In this present work, investigators determine how particle temperature, combustion pressure, and heat treatment affect the porosity, oxide content, and tensile properties of warm-sprayed titanium. Coatings were deposited with nitrogen flow rates ranging from 0.5 to 1.5 m 3 /min and combustion pressures of 1 and 4 MPa. Optimal coating properties were found for specimens formed at a nitrogen flow rate of 0.75 m 3 /min and a combustion pressure of 4 MPa. Post-spray heat treatment was found to improve bonding between deposited particles, significantly increasing the strength and ductility of the titanium coatings.

A new high-pressure warm spray gun was designed with the aim of increasing particle velocities to 1000 m/s for 30 µm Ti particle at 1000 °C or below. Nozzle geometry and combustion chamber pressure were optimized based on one-dimensional simulations. The flow rate of nitrogen gas injected into a mixing chamber was determined by calculation. The fuel injector was developed experimentally, its geometry optimized to spay small well-diffused droplets of kerosene into a 4 MPa chamber.

This study investigates particle velocities achieved by high-pressure warm spraying. Commercially pure titanium (CP-Ti) and Ti-6Al-4V powders were deposited on different substrates while varying spray parameters to determine their effect on particle velocity and coating quality. Particle image velocimetry was used to measure particle velocity, which peaked at 1,000 m/s. Coatings obtained under optimized conditions were characterized based on porosity, oxygen content, and hardness. The results show that the increased velocity of high-pressure warm spraying has significant beneficial effects in terms of improving density and controlling porosity and oxygen content.

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.

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.

It is well known that thermal spray condition affects the coating properties such as porosity, elastic modulus, coefficient of thermal expansion (CTE), coating fracture strength and coating cohesive strength. Therefore, residual stress formed in the sprayed coating and coating stress generated during in-service is dramatically changed with the thermal spray condition. In this study, effect of several kinds of thermal spray conditions on these properties of the coating was examined experimentally. Typical thermal barrier coating system composed of a partially stabilized zirconia (its chemical composition is 8wt%Y 2 O 3 -ZrO 2 ) and CoNiCrAlY bond coating was selected herein. In-flight particle velocity and temperature, and the substrate temperature were changed as the thermal spraying process parameters varied. For the ceramic coating layer, the coating properties such as porosity, Vickers hardness, CTE, elastic modulus, bending fracture strength, fracture toughness of splat boundary and then coating residual stress were measured systematically.

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.

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.

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.

It is essential to understand and clarify what causes adhesive strength between the thermally sprayed deposit and the substrate. It is known that the adhesive bond strength is strongly related to the roughness of the substrate surface, however, we are yet to know why and how the roughness affects the strength. Although the roughness works as a socalled anchor or interlocking effect, we cannot show how the effect quantitatively relates to the strength. Before we use the word “roughness”, we need to define roughness as a strict meaning in connection with the adhesive strength to avoid any ambiguity in expressing the roughness itself. Seeking the true roughness to relate to the bond strength, we have introduced a newly developed roughness indicator correlated closely with the adhesive strength. This indicator is derived by theory on the basis of the assumption that the bond strength is primarily caused by the mechanical friction between the deposit and the substrate. It was supposed that chemical and physical adhesions were secondary effects except that the molten particle, the substrate temperature or both of them are high enough to form these bonds by thermal interaction. The increase in contact area between the splat and the substrate could raise the friction force between them. The higher substrate temperature could increase the contact area when the molten particle interacts with the substrate, because the molten particle could wet better on the high temperature substrate. The substrate temperature influence on the adhesive strength was also investigated.

Although substrates are often grit blasted prior to thermal spraying to improve coating adhesion, the relationship between surface roughness and bond strength is not well understood. This paper proposes a new method for characterizing the surface roughness of substrates on the hypothesis that mechanical friction between the coating and substrate primarily determines adhesive strength. The authors explain how they measure 3D surface profiles using a confocal scanning microscope and develop the necessary math to calculate friction forces and coefficients. They also present experimental results comparing the roughness of stainless steel and aluminum substrates with the adhesive strength of plasma-sprayed Al 2 0 3 layers. Paper includes a German-language abstract.