Deposition efficiency is an important metric for thermal spraying, but it can be difficult to measure in some cases. In this study, a volume-based deposition factor is developed that can be used to estimate deposition efficiency simply by evaluating process parameters, coating thickness, and porosity. Coating thickness or thickness per pass is measured from the sample and parameters like feed rate, step size, and surface speed are taken into account. The correlation to deposition efficiency is demonstrated for various plasma and HVOF processes using powder and suspension feedstock materials. An example is also given showing how the method can be used to evaluate older data sets collected without regard to deposition efficiency.

Polymer cold spray has yielded lower deposition efficiency (DE) and quality deposits compared to metal cold spray. The disparity stems from metals being studied far longer than polymers in cold spray; in addition, polymers exhibit richer thermo-mechanical behavior. An experimental study was conducted to examine the effects of polymer feedstock degree of crystallinity (D) on cold sprayed deposits of polyetherketoneketone (PEKK), a thermoplastic used in aerospace and other high-performance applications. As deposition relies on the plastic deformation of the impacting particle, polymers with high D may inhibit deposition, reducing deposit quality and efficiency. This study evaluates three PEKK grades produced using different ratios of terephthalic (T) to isophthalic (I) monomer moieties (T/I = 60/40, 70/30, 80/20). The ratios control D, with higher proportions of T monomers corresponding to higher crystallization rates and degrees of crystallinity. A parametric study was completed to evaluate functional process set points of system carrier gas temperature and powder mass flow rate. Using operational parameters common among the PEKK grades, spray cycles were completed for each material and quantitative responses to variation in crystallinity were evaluated through a suite of analyses. DE of the materials was assessed gravimetrically, deposit porosity was evaluated by scanning electron microscopy, and thermophysical changes to the feedstock during the spray cycle were determined by differential scanning calorimetry. Overall, we found that cold spray processing of powders of lower D formed less porous deposits with a higher DE than more crystalline powders sprayed at the same process conditions. PEKK grades with lower T/I ratios achieved DEs in the range of 60-75%, whereas the most T enriched grade only reached ~10% DE.

In cold spray (CS) additive manufacturing process, micrometer scale particles accelerated through a supersonic nozzle are targeted on a surface with velocities in the rage of 300-1500 m/s in solid state. The impact energy of the particles leads them to deform plastically with high shear energy near the impact interface and adhere to the surface metallurgically, mechanically, and chemically. Using CS, deposition of metals, metal matrix composites, and polymers are achieved with high adhesive/cohesive strength and low porosity. Sensitivity of the CS additive manufacturing process to the variabilities in the process parameters are still being understood. Among the process parameters, particle morphology can have significant implications on drag forces, and therefore, on the particle impact velocity. This in turn affects the deposition efficiency (DE) and the quality of products. In this work, a new approach is introduced for computing DE by incorporating particle sphericity and its variation into one-dimensional numerical models. Size, sphericity, and the variability of size and sphericity of aluminum, copper, titanium, and tantalum particles are measured from static optical microscope images. The data is used for predicting impact velocity, temperature, and DE. The model results are then compared with particle velocity measurements.

To improve the deposition efficiency of copper particles, especially fine particle, in cold spray process onto metallic substrate, optimization of nozzle shape and dimension was performed by numerical simulation. Maximum velocity of the particle reached up to 685 m/s under the optimum conditions by using self-designed nozzle based on the simulation results. In the spraying of copper particle onto steel substrate, lamellar-like unique micro-structure was observed near the interface region of the steel substrate. Correspondingly, hardness increase in this region of steel substrate was recognized. Work hardening was induced in the steel substrate due to the higher velocity of copper particles. Furthermore, to reduce the bow shock effect on the substrate surface region in cold spray process, special shaped nozzle was newly developed. While nominal particle velocity decreased in the special nozzle, deposition efficiency, Vickers hardness and adhesion strength increased significantly especially in the case of fine particle and higher pressure levels of the working gas. Numerical simulation showed that pressure level on the substrate surface decreased effectively in the special nozzle. In the observation of a cold sprayed individual particle onto mirror polished substrate, extended metal jet was recognized at particle’s periphery when the particle was sprayed by the special nozzle. The results indicate that the decrease of particles velocity by bow shock was suppressed effectively in the special nozzle as compared with conventional nozzle.

This work assesses the influence of powder characteristics on the deposition efficiency, microstructure, and tribological properties of Cr 3 C 2 -NiCr coatings. Four commercial powders prepared by different methods were used for the study. All have a spherical morphology but vary in terms of porosity, carbide grain size, and flowability. The feedstocks were deposited on flat low-carbon steel substrates using a liquid-fueled HVOF torch mounted on an industrial robot. Deposition efficiency was measured along with coating hardness, Young’s modulus, and abrasive wear resistance. In addition, some of the coatings were heat treated and changes in microstructure and hardness were recorded.

The coatings of zinc and its alloys are broadly used to prevent the rusting of substrate surfaces such as steel. Cold gas dynamic spray (CGDS) is an innovative coating technique in which the deposition of solid powder particles depends upon the kinetic energy of the particles rather than thermal energy. Therefore, application of cold spray is to provide superior rust resistance by depositing more materials, formation of passivation layer, and cathodic protection. In this study, numerical investigations on zinc micro and nano size particles in CGDS were carried out. The height of the injector, the expansion ratio and the diameter of the inlet of the de-Laval nozzle was varied systematically by optimizing the stand-off distance using the two-dimensional axisymmetric models of CGDS, to study their effects on the velocity and the distribution of the particles. Prediction of the deposition efficiency was carried out using the various critical and erosion velocity models.

In this work, fully dense titanium parts are fabricated by cold gas dynamic spraying (CGDS). Titanium powder is deposited using a low-pressure CGDS process with nitrogen as the carrier gas. The density, porosity, hardness, and tensile properties of the parts produced are determined and discussed.

This work addresses some of the challenges associated with depositing copper on PEEK by cold spraying. Getting copper powder to adhere to the PEEK substrate is not difficult at first, but deposition efficiency falls rapidly during coating build-up. Without heating the propellant gas, a copper coating will not form, even at the highest gas pressures. Increasing prechamber pressure is necessary, but requires an increase in gas temperature to 400°C to reach a deposition efficiency of 70%. Subjecting PEEK to such heat causes delamination issues that offset the deposition efficiency gained.

Thermal sprayed nanoscale materials produce coatings with enhanced properties. Thermally spraying nanopowders, in non-agglomerated form, include several potential advantages. These advantages can not be tested because nanoparticles do not have the inertia required to cross streamlines in typical plasma spray flows. Hence, they are swept away without depositing on the substrate. Analysis of inertial deposition reveals that the non-dimensional Stokes number largely characterizes deposition efficiency. Under low pressure plasma spray conditions, with reduced stand-off distance, the Stokes number of nanopowders can be made to approach the Stokes numbers of typical thermal spray powders. This paper explores the conditions that make the inertial deposition efficiency of nanoparticles quantitatively similar to the typical inertial deposition efficiency of microparticles. Paper includes a German-language abstract.

In the cold spraying process, particle velocity is commonly regarded as the key factor that influences the deposition efficiency and properties of the coating. The present paper reports on a study in which the velocity of in-flight particles was measured using a DPV-2000 system. The influences of He and N 2 gas pressure and temperature and particle morphology on the particle velocity and deposition efficiency of the coating using stainless steel 316L powders were studied. The microstructure of the coating was examined using optical microscopy. The critical velocity of stainless steel 316L powders was estimated according to the particle velocity distribution and deposition efficiency of the coating. The experiment results suggested that the gas pressure has a more significant influence on the particle velocity and deposition efficiency of the coating than the gas temperature. The particle morphology also has significant influence on the particle velocity. The critical velocity of stainless steel 316L powders was in the range of 630 to 680 m/s, and it decreased slightly with the gas temperature.

Results concerning the relationship between deposition efficiency and mechanical properties with coating roughness are presented. A commercial partially stabilized zirconia powder was plasma sprayed at different power levels, H2/Ar ratio and spray distance. Coatings sprayed at high deposition efficiencies demonstrated improved mechanical properties. The deposition efficiency is also proportional to the coating roughness. When prepared with a high plasma power and a high H2/Ar ratio, and at short spray distances, these coatings exhibited the highest deposition efficiency and coating roughness. The high coating roughness associated with a high deposition efficiency arises because coarse powder particles are likely to be melted at higher plasma power. However, opposite trends were also observed within different regimes of powder size distribution and torch power. Another objective of this work was to demonstrate that a surface profilometer can be used to understand the deposition efficiency and mechanical properties of thermal spray coatings. Key words: Roughness, Mechanical Properties, DE.

The gasturbine industry is performing utmost efforts to allow increased operation temperatures to improve engine efficiency by a reduced cooling air/fuel-ratio. The reason is to save fuel and at the same time lower emission rates. A possible solution to improve the insulation of the Thermal Barrier Coating (TBC) is followed by the strategy of developing reliable thick TBCs. To be successful concerning the coating quality and to apply thick TBCs at reasonable costs and spray time it is necessary to apply the coating consistently by high deposition rates. The plasma spraying of thicker coatings leads to long spray periods which demands for a process control to keep the process parameters within the tolerances during the entire spray procedure. A major property is the control of substrate temperature with pyrometer systems and to define the process tolerances. Furthermore, a high deposition efficiency is of prime importance to shorten spray times. The optimisation of the gas composition in terms of viscosity and thermal conductivity (SPRAL 22) enables to increase dramatically this efficiency by 30% to 70%. Moreover, in order to reduce thermal expansion mismatch between substrate and ceramic top coat, a high amount of porosity is beneficial to lower young's modulus of the ceramic top. This can be achieved by adding a polymer powder to the yttria partly stabilised zirconia powder. It also leads to an increase in deposition efficiency.

In this study, a novel strategy to manufacture high strength cold-sprayed Al coating by using powder with wide size distribution is proposed. The microstructure and mechanical properties of deposited coating sprayed at three typical impact velocities before and after heat treatment are investigated. Furthermore, the deposition and strengthening mechanisms of the coating sprayed at various impact velocities are clarified. The results show that the coating with higher density and mechanical properties can be successfully fabricated by cold spray at comparatively low particle impact velocity. The mechanical properties were enhanced with the contribution of heat treatment process. It is the in-process tamping effect induced by larger powder that results in the severe plastic deformation thus leads to densification and excellent mechanical properties of the cold-sprayed Al coating.

High Velocity Oxy-Fuel (HVOF) method using propylene as a fuel gas was employed to spray alumina particles. In order to improve the coating characteristics such as the deposition efficiency and the hardness, three HVOF gun nozzles of varying geometry were designed and tested experimentally. The spraying process was also simulated numerically for each of the nozzle geometries to understand their effectiveness in influencing the velocity and temperature of the sprayed particles. The coating was characterized using optical and scanning electron microscopy (SEM), micro-vickers hardness test and X-ray diffractometry (XRD). Results showed that with the use of a convergent and divergent type gun nozzle, similar to that of a Laval nozzle, the extent of melting of the alumina particles could be increased. This was exhibited by an increase in the deposition efficiency to the extent of 45%. However, the sharp changes in the convergent and divergent nozzle geometry, resulted in fusion and agglomeration of alumina particles leading to spitting during the spraying process. The results clearly showed that alumina coatings of excellent hardness in the range of 920-1290 HV, with a relatively dense microstructure could be obtained in HVOF method irrespective of the gun nozzle geometry, provided the spraying parameters are properly controlled.

It is well accepted that the morphology and microstructure of the splats have a strong influence on the characteristics and properties of thermally sprayed coatings. McPherson has made pioneering and outstanding contributions in the above area, especially for plasma sprayed coatings. Recently, splat morphology - microstructure - properties correlation has also been attempted in the case of HVOF thermal spray coatings. However, only limited data is available in the case of detonation sprayed coatings inspite of the fact that DS coatings have been available commercially for a long time. In the present work, the influence of particle velocity and temperature on the splat morphology and also area coverage of the splat has been studied for detonation sprayed Al 2 O 3 particles on a mild steel substrate. Further, the effect of two detonation spray process variables namely, oxy fuel ratio and shot frequency on splat morphology and splat area coverage has been evaluated. The above correlation has then been utilized to understand the variation of deposition efficiency of detonation sprayed Al 2 O 3 coatings on mild steel as a function of spray process parameters.

A typical feature of plasma sprayed zirconia-based coatings is that their depostion efficiencies are low. Thus, from the economic view of reducing cost, how to improve the depostion efficiency of plasma sprayed zirconia-based coatings is very important for practical application. In this paper, spray-dried nanostructured powders and conventional powders were used as feedstocks to deposit nanostructured and conventional zirconia coatings by atmospheric plasma spraying(APS). Deposition efficiencies for both nanostructured and conventional zirconia coatings deposited under different spraying parameters were comparatively measured. The obtained results revealed that the spray-dried nanostructured powders possessed higher deposition efficiency than that of conventional powders. In addition, it was found that spraying parameters had strongly influence on microstructure and microhardness of zirconia coatings. Therefore, in order to obtain high quality zirconia coating, proper spraying parameters must be considered.

In plasma spraying, generally, powders are radially injected into plasma jet. However, percentage of particles which have enough enthalpy to melt is not high in radial injection, possibly leading to low deposit efficiency especially for high melting point powders. In this research, powder port for radial injection was attached to an axial injection plasma torch in order to compare the plasma flame in both cases of axial and radial powder injections. The powder trajectory and the shape of the plasma plume were observed, and particle flux density, temperature, velocity, and deposit efficiency of alumina powders were measured and the results were compared between the two powder injection methods.

Computer simulations of thermal spray coating processes can accelerate the development of new products by minimizing the need for prototypes. An important aim of these simulations is the calculation of the coating distribution on the surface of a given workpiece with respect to a given movement path of the spray gun. In this paper, a novel approach for computing coating distribution on arbitrarily complex freeform surfaces is presented. In contrast to approaches that implement symmetric deposition models, the presented concept is based on a rotationally asymmetric model, making it particularly well suited for wire arc spraying as turbulences caused by electrodes in the gas flow often result in asymmetric coating distributions. In order to obtain the required knowledge base to derive the deposition model and verify the simulation, basic experiments were made. The simulation concept and experimental setup are presented in the paper along with the results.

A typical feature of cold spray process is that a deposit can be formed without change of the original structure and compositions of spray materials. Only particles which reach the velocity higher than the critical velocity can be deposited on a substrate in cold spraying. When the spray particle impacts on the substrate at an off-normal angle, the normal component of particle impact velocity will change with the approaching angle of spray particle to substrate. In the present study, copper and titanium powders are used to deposit coating using cold spray process at different impact angles with regard to substrate. The deposition characteristics of spray materials are examined. The results show that the impact angle has a significant influence on the deposition characteristics. The relative deposition efficiency changes with the spray angle. It has been found that there is a critical impact angle at certain particle conditions below which no deposition occurs. The relation between spray angle and relative deposition efficiency can be divided into three spray angle ranges: maximum deposition angle range, transient angle range and no deposition angle range. In the transient angle range, the relative deposition efficiency increases with an increase in spray angle from zero at the critical spray angle to 100%. The transient angle range depends on the particle velocity distribution. A model is proposed to explain the relation between the spray angle and the relative deposition efficiency.

In this study, experiments investigated the influence of substrate conditions such as substrate thickness, substrate surface conditions (blasted or polished), substrate temperature, number of overlaps and gun traverse speed on cold sprayed copper and titanium coatings. The influence of substrate thickness, substrate temperature and number of overlaps were different by materials of feedstock powder (copper and titanium). In another words, titanium deposition efficiency increase slightly while copper deposition efficiency decrease slightly with increasing the substrate thickness, the preheated substrate temperature and number of overlaps and gun traverse speed. On the other hands, both deposition efficiency of copper and titanium coating increase slightly with an increase in surface roughness from 0.2 μm Ra (polished surface) to 7 μm Ra (blasted surface). The deposition efficiency of titanium and copper decrease rapidly with increasing gun traverse speed.