In this paper, yttria-stabilized zirconia (YSZ) coatings were prepared by plasma spraying of ready-to-spray suspensions provided by three different manufacturers. High-enthalpy hybrid water-argon plasma torch WSPH 500 was successfully used for deposition of coatings with porous and columnar microstructure consisting of tetragonal non-transformable phase. Sensitivity of the deposition process to variation of deposition conditions was also evaluated by the change of suspension injection point position. Slight differences in the microstructures of the deposited coatings (in particular character of porosity and mutual bonding of the microsplats) were reflected in slight but measurable differences in hardness and wear resistance of the coatings indicating changes in the coating cohesion. Tensile adhesion/cohesion strength of the coatings was found to be in the range of 9 to 15 MPa. High coating porosity desirable for low thermal conductivity combined with high suspension feed rate (from about 100 to 120 ml/min in this study) makes the WSP-H coatings promising for further development for example in thermal barrier coatings applications.

The high wear resistance of Cr 3 C 2 -NiCr coatings is reliant on the formation of a dense coating containing a high percentage of carbide grains, with minimal carbide degradation. Such coating characteristics are typically achieved through the use of high velocity oxygen fuel (HVOF) spraying. The propane fuelled, manually operated HIPOJET 2700 HVOF system is one of a suite of smaller sized commercial HVOF systems recommended for smaller job shops. However, few works have characterised the properties of carbide composite coatings produced with this system. In this work a full factorial design of experiment analysis was used to assess the effect of key operating parameters on the quality of Cr 3 C 2 -NiCr coatings. The combustion parameters (fuel and oxygen flows) were fixed at the manufacturers recommended settings in order to focus on the effect of nozzle length, powder feed rate and, spray distance. The effect of these variables on the porosity/oxide content, carbide content, microhardness, coating thickness, and relative deposit efficiency is discussed.

This study investigates cold spray particle velocities achieved at different pressures and particle feed rates using particle image velocimetry (PIV). Particle dispersion and velocity evolution along the jet axis were investigated for several feedstock materials. It was found that average particle velocity decreases with increasing particulate loading. The effect is aggravated at lower pressures, but mainly depends on feedstock material, which implies more complex, volume-fraction related physics playing a role. Velocity distribution and particle dispersion were also found to be influenced by particle feed rate, depending on the material. Increased particle feed rates affect the magnitude and distribution of impact velocity and consequently the efficiency of cold spraying.

The triplex-torch design providing three stable arcs in power and geometry promises a stable, quasi-laminar plasma jet leading to an uniform plasma and thus an uniform particle distribution. Additionally, the three powder injectors lead to an extended powder feed rate, resulting in a lower spraying time. This paper deals with particle properties of YSZ (SM 204NS) with respect to their injection location and powder feed rate and the resulting coating microstructure and porosity levels. Concerning the injection conditions, the three powder jets can be clearly identified. Their location depends on the spraying distance and less pronounced on the injection velocity. Interestingly, if the different powder-sizes are grouped and the jet cross-section is plotted for the different size cuts, the position of each size cut varies. Thus, a local separation of the powder takes place in the jet. The powder feed rate was varied from 30 to 320 g/min. Accordingly to this variation, the particles mean temperature was lowered from 3050°C down to 2650°C, and the velocity values dropped from 230 m/s to 155 m/s. Regarding the achieved coating thickness, up to a powder feed rate of approximately 210 g/min the thickness is increasing with no dramatic change in the porosity values. More powder does not lead to a thicker coating but results in a significantly higher porosity. Moreover, the microstructure of the coatings obtained by using a higher feed rate exhibits more segmentation cracks.

