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.

The in-flight particle temperature and velocity are very important factors that determine the deposition characteristics and coating qualities in plasma spray. Therefore, predicting the in-flight particle temperature and velocity with spray conditions is benefit to control the prepared coating qualities. In the present study, a steady-state 3D model was used to calculate the gas flow field of a plasma jet. The plasma gas temperature and velocity profiles applied to the plasma jet as the boundary conditions were obtained with our previous studies of plasma arc behaviors inside plasma torch. After obtained the gas temperature and velocity of the plasma jet, the in-flight particle trajectories, temperature, and velocity were investigated with numerical calculation method. Also the inflight particle temperature and velocity were verified by experimental measurements.

Inconel 718 is a precipitation-hardenable nickel-base superalloy with decent corrosion resistance, high strength at ambient temperature, and excellent creep and fatigue strength at high temperature. In this study, Inconel 718 was deposited by cold spraying with nitrogen and helium gas. Particle velocities were measured and splat morphology and coating microstructure were observed. Mechanical properties, including hardness, bond strength, and tensile strength, were also investigated. Although the deposits sprayed with helium had slightly better mechanical properties, nitrogen-sprayed Inconel 718, post heat treatment, exhibited mechanical properties similar to those of the bulk material.

Resonant ultrasound spectroscopy was applied to analyze the elastic anisotropy of thick copper, aluminum, titanium, and nickel coatings prepared by cold spraying and to determine the respective elastic moduli. The results show that the coatings exhibit only weak deviations from perfect isotropy, and the obtained elastic moduli are comparable with those of the corresponding polycrystalline bulks. The increased internal friction observed in some of the studied coatings may indicate grain refinement and consequent grain boundary sliding.

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.

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.

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.