In this study, atmospheric pressure microwave plasma spraying is evaluated as a potential coating process for heat susceptible materials. The work was carried out using an experimental setup consisting of a 2.45 GHz microwave generator and a modified plasma torch. To characterize the spraying process, investigators measured plasma temperature, plume shape, and particle velocities for different gas flow rates, nozzle diameters, and spray distances and correlated the results with the flattening behavior of particles as captured in SEM images. Spray trials were then conducted to optimize the deposition of hard chrome on carbon-fiber reinforced polymer substrates and TiO2 powder on stainless steel. In both cases, the coatings were successfully applied; the former by decreasing the nozzle diameter, the latter by reducing heat input to spray particles.

Flattening mechanism of thermally sprayed single particles onto the flat substrate surface was investigated by using a metallic powder and stainless steel substrates. As the flattening pattern of the particles occurs quickly after the impact with the substrate surface, “pattern formation” mechanism was investigated. To perform this investigation, particles were thermally sprayed onto flat substrates through controlled substrate temperature and ambient pressures. The top surface, bottom surface and cross section microstructure of the splats were systematically investigated. By summarizing the results obtained, it was inferred that the initial solidification in peripheral region (starting at splat-substrate interface) might have a role on the formation of disk shaped splat.

Reactive plasma spraying (RPS) has been considered as a promising technique for in situ formation of aluminum nitride (AlN) based thick coatings. This study investigated the reactive plasma spraying of AlN coating with using Al 2 O 3 powder and N 2 /H 2 plasma. It was possible to fabricate a cubic- AlN (c-AlN) based coating. The phase composition of the coating consists of c-AlN, α-Al 2 O 3 , Al 5 O 6 N and γ-Al 2 O 3 . Understanding the nitriding process during coating deposition is essential to control the process and improve the coating quality. The nitriding process was performed by spraying, collecting the particles into a water bath (to maintain its particle features) and observing their microstructures and cross sections. During the coating process, the sprayed particles were melted, spheroidized and nitrided in the N 2 /H 2 plasma to form the cubic aluminum oxynitride (Al 5 O 6 N). The particles collided, flattened, and rapidly solidified on the substrate surface. The Al 5 O 6 N is easily transformed to c-AlN phase (same cubic symmetry) by continuous reaction through plasma environment. Improving the specific surface area by using smaller particle sizes enhances the surface nitriding reaction and improves the nitriding conversion. Furthermore, using AlN additives enhances the nitride content in the coatings. It was possible to fabricate thick and uniform coatings with high AlN content by spraying fine Al 2 O 3 /AlN mixture. Furthermore, the N 2 gas flow rate improved the nitriding conversion and the coating thickness.

Aiming at clarifying the individual splat formation mechanism in thermal spray process, commercially available metallic powders were thermally sprayed onto AISI304 substrate surface. The splats changed from a distorted shape with splash to a disk-shaped splat in flattening after collision onto substrate surface, through substrate preheating and/or reducing the ambient pressure. Accordingly, both substrate temperature and ambient pressure have an equivalent effect on the shape transition. The observation on the bottom surface morphology of single splat indicated that the ring-shaped initial solidification might play an important role during splat formation process. As a simulation of the real thermal spray process, free falling experiment has been conducted. The thermal history of the free falling metal droplet onto AISI304 substrate indicated that the flattening pattern is decided so quicky just after collision onto solid surface, which is enough earlier to the finalization of the flattening.

It was possible to fabricate cubic-AlN (c-AlN) based coating through the reaction between Al powder and the N 2 /H 2 plasma in APS system. The fabricated coating was about 100 μm with hardness about 540 Hv, which is much higher than the hardness of Al. The formation of the cubic phase in APS is directly related to the rapid solidification phenomena of plasma spraying process. The sprayed powder particles show rapid cooling rates upon impact with the substrate, which prevent its complete crystal growth. The nitride content increased with the spray distance due to increase the flight time of Al particles in the N 2 plasma. Using smaller particle size improved the nitriding reaction at short spray distance due to increasing the particle temperature. However increasing the particle temperature leads to excessive vaporization of Al particles and completing nitriding reaction during flight, therefore suppressing the coatings thickness. The nitriding reaction of the larger particle size Al powders can be enhanced through NH4Cl powders addition. That NH4Cl addition changed the reaction pass way from liquid-gas to vapor-phase intermediate mechanism.

