Motohiro Yamada

Cold Spray Coating Deposition Mechanism on the Thermoplastic and Thermosetting Polymer Substrates

Cold spraying is a successful and promising coating technique for many engineering applications due to its high-rate and high-dense coating development abilities. Nevertheless, their practical use in polymer substrate is still in the fledgling phase. There are very few articles about the cold spray coating on polymers; however, the interaction of metallic particle with the polymer substrate is poorly understood, and thus a thick coating has not successfully been developed on the polymer substrate. In order to rationalize as full as possible the entire behavior of the high velocity particle with the polymer substrate, we used thermoplastic and thermosetting polymer materials as substrates. The particle behaviors with the substrate were observed under various gas pressure and temperature, and with various particles feed rate. The result showed that the particle behaviors were unique with respect to the substrate. Also it was clearly understood that the metal particles not experienced any plastic deformation due to the soft nature of the polymer substrates. The particles attached to the thermoplastic substrate either through adhesive bonding and/or mechanical inter locking, whereas only pure localized fracture observed on the thermosetting substrate and thus no particles attached firmly on the substrate.

Influence of Substrate Temperature on Adhesion Strength of Cold-Sprayed Coatings

Cold spraying is a promising process for fabricating functional coatings. Because of solid state particle deposition, the electrical and chemical properties of cold-sprayed coatings are almost similar to the bulk materials. The most important factor in depositing a particle through cold spraying is known as the critical velocity. When the particle velocity is beyond the critical velocity, the plastic deformation of both the sprayed metallic particle and the metallic substrate causes the removal of surface oxide layers and newly-formed surfaces are exposed. The newly-formed surface yields the metallic bond, and then cold spray deposition can occur. Therefore, the substrate conditions must also be considered in order to understand the cold spray process. In this study, the influence of substrate temperature on the adhesion strength of coatings was investigated by substrate pre-heating. The adhesion strength was examined by a shear adhesion test. This showed that substrate pre-heating enhances the adhesion strength with a specific combination of coating and substrate materials. Thermal stress and substrate surface oxidation have been suggested ss the causes of these.

The Effect of CFRP Surface Treatment on the Splat Morphology and Coating Adhesion Strength

Metallization of Carbon Fiber-Reinforced Polymer (CFRP) composites aggrandized their application to aircraft, automobile, and wind power industries. Recently, the metallization of CFRP surface using thermal spray technique, especially the cold spray, a solid state deposition technique, is a topic of research. However, a direct cold spray deposition on the CFRP substrate often imposes severe erosion on the surface owing to the high-impact energy of the sprayed particles. This urges the requirement of an interlayer on the CFRP surface. In the present study, the effect of surface treatment on the interlayer adhesion strength is evaluated. The CFRP samples were initially treated mechanically, chemically, and thermally and then an interlayer was developed by atmospheric plasma spray system. The quality of the coating is highly dependent on the splat taxonomy; therefore the present work also devoted to study the splat formation behavior using the splat-collection experiments, where the molten Cu particles impinged on the treated CFRP substrates. These results were correlated with the coating adhesion strength. The coating adhesion strength was measured by pull-out test. The results showed that the surface treatment, particularly the chemical treatment, was fairly successful in improving the adhesion strength.

Challenges Upon Reactive Plasma Spray Nitriding: Al Powders and Fabrication of AlN Coatings as a Case Study

Reactive plasma spraying (RPS) is a promising technology for the in situ formation of several ceramic coatings. The focus of this paper is to summarize the state of our current knowledge about the RPS process and using the nitriding of Al particles and the fabrication of aluminum nitride coatings, as a case study. The aspects and challenges in this process such as the influence of the plasma power, in-flight time, particle size, nitriding mechanism, splat morphology, in-flight particle diagnostics, N2 plasma gas, and the feeding rate on the RPS process are analyzed and discussed.

