The flattening of droplet impacting on a substrate is one of the most important basic processes during the formation of a thermal spray coating. The structure of splat will determine the structure of coating, and consequently the adhesion and properties of coating. This paper investigates the dependency of the change of the morphologies of splats from irregularly complicated form to regular disk form by the preheating of substrate on the types of evaporable substances adsorbed on substrate surface in order to provide farther evidence for the evaporated gas induced splashing mechanism. Paper includes a German-language abstract.

Flattening mechanism of thermally sprayed single particles onto the flat substrate surface was investigated by using a metallic powder and stainless steel substrates. As the flattening pattern of the particles occurs quickly after the impact with the substrate surface, “pattern formation” mechanism was investigated. To perform this investigation, particles were thermally sprayed onto flat substrates through controlled substrate temperature and ambient pressures. The top surface, bottom surface and cross section microstructure of the splats were systematically investigated. By summarizing the results obtained, it was inferred that the initial solidification in peripheral region (starting at splat-substrate interface) might have a role on the formation of disk shaped splat.

A transition in the flattening behavior of thermally sprayed metals has been observed in previous studies. It has been proposed that ultra-rapid cooled chill structure preferentially formed at the bottom part of the splat may play a role in the generation of disk-shaped metallic splats. The applicability of this hypothesis to other materials was verified experimentally for several ceramic oxides. To accomplish this, Al 2 O 3 , Y 2 O 3 , and YSZ particles were plasma sprayed onto stainless steel substrates and the fraction of disk-shaped splats was measured as a function of substrate temperature. Splat microstructure was also examined. Unique amorphous and chill structures were observed in the bottom portion of Al 2 O 3 and Y 2 O 3 splats, indicating that similar formation mechanisms may be at work. However, only a columnar microstructure was observed in the YSZ splats, which calls for additional study.

Aiming at clarifying the individual splat formation mechanism in thermal spray process, commercially available metallic powders were thermally sprayed onto AISI304 substrate surface. The splats changed from a distorted shape with splash to a disk-shaped splat in flattening after collision onto substrate surface, through substrate preheating and/or reducing the ambient pressure. Accordingly, both substrate temperature and ambient pressure have an equivalent effect on the shape transition. The observation on the bottom surface morphology of single splat indicated that the ring-shaped initial solidification might play an important role during splat formation process. As a simulation of the real thermal spray process, free falling experiment has been conducted. The thermal history of the free falling metal droplet onto AISI304 substrate indicated that the flattening pattern is decided so quicky just after collision onto solid surface, which is enough earlier to the finalization of the flattening.

This paper investigates the oxidation that occurs during the flight movement of a powder particle and during the spatter solidification in the thermal spray process. The effects of oxidation on droplet flattening, on the mechanical and thermal interactions between spatter and substrate, on spatter morphology, on porosity, and on adhesion are studied. The influence of wetting and oxygen dissolution is analyzed. The experimental results show that during High Velocity Oxy-Fuel spraying of the chromium carbide-nickel-chromium powder, the relative mass of chromium oxide in the coating is about 4.95%. The theoretical results agree well with the experimental observations. Paper includes a German-language abstract.

The paper reports the results of structure examination of intermetallic Fe-Al type coatings obtained by the detonation gun spraying on a C45 plain carbon steel substrate. The structure was analysed with scanning (SEM), transmission (TEM) electron microscopy techniques and electron (SAE) and X-ray diffraction methods as well as quantitative inspection of composition in microareas (EDX). Special attention was paid to the interface between the coating and the substrate analyzing particularly the substructure of the individual grains contained up to 15μm away from the substrate surface layer. The results allowed explaining the formation mechanism of the coating morphology with a contribution of intermetallic phases Fe 3 Al, FeAl, FeAl 2 and Fe 2 Al 5 as well as the ε phase taking into consideration the influence of velocity, temperature and pressure on the powder particles during the D-gun spraying. It was established that the coating produced with the DGS method had sublayer morphology of alternate flattened and partially melted grains with wide range of Al content from 39 up to 63 at.%. Partial melting of the individual powder particles brought about the appearance of the amorphous grains and subsequently columnar crystals of the Fe-Al type phases formed sequentially at the interface area coating and cold substrate surface layer material, which was essential in the mechanism of the Fe- Al coating formation. It was established, that in the area of the polycrystalline dispersive structure formed from the highly plasticized FeAl grains during D-gun spraying, complex oxide films identified as Al 2 O 3 -γ formed, serving as specific composite reinforcement in the intermetallic Fe-Al coating. A mechanism of crystallization of partially melted Fe-Al particle containing nominally 63 at.% Al was carried out in the work in an attempt to explain the formation of different sub-layers within the Fe-Al intermetallic coating at the interface 045 steel surface layer.

This paper presents a one-dimensional heat transfer model which predicts the solidification and cooling of a plasma-sprayed alumina splat after the flattening process is completed. A heterogeneous nucleation process taking place on the substrate surface was assumed. The density and average size of the formed nuclei were determined from the integration of the nucleation rate calculated from the classical kinetic theory for nucleation. This rate depends on the activation energy required for nucleation which takes into account the effect of the surface via a wetting angle between the growing nucleus and the catalytic surface. This contact angle was estimated from the comparison of the computed grain density with the density observed on splat surface using an atomic force microscope. When 67% of the splat surface in contact with the substrate are covered by grains, a planar solidification front was assumed to move through the melt. The theoretical model accounted also for the selection of the crystalline phase. Calculations were performed for various substrate materials at different initial temperatures. Results are expressed in terms of nucleation temperature, nucleation rate, density and grain size distribution.

The splashing usually occurs when a droplet impact on a substrate surface during thermal spraying, which results in the formation of splat with irregularly complicated morphology. In present study splats are formed on polished stainless steel substrate surface covered with different organic substances with different boiling points by plasma spraying under different preheating temperature of substrate in order to clarify the factors which control the splashing during droplet flattening in thermal spray process. The droplet materials used are aluminum, nickel, copper, Al2O3 and molybdenum. Three kinds of organic substances used are xylene, glycol and glycerol which are brushed on the surface of substrate before spraying. It is found that when the preheating temperature exceeds 50°C over the boiling point of organic substance brushed on substrate surface the regular disk type splats are formed in the case that no substrate melting occurs by molten droplet. When the flattening of droplet causes the melting of substrate such as the combination of Mo droplet with stainless steel substrate, the preheating of substrate has no influence on splat morphology. The evaporated gas induced splashing and substrate surface melting induced splashing models are proposed to interpret the formation of the annulus-ringed splat.

The structure and morphology of plasma sprayed splats are experimentally investigated using different droplet materials and substrate materials. Droplet materials include aluminum, copper, nickel and refractory metals such as molybdenum and tungsten, and substrate materials include aluminum, stainless steel, and molybdenum plates. The results show that the splashing occurs during the splatting of a completely molten droplet. Most splats formed by droplets molten completely are only central part of the ideal disk type ones, which are defined as the annulus-ringed disk-like splat. It is found that the morphology of such annulus-ringed disk-like splat is greatly influenced by the combination of droplet and substrate materials depending on whether substrate melting occurs. With the combinations of droplet and substrate materials which are of similar thermal properties the splashing of central area of splat tends to occur to present a honeycomb structure at the center of splat. When droplet impacting can cause melting of substrate annulus-ringed splat prefers to present a split type. The flattening ratio of an annulus-ringed disk splat is typically less than 2.

The major problems with plasma sprayed hydroxyapatite (HA) coatings for hard tissue replacement are severe HA decomposition and insufficient mechanical properties of the coatings. The loss of crystalline HA after high temperature spraying is due mainly to the loss of OH- in terms of water. The present study employed steam to treat HA droplets and coatings during both in-flight and flattening stages. The microstructure of the HA coatings and splats was characterized using scanning electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffractometry (XRD). Results showed that a significant increase in crystallinity of the HA coating was achieved through the steam treatment (e.g., from 58% to 79%). The Raman spectroscopy analyses on the individual splats and coatings indicate that the mechanism involves entrapping of water molecules by the individual HA droplets upon their impingement. It further suggests that the HA decomposition has already taken place before the impingement of the droplets on pre-deposited materials or the substrate. The improvement in crystallinity and phases, e.g., from tricalcium phosphate and amorphous calcium phosphate to HA, was achieved by reversing the HA decomposition through providing extra OH-ions. Furthermore, the steam treatment during the spraying also accounts for remarkably increased adhesion strength from 9.09 MPa to 23.13 MPa.

Three-dimensional molecular dynamics simulation was conducted to clarify at an atomic level the flattening process of a high-temperature droplet impacting a substrate at high speed. The droplet and the substrate were assumed to consist of pure aluminum, and the Morse potential was postulated between a pair of aluminum atoms. In this report, the influences of the impact parameters, such as the droplet velocity and the droplet diameter on its flattening behavior were analyzed. As a result, following representative conclusions were obtained: (1) the flattening ratio increases in proportion to the droplet velocity and the droplet diameter; (2) the flattening ratio for nanosized droplet can be rearranged by the same dimensionless parameters of the proper physical properties, such as the viscosity and the surface tension, as those used in the macroscopic flattening process.

This study investigates the deposition behavior of cold sprayed copper particles on flat surfaces. In the experiments, a Laval-barrel nozzle was used to spray water atomized spherical copper particles with a mean diameter of 5 µm onto mirror-polished stainless steel. The particles were similar in morphology regardless of spraying conditions with an average bonding strength of 60 MPa as determined by nano scratch (shear) testing. An amorphous-like layer at the particle-substrate interface indicates that the deformation of the particles initially destroys their surface oxide, revealing an active fresh surface that facilitates metallic bonding. At higher spray velocities, metal jetting is observed at the periphery of flattened particles and its relationship with deposition efficiency is statistically analyzed and put forth as a potential method for controlling the cold spray process.

This study aims to develop a metal-based compatibilizing sublayer on a Carbon Fiber-Reinforced Polymer (CFRP) composite to overcome the erosion issue of polymer substrate using the cold spray deposition technique. The objective is to contribute to the in-situ repair of aircraft structures. Two cases of sublayers, i.e., Al-based sublayer (1126 μm thick) and Cu-based sublayer (547 μm thick), have been prepared and co-cured with the CFRP substrates by pressure assisted molding process. Gas-atomized copper powders were deposited on a reference sample of aluminum panel (A-0) and on two functionalized composite substrates (A-1 and C-1) by a high-pressure cold spraying (HPCS) process. The results show that cold spray deposition onto the Al-based sublayer leads to a coating formation whereas the Cu-based sublayer is strongly eroded by the supersonic collision of copper powders. Scanning electronic microscope (SEM) morphologies were used to investigate the HPCS deposition mechanisms on various configurations of substrates. It was found that the high deposition efficiency of case Cu/A-0 was achieved by metallic bonding, evidenced by the significant flattening powders and agglomeration phenomenon of multiple particles. The copper particles of case Cu/A-1, encapsulated by the deformed aluminum powders, could anchor to the substrate via mechanical interlocking, whereas only pure localized fracture of epoxy and exposed broken carbon fibers were observed on the substrate of case Cu/C-1. The results demonstrated the feasibility of an Al-based sublayer-assisted cold spray process for the thermosetting CFRP composite to achieve a successful deposition of copper powders, which also emphasized the necessity to search an optimal material coupling between sublayers and coatings.

In order to investigate the potentials to improve the deposition efficiency and to functionalize the polymer-based substrates, six configurations of microparticles Sn, Zn, Al, Sn+Al 2 O 3 , Al+Al 2 O 3 , Cu+Al 2 O 3 were cold sprayed on the substrate of Carbon Fiber Reinforced Polymer (CFRP) composites equipped with Cu-based sublayer or Al-based sublayer. The process conditions were kept unchanged. Microanalysis of sublayers and coatings was performed via a Scanning Electronic Microscope (SEM), the deposition mechanisms of different powders couplings on CFRP substrate were then discussed. The results indicated that although the deposition efficiencies were negative, the systems of Zn, Al and Al+Al 2 O 3 perform better among all the configurations. It was found that the addition of alumina led to a lower deposition efficiency (DE), compared to the corresponding pure coatings. For single-component Sn, Zn and Al powders, they all showed an increasing trend of DE when changing the substrate from Cu-based systems to Al-based systems. The aim of this present work is to elaborate the intrinsic causes of erosion issues and to provide a reference value for picking spraying materials and preparing functionalized CFRP substrates. According to the SEM analysis, the insufficient deformation and escape behaviours of spherical copper powders explained for the difficulty of coating formation. It was noticeable that the surfaces of Al-based systems were more uniform than those of Cu-based ones, due to their desirable deformation abilities. Besides, the significant flattened particles, material mixing and melting phenomenon were observed in Al-involved systems, which would definitely contribute to the adhesive bonding between coating and substrate.

The bonding between flattened particles in plasma-sprayed metal coatings dominates their corrosion behavior by influencing the porosity in coatings, especially the porosity connected to the substrate for coatings used in a corrosive environment. Therefore, how to efficiently enhance the lamellar interface bonding in metallic coatings has been an important issue which has not been settled effectively. In this study, a shell-core structured powder particle designing with cladding spherical Ni20Cr powders with refractory molybdenum as alloying element is proposed to limit the evaporation of low melting point elements and subsequently raise particle temperature significantly high enough to cause impact melting. Results show that a dense coating with much low porosity was obtained due to the improved lamellar interface bonding by gas shrouded plasma spraying of the composite NiCr -Mo particles. Electrochemical method was employed to evaluate the polarization behavior of the NiCr - Mo coating to estimate its connected porosity. It was revealed that NiCr-Mo coating of excellent corrosion resistance with low connected porosity can be obtained by designing the shell-core-structured powder.

Although thermal spraying technique is used in many industries, it suffers from several problems. For example, the hardness of the coatings is lower than that of sintered material for the incompleted cohesiveness. An yttrium aluminum garnet (YAG) laser was used during HVOF spraying to improve the properties of the applied coating. Several carbide powders were used as thermal spray materials, and stainless steel (SUS304) was used as substrate. Coatings were sprayed by hybrid spraying method, which was combined HVOF spraying with YAG laser. The hardness of coatings sprayed by hybrid spraying was higher, and the weight loss in a blast-erosion test was smaller than that of coatings applied by HVOF spraying only. The particles deposited in the coatings obtained by hybrid spraying were very fine. Laser irradiation to the HVOF flame improve the adhesion strength between particles and the deposition of fine carbide particles in the coating. It was considered that mechanism of coating strengthening in hybrid spraying was resulted from strengthening of cohesiveness by heating effect and decreasing of porosity by flattening effect according to observation results of Ni splats sprayed by hybrid spraying method.

Residual stress can be developed in most thermally sprayed coatings due to the momentum of molten particles during impact, and heat transfer during solidification of the splats. Another reason for residual stress built-up in thermally sprayed coatings is due to splat curl-up during solidification and the differences in thermal expansion coefficients between the coating and the substrate. However, in the cold spraying process, it is believed that the main reason for residual stress formation is plastic deformation during impact and flattening of solid particles. Residual stresses can drastically influence coating quality and reduce its service time. In this study, residual stress is measured for two well-known nickel based super alloys (Inconel 625 and Inconel 718) deposited on 7074 aluminum alloy substrates by the cold spraying technique. Residual stress in Inconel 625 was found to be highly tensile on the surface and compressive on the subsurfaces. After heat treatment the residual stress was relieved and was compressive in nature. Whereas for Inconel 718, residual stress was compressive on the surface and tensile on the subsurfaces in the as-sprayed condition. After heat treatment, the residual stress was compressive with increased magnitude. The heat treatment at 800°C made the residual stress more compressive. The porosities of both Inconel 625 and Inconel 718 were reduced after heat treatment.

Atmospheric Plasma Spray is widely used for tens of years to elaborate protective coatings on parts for several applications. However, our understanding of the APS process can still be improved, requiring a fine modeling of the process in parallel with some corresponding experiments. In the present work, a complete series of models was applied to reinforce our knowledge of the process: the case of an alumina coating was considered. A 3D CFD model was first used to study the internal arc within the torch. Interactions between the external plasma jet and the injected particles were then computed in a second step. At this level, the predicted in-flight particle characteristics were compared with some corresponding measurements recorded with the DPV 2000 diagnostic tool. A third model was then applied to investigate the particle flattening on the substrate/coating material. SEM pictures of coating cross-sections were then captured and a last model was finally applied to estimate the coating effective thermos-mechanical properties based on calculations performed directly on the SEM micrographs. This set of models allows investigating the APS process from the DC arc within the torch to the coating properties.

Copper splats are deposited on the flat stainless steel surface at the ambient and preheated conditions. The splashing occurs as the splats are deposited at an ambient atmosphere. The characteristics of the splashing occurring at different splat regions during spreading of the droplet are examined. The splashing can be classified into two types according to the splashing mechanisms. At the surrounding region of the splat larger than flattening ratio about 1.5 to 2, the radial splashing takes place by jetting-away of splat materials, which leads to the formation of a splat with a reduced diameter. At the central area of the reduced splat, the upward splashing occurs through the blowing up of the top surface layer which results from the high pressure of gas bubbles. At the preheated condition which can remove surface adsorbates, no evident splashing occurs under the normal spray conditions. Two types of splashing can be explained by the gases evolved through evaporation of the adsorbates resulting from the heating of the high temperature droplet. The spreading of the droplet involved in the wave urging flow is presented.

As underlined in 1981 by Mc Pherson, thermo-mechanical properties of plasma-sprayed coatings depend not only on the way particles flatten and resulting splats solidify and cool down, but also on the thermal history of particle layering at the same location. To illustrate what is our present knowledge in that field, plasma-sprayed alumina coatings will be considered through modelings and measurements. The first part of this paper discusses the phenomena linked with particle impact and splat formation: splashing, spreading, solidification and grain growth, angle of impact in conjunction with particle parameters at impact and substrate surface parameters (chemistry, phase structure and roughness, temperature). The second part examines splats layering. It addresses the influence of plasma jet heat flux, relative velocity torch-substrate, powder flow rate and deposition efficiency on splat time-temperature evolution and resulting quenching stress, coating adhesion/cohesion and microstructure. The shadow effect when spraying off normal angle is also discussed. The last part deals with the effect of the successive cooling and reheating of passes on coating properties, and condensation of the vapor issued from evaporating particles.