Residual stress and adhesive/cohesive strength in cold-sprayed coatings are extremely important factors, and the balance between them can be a determining factor in coating failure, such as by delamination or cracking. Dominated residual stress to bonding stress should lead to coating peel off from its substrate. Up to now, it is still difficult to measure the residual stress of cold-sprayed coating especially inside it. In the present study, a high-energy X-rayed beam was utilized to penetrate the cold-sprayed coatings and the change of its diffraction angle can be detected. This gave a way to measure the residual strain inside the cold-sprayed coatings. With the scanning of the X-rayed beam and the detector at different locations, the strain of cold-sprayed coating at 2 directions can be obtained. Moreover, the residual stress of cold-sprayed coating can be calculated with the measured strain.

WC-Co coatings are primarily deposited by using the high velocity oxy-fuel (HVOF) spray process. However, the decomposition and decarburization of carbides during spraying result in the degradation of coating wear performance. In this paper, a novel high pressure HVOF with the characteristics of lower particle temperature and higher particle velocity was developed. It exhibits combustion chamber pressures up to 3.0 MPa. The influence of combustion chamber pressure and oxygen/fuel equivalence ratio on WC-Co particle velocity and temperature levels were analyzed by numerical simulation. The experiment results show that the combustion chamber pressure and the oxygen/fuel equivalence ratio have a significant influence on the particle velocity and melt degrees, as well as, on the coating microstructure and microhardness. High velocity WCCo particles in different states, i.e., molten, semi-molten and non-molten can be readily obtained by changing the spray conditions. A comparison to the conventional JP-5000 was also executed.

Cold spray is a new emerging coating technology in which particles in a solid state are deposited via plastic impact on a substrate at a high velocity and a temperature that is much lower than the melting point of the starting powder. Compared to the conventional thermal spray processes, dense coatings without any degradation can be obtained by cold spray process with high deposition efficiency. CoNiCrAlY coatings are widely used for land-based gas turbines to resist high-temperature oxidation and hot corrosion. Owing to the high cost of the low-pressure plasma spray (LPPS) or some degradation in the hyper-velocity oxy-fuel (HVOF) spray process, cold spray process is a prospective candidate for coating preparation. In the current study, CoNiCrAlY coatings were prepared by cold spray and LPPS processes, and a comparison of the coating’s properties between the LPPS and cold spray process was carried out. The spray conditions of cold spray were optimized by the measurements of deposition efficiency and the observations of microstructure.

In this investigation, Inconel 718, a material known to cause nozzle clogging upon cold spraying, was cold spray formed to 6 mm-thick using the Plasma Giken cold spray system PCS- 1000. This was made possible due to the novel non-clogging nozzle material combined with a nozzle water cooling system. Coatings were as-spray formed using both nitrogen and helium as the propelling gasses. The resulting microstructures as well as the corresponding mechanical properties were studied. In addition, the effect of post-heat treatments was also investigated. It was found that for a given propelling gas used, the coating porosity level remained relatively similar (about 2.4% for nitrogen and 3.6% for helium) regardless of the coating treatment (as-sprayed or heat treated). Visual inspection from SEM micrographs showed a higher fraction of inter-particle metallurgical bonds for nitrogen gas sprayed coatings heat treated at 1250°C for 1 hour due to some sintering effect. This significantly affected its tensile properties with an average resulting ductility of 24.7%.

The conventional high-velocity oxy-fuel (HVOF) process has characteristics of high flame velocity and moderate temperature, and is widely used to deposit cements, metals and alloys coatings such as WC-Co, nickel and stainless steel. In this paper, a high pressure HVOF system with combustion chamber pressure up to 3.0MPa, and with characteristics of higher flame velocity and lower temperature was developed. In-flight particle velocity was measured using the DPV-2000 system at combustion chamber pressures from 1.0 to 3.0MPa, and stainless steel 316L powder was deposited at a combustion chamber pressure of 3.0MPa. The influence of spray conditions on the coating microstructure, deposition efficiency and micro-hardness were investigated. It was shown that the combustion chamber pressure has significant influence on particle velocity. Dense coatings composed of unmolten or partially molten particles could be deposited by varying the spray parameters. In the experiment, deposition efficiency up to 90% was achieved at the optimized spray conditions.

The adhesion mechanism of deposits/substrate interface prepared by cold spray method has not been fully understood up to now. It seems that the adhesion strength is mainly determined by the mechanical (including the plastic deformation of particle and substrate) and thermal interaction between the particle and substrate when the particles impact onto the substrate with a high velocity. In order to understand the adhesion mechanism, the influences of particle impact velocity on the adhesion strength were investigated in this study. The particle velocity was obtained with DPV-2000 measurement and CFD simulation. The relationships between the adhesion strength of deposits/substrate interface and particle velocity were discussed. The results show that greater adhesion strength can be obtained with the increase of particle velocity. There are two available ways to improve the adhesion strength. One is to increase the temperature of working gas, and another is to employ helium gas as the working gas instead of nitrogen gas.

