In-flight Particle Velocity Research Articles

Numerical Study of In-flight Particle Parameters in Low-Pressure Cold Spray Process

A 2-D model of the low-pressure cold spray with a radial powder feeding was established using CFD software in this study. The flow field was simulated for both propellant gases of nitrogen and helium. To predict the in-flight particle velocity and temperature, discrete phase model was introduced to simulate the interaction of particle and the supersonic gas jet. The experimental velocity of copper powder with different sizes was used to validate the calculated one for low-pressure cold spray process. The results show that the computational model can provide a satisfactory prediction of the supersonic gas flow, which is consistent with the experimental Schlieren photos. It was found that similar velocity was obtained with the drag coefficient formula of Henderson and with that of Morsi and Alexander. As the shape factor was estimated, the reasonable prediction of velocity for non-spherical particle can be obtained, to compare with the experimental results.

Comprehensive process maps to synthesize high density plasma sprayed aluminum oxide composite coatings with varying carbon nanotube content

Comprehensive process maps have been developed to synthesize “high density” plasma sprayed Aluminum oxide (Al 2O 3) composite coatings with 0, 4 and 8 wt.% carbon nanotube (CNT) reinforcement. These process maps correlate the key processing parameters such as plasma power, powder feed rate, primary gas flow rate, and stand-off distance to the temperature and velocity of the in-flight particles and finally, to the porosity and microhardness of coatings. Relative importance of each significant process parameters was also investigated by making “ Pareto diagrams”. Stand-off distance followed by the plasma power were found to be most sensitive parameters which affects the porosity of the plasma sprayed Al 2O 3–CNT composite coatings. Process maps showed that CNTs have a significant effect on altering the temperature and velocity of in-flight particles. Higher thermal conductivity of CNTs alters the heat transfer phenomena during the coating build up which assists in densification.

The Effect of HVOF Particle-Substrate Interactions on Local Variations in the Coating Microstructure and the Corrosion Resistance

Splashing and redeposition of droplets occur during thermal spray processing, which affects the coating porosity and morphology. Therefore, this phenomenon is important from a practical point of view such as corrosion. Particle interaction with substrate is a function of the particle velocity, viscosity, temperature, as well as the substrate temperature, chemistry, roughness, and geometry. In the present study, the splashing phenomenon was studied on CrC-NiCr and stainless steel materials deposited using the high velocity oxygen fuel process. The effect of particle splashing on the coating microstructure was investigated with respect to the corrosion properties. Particle behavior during impact was explained based on in-flight particle velocity and temperature measurements. It was found that the conditions that favor particle splashing promote occurrence of localized porosity. The localized porosity was a strong function of the substrate curvature and originated from the substrate asperities.

Phase Formation and Transformation in Alumina/YSZ Nanocomposite Coating Deposited by Suspension Plasma Spray Process

Suspension Plasma Spray process was used for deposition of pseudo-eutectic composition of alumina-yttria-stabilized zirconia as a potential thermal barrier coating using Mettech axial III torch. Process variables including feed and plasma parameters were altered to find their effects on the formation of phases in the composite coating. The in-flight particle velocity was found to be the crucial parameter on phase formation in the resulting coatings. Low particle velocities below 650 m/s result in the formation of stable phases i.e., α-alumina and tetragonal zirconia. In contrast, high particle velocities more than 750 m/s favor the metastable γ-alumina and cubic zirconia phases as dominant structures in as-deposited coatings. Accordingly, the plasma auxiliary gas and plasma power as influential parameters on the particle velocity were found to be reliable tools in controlling the resulting coating structure thus, the consequent properties. The noncrystalline portion of the coatings was also studied. It was revealed that upon heating, the amorphous phase prefers to crystallize into pre-existing crystalline phases in the as-deposited coating. Thus, the ultimate crystalline structure can be designed using the parameters that control the particle velocity during plasma spray coating.