Fabrication of AlN/Al2O3 Coatings via Atmospheric Reactive Plasma Nitriding of Al2O3 Powders

Plasma sprayed alumina (Al2O3) coatings has showed a great effect in service the life of engineering tools, while its low thermal conductivity limits its application in heat exchange applications. Aluminum nitride (AlN) is a promising material to improve the thermal conductivity of Al2O3 coatings. This paper proposed a new way to fabricate AlN/Al2O3 coatings through reactive plasma nitriding of Al2O3 powders. It was possible to fabricate cubic-AlN/Al2O3 coatings by spraying Al2O3 powders in atmospheric plasma spray process (APS). During plasma Al2O3 powder reacted in high temperature N2/H2 plasma to form aluminum oxynitride and it easily nitride to produce the cubic AlN phase. Thus, both of Al5O6N and c-AlN phrases have the same cubic symmetry (cubic and closely packed crystal structure). Then, the particles collide, flatten, and rapidly　solidified on substrate surface. The high quenching rate of the plasma flame prevents the crystal growth and formation of hexagonal AlN phase. The fabricated coatings consist of c-AlN, α-Al2O3, Al5O6N and γ-Al2O3. The AlN content was improved with increasing the flight time (spray distance) due to increasing the reaction time between Al2O3 particles and the surrounding N2 plasma. It was possible to fabricate AlN/Al2O3 coating consist of 97 wt. % of AlN phase at the spray distance of 300 mm.

Fabrication of Si Nitride Coating onto Metal Substrate by Reactive RF Plasma Spraying

Si 3N 4 thick coating was fabricated by reactive RF plasma spraying, in which elemental Si reacted with surrounding nitrogen plasma. It was possible to fabricate the Si 3N 4 coating by reactive spraying on a graphite substrate. As for the substrate, however, graphite is difficult to apply to the practical structural parts because of its low mechanical strength. Thus, it was necessary to realize Si 3N 4 coating onto the metal substrate. In this research, Ti alloy, carbon steel and Ni alloy were used as the metal substrates. Ti alloy was difficult to apply to the formation of Si 3N 4 coating because the preferential reaction of Ti-N prevented the nitriding reaction of Si-N. On the carbon steel substrate, pure Si coating was fabricated. However, nitriding reaction was difficult to occur on this substrate because the melting point of carbon steel was lower than the reaction temperature of Si-N. Ni alloy, one of the useful heat-resistant alloys, was also tried as the substrate. Finally, the feasibility of some kinds of interlayer between Si 3N 4 and steel substrate was clarified to improve the bonding property between Si 3N 4 and metal substrate.

REACTIVE PLASMA NITRIDING OF AL2O3 POWDER IN THERMAL SPRAY

Among advanced ceramics, aluminum nitride (AlN) had attracted much attention in the field of electrical and structural applications due to its outstanding properties. However, it is difficult to fabricate AlN coating by conventional thermal spray processes directly. Due to the thermal decomposition of feedstock AlN powder during spraying without a stable melting phase (which is required for deposition in thermal spray). Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of AlN thermally sprayed coatings. In this study the possibility of fabrication of AlN coating by reactive plasma nitriding of alumina (Al2O3) powder using N2/H2 plasma was investigated. It was possible to fabricate a cubic-AlN (c-AlN) based coating and the fabricated coating consists of c-AlN, α-Al2O3, Al5O6N and γ-Al2O3. It was difficult to understand the nitriding process from the fabricated coatings. Therefore, the Al2O3 powders were sprayed and collected in water. The microstructure observation of the collected powder and its cross section indicate that the reaction started from the surface. Thus, the sprayed particles were melted and reacted in high temperature reactive plasma and formed aluminum oxynitride which has cubic structure and easily nitride to c-AlN. During the coatings process the particles collide, flatten, and rapidly solidified on a substrate surface. The rapid solidification on the substrate surface due to the high quenching rate of the plasma flame prevents AlN crystal growth to form the hexagonal phase. Therefore, it was possible to fabricate c-AlN/Al2O3 based coatings through reactive plasma nitriding reaction of Al2O3 powder in thermal spray.