Effect of substrate temperature and ambient pressure of deposition chamber on contact condition between free falling droplet and substrate surface were investigated. Cu droplet and SUS304 substrate were used for the experiment. To estimate the contact condition, temperature history of molten droplet was measured at splat-substrate interface by thermocouples embedded in the substrate and high frequency data logging system. Quantitative porosity on splat bottom surface and degree of circularity of splat shape was evaluated. Splat adhesion strength experiment was conducted and dynamic wetting behavior of substrate by molten droplet was captured by ultra-high speed video. Heating substrate and vacuuming chamber pressure enhanced the heat transfer from splat to substrate, which can be attributed to the good contact at splat bottom surface. Adhesion strength of the splat to the substrate corresponded well with degree of circularity. Consequently, substrate temperature and ambient pressure of deposition chamber have an equivalent effect to contact condition at interface between droplet and substrate surface.

Aluminum nitride (AlN) and iron nitride (Fe 4 N) coatings were fabricated by reactive plasma spraying using fine feedstock powders. Reactive plasma spraying, in which element particles react with surrounding active species in the plasma, enables to fabricate nitride ceramics which decompose without stable melting phase. However, it is difficult to fabricate the coatings which include higher concentration of nitride phase by reactive plasma spraying using conventional particle size of feedstock powders. Therefore, fine feedstock powders were used in order to enhance the nitriding reaction during spraying. Aluminum or iron particles were injected into Ar/N 2 plasma and were deposited onto graphite substrates. It was possible not only to increase the nitride phase content in the coatings but also to densify the microstructure in both materials. Thus, it became clear that using fine feedstock powders are useful for fabrication of nitride ceramic coatings by reactive plasma spraying.

A variety of metallic powder particles were thermally sprayed onto the mirror polished metallic substrate surface and the effect of both substrate temperature and ambient pressure on the flattening behavior of the particle was systematically investigated. In the flattening behavior of the sprayed particle onto the substrate surface, critical conditions were recognized both in the substrate temperature and ambient pressure. That is, the flattening behavior changed transitionally on that critical temperature and pressure range, respectively. A transition temperature, Tt, and transition pressure, Pt, were defined and introduced, respectively for those critical conditions. The fact that the dependence both of transition temperature and transition pressure on the sprayed particle material had similar tendency indicated that the wetting of the substrate by the molten particles seemed to be domination in the flattening. Three dimensional transition curvature by combining both transition temperature and transition pressure dependence was proposed as a practical and effective controlling principle of the thermal spray process.

Aluminum nitride (AlN) is one of the attractive ceramics with respect to its excellent mechanical and electrical properties. In this study, AlN coatings were fabricated and the influence of feedstock powders was investigated by reactive RF (Radio Frequency) plasma spraying. Two different particle sizes of commercial aluminum (Al) powders and Al/AlN mixed powders were used as the feedstock powder. The feedstock powder was injected into a RF plasma, and sprayed particles were deposited onto carbon steel or quartz substrates. As a result, it was possible to fabricate thick and dense AlN coating using smaller particle size of Al powders and quartz substrate. However, many agglomerates were formed in the coatings. On the other hand, 50 wt% or above of AlN addition in the feedstock powders was effective to prevent the formation of the agglomerates. Therefore, Al/AlN mixed powder with smaller particle size was useful for fabrication of AlN coatings by reactive RF plasma spraying.

We have confirmed that in the thermal spraying of practical powder materials, the splat shape changes with increasing substrate temperature to a circular disk shape from a fringe shape with splashing at a critical substrate temperature, Tt. The increase of substrate temperature may accompany a kind of essential change on substrate surface, because the effect is maintained until the substrate is cooled down to room temperature. However, the nature of the substrate surface change due to the heating has not been clearly understood yet. In this study, AISI304 stainless steel was employed as a substrate material, and the substrate was heated in an air atmosphere or laser treated as a pretreatment. Substrate surface topography was analyzed precisely by atomic force microscopy, AFM. We discuss the relationship between surface topography in nano-meter scale and splat morphology. Moreover, in order to evaluate the effect of chemical composition of the substrate surface, gold was coated onto the substrate surface by PVD method after the heat treatment. The effect of adsorbate/condensate on the substrate surface on the flattening behavior of thermal sprayed particles was also verified.