プラズマ溶射によって成膜された遮熱コーティング材の非弾性変形(<小特集>ガスタービン用遮熱/耐環境コーティング技術)

A fundamental study for an inelastic deformation of freestanding plasma-sprayed thermal barrier coatings (TBCs) has been conducted. Cantilever-type bending tests are carried out to obtain a stress-strain curve of the freestanding ceramic coating peeled from the TBC coated sample by an electrochemical treatment. In order to investigate about an inelastic deformation and its mechanism appeared in the sample, in-situ scanning electron microscope (SEM) observation is performed by means of a small tensile testing device that can be inserted into the SEM vacuum chamber. The bending test result indicated that the coating deforms with a nonlinear behavior under a monotonic loading and with a hysteresis loop under cyclic loading, in spite of that it is ceramic material. In-flight particle velocity in the spraying parameter affected the stress-strain curve significantly. In-situ SEM observation during the bending test revealed that sliding at boundary between splats plays an important role in an inelastic deformation.

Inelastic Deformation of Freestanding Plasma-sprayed Thermal Barrier Coatings

A fundamental study for an inelastic deformation of freestanding plasma-sprayed thermal barrier coatings (TBCs) has been conducted. Cantilever-type bending tests are carried out to obtain a stress-strain curve of the freestanding ceramic coating peeled from the TBC coated sample by an electrochemical treatment. In order to investigate about an inelastic deformation and its mechanism appeared in the sample, in-situ scanning electron microscope (SEM) observation is performed by means of a small tensile testing device that can be inserted into the SEM vacuum chamber. The bending test result indicated that the coating deforms with a nonlinear behavior under a monotonic loading and with a hysteresis loop under cyclic loading, in spite of that it is ceramic material. In-flight particle velocity in the spraying parameter affected the stress-strain curve significantly. In-situ SEM observation during the bending test revealed that sliding at boundary between splats plays an important role in an inelastic deformation.

Atmospheric plasma spraying of yttria-stabilized zirconia coatings with specific porosity

The interdependence between plasma spray process parameters and porosity of YSZ coating microstructures was investigated with simultaneous consideration of the deposition efficiency. Based on a factorial experimental plan, the argon plasma gas flow, the current, the interaction of argon flow and current, and the spray distance for the Triplex II plasma gun were found to yield the main contributions to porosity as well as to deposition efficiency. Each of these three process parameters has a significant individual effect on the in-flight particle velocities and temperatures. The contribution to the effects on porosity arises almost exclusively from the particle temperature. Regarding the deposition efficiency, the larger contribution originates from the particle velocity. To achieve a targeted high porosity at reasonable deposition efficiency a simple linear regression model was applied yielding an argon flow of 50 slpm and a current of 470 A at a spray distance of 200 mm as the optimum parameter set. The average particle temperature estimated for this optimum is just above the melting temperature. At this setting, a porosity of 17.7% and a deposition efficiency of 32.5% may be expected. At a greater spray distance and lower power density (lower current and/or higher argon plasma gas flow) the deposition efficiency was observed to drop considerably. The cooling of the particles here becomes critical, i.e. the particles are only partly molten. This was verified by an analysis of the density distributions of measured in-flight particle temperatures.

Wear behavior of plasma sprayed composite coatings with in situ formed Al 2O 3

In the present study, the wear behavior of in situ formed Al 2O 3 reinforced hypereutectic Al–18Si matrix composite coatings have been investigated. These coatings were successfully fabricated with mechanically alloyed Al–12Si and SiO 2 powder deposited on aluminum substrates by atmospheric plasma spraying (APS). The produced samples were characterized by means of microscopic examinations, hardness measurements and wear tests. The obtained results pointed out that the amount of in situ formed Al 2O 3 particles increased with increasing spray distance and decreasing in-flight particle velocity and temperature, which was accompanied by an improvement in hardness and wear resistance.

