Decomposition of the nitride ceramic particles like aluminium nitride (AlN) during conventional thermal spray process prevents their deposition on the substrate. Reactive plasma spraying (RPS) is a promising solution to fabricate AlN coatings. It is based on reaction and deposition of molten particles in active nitrogen ambient to form the AlN phase. Several thick AlN based coatings were fabricate successfully by reactive plasma spraying of aluminium and/or alumina particles. This study shows our recent achievements of fabrication of AlN coatings with improved conductivity. It was possible to fabricate AlN based coatings through reactive spraying of fine alumina particles mixed with fine AlN additives. Using small particle size powders improved the particles melting, surface area, therefore nitriding conversion and the AlN content. The fabricated AlN based coating contains several of oxide phases, with low density and high porosity, therefore its thermal conductivity was very low (about 2.6 W/m.K). To fabricate AlN coatings with high thermal conductivity, a liquid phase promoting additive (yttria) was added to the feedstock powder. It assists the formation of the yttrium aluminate (Y-Al-O) phase and therefore the sintering of the coatings during heat treatment. Finally, AlN coating with improved thermal conductivity (above 90 W/m.K) was developed.

This study investigates the feasibility of spraying aluminum nitride-alumina-yttria mixtures in a nitrogen plasma ambient. Coatings consisting of h-AlN, c-AlN, Al 5 O 6 N, and γ-Al 2 O 3 with small amounts of α-Al 2 O 3 and aluminum-yttrium oxide phases were produced. Although using the Y 2 O 3 additives significantly affected the process and microstructure, it did not achieve the high thermal conductivity desired in as-sprayed coatings. However, a high thermal conductivity (>90 W/m·K) AlN coating was fabricated by increasing the AlN content and enhancing sintering during heat treatment.

This study investigates the influence of particle temperature and velocity during reactive plasma spraying and the effect of plasma gases on coating properties. Using hydrogen gas with low flow rate was found to be better for reactive plasma spraying of fine Al 2 O 3 -AlN mixtures. The H 2 gas increased in-flight particle temperature, affecting in-flight vaporization, AlN content, phase transformation, deposition efficiency, and coating thickness. N 2 gas, on the other hand, increased particle velocity, thereby reducing particle residence time in the plasma, which affects melting, nitride conversion, and phase transformation.

The aim of this study is to improve the releasability of carbon fiber reinforced plastic (C-FRP) molds by depositing a plasma-sprayed Y 2 O 3 coating on working surfaces. Composite molds are considered for use in IC encapsulation. To promote adhesion, an interlayer, either copper or gradient yttria, is applied between the substrate and coating. The coatings deposited over gradient Y 2 O 3 performed better than coatings applied over copper due to the large reduction in thermal mismatch achieved with a graded thermal expansion coefficient. Preliminary releasability results show that the yttria coatings have high adhesion strength and the potential to uniformly reduce release forces between C-FRP surfaces and epoxy resins.

This study investigates the influence of substrate temperature on the adhesion strength of cold spray coatings. Copper, aluminum, and iron powders were deposited on preheated substrates similar in composition with the respective powder. Adhesion strength was determined by shear adhesion testing. The results show that substrate preheating improves adhesion strength for specific combinations of coating and substrate materials, likely due to the relief of thermal stress and the formation of an oxide layer on the substrate surface.

Cold spraying is a promising process to fabricate functional coatings. Because of the solid-state particle deposition, the electrical and chemical properties of the coatings are similar to those of bulk materials. This is not the case with mechanical properties, however, which differ between the coating and bulk due to severe plastic deformation in the particles. The residual stress and bonding state of particles in cold-spray coatings are investigated in this study as to their influence on mechanical anisotropy. Bending tests on cold spray copper coatings show that loading direction has a significant effect, which may be related to the distribution of microvoids and pores. Traverse patterns and process gas temperature are also examined and are shown to play a role.

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.

Titanium dioxide (TiO 2 ) is a unique material that can be used as an excellent photocatalyst, hence it will be very useful to solve the current world pollution problem. For photocatalytic application, the cold spray process, which uses supersonic high velocity technique at moderate temperature is considered to be a suitable technique as it will inhibit the phase transformation of TiO 2 . It is known that obtaining a thick coating of TiO 2 is difficult as it is a hard ceramic material. However, we have successfully synthesised TiO 2 powders that can be cold sprayed. Our synthesis technique is a simple hydrolysis method of titanyl sulphate (TiOSO 4 ) in distilled water. It was discovered that the cold sprayability of TiO 2 powders is depended on the powder morphology and using our synthesis method, powder morphology can be easily altered. Addition of (NH 4 ) 2 SO 4 during hydrolysis and post synthesis hydