Short Spray Distance Research Articles

Impact of Precursor Thermal Decomposition Reactions on YSZ Coatings Deposited with a Low-Power Plasma Torch at Short Spray Distance

Impact of Precursor Thermal Decomposition Reactions on YSZ Coatings Deposited with a Low-Power Plasma Torch at Short Spray Distance

Low power plasma spray assisted thermal barrier coating repair without the plugging of cooling holes

Abstract Currently operating new generation aircraft engines are equipped with multi-hole combustion chambers, coated with a thick ZrO2-4 mol% Y2O3 (YpSZ) thermal barrier coating (TBC). The local repair of such multi-hole combustors requires the development of specific deposition methods. This paper describes a modified version of a low power plasma reactor (LPPR), upgraded with two opposite gas flows to prevent cooling channel plugging. The influence of some experimental parameters (plasma power density, spray distance and precursor concentration) on the repaired TBC microstructure is examined. A short spray distance, associated with a low power density and a low concentration of nitrate precursors, led to a suitable microstructure of the deposited YpSZ. Moreover, FTIR spectroscopy and SEM-EDS analysis revealed the efficient conversion of the nitrate precursors into oxide and the good infiltration of the deposited YpSZ into the damaged area. Finally, it is shown that, once the deposition parameters have been optimized, a dual flow system effectively allows local repair to be carried out without plugging the cooling system.

A new mathematical model for accurate quantification of cryogen spray cooling in cutaneous laser surgery using realistic boundary conditions.

Laser dermatologic surgery is widely used for various skin diseases. Prior to the laser treatment, transient cryogen spray cooling (CSC) is employed to prevent unwanted thermal damage of skin tissue due to melanin absorption in the epidermis. Accurate quantification on the cooling effect of CSC is essential in the subsequent laser energy dosage prediction. Based on the existing experimental data, we present a new dynamic correlation in terms of non-dimensional heat convection coefficient as a function of the dimensionless time and dimensionless spatial location. The effects of transient spray cooling and the thermal properties of experimental substrate are included in the correlation. The heat transfer within the skin tissue is calculated by a heat conduction equation. Meanwhile, the dynamic correlation of heat convection coefficient is incorporated into the heat conduction equation as a convection boundary condition. Simulation results demonstrate that the correlation correctly reflects several important characteristics of CSC and the model predictions are in consistent with the experimental observations. Numerical results indicate that the CSC time duration should be short enough to maintain selective cooling in epidermis while long enough to avoid the non-uniform spatial cooling on skin surface. A shorter spray distance (for example z = 30mm) is found to have better cooling effect than a longer spray distance (z = 60mm). The dynamic correlation in terms of non-dimensional heat convection coefficient proposed in this study can be used in accurate quantification of the effect of CSC. Optimal cooling duration of 75-100ms and short spray distance of 30mm are recommended for CSC clinical application.

Research on the Significant Effect of Nozzle’s Structure on Air Outlet Speed Based on Numerical Simulation

Aiming at the problems of short spray distance and poor atomization effect in pesticide spraying. In addition, in order to enhance the initial velocity of the droplets and the secondary atomization of the droplets, a new air-driven combined electrostatic nozzle based on the Venturi effect is designed in this paper. And a method for accelerating and atomizing mist droplets using the Venturi effect is proposed. Combined with the distribution of airflow from the venturi, the acceleration and atomization effects of the nozzle were verified and analyzed. Using the factor design test method, three structural parameters, namely the distance A from the nozzle outlet to the venturi mouth, the retracting radius B of the venturi mouth, and the extension distance C of the venturi mouth were tested, and the following conclusions were drawn. At the 1% significance level, the single factors A, B, and C have significant effects on the average velocity of the venturi throat. The interaction of two factors, AB, AC, and BC, had no significant effect on the average velocity of the venturi throat. The interaction of three factors ABC has a significant effect on the average velocity of the venturi throat.