Thick titanium coatings were prepared by warm spraying (WS) and cold spraying (CS) process to investigate the oxidation and microstructure of the coating layers. Prior to the coating formations, the temperature and velocity of in-flight titanium powder particle were numerically calculated. Significant oxidation occurred in WS process using higher gas temperature conditions with low nitrogen flow rate, which is mixed to the flame jet of an HVOF spray gun in order to control the temperature of the propellant gas. Oxidation, however, decreased strikingly as the nitrogen flow rate increased. In CS process using nitrogen or helium as a propellant gas, little oxidation was observed. Although most of the cross-sections of the coating layers prepared by conventional mechanical polishing looked dense, coating cross sections prepared by an ion-milling method revealed the actual microstructures containing small pores and unbounded interfaces between deposited particles. Even when scanning electron microscopy or x-ray diffraction method did not detect oxides in the coating layers by WS using high nitrogen flow rate or CS using helium, the inert gas fusion method revealed minor increase of oxygen content below 0.3 wt%.

It has been well known that the coating quality of plasma spraying is strongly influenced by instability of jets in plasma spray due to the arc root fluctuation. A three dimensional (3D) unsteady modeling was employed in the research to analyze the arc root fluctuation in a DC non-transferred plasma torch. Numerical calculations on the distribution of gas temperature and velocity in plasma torch were carried out using argon as plasma gas. The electrical current density and potential were also discussed. The results indicate that the fluctuation of arc inside the plasma torch is mainly induced by the movement of the arc root on the anode surface. The arc root moves downstream with the flow of gas, and the arc will warp from the electromagnetic force simultaneous to the movement. While the arc warps close to the anode boundary, a new arc root is formed somewhere upstream of the original attachment. This article represents nature of fluctuation of arc root, also in this paper we will present that the voltage-drop calculated is larger than that measured experimentally based on the hypothesis of local thermodynamic equilibrium.

This paper describes the development and use of a gas heating system for cold spraying. By heating helium and nitrogen gases to temperatures of up to 900 °C, deposition efficiencies of more than 90% have been achieved for titanium, niobium, and stainless steel as well as aluminum, copper, and nickel. Numerous coating examples are presented and discussed.

In this study, aluminum powder is cold sprayed onto copper, Al 5052, and 304 stainless steel substrates at working gas temperatures of 200, 300, and 400 °C. In-flight particle temperatures and velocities are determined theoretically and compared with experimentally measured values. It is shown that particle velocity increases with increasing gas temperature, while critical velocity falls. At 400 °C, a deposition efficiency of more than 95% was achieved. In contrast, the bonding strength at the coating-substrate interface is influenced much more by the substrate material than gas temperature.

This study investigates the influence of high-frequency pulse detonation (HFPD) spraying parameters on particle characteristics and coating quality. A dual-slit velocimeter was used to measure the velocity and temperature of Al 2 O 3 particles as fuel flow ratio and spray distance were adjusted. Experimental results show that particle velocity varied from 600 to 820 m/s as the fuel flow ratio was changed, but particle temperature remained relatively constant at about 2200 °C. Spraying distance had essentially no effect on in-flight particle properties although it influenced deposition efficiency, as did fuel flow ratio.

The velocity of particles prior to the impact on the substrate surface is a very important factor that determines the deposition characteristics and coating quality in cold spray. The DPV-2000 system is an on-line diagnostic system that simultaneously measures the velocity, temperature and diameter of thermally sprayed particles. In the present study, the DPV-2000 system was employed to measure the velocity and diameter of cold sprayed particles. The effects of pre-setting software parameters of the DPV-2000 system on the measured particle velocities were investigated. The experimental results showed that the pre-setting values of the software parameters produced a significant influence on the accuracy of measured results.

Cold spray is a relatively recent spray coating technology in which metal or alloy particles are plastically deformed by the kinetic energy of the particles accelerated in a supersonic gas flow through a convergent-divergent nozzle before hitting the substrate. The particle velocity at impact onto the substrate is a key factor in determining the characteristics of the cold spray deposit. Therefore, various studies have been carried out on particle acceleration with the aim of obtaining faster cold spray particle velocities. Mathematical modeling has also been carried out on spherical particle acceleration in a supersonic gas flow in a Laval nozzle. To understand better how a non-spherical particle behaves in a supersonic gas flow, experiments were carried out on the effect of morphology on particle acceleration in cold spray. Two types of powder morphology were used for the experiment, one was spherical and the other was angular and jagged. The particle size distributions were almost the same. In-flight particle velocities of the spherical and angular particles were measured with a DPV-2000. It was found that the particle morphology greatly influenced the in-flight particle velocity and deposit efficiency.