Controlling graphite content in plasma sprayed cast iron coatings via in-flight particle diagnostic

Graphite formation and degradation in thermally sprayed cast iron coatings is a technological barrier for achieving superior wear resistant coatings. Therefore, there is a need to understand the in-flight particle behavior of cast iron powder and introduce new approaches to control the graphite content. In this study, it has been demonstrated that the graphite content can be controlled by means of in-flight particle diagnostic. For this purpose, cast iron coatings were plasma sprayed under a variety of spray conditions and characterized by using an optical microscope, X-ray diffractometer and electron probe micro-analyzer. As a result, a significant amount of graphite with respect to a wide range of in-flight particle temperature and velocity was preserved in cast iron coatings.

Evaluation of hillock-like deposition behaviour of HVOF thermal spray coatings using rotary shutter procedure

Evaluation of hillock-like deposition behaviour of WC/Co coatings was performed using a rotary shutter procedure coupled with 100 passes of high velocity oxy fuel thermal spraying. During the spraying procedure, a DPV 2000 in situ diagnostic instrument was used to monitor the temperature and velocity of inflight particles. The microstructure of the coatings was analysed using an optical microscope to monitor the porosity and thickness distributions at various positions of the hillock-like coatings. A 7·1% porosity was observed at the centre of the hillock-like coatings, and a 10% porosity was observed at the edge, each at a spraying distance of 200 mm. The centre area of hillock-like coatings exhibited almost a 30% decrease in porosity compared to the edge area. The results indicate a higher quality structure at the centre area of the hillock-like coatings.

In-situ TiB 2–Al 2O 3 formed composite coatings by atmospheric plasma spraying: Influence of process parameters and in-flight particle characteristics

In the present study, mechanically alloyed Al–12Si, B 2O 3 and TiO 2 powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). The effects of mechanical alloying (MA) time and plasma parameters (arc current and primary/secondary/carrier gas flow rate) on in-situ reaction intensity and in-flight particle characteristics (temperature and velocity) have been investigated. It has been observed that MA time has a remarkable effect on powder morphology and relative amount of in-situ formed TiB 2 and γ-Al 2O 3. In-flight particle diagnostic measurements demonstrate that among the plasma parameters arc current has the strongest effect on in-flight particle velocity and temperature. Also, results indicate that in-flight particle velocity is more dominant than temperature on the relative amount of in-situ formed phases.

Effective Parameters in Axial Injection Suspension Plasma Spray Process of Alumina-Zirconia Ceramics

Suspension plasma spray (SPS) is a novel process for producing nano-structured coatings with metastable phases using significantly smaller particles as compared to conventional thermal spraying. Considering the complexity of the system there is an extensive need to better understand the relationship between plasma spray conditions and resulting coating microstructure and defects. In this study, an alumina/8 wt.% yttria-stabilized zirconia was deposited by axial injection SPS process. The effects of principal deposition parameters on the microstructural features are evaluated using the Taguchi design of experiment. The microstructural features include microcracks, porosities, and deposition rate. To better understand the role of the spray parameters, in-flight particle characteristics, i.e., temperature and velocity were also measured. The role of the porosity in this multicomponent structure is studied as well. The results indicate that thermal diffusivity of the coatings, an important property for potential thermal barrier applications, is barely affected by the changes in porosity content.

Suspension HVOF Spraying of Reduced Temperature Solid Oxide Fuel Cell Electrolytes

Metal-supported solid oxide fuel cells (SOFCs) composed of a Ce0.8Sm0.2O2−δ (SDC) electrolyte layer and Ni-Ce0.8Sm0.2O2−δ (Ni-SDC) cermet anode were fabricated by suspension thermal spraying on Hastelloy × substrates. The cathode, a Sm0.5Sr0.5CoO3 (SSCo)-SDC composite, was screen-printed and fired in situ. The anode was produced by suspension plasma spraying (SPS) using an axial injection plasma torch. The SDC electrolyte was produced by high-velocity oxy-fuel (HVOF) spraying of liquid suspension feedstock, using propylene fuel (DJ-2700). The emerging technology of HVOF suspension spraying was explored here to produce thin and low-porosity electrolytes in an effort to develop a cost-effective and scalable fabrication technique for high-performance, metal-supported SOFCs. In-flight particle temperature and velocity were measured for a number of different gun operating conditions and standoff distances and related to the resulting microstructures. At optimized conditions, this approach was found to limit material decomposition, enhance deposition efficiency, and reduce defect density in the resulting coating, as compared to previous results reported with SPS. Produced button cells showed highly promising performance with a maximum power density (MPD) of 0.5 W cm−2 at 600 °C and above 0.9 W cm−2 at 700 °C, with humidified hydrogen as fuel and air as oxidant. The potential of this deposition technique to scale-up the substrate size to 50 × 50 mm was demonstrated.