Splash involved deposition behavior and erosion mechanism of long laminar plasma sprayed NiCrBSi coatings

The behavior of low-velocity NiCrBSi particles with a high surface temperature of 1800 °C during the long laminar plasma spraying process was studied. During the experiment, the maximum length of the laminar plasma jet exceeded 600 mm, and coatings were obtained using a laminar plasma spray at a wide spray distance ranging from 250 to 500 mm. Interestingly, the coatings obtained at a short spray distance of 250 mm showed special alternate porous and compact lamellar structure with island-like bulges that were evenly distributed on the coating surface. This type of structure was possibly formed by a low-velocity splashes deposition process intensified by the furry-like structure, which can be attributed to vapor phase deposition. The coatings obtained at an extremely long spray distance of 500 mm showed massive cracks. Moreover, the medium-distance sprayed coatings of 350–450 mm showed similar lamellar-like microstructure and mechanical performance because of the similarity in the particle states in the low-energy-gradient laminar plasma jet. Thus, a laminar plasma spray shows great reproducibility and potential for improving the spraying efficiency on an uneven plane.

The microstructure and selected mechanical properties of Al2O3 + 3 wt.% TiO2 plasma sprayed coatings


 
 
 The Al2O3+TiO2 coatings are of the interest of surface engineering due to their high hardness and wear resistance but also increased toughness, when compared to pure Al2O3 ones. This article describes the deposition of Al2O3+3 wt.% TiO2 coatings by Atmospheric Plasma Spraying (APS) technique. The commercial AMI 6300.1 powder (-45 + 22 μm) was used as a feedstock. The 2k+1 spraying experiment, based on two variables, namely spray distance and torch velocity, was designed. The samples were characterized in the terms of morphology, microstructure, microhardness and roughness. It was observed that the shorter spray distance resulted in lower porosity, higher microhardness and lower roughness of coatings.
 
 

Segmented Thermal Barrier Coatings for ID and OD Components Using the SinplexPro Plasma Torch

Thermal barrier coatings (TBCs) with high thermal strain tolerance and erosion resistance are commonly applied onto the inner and outer diameters of hot sections of gas-turbine engine components. In this work, strain-tolerant, segmented TBCs were developed using the SinplexPro-cascaded torch. Design of experiments were carried out to study the effect of process variables such as plasma power, powder feeding rate, spraying distance and surface speed on the coating microstructure and properties. Optimized process parameters for the segmented coating microstructures at shorter spray distance ( 114mm) are achieved, which are targeted for spraying inner diameter (ID) and outer diameter (OD) engine components, respectively. The plasma torch hardware life was evaluated by torch cycle duration runs. Examples of highly strain-tolerant TBCs onto the ID and OD engine components were demonstrated, highlighting the wide versatility and process range of the SinplexPro.

Effects of spraying conditions on the microstructure and properties of NiCrBSi coatings prepared by internal rotating plasma spraying

NiCrBSi alloy coatings were fabricated via internal rotating plasma spraying (IRPS) inside the cylinder liner and in open space using the same processing parameters. The effects of spraying condition on the microstructural, mechanical and tribological properties of the IRPS coatings were investigated. Simultaneously, the effects of the flow rate of primary gas (FRPG) on the coatings at a short spray distance were thoroughly evaluated. The microstructure, Vickers hardness, bond strength and tribological properties of the coatings were experimentally determined and characterized in detail. The results demonstrated that all coatings fabricated by IRPS had a dense structure and high microhardness. The bond strength and tribological properties were effectively promoted inside the cylinder, in contrast to open space condition. It is also found that coefficient of friction and wear volume of the coatings declined with the increasing of FRPG. Especially, the coatings prepared with FRPG at 100 L⸱min−1 exhibited the lowest friction and best wear resistance despite their higher porosity. The main wear mechanisms were abrasive and fatigue wear for the coatings deposited in open space, while the dominant wear mechanisms for the coatings prepared inside the cylinder were adhesive wear and tribolayer delamination.

TRIBOLOGICAL PROPERTIES OF Al2O3 + TiO2 COATINGS MANUFACTURED BY PLASMA SPRAYING

In the paper the results of tribological, microscopic, and mechanical research of Al2O3 + TiO2 coatings manufactured by plasma spraying are presented. The feeding material was a powder Al2O3 + 13 wt.% TiO2 (Metco 6221, OerlikonMetco) with grain size – 45 ± 15 μm. The cylinder substrates made from stainless steel (X5CrNi18-10) had a diameter equal to 25 mm and 2 mm of thickness. The variable spray parameters were plasma torch velocity in terms of substrate and spray distance. The morphology of obtained coatings was tested by scanning electron microscope (SEM), and the microstructure was investigated by light optical microscopy (LOM) and SEM. The results of mechanical properties examinations revealed the dependence of the microhardness and fracture toughness on the spray parameters. Tribological examinations were made in the ball-on-disc mode in technical dry friction conditions. Two loads were used, 5 N and 10 N. Based on the carried out tests, it could be concluded that a shorter spray distance and a reduction of the torch velocity allows one to achieve a more compact structure, which is characterized by good adhesion at the coating-substrate interface (in range from 11 to 14 MPa) and good wear resistance.