The method of simulating the wear performance under working conditions using a high-temperature ultra-high-speed wear testing machine was adopted to study the effect of feed rate variation on the wear behavior and scraping performance of the AlSi/hBN sealing coating and TC4 simulated blades. The macro and micro morphology of the coating and blades were analyzed by stereomicroscope and scanning electron microscope (SEM). The phase composition of the coating was analyzed by energy dispersive spectrometer (EDS) and X-ray diffraction. The results showed that, under the conditions of temperature of 450°C, line velocity of 300m/s, and feed depth of 500μm, the change in feed rate significantly affected the macro and micro morphology and wear mechanism of the AlSi/hBN sealing coating-TC4 simulated blades. At low feed rates, severe wear occurred, mainly manifested as grooving, adhesion transfer, and overheating mechanisms. At medium to high feed rates, good machinability was observed, mainly manifested as cutting and transfer of coating material to the blades.

Traditional fused and crushed (F&C) titania feedstock powders have relatively poor flow and are limited in the feed-rates that can be achieved before deposition efficiencies (DEs) start to decline. In addition, the coatings made with F&C powders generally have high stiffness and limited fracture toughness. Such coatings bear the risk of cracking/delaminating particularly in the case of thick coatings. To address these issues, a new agglomerated and sintered (A&S) titania feedstock powder has been developed and compared in side-by-side spray tests to the traditional F&C titania powder of comparable particle size. The spray behavior, achievable feed-rates and DEs, as well as the resulting coating characteristics were evaluated from the view point of their application as thick, electrically conductive coatings. The new A&S powder yields up to approximately 200% improvement in DE while producing coatings with more suitable microstructure, lower electrical resistivity and higher thickness. Furthermore, for a given set of process parameters, the DEs obtained with this new powder show little sensitivity to the powder feed-rates, thus allowing spraying at higher feed-rates without compromising coating DEs. This feature of the powder has significant commercial advantages for thick coatings when combined with high throughput guns such as TriplexPro-200. Preliminary results of particle diagnostics towards understanding of the fundamental principles behind these improvements are also discussed.

In this work, a novel liquid fuel HVOF process fueled with ethanol was used to prepare 75wt%Cr 3 C 2 –25wt%NiCr coatings on AISI304 stainless steel substrate. Taguchi method was employed to optimize the spray parameters (ethanol flow rate, oxygen flow rate, powder feed rate and standoff distance) to achieve better erosion resistance at 90° impact angle. The results indicated that ethanol flow rate and oxygen flow rate were identified as the highly contributing parameters on the erosion wear loss. The important sequence of the spray parameter is ethanol flow rate > oxygen flow rate > standoff distance > powder feed rate. The optimal spray parameter (OSP) for minimum erosion wear loss was obtained under ethanol flow rate of 28slph, oxygen flow rate of 420slpm, powder feed rate of 76.7 g/min and standoff distance of 300mm. The phase composition, microstructure, hardness, porosities, and the erosion wear behaviors of the coatings have been studied in detail. Besides, erosion wear testing of the optimized coating was conducted at 30°, 60° and 90° impact angle using air jet erosion testing machine. The SEM images of the erodent samples were taken to analyze the erosion mechanism.

HVOF sprayed tungsten carbide (WC) thermal spray coatings are finding increasing acceptance for replacement of electroplated chromium on aircraft landing gear parts. In order to replace chrome plating by an HVOF WC coating, the latter should exhibit wear and fatigue characteristics at least as good as those of chrome plating. Sliding wear performance and fatigue life of tungsten carbide coated parts depend on morphology and phase composition of the coating which in turn depend on spray parameters such as powder characteristics, powder feed rate, gas flow rates, and spray distance. A Design of Experiments (DOE) approach for a Jet Kote™ HVOF spray system was used to identify optimal spray parameters for WC-Co and WC-Co-Cr coatings based on best sliding wear and best fatigue characteristics. Best sliding wear was defined as minimum block loss in the standard ASTM-G77 wear test in which a rotating WC coated ring is tested against a stationary Al-Ni-Bz block. Best fatigue characteristics were defined as compressive residual stresses in the coating in the range 250 to 450 MPa. Spray parameters found to strongly affect wear were powder feed rate, oxygen flow rate, powder size, and a powder type-powder size interaction. Spray parameters strongly affecting residual stresses include powder type, hydrogen flow rate, and powder feed rate. Some spray parameters were found to be orthogonal with respect to wear behavior and residual stress. This means some parameters can be adjusted to maximize one performance criterion (wear or residual stress) without adversely affecting the other.

The paper reports numerical simulation results of a direct current (DC) suspension plasma spray with an axial injection system. In the numerical modelling, a two-way coupled Eulerian-Lagrangian approach was employed to simulate plasma flow and suspension behavior. As effects of two-way interaction, momentum transfer and energy transfer were chosen. The plasma spray was assumed to have a rotationally symmetric, two-dimensional shape around the injection axis of suspensions (the central axis), and therefore the axisymmetric two-dimensional approximation was adopted in the numerical modelling. Working gas was pure argon and was supplied from both the anode-side and the cathode-side ports. A total volume flow rate of the working gas from anode and cathode was set to 46.5 slm. A feed rate of suspension was parametrically set to 0, 15, 25, or 35 g/min. Numerical results indicate that the temperature of a plasma hot-core region and the velocity of a plasma jet around the central axis drop more with increasing feed rate of suspension mainly because of a decrease in Joule heating with increasing it. The numerical results also suggest that the increase in feed rate of suspension leads to practically lengthening flight distance of suspensions required for completing evaporation process of disperse medium.

In the present work thin Al 2 O 3 coating was obtained mainly by low pressure plasma spray (LPPS). Low-porosity and low-roughness deposits resulted from the optimized spray conditions, i.e. plasma parameters, grit blasting, powder feed rate and specimen rotation speed. Results showed that LPPS processing was highly beneficial for densifying the ceramic coatings, especially when coupled to a moderate powder feed rate. Coating average surface roughness (Ra) ranged from 1.5 to 2.5 µm for a coating thickness of less than 30µm and an original substrate Ra of 1.1µm. The spray conditions were optimized particularly for a low feed rate and a high specimen rotating speed to lower surface roughness. Moreover, a specific atmosphere/temperature control device was developed (using local gas injection close to the specimen to be coated). This resulted in improving cooling efficiency, which reduced microcracking in the deposits. Mechanical pull-off adhesion test was also carried out to evaluate these low-roughness thin coatings. Adhesion was shown to be satisfactory for direct coating (i.e. without any bond coat) of a low-roughness (Ra=1.1µm) AISI 316L substrate.

SUS316L coatings were sprayed by a high-velocity air fuel (HVAF) system to reduce oxidation and thereby improve the corrosion behavior of stainless coatings. The effects of powder feed rate and particle size on the microstructure, oxide amount and adhesion strength of the coatings were investigated. The corrosion resistance of the coatings was evaluated by conducting salt spray tests. The oxide amount in the coatings sprayed by the HVAF process is below 7 % and adhesion strength is below 34 MPa. In comparison with those of coatings sprayed by a high velocity oxygen fuel (HVOF) system, the oxide amount and adhesion strength are decreased because the particles heated below the melting point of the alloy and insufficient softened in HVAF process. The coatings deposited are original porous, and they become denser through the impinging effect caused by the following sprayed particles. With the increase of powder feed rate and particle size, there is a tendency of reduction in oxides, and an obvious decrease in adhesion strength. Corrosion resistance of the unsealed coatings is insufficient, and this becomes notable with increasing powder feed rate and particle size. The sealed HVAF coating sprayed with the largest particles shows the best corrosion resistance.

The applications of Wire Arc Spraying (WAS) include large-area corrosion protection coatings e.g. the protection of off-shore wind power plants. While WAS is cost efficient and well-known, the inherent instabilities can lead to coating defects and subsequent vulnerabilities in the corrosion protection coating. The occurrence of these process-related fluctuations cannot be predicted by deterministic models. However, these fluctuations can be monitored in situ, analyzed and finally minimized. A sensor unit is set up on the free jet of a WAS process using ZnAl15 wire. Voltage, amperage, noise and wire feed rate are measured in situ at a sampling rate of 80 MHz. Following a design of experiments approach, 64 different parameter settings are run and measured. For that purpose, voltage, atomizing gas and wire feed rate of the free gas jet have been varied. A generalized linear model (GLM) is trained on the dataset. A Fast Fourier Transformation (FFT) in conjunction with smoothing filters is conducted. Adopting the GLM enabled the calculation of parameters that minimize process fluctuations. Plots in the form of response surfaces depict the influence of the varied parameters on the process stability. A signal analysis using FFT revealed major periodic changes of the voltage in the range of 0.5-1 kHz next to process control-related frequencies at 20 kHz. The mounting and structuring of the data as well as the calculation of key figures is fully automated. Due to the high degree of automation, large quantities of data can be processed. In the future, a simplified version of the adopted sensor unit may be adopted to optimize parameters in an autonomous way. This can ensure not only the minimization of process fluctuations for any chosen feed wire, but also indicate irregularities in the process. The high-resolution recording and automated analysis of the data allows the determination of optimized parameters as well as major underlying frequencies.

Plasma spraying with axial powder injection inside the anode nozzle is a method considered to markedly enhance deposition efficiency and reduce plasma power compared to the most general method of injecting powder at the nozzle exit. However, powder injecting inside the nozzle will also likely cause problems from powder deposition on the nozzle wall and clogging of the particle feed channels. In this study, spherical stainless steel 316 and angular alumina powders with a mean size of 20-40 μm are used to deposit coatings via axial powder injection. The effect of powder feed rate, gas flow rate, and plasma power on deposition efficiency and particle clogging are investigated. The results show that particle clogging can be avoided by reducing powder feed rate and increasing the velocity of the plasma jet.

The use of a liquid feedstock carrier in suspension plasma spray (SPS) permits injection of fine powders, providing the possibility of producing sprayed coatings that are both thin and dense and have fine microstructures. These characteristics make SPS an attractive process for depositing highly efficient electrodes and electrolytes for solid oxide fuel cell (SOFC) applications. In the present study, NiO-yttria stabilized zirconia (YSZ) anode and YSZ electrolyte half cells were successfully deposited on porous Hastelloy X substrates by SPS. The NiO-YSZ anode deposition process was optimized by design of experiment. The YSZ electrolyte spray process was examined by changing one parameter at a time. The results from the design-of-experiment trials indicate that the porosity of the as-deposited coatings increased with an increase of suspension feed rate while it decreased with an increase of total plasma gas flow rate and standoff distance. The deposition efficiency increased with an increase of total plasma gas flow rate, suspension feed rate and standoff distance. The microstructure examination by SEM shows that the NiO and YSZ phases were homogeneously distributed and that the YSZ phase had a lamellar structure. It was observed that the density of the YSZ electrolyte layer increased as input power of the plasma torch increased.

Nanostructured WC-12%Co coatings were deposited by suspension plasma spraying of submicron feedstock powders, using an internal injection plasma torch. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a fine surface finish. A PEI (polyethylene-imine) dispersant was used to stabilize the colloidal suspension in an ethanol carrier. In-flight particle states were measured for a number of operating conditions of varying plasma gas flow rates, feed rates and standoff distances, and related to the resulting microstructure, phase composition (EDS, SEM, XRD) and Vickers hardness. High in-flight particle velocities, in excess of 800 m/sec, were generated, leading to dense coatings. It was observed that the coating quality was generally compromised by the high temperature and reactivity of the small particles. To compensate for this shortcoming, the suspension feed rate was adjusted, thereby varying the thermal load on the plasma. Results showed that a slightly larger agglomerate size, in conjunction with low particle jet temperatures, could somewhat limit the decomposition of WC into brittle W2C/W3C and amorphous cobalt containing binder phases.