Cermet coatings prepared by pulsed gas dynamic spraying process: Effect of the process parameters

The work presented is part of a research program aimed at exploring the possible advantages of the pulsed gas dynamic spraying (PGDS) process, as an alternative production technique of WC-based cermet coatings. In the PGDS process the feedstock particles are accelerated and heated in a high velocity flow before impacting the substrate to be coated. In this study the effect of the processing conditions, in particular the gas temperature, the spray pressure, the standoff distance, and the initial particle temperature on the cermet coating microstructure is investigated. Monitoring of the particle velocity at the gun exit is conducted using a laser diagnostic system in order to correlate it with the various spray parameters. Different analysis techniques (OM, SEM, XRD, and microhardness) are used to characterize the coatings produced. Detailed comparisons of powders and coating microstructure, phase composition, and microhardness are presented and discussed. The results show that an increase in spray temperature and in-flight particle velocity, obtained by increasing the gas temperature and pressure, leads to a decline in the volume fraction of defects within the coating as well as improvement of the substrate/coating interface. High deposition efficiency with low coating porosity level was obtained by decreasing the standoff distance and preheating the feedstock powder. Spraying onto Al-6061 and SS-304L type of substrates did not reveal a significant effect on the coating microstructure.

Study on gas permeation behaviour through atmospheric plasma-sprayed yttria stabilized zirconia coating

Gas permeation behaviour through atmospheric plasma-sprayed 8 mol% yttria stabilized zirconia (YSZ) electrolyte coating was studied experimentally. YSZ coatings were fabricated using different powder feedstock. The temperature and velocity of in-flight particles during spraying were measured with a diagnostic system. The results showed that particle temperature and velocity were significantly influenced by the size of powders. The gas permeability of these coatings was estimated by a specific instrument with pure O 2, N 2 and H 2. It was found that the gas permeability was reduced by decreasing the size of powder. Gas permeation behaviour through plasma-sprayed YSZ coating was studied. Transition flow was compatible to gas permeation behaviour for all three plasma-sprayed YSZ coatings. The relationship between gas permeation behaviour and coating microstructure is discussed in this article.

Optimization of Atmospheric Plasma Spray Process Parameters using a Design of Experiment for Alloy 625 coatings

Alloy 625 is a Ni-based superalloy which is often a good solution to surface engineering problems involving high temperature corrosion, wear, and thermal degradation. Coatings of alloy 625 can be efficiently deposited by thermal spray methods such as Air Plasma Spraying. As in all thermal spray processes, the final properties of the coatings are determined by the spraying parameters. In the present study, a D-optimal experimental design was used to characterize the effects of the APS process parameters on in-flight particle temperature and velocity, and on the oxide content and porosity in the coatings. These results were used to create an empirical model to predict the optimum deposition conditions. A second set of coatings was then deposited to test the model predictions. The optimum spraying conditions produced a coating with less than 4% oxide and less than 2.5% porosity. The process parameters which exhibited the most important effects directly on the oxide content in the coating were particle size, spray distance, and Ar flow rate. The parameters with the largest effects directly on porosity were spray distance, particle size, and current. The particle size, current, and Ar flow rate have an influence on particle velocity and temperature but spray distance did not have a significant effect on either of those characteristics. Thus, knowledge of the in-flight particle characteristics alone was not sufficient to control the final microstructure. The oxidation index and the melting index incorporate all the parameters that were found to be significant in the statistical analyses and correlate well with the measured oxide content and porosity in the coatings.