Cold gas spraying of Ti-48Al-2Cr-2Nb intermetallic for jet engine applications

The present article describes aspects of the cold gas spray processability of the intermetallic Ti-48Al-2Cr-2Nb (at. %) alloy, which is employed as a structural material in gas turbine engines. The effects of processing parameters, namely, gas pressure, gas temperature, spray distance, as well as the gas atomized feedstock particle size (d50 = 30 and 42 μm, respectively) and phase composition on deposition, were investigated. The results showed that when the highest available gas pressure (40 bar) and temperature (950 °C) were combined with a short spray distance (20 mm), well-adhering coatings could be deposited regardless of the investigated particle size. However, the maximum coating thickness could be achieved was about 30 μm with a deposition efficiency of 1%. Phase composition of the gas atomized feedstock was investigated with HT-XRD and according to the findings, heat treatment of the feedstock under vacuum was carried out. With this treatment, non-equilibrium, disordered α phase of the atomized powder was transformed into an α, α2 and γ phase mixture. At the same time, an increase in the hardness and oxygen content of the powder was detected. Swipe test performed with the heat treated powder revealed no improvement in terms of deposition, in fact, the number of adhering particles on the substrate was decreased in comparison with that of the untreated powder.

Effects of spray parameters on the adhesion between plasma-sprayed cast iron splat and aluminium substrate

For a thermally sprayed coating, a strong adhesion between the coating and the substrate is critical for its service performance. Furthermore, the adhesion between the coating and the substrate is mainly controlled by the splat-substrate adhesion. Therefore, enhancing the splat-substrate adhesion is important for the preparation and application of thermally sprayed coating. In the present work, individual cast iron splats were deposited on polished aluminium surfaces by atmospheric plasma spraying technique (APS) with varying plasma arc powers and spray distances. After spraying, the splat-substrate interfacial structure was examined by a field emission scanning electron microscope (FESEM). The effects of the arc power and spray distance on the morphology of the splats were studied. It was found that high arc power promoted splat-substrate adhesion by elevating the temperature and velocity of the impinging particles compared to low arc power. Moreover, a short spray distance promoted splat-substrate adhesion due to the heating of the substrate surface by the plasma jet and the high impinging velocity of the particles compared to a long spray distance.

Dense yttria-stabilized zirconia coatings fabricated by plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition (PS-PVD) is a novel technique which can offer new opportunities to obtain advanced microstructures. In this study, dense yttria-stabilized zirconia (YSZ) coatings were deposited using PS-PVD with proper parameters. The dependencies of the microstructure of the as-sprayed coatings on axial spray distance and radial position were discussed. Four typical coatings were selected for the current work. The corresponding microstructures, phase compositions and mechanical properties were studied in detail. These as-sprayed coatings had dense lamellar or lamellar/columnar hybrid microstructures, the lowest porosity was 0.44%. Both monoclinic and tetragonal zirconia were detected in the as-sprayed coatings. The distribution of yttrium was not homogeneous, especially in the coatings deposited at shorter spray distance. Furthermore, oxygen got lost partly during the spraying process with decreasing spray distance. The hardness (H) and Young's modulus (E) of the coatings changed simultaneously with the different microstructures. The maximum values of H and E were 16.6±0.6 and 234.3±10.1GPa respectively, which were found in the densest coatings. The dense PS-PVD ceramic coatings might be applicable in anti-corrosion fields (such as alkali corrosion, marine corrosion and hot corrosion).

