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.

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.

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.

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.

Two kinds of thermal barrier coatings with NiCoCrAlY bond coatings (BCs) deposited by electron beam-physical vapor deposition (EB-PVD) and high velocity oxy-fuel thermal spraying (HVOF), respectively, as well as their top 8wt.%Y 2 O 3 -ZrO 2 (YSZ) ceramic layers deposited in one batch by EB-PVD were prepared on near-α titanium alloys. The field emission scanning electronic microscopy and microhardness indentation are used in comparatively study of microstructures, microhardness of samples. Cracking modes and crack characteristics in TBCs are investigated after thermal cycling in atmosphere, along with the discussion of roles of residual stresses, bonding strengths and mechanical properties of bond coatings in different failure extents. It is found that morphologies of BCs deposited by different methods (EB-PVD and HVOF) result in the different microstructures and microhardness of their upper YSZ. The denser and more homogeneous BC prepared by EB-PVD leads to the YSZ with finer and denser columnar clusters and higher microhardness, and the inhomogeneous and porous latter results in the upper YSZ with coarser and loosely bonded columnar grains and lower microhardness, and the TBC with BC deposited by EB-PVD is more protective, which is synthetically induced by residual stresses, bonding strengths and mechanical properties of bond coatings.

High temperature titanium alloys are considered as good candidate materials for many aerospace applications. In order to increase the usable temperatures and oxidation resistance of titanium alloys, plasma spraying thermal barrier coatings on the titanium alloys is considered as an effective method. The effect of plasma spraying process on microstructure and microhardness of the titanium alloy was investigated by scanning electron microscope, energy dispersion analytical X-ray spectroscopy and microhardness test. The results show that the microstructure of the titanium alloy inside the substrate keeps unchanged after plasma spraying, and no interaction and atomic diffusion happen evidently at the bond coat/substrate interface. However there exists a thin layer of plastic deformation zone in the substrate beneath the bond coat/substrate interface after plasma spraying. The residual stresses are induced inside the titanium alloy due to the thermal expansion mismatch and the temperature gradient inside the substrate during plasma spraying, and lead to generating microcracks in the surface beneath the bond coat/substrate interface and the increase of microhardness in the substrate.

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.

Cu coating was deposited by microplasma spraying system under a low power of 2.8 to 4.2kW. The effects of the main processing parameters including plasma arc power, operating gas flow and spray distance on particle velocity during spraying, and the microstructure and properties of the coating were investigated. The coating microstructure was examined with optical microscopy. The coating properties were characterized by cross sectional microhardness. The particle velocity during in-flight was examined using a particle velocity/temperature measurement system based on thermal radiation. The experiment results showed that particle velocity was increased with the increase in operating gas flow, and was not influenced significantly by plasma arc power and spray distance. Moreover, the microhardness of the coating was increased with the increase in arc power and with the decrease in spray distance. The operating gas flow showed no significant influence on the microhardness of the coating. The analysis suggested that the microhardness of the coating is influenced significantly by particle temperature. The comparison showed that the microhardness of the Cu coating deposited by microplasma spray is comparable to that of the coating deposited by conventional plasma spray system at power level of 30kW.