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.

A special HVOF gun is used for aerodynamic research on internal flows of gas and particles in HVOF gun. The gun has rectangular cross-sectional area and has sidewalls of optical glass or transparent acrylic resin. Compressed air is used as process gas instead of combustion gas to visualize internal flow of the gun. The high-speed gas flows including shock waves in the gun are visualized by Schlieren technique. Particle trajectories in the gun are also visualized by high-speed digital video camera. The observation of erosion pattern created by particle collision on the barrel wall helps understand the particle trajectories throughout the barrel.

Titanium has an excellent corrosion property in chloride containing environments such as seawater. A modified HVOF spray process was developed by introducing a mixing chamber between the combustion chamber and the powder feed port. Nitrogen gas was fed into the mixing chamber to control the temperature of the combustion gas generated in the combustion chamber. By controlling the flow rate of nitrogen, various Ti coatings with different degree of oxidation and porosity could be fabricated. The densest coating produced by this process with surface polishing treatment maintained excellent corrosion protection over a steel substrate in artificial seawater in a laboratory test over 1 month.

This paper presents the adhesive strength results of copper and titanium deposits produced by cold spray processes on steel, stainless steel, aluminum and copper substrates. We investigated how the combinations of the particles and the substrate, and the pressure in the cold spray nozzle chamber affect the adhesive strength between the deposits and substrates. We used nitrogen and helium as cold spray process and powder carrier gasses in the processes. We found that helium gas produced much higher adhesive strength than nitrogen and that the strength of the deposits produced by using both helium and nitrogen gases was almost proportional to the chamber pressure. The adhesion produced by cold spray processes appeared to be dependent on the combination of hardness of powder and substrate metals. We also present cross section micrographs of the deposits and substrate observed after tensile strength tests. They show the substrate which is soft or easily deformed by particle collisions generates strong adhesion.

In the present study, the zinc powder (-48 µm) was used to deposit coating by cold spraying using nitrogen as driving gas at different operating temperatures. The microstructure of the deposited coating was characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM) to reveal the occurrence of fusion during the impacting of spray particles. The selected area electron diffraction analysis was used to examine the microstructural feature at the near interface areas between the deposited particles in zinc coating. Numerical simulation was carried out to estimate the particle temperature increment during the impacting process. The simulation result suggests a possibility of the melting of zinc particles at the localized contact region on impact. The examination of the coating surface provided the evidence for the occurrence of the melting of spray particles on impact. The experimental results showed that the cold-sprayed zinc coating presented a dense microstructure. The nano-structural phase was formed at the near interface areas between deposited particles in zinc coating, while the grains in the size of micrometers similar to that in the powder were retained in the inside of the particles in the coating. Moreover, the TEM observation evidently showed that the amorphous phase was formed at the interface areas between the particles. It can be considered that the amorphous phase in the coating was formed through subsequent rapid solidification of the melted material on impact. This fact provided further evidence to the occurrence of localized melting during impacting of spray particles.

The low temperature characteristic of cold spraying makes it possible to deposit the coating of temperature sensitive materials, such as nanostructured material, without any significant change in the microstructure of feedstock. In the present study, the Fe and Si powders of particle size less than 75 µm were mixed at a composition of 10wt%Si and ball-milled to produce the nanostructured feedstock. Cold spraying process was used to deposit coating with nitrogen as a driving gas at different temperatures. The microstructure of the as-sprayed nanostructured Fe-Si coating was characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The grain sizes of the feedstock and as-sprayed coating were estimated based on X-ray diffraction analysis. The results showed that the nanostructured Fe-Si coating can be deposited by cold spraying using the ball-milled powders as feedstock. The as-sprayed coating presented a dense microstructure. The average grain size of the as-sprayed coating was comparable to that of the corresponding milled feedstock. No significant effect of the temperature of driving gas on the microstructure of cold-sprayed nanostructured Fe-Si coating was recognized. Moreover, TEM analysis showed that the amorphous phase was present in the as-milled powders and the as-sprayed coating along with the nanocrystalline.

This paper analyzes the behavior of coating particles as well as the gas flow in a High-Velocity Oxy-Fuel (HVOF) gun by using numerical simulation. Special attention is paid to the particle behavior in turbulent boundary-layer inside the barrel.