosity decreased from 56% to 38% due to the modification by micro-arc discharges, whereas the microhardness increased from 1013 ± 150 HV to 1639 ± 31 HV.

NANOSTRUCTURAL MECHANICALLY ALLOYED DISPERSION STRENGTHENED MATERIALS BASED ON METALS

Development of modern industries requires creation of new materials, which have, along with special properties, high strength and heat resistance. The most difficult is the provision of the latter. To increase it, it is necessary to reduce the amount of free movement of dislocations to a maximum permissible value, localize movement of defects of the crystal structure within the grain (subgrain), minimize the runoff of defects of the crystal structure at grain boundaries, exclude intergranular sliding, which can be successfully realized in dispersion-hardened materials. To obtain them, a promising technology is based on the reaction mechanical alloying. The article presents the results of the author’s work, which made it possible to establish the regularities in the formation of phase composition, structure and properties at all stages of obtaining mechanically alloyed dispersed-hardened materials based on metals, which are the basis for the production of basic structural alloys-aluminum, copper, iron and nickel. These alloys are used for products operating under severe temperature-strength conditions – at temperatures reaching 0.85T ml of the substrate. They have tensile strength 1.2–1.5 times higher than that of analogues, and on their basis a new area of materials science of structural materials was created – nanostructured mechanically alloyed dispersed-hardened materials based on metals. The most promising areas of application of technologies based on reaction mechanical alloying in the field of materials science are: 1) high-strength materials for machine parts, tooling and products of various functional purposes operating under severe temperature and force conditions; 2) powders for gas-thermal spraying and coating of them; 3) nanocrystalline modifying ligatures and modifiers. The results of industrial introduction are given – two small-scale enterprises for the production of import-substituting science-intensive products were created.

Novel Method of Aluminum to Copper Bonding by Cold Spray

Cold spray bonding (CSB) has been proposed as a new method for joining aluminum and copper. At high speeds, solid Al particles impacted the groove between the two substrates to form a bond between Al and Cu. Compared to traditional welding technologies, CSB does not form distinct intermetallic compounds. Large stainless steel particles were introduced into the spray powders as in situ shot peen particles to create a dense Al deposit and to improve the bond strength of joints. It was discovered that introducing shot peen particles significantly improved the flattening ratio of the deposited Al particles. Increasing the proportion of shot peen particles from 0 to 70 vol.% decreased the porosity of the deposits from 12.4 to 0.2%, while the shear strength of joints significantly increased. The tensile test results of the Al-Cu joints demonstrated that cracks were initiated at the interface between the Al and the deposit. The average tensile strength was 71.4 MPa and could reach 81% of the tensile strength of pure Al.