This study investigates the influence of powder feed rate on the deposition efficiency of HVOF sprayed materials, including NiCr, FeNiCrMoCSi, 316L stainless, Cr 3 C 2 /NiCr, WC-Co/Cr, and WC-Cr 3 C 2 /Ni. A liquid-fuel HVOF spray gun is used in combination with a modified injector block that increases the number of powder injection ports from two to four. In the experiments, powder feed rates were incremented from a baseline of 100 g/min to 400 g/min for the metal powders and up to 500 g/min for the cermets, while measuring deposition efficiency for each run. Coating samples were also produced for metallographic analysis, hardness testing, and the evaluation of porosity and roughness. All results are presented and discussed along with potential implications on coating costs.

This work focuses on the laser cladding process and the behavior or interaction between the powder particles and the laser beam, specifically examining how various process parameters might affect the creation of melt pool formations. The experiment focused on examining the influence of laser intensity and other important factors on the amount of metal in the substrate of 316L stainless steel, particularly while utilizing Inconel 625 powder. The study was conducted by utilizing cross-sectional images and quantifying the ratio of areas of the melted substrate material across a sliced cross-sectional area. The study also investigated the influence of recirculation patterns resulting from the Marangoni convection force on the formation of the melt region. The study's results indicate that a low powder feed rate is preferable, which in this study was 5 g/min, and provides better results with a symmetrical and profound melt profile. The melt shifts to asymmetrical profiles when the feed rate increases significantly over this value. The primary cause of this phenomenon is attributed to the Marangoni forces and the momentum transfer generated by the powder jets. The investigation also emphasizes the complex interplay among the process factors and highlights the crucial role of laser source power in triggering a fast escalation in the volume of melted material. In addition, the study supports the idea that maintaining the laser energy input as a constant helps to create a consistency in the total melt area even when the cladding speed is increased.

Saint-Gobain Coating Solutions new ProPlasma HP gun, based on conventional plasma gun design (single anode-single cathode), has been evaluated for typical demanding applications of the thermal spray industry. A general overview of this gun main features is presented. Aim of its development was to fulfill the whole range of applications of plasma spraying, from high feed rate –high deposit efficiency (DE) to high velocity. First, plasma spraying performance of ProPlasma is exposed through 2 typical examples of materials extensively sprayed: yttria stabilized zirconia (YSZ) and chromia, both needing high DE at high feed rates. A set of spraying parameters was developed for YSZ Saint-Gobain #204 hollow spherical powder showing 57% DE at 150g/min and exhibiting 13 to 15% porosity as typically requested for thick TBCs. Two sets of parameters were developed for chromia at 120g/min leading either to 3% porosity - 51% DE or 2% porosity - 42% DE. Then, the possibility to reach a high velocity mode is shown through the presentation of the anode development made for a typical high velocity application material: WC-Co. After a first design showed experimental promising results with low porosity, the anode design has been further improved based on experiment and CFD modeling.

The interaction between thermal spray parameters and coating characteristics of atmospheric plasma sprayed WC-12Co was investigated by means of statistically designed experiments. A central composite design was employed to study the influence of the process parameters’ ratio Ar/H 2 , plasma gas volume flow, powder feed rate and stand-off distance on the deposition efficiency, tensile strength, porosity, roughness, particle temperature, particle velocity and the percentage of molten carbides. The process parameters’ volume flow and powder feed rate showed a considerable impact on the microstructure and morphology of the coatings, which could be traced back to a change in the melting degree of the powders as well as a change in particle temperature and velocity. On the basis of the statistical experiments, mathematical expressions were found to describe the coating characteristics. The quality of the mathematic model was scrutinized in a subsequent evaluation stage.

The effects of powder loading on temperature distributions of yttria-stabilized zirconia particles during flight, which can be used as a melting status indicator, were investigated in atmospheric dc plasma spraying. Commercially available diagnostic systems were utilized to measure the state of in-flight particle parameters. As the powder feed rate was increased, the intensity of a peak related to the latent heat increased, suggesting the increase of semi molten particles. Interesting findings are that the deposition efficiency of the coating actually increased in some conditions at higher powder feed rates. This implies that higher molten degrees of particles do not always give higher deposition efficiencies. The loading effects also affected the result of diagnostics, which requires special care when the diagnostic condition is different from the actual spray conditions.