Warm Spray (WS) process, which can control the temperature of a combustion gas jet used to propel powder, has been successfully applied to deposit WC-Co coatings. Detrimental reactions resulting from dissolution of WC into Co binder and decarburization were suppressed effectively by keeping the WC-Co particles’ temperature below the m.p. of the binder phase. In this study, three nano-structured WC-12Co powders with different particle strength were prepared by changing the sintering conditions of spray-dried powder and were deposited by WS. The deposition efficiency and porosity of the coatings decreased with increasing the particle strength. The coating deposited from the powder with very low particle strength showed significant phase changes, while those deposited from the higher particle strengths showed almost no change. Particle Image Velocimetry revealed significant disintegration of the weakest powder, which explains the changes observed. The hardness and wear properties of the former coating, therefore, were inferior to the other two.

This study investigates the effect of alloying additions on the oxidation behavior of iron (Fe) and nickel-chromium (NiCr) powders during atmosphere plasma spraying. The chemical composition and phases of oxides in the particles as well as in the coatings are assessed for different powder mixtures and spraying parameters. The results show that oxygen content can be significantly reduced by adding silicon (Si) and boron (B) to iron powders and Si, B, and carbon (C) to NiCr. The preferential oxidation and subsequent vaporization of Si, B, and C from the surface of the sprayed particles are believed to play a major role in controlling oxidation in the APS process.

The high-velocity oxy-fuel (HVOF) process is commonly used to deposit WC-Co coatings. There are some problems with this process, especially the decomposition and decarburization of WC during the spraying. To eliminate these degradation, the warm-spray (WS) process originally developed by our group, which provides a possibility to control the flame temperature and the fabrication of WC-Co coatings can be made at lower temperature ranges that those of HVOF process, was applied to deposit WC-Co coatings. Microstructural characterization and phase analysis were carried out on deposited coatings by SEM and XRD. The mechanical properties such as hardness, fracture toughness, and wear properties were investigated. The results showed that WS coatings did not contain any detrimental phase such as W 2 C and W, which are usually observed in HVOF coatings. The hardness of WS coatings were lower than those of HVOF coatings, however, the relation of hardness-Co content of WS coatings showed the similar trend as that of the sintered WC-Co. The improvement of wear behavior was also observed in WS coatings.

The warm spray gun was developed to make a coating of temperature-sensitive material, such as titanium, on a substrate. The gun has a combustion chamber followed by a mixing chamber, in which the combustion gas is mixed with the nitrogen gas at room temperature. The temperature in the gun can be controlled in the range of about 1500 - 2500 K by adjusting the mass flow rate of nitrogen gas. The mixed gas is accelerated to supersonic speed through a converging-diverging nozzle followed by a straight passage. In this paper, the performance of the warm spray gun is investigated by the simulation program in order to deeply understand the performance of the warm spray gun. The gas flow as well as the velocity and temperature of titanium particle inside and outside the gun are predicted by the numerical simulation.

Titanium particles were deposited on a steel substrate by the impact of high velocity in warm spraying. In the process, nitrogen gas at various flow rates was mixed to control the temperature of a supersonic gas flow generated by combustion. TEM and other techniques were used to analyze the microstructure of the interface between the titanium coatings and the substrate. At the lower nitrogen flow rate, thick oxide double layers in the interface region were observed. The adhesive strength of the coating was high even at lower particles’ velocity possibly because the mechanical interlocking between the titanium particle and the substrate could be enhanced by the high deformability of heated particles. As the nitrogen flow rate increased, however, just a little oxide and a very thin oxide layer covering on the titanium splats were locally detected. The highly localized pressure and the resultant intensive shear stress generated within a titanium particle by the impact could reveal the fresh metal surface through break-up of the thin oxide films on the particle and the substrate. As a result, the metallic bonding between the deposited particle and the substrate was formed and increased the adhesive strength remarkably beyond a certain impact velocity.

Warm Spray has demonstrated that it could fabricate comparatively dense metal coatings keeping with high purity during the atmospheric process. Its key technology is the control of the temperature of the supersonic combustion jet prior to supplying feedstock. So far, even titanium (Ti), known as one of materials difficult for the atmospheric process, could be deposited with less oxidation and higher density of the resulting coatings. For instance, the porosity and oxygen content of two coatings obtained were 2.3 vol% and 0.28mass%, and 1.1vol% and 0.92mass%, respectively. Further densification of Ti coatings was achieved by bi-modal size distribution of feedstock powder upon Warm Spraying in this study. When bigger Ti particles were mixed with the usual feedstock powder under 45 µm, the coating porosity was decreased to 0.8vol% simultaneously with the low oxygen content of 0.26mass%, which was comparable to the level of feedstock powder. This densification is caused by the balance of the enhancement of the peening effect by big particles and of optimization of the filling rate of the big and small particles.

Thermal spraying of dense titanium coatings in the air atmosphere was achieved by using a two-stage high velocity oxy-fuel process (HVOF) called the Warm Spray Process. In the process nitrogen gas is mixed with the combustion gas to lower the gas temperature. Gas dynamics modeling of the flow field of the gas in the spray apparatus as well as the acceleration and heating of titanium powder injected from the powder feed ports were conducted. Based on the obtained temperature history of a titanium powder particle, its oxidation during flight was also predicted by using a Wagner-type oxidation model. These results were compared with measured velocity and temperature of sprayed particles by DPV2000 and the properties of deposited coatings. Significant discrepancy in the temperature of sprayed particles was found between the calculation and measurement whereas the measured velocity was closer to the model calculation. The model prediction of oxygen content was in a good agreement with the analysis of actual coatings. Furthermore, properties of the sprayed coatings such as porosity, oxygen content were correlated with the particle velocities and temperatures. Nitrogen gas was highly effective in lowering the oxygen content, but excessive nitrogen addition caused the coating porosity to increase due to insufficient particle temperatures.

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.

Coatings of HastelloyC fabricated by HVOF spraying with a gas shroud (GS) have shown the superior barrier characteristic and corrosion resistance in seawater environment. During immersion of these coatings in artificial seawater, however, vibrational behaviour of the corrosion potential was observed. Some types of surface modification of the sprayed coatings and changing of the spray condition were carried out and their effect on the corrosion potential was investigated. It was revealed that the vibration is related to surface oxides formed during the spray process. Surface modification of the sprayed coatings and changing of the spray condition could reduce the vibration effectively.

Improved HVOF spraying with a gas shroud has been developed to fabricate environmental barrier coatings of corrosion resistant alloys such as HastelloyC. For such coatings, control of oxidation of the powder material during spraying is very important and the gas shroud has been effective to lower oxygen content to 0.19mass%. In the present study, further reduction of oxygen content to 0.063mass% was achieved by changing the composition of combustion gas by introducing nitrogen into the combustion chamber. This value is almost comparable to the oxygen content 0.042mass% of the feedstock powder but the porosity of the coating increased. Introduction of nitrogen to the combustion chamber lowered the temperature of the spray particles in flight while maintaining their high velocity. Another coating with 0.14mass% was obtained with open porosity below 0.1vol% by changing the mixing ratio of nitrogen, which exhibited improved environmental barrier property in artificial seawater.

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.

The thermal spray committee of the Japan Association of Corrosion Control (JACC) has been conducting a marine corrosion test of thermal sprayed Zn, Al and Zn-Al coatings since 1985. Twelve kinds of sprayed coating were deposited onto steel pipes by arc- and flame-spraying to varied thickness and subjected to various post-spray treatments. The samples were set vertically into seawater at a port 80 km south from Tokyo. Corrosion performance of these coatings has been inspected annually by recording the appearance and coatings’ thickness at sea air-, splash- and tidal-zones. No significant changes were observed for five years exposure. After 7 years, however, Zn coatings with and without sealing started to suffer degradation in the immersed portion. Contrary to this, Al and Zn-Al coatings still exhibit superb corrosion performance. The test will continue till 2006 to complete the test period of 20 years. In order to place the test into a proper perspective, we also conducted a survey on the corrosion prevention by thermal spray technology in 2001, collecting more than 170 published reports and asking experts to contribute reviews on various aspects of the technology. This paper first describes several topics from the survey report to explain the past and the present situation of thermal spraying for corrosion prevention in Japan. Then, the corrosion performance of sprayed coatings in the exposure test during 15 years will be summarized.