The Synthesis of Titanium Dioxide (TiO2) Powder for Cold Spray Process

Titanium dioxide (TiO2) has attracted numerous attentions due to its high photocatalytic property. TiO2 as photocatalyst is best to be coated over large area substrate (walls, windows, etc.). In this case, cold spray process is the most suitable technique, not only because its simplicity and efficiency for large area deposition, but also because it is a low temperature process that would retain the high photocatalytic anatase phase of TiO2 in the coating. However, cold spraying of hard ceramic materials, such as TiO2, is widely known to be difficult. In this study, we are reporting a novel synthesis method of TiO2 powder for cold spray via the hydrolysis of titanyl sulphate (TiOSO4) with addition of ammonium sulphate ((NH4)2SO4). It is a relatively simple synthesis method that can obtain pure anatase structure even without high temperature treatment, although post treatments, such as hydrothermal and calcination further increase the crystallinity and change the morphology of synthesised TiO2. It is believed that the synergetic effect of post treatments and adsorption of NH4+ and SO42− ions on the surface of TiO2 causes the alteration in morphology of TiO2 powder. Finally, the cold spray spot test shows that the TiO2 powders can be cold sprayed. This is a remarkable finding as the difficulty of cold spraying TiO2 is widely known.

Controlling of Nitriding Process on Reactive Plasma Spraying of Al Particles

Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of aluminum nitride (AlN) thermally sprayed coatings. To fabricate thick A lN coatings in RPS process, controlling and improving the in-flight nitriding reaction of Al particles is required. In this study, it was possible to control the nitriding reaction by using ammonium chloride (NH4Cl) powders. Thick and dense AlN coating (more than 300 μm thickness) was successfully fabricated with small addition of NH4Cl powders. Thus, addition of NH4Cl prevented the Al aggregation by changing the reaction pathway to a mild way with no explosive mode (relatively low heating rates) and it acts as a catalyst, nitrogen source and diluent agent.

Reactive atmospheric plasma spray of Al powder

Reactive plasma spraying (RPS) is based on using reactive plasma to react with feedstock powders. RPS is a promising technology of material processing and coating manufacturing, especial for ceramic coatings like AlN which is expected to contribute to semiconductor equipments and so on, due to its outstanding properties [1]. However, fabrication of AlN coatings by conventional thermal spray processes was impossible due to thermal decomposition of AlN powder during spraying. In our previous study we fabricated AlN coatings through nitriding of Al powder in evacuated Radio Frequency (RF) plasma spray equipments using Ar/N2 mixture as a plasma gas [2,3]. In this study we fabricated AlN coatings through nitriding of Al powder in the air by Atmospheric plasma spray (APS) process, using N2/H2 as plasma gases. Al particles are injected in the plasma jet created by the passage of N2/H2 gases through an electric arc under high current conditions. Al particles continuously heated up (At high temperatures: above its melting point) and interacts with the plasma jet during flight to reach a certain energy state which is composed of thermal energy and kinetic energy. Al particles are fused and simultaneously accelerated by the plasma flow and gain their velocity and temperature by thermal and kinematic transfers towards SS400 substrate to be coated. Firstly we used carrier gas Ar and we found that the spray distance is an important parameter for nitriding reaction of Al powder. At small spray distance the coatings contain both Al and AlN but by increasing the spray distance the amount of AlN increases gradually, due to increase the reaction time so the possibility of Al nitriding increases, that at 300 mm spray distance the coatings is pure AlN. However, the coatings thickness decreased gradually from 100 to 20 µm with increasing the spray distance from 150 to 300 mm, thus particles temperature decrease by increasing the spray distance so its deposition efficiency decreased. To fabricate thick AlN coatings we used N2 as carrier gas and the coating mainly AlN with thickness 100 µm at spray distance 100 mm.