Development of hydroxyapatite coatings by solution precursor plasma spray process and their microstructural characterization

Solution precursor plasma spraying (SPPS) is a promising method of obtaining coatings with unique microstructural features. The present study reports about the development of hydroxyapatite (HA) coatings by SPPS using water-based calcium-phosphate solution and about the analyses of their microstructure. X-ray diffraction (XRD) analysis showed that HA was the major crystal phase of the coatings, being present in the range from 95% to 98%. The coatings of greater HA content were obtained while spraying using shorter spray distance. The coating crystallinity ranges from 24% to 69% depending on the temperature at deposition. An increase of the temperature results in greater crystallinity. The obtained coatings' microstructures may be described by dome-like surface and by the columnar-like cross-section when using precursor with low concentration. The use of concentrated solution precursor results in cauliflower-like structures composed of agglomerated fragmented shells and fine spherical particles. The possible phenomena occurring in droplets in contact with the plasma jet and the coating build-up mechanisms were tentatively explained.

Effect of spray parameters on the splashing of plasma-sprayed cast iron particles

To understand the effects of spray parameters on the splashing, cast iron particles were plasma-sprayed onto polished surfaces of aluminum substrate to form single splats. Various plasma arc powers and spray distances were applied to adjust the morphology of the splats which was studied using a field emission scanning electron microscope (FESEM). The experimental results showed that the splashing of impinging droplets was significantly restrained for the splats deposited with high arc power (30 kW) and short spray distance (80 mm). This finding would be beneficial to improving the adhesive strength of the coating.

Technical Study of Laminated Template Electroforming in Fabrication of Metal Parts

Laminated templates electroforming (LTE) is one kind of metal-parts directly forming technologies which are based on discrete/accumulation theory. This paper introduces the forming principle, equipment configuration and experimental studies. By using templates as auxiliary tool and jet electroforming as filling method, the current density of electroforming was significantly improved and a group of copper parts in specific shape were fabricated. Experimental results show that on-load voltage, nozzle diameter, spray distance, spray flow velocity have direct influence on processing speed and locality. A small diameter of nozzle and short spray distance helps to achieve a precise locality and good dimensional accuracy, after process parameters have been optimized.

An experimental study on the spray and thermal characteristics of R134a two-phase flashing spray

Flashing spray of volatile liquids is a common phenomenon observed in many industrial applications such as fuel injection of engines, accidental release of flammable and toxic pressure-liquefied gases, failure of a vessel or pipe in the form of a small hole in chemical industry, and cryogenic spray cooling in laser dermatology, etc. In flashing spray, the volatile liquid is depressurized rapidly at the exit of a nozzle (or a hole in a vessel) and becomes superheated. Such superheated liquid (in the form of either a jet or droplets) will lead to explosive atomization with fine droplet and a short spray distance. This paper presents an experimental investigation to the spray and thermal characteristics of flashing spray using cryogen R134a. A photographic study of the spray is firstly conducted to visualize the spray formation and the dynamic characteristics of the spray. Afterwards, the spray characteristics are measured by the phase Doppler Particle Analyzer (PDPA). The distributions of the diameter reveals the dramatic dynamic variation of the liquid droplets due to explosive atomization of large droplets in the region near the exit of nozzle, while the self-similar velocity profiles are fitted by two empirical correlations to describe the non-dimensional axial and radial velocities, respectively. The temperature field within the spray is measured by a small thermocouple. The temperature measurements provide detailed quantitative information of both radial and axial temperature distributions of droplets within the spray. These experimental results provide deep understanding into the whole characteristics of two-phase flashing spray of volatile liquids.

Architecture and phases composition of suspension plasma sprayed alumina–titania sub-micrometer-sized coatings

Sub-micrometer-sized Al 2O 3–TiO 2 plasma-sprayed coatings exhibit superior performances compared to micrometer-sized ones. Two routes can be implemented to manufacture such finely structured coatings: i) spraying micrometer-sized agglomerates of nanometer-sized particles which results in a two-scale coating architecture and ii) spraying a suspension of sub-micrometer-sized particles (suspension plasma spraying, SPS). SPS was implemented in this study and Al 2O 3-base coatings with 13 and 60 wt.% of titania, respectively, were manufactured by spraying a suspension made of a mixture of Al 2O 3 and TiO 2 particles both of 300 nm, average diameter. Coating structural features and phase contents were studied. Results show that the coatings exhibit a very fine lamellar structure with a homogeneous repartition of Al and Ti. Complex phases, made of intermediate Al, Ti, and O oxides, have been also identified. Indeed, coatings formation results from rapid solidification rates and high transient thermal fluxes imparted by the plasma flow to the substrate due to the short spray distance encountered in SPS (in the order of 30 mm) requested by the small kinetics and thermal inertia of sub-micrometer-sized particles.

Influence of NH4Cl Powder Addition for Fabrication of Aluminum Nitride Coating in Reactive Atmospheric Plasma Spray Process

Reactive plasma spray is the key to fabricating aluminum nitride (AlN) thermally sprayed coatings. It was possible to fabricate AlN/Al composite coatings using atmospheric plasma spray process through plasma nitriding of Al powders (Al 30 μm). The nitriding reaction and the AlN content could be improved by controlling the spray distance and the feedstock powder particle size. Increasing the spray distance and/or using smaller particle size of Al powders improved the in-flight nitriding reaction. However, it was difficult to fabricate thick and dense AlN coatings with an increase in the spray distance and/or when using fine particles. Thus, the coatings thickness was suppressed because of the complete nitriding of some particles (formation of AlN particles) during flight, which prevents the particle deposition. Furthermore, the excessive vaporization of Al fine particles (due to increased particle temperature) decreased the deposition efficiency. To fabricate thick AlN coatings in the reactive plasma spray process, improving the nitriding reaction of the large Al particles at short spray distance is required to decrease the vaporization of Al particles during flight. This study investigated the influence of adding ammonium chloride (NH4Cl) powders on the nitriding process of large Al powders and on the microstructure of the fabricated coatings. It was possible to fabricate thick AlN coatings at 100 mm spray distance with small addition of NH4Cl powders to the Al feedstock powders (30 μm). Addition of NH4Cl to the starting Al powders promoted the formation of AlN through changing the reaction path to vapor-phase nitridation chlorination-nitridation sequences as confirmed by the thermodynamic analysis of possible intermediate reactions. This changes the nitriding reaction to a mild way, so it is more controlled with no explosive mode and with relatively low heating rates. Thus, NH4Cl acts as a catalyst, nitrogen source, and diluent agent. Furthermore, the evolved gases from the sublimation or decomposition of NH4Cl can prevent the Al particles coalescing after melting.

Fabrication of AlN Coatings by Reactive Atmospheric Plasma Spray Nitriding of Al Powders

Atmospheric Plasma Spray (APS) is well-established process and is widely used to produce various coatings with high deposition rate for structural and functional applications. Aluminum nitride is a promising material due to its outstanding properties. In this study, AlN coatings were synthesized by APS process through reactive plasma nitriding of Al powders by nitrogen plasma. Al powders were supplied into the plasma stream by Ar carrier gas and reacted with the surrounding plasma, then were deposited onto substrate. The obtained coatings were cubic-AlN/Al mixture with 100 μm of thickness at 150 mm of spray distance. Since the nitride content increases gradually with spray distance, the coating almost consists of AlN at 300 mm. However, the thickness of the coatings decreased with increasing spray distance, it was less than 10 μm at 300 mm spray distance. The nitriding reaction was investigated and it indicated that Al particles may react during flight and after deposition on the substrate surface. Completing the nitriding during flight (formation of AlN in-flight) decreases the coatings thickness. We can conclude that enhancing the nitriding reaction after deposition at short spray distance may enable to fabricate thick AlN based coatings.

Plasma Spraying and Characterization of Tungsten Carbide‐Cobalt Coatings by the Water‐Stabilized System WSP

Tungsten carbide‐cobalt powders (WC‐17wt% Co) were plasma sprayed by a water‐stabilized system WSP. Experiments with variable feeding distances and spray distances were carried out. Thinner coatings were deposited on carbon steel substrates and thicker coatings on stainless steel substrates to compare different cooling conditions. Basic characterization of coatings was done by XRD, SEM, and light microscopy plus image analysis. Microhardness was measured on polished cross‐sections. The main focus of investigation was resistance against wear in dry as well as wet conditions. The appropriate tests were performed with set‐ups based on ASTM G65 and G75, respectively. The influence of spray parameters onto coating wear performance was observed. The results of mechanical tests were discussed in connection with changes of phase composition and with the quality of the coating′s microstructure. The results show that for obtaining the best possible WC‐17Co coating with WSP process, from the viewpoint of wear resistance, the desired parameters combination is long feeding distance combined with short spray distance.