For 6 years, we have developed corrosion resistant coatings for the marine structural steels by using a thermal spray technique. Such a coating requires primarily impermeability and secondarily cleanliness. In order to make denser and highly corrosion resistant coatings, we selected spray materials and improved fabrication processes. In addition, some new methods were designed to evaluate the sprayed particle’s state and coating properties with high accuracy and sensitivity. An inert gas shroud system was attached with the commercial HVOF apparatus and this attachment enabled inflight spray particles to be accelerated over 750 m·s-1 and simultaneously to avoid to be oxidized. The coating of HastelloyC nickel base alloy by this process had zero through porosity and 0.2 mass% of oxygen content, leading to be comparable to the bulk material of HastelloyC in terms of corrosion resistance. This coating, formed on steel, demonstrated an excellent protective performance over 10 months in the marine exposure test.

For thermal sprayed coatings, compactness of their constituent particles is required in many applications, e.g. to obtain impermeable anticorrosion coating in marine use. We investigated key factors to improve compactibility of deposited particles in HVOF sprayed coatings by condition measurements of spray particles. The results revealed that plastic deformability of the sprayed particles as well as their molten fraction was important to obtain the dense VHOF coatings.

This study evaluates the through-porosity of HVOF-sprayed Hastelloy C coatings on carbon steel with respect to coating thickness and combustion pressure. The amount of through-porosity in the NiCrMo alloy layer is determined by chemical analysis, using ICP emission spectroscopy, and compared with electrochemical measurements. Paper includes a German-language abstract.

This study examines stainless steel and Hastelloy C coatings sprayed using commercial HVOF equipment. Porosity and oxygen content have been measured under various spray conditions and laboratory corrosion tests have been carried out using electrochemical techniques. The report first summarizes major results obtained in the laboratory evaluation, then presents the results of marine exposure testing for up to six months. It concludes with detailed comparisons between the two. Paper includes a German-language abstract.

Previous studies have shown that gas shrouding is an effective means for controlling oxidation during HVOF spraying. In this present work, the authors attach a gas shroud to an oxyfuel torch with a longer barrel to further investigate the correlation between the state of HVOF sprayed particles and the density and oxygen content of the resulting layers. It is shown that with gas shielding, extended barrel length, and optimized spraying parameters, it is possible to accelerate powder particles to a velocity of over 750 m/sec with maintaining a high molten fraction, thereby producing very dense (zero porosity) stainless steel layers with oxygen contents less than 0.2% by weight. Paper includes a German-language abstract.

Importance of coating adhesion in a corrosive environment was studied experimentally. Tensile adhesion strength of HVOF sprayed 316L stainless steel and Hastelloy C coatings were tested in as-sprayed condition as well as after immersion in seawater. It was found that the adhesion strength of the stainless steel coatings degraded rapidly whereas that of the Hastelloy coatings remained almost intact. Specimens with an artificial defect were also immersed in seawater. The cross sectional observation after the test revealed that the corrosion at the coating-substrate interface proceeded much faster with the stainless steel coating as compared to the Ni-base alloy coating. A model experiment to simulate the galvanic corrosion of a coating-substrate couple was carried out and no significant difference in the galvanic current density was found between the two coatings when coupled with the steel substrate. The tightness of the coating-substrate interface was then tested with a fluorescent dye penetration test. The dye could penetrate the boundary between the stainless steel coating and the substrate whereas the boundary between the Ni-base alloy coating and the substrate was so tight that no penetration occurred. The size of the micro-gaps at the coating-substrate boundary was discussed from the viewpoint of classical Washburn-Ridiel theory. It was concluded that such micro-gaps between the coating and substrate must be eliminated for these barrier-type coatings to be used in corrosive environments. Heat treatment was highly effective for suppressing the preferential corrosion at the coating-substrate boundary.

The HVOF sprayed coatings of SUS316L stainless steel and Hastelloy C nickel-based alloy were studies with respect to corrosion behaviour in seawater. Corrosion took place at the small crack, which was formed by insufficient filling up of sprayed particles. The corrosion mechanism of the sprayed film was similar to the crevice corrosion. Some treatments for decrease of the crack resulted in considerable improvement of the corrosion resistance.