It has been confirmed in the flattening behavior of the thermal sprayed particles that the splat shape on the flat substrate surface changes to a disk type from a splash type at relatively narrow temperature range with increasing the substrate temperature. The temperature increasing actually causes a certain non-reversible change on the substrate surface, because the changing effect is maintained till the substrate is cooled down to the room temperature. We have pointed out that this non-reversible change on substrate surface due to the heating might be the possible domination for the transition phenomenon of the thermal sprayed particles, that is, the essence of the substrate surface change due to the heating is surface roughness in nano-meter range. In this study, several kinds of substrates once heated to some elevated temperature were analyzed precisely by atomic force microscope (AFM) to characterize the change in their surface roughness character. The fundamental static wetting behavior of the substrate surface by a water droplet was investigated for the reference in the present study. The results obtained revealed that the change of the substrate surface topography in nano-meter range affect most effectively the wetting behavior of the droplet or splat on the surface. Hence, the substrate heating may bring about the change in the physical way on the substrate surface and this change induces the transition phenomenon.

In the thermal spraying of powder materials, it has been observed in practice that the splat shape changes to a disk type from a splash type with increases in the substrate temperature. However, the details of the substrate surface change due to the heating has not been fully characterized. In this study, an AISI 304 stainless steel substrate surface heated to 673K was analyzed precisely by atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy. The results obtained revealed that the change of the substrate surface occurred not in the chemical composition but mainly in the surface roughness, especially surface morphology in nano-meter scale. Hence, substrate heating may bring about the change in substrate surface and this change induces the transition phenomenon.

Aluminum nitride (AlN) is one of the attractive ceramics applicable to the surface modification because of its excellent properties in chemical stability and thermal conductivity. In this research AlN coating was fabricated by reactive RF (Radio Frequency) plasma spray process, a kind of thermal spraying techniques. Reactive plasma spraying, in which metal element reacts with surrounding active species in plasma, has been considered to be an useful process for the formation of non-oxide ceramics thick coatings. By increasing nitrogen content in plasma gas, AlN coating without pure Al part was attained while the coating microstructure was heterogeneous, brittle and quite porous. By decreasing nitrogen content in plasma gas, on the other hand, Al/AlN composite coating with more homogeneous, less porous microstructure could be attained. Changing nitrogen fraction in plasma gas may be effective for controlling AlN content in Al/AlN composite coating. Nitriding process of aluminum in reactive RF plasma spraying was also investigated in this study. It could be considered that nitridation process of Al was occured during the particle flight in plasma or after the particle deposited onto the substrate. Nitriding reaction process of Al in the reactive plasma spray process was verified in the study.

Electromagnetic acceleration plasma generators, which are called Magneto-Plasma-Dynamic (MPD) arcjet generators, can produce higher-velocity, higher-temperature and higher-density plasmas than those of conventional thermal plasma torches, because MPD arcjet plasma is efficiently accelerated by electromagnetic body forces in MW-class input power operation. These properties are effective for deposition of rigid coatings adhering strongly to substrate surfaces. In the present study, we newly developed an ablation type MPD arcjet generator for titanium nitride (TiN) reactive spray coatings. The coatings were deposited onto steel substrate. The phase structure and the composition of the coatings were analyzed by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD), and their Vickers hardness were measured. These analyses showed that the MPD spray process could successfully form dense and uniform titanium nitride coatings. The properties of the titanium nitride coatings were highly sensitive to the titanium cathode diameter and discharge current.

The quasi-steady magneto-plasma-dynamic (MPD) arcjet generator is a promising plasma accelerator, which has a coaxial electrode structure similar to those of conventional plasma torches. The MPD arcjet generator utilizes principally electromagnetic acceleration of the interaction between the discharge current of kiloamperes and the azimuthal magnetic field induced by the discharge current, although the working gas is accelerated aerodynamically through a nozzle in a thermal arcjet generator. In this paper, ablation-type MPD arcjet generators are developed for ceramic coatings. Discharge voltages and ablation rates of ceramic materials are examined, and front velocities of ablated atoms of ceramic component are also estimated using a streak camera. The Vickers hardness of coating is measured. Their cross sections are observed with a scanning electron microscope, and their surfaces are analyzed by means of x-ray diffraction and x-ray photoelectron spectroscopy. Paper includes a German-language abstract.

Ammonia and a mixture of nitrogen and hydrogen are used for material processing. Since these gases are chemically active, the processing efficiency is enhanced. This article describes a study to understand the physical properties of ammonia and a mixture of nitrogen and hydrogen plasmas inside and outside an arcjet generator. Spectroscopic measurement is made, and several plasma properties are determined from the data. The result shows that the H-atom electronic excitation temperature and the nitrogen rotational excitation temperature decreased from 7000-11000 K in the constrictor to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit with mass flow rates of 0.1-0.2 g/s at input powers of 7-12 kW. However, the NH rotational excitation temperature did not show a significant axial decrease even in the downstream plume. Paper includes a German-language abstract.