Investigation of Al-Al2O3 Cold Spray Coating Formation and Properties

Coating build-up mechanisms and properties of cold-sprayed aluminum-alumina cermets were investigated using two spherical aluminum powders having average diameters of 36 and 81 μm. Those powders were blended with alumina at several concentrations. Coatings were produced using a commercial low-pressure cold spray system. Powders and coatings were characterized by electronic microscopy and microhardness measurements. In-flight particle velocities were monitored for all powders. The deposition efficiency was measured for all experimental conditions. Coating performance and properties were investigated by performing bond strength test, abrasion test, and corrosion tests, namely, salt spray and alternated immersion in saltwater tests. These coating properties were correlated to the alumina fraction either in the starting powder or in the coating.

Effect of in-flight particle velocity on the performance of plasma-sprayed YSZ electrolyte coating for solid oxide fuel cells

In this paper, the effect of particle velocity on the characteristics of atmospheric plasma-sprayed yttria stabilized zirconia was investigated through adjusting auxiliary helium flow rate. The temperature and velocity of in-flight particles were measured with the DPV2000 diagnostic system. The results showed that helium flow rate significantly influenced particle velocity and less distinctly influenced particle temperature. The microstructure of the coatings was characterized by scanning electron microscopy and X-ray diffraction analyzer. The ionic conductivity of the deposits through thickness direction was measured by a potentiostat/galvanostat based on three-electrode assembly approach in a temperature range of 500–1000 °C. The specific gas permeability was estimated through measuring the coating gas flow rate with a home-made experimental setup. The results showed that the gas permeability was improved by increasing the in-flight particle velocity. However, the in-flight particle velocity had little effect on the ionic conductivity of specimens.

Influence of particle temperature and velocity on the microstructure and mechanical behaviour of high velocity oxy-fuel (HVOF)-sprayed nanostructured titania coatings

Nanostructured titania feedstock powders were deposited via high velocity oxy-fuel (HVOF) spraying onto Ti–6Al–4V substrates. Using in-flight particle diagnostics, different particle temperatures and velocities were employed in order to reveal their effects on microstructure and mechanical properties of the coatings. A series of linear dependencies were observed involving processing conditions (i.e., in-flight particle temperature and velocity) and characteristics of the resulting coating microstructural features and properties, such as, phase composition, Vickers microhardness and the deflection of Almen strips (residual stress). High-bond strength values were observed when compared with other ceramic thermal spray coatings available in the literature. This study provides different levels of information on the processing of nanostructured ceramic powders via HVOF spraying and opens possibilities for development and application of HVOF-sprayed nanostructured titania coatings in the biomedical field and other disciplines, where superior mechanical behaviour is required.

Oxidation control in atmospheric plasma spraying coating

Thermal spray coatings are formed by successive impingements and interbonding materials among the splats, solidified individual molten particles. Depending on the processing conditions employed during the spray process, deposits are produced with an assortment of microstructures and properties. This study highlights how the coating oxidation differences are influenced by the mechanisms involved during the spray process. The commercial steel powder referenced Amdry XPT 512 is chosen for a systematic study of comparison across different spraying techniques. Steel particles were sprayed with a F4 plasma torch and with a shrouded plasma spray process used in order to protect particles against atmospheric oxidation. The plasma jet was successively shielding by an inert gas shroud and by an oxidizing gas shroud. In-flight oxidation and post impact oxidation present in coatings are discussed in detail and the effects of these mechanisms on coating properties are addressed. The comparison was made on in-flight particle characteristics and on coating properties in terms of oxide content and porosity level. Using shrouded gas, in-flight characteristics are quite similarly independent of the nature of the shrouded gas. This way, the comparison of oxide contents present in the coatings corresponds to in-flight oxidation and is completely dependent on the nature of the shrouded gas. Comparing these results to those obtained by APS, a decrease in both velocity and temperature of in-flight particles was observed leading also to a significant decrease in oxide contents and to a slight increase in porosity level compared to coatings sprayed with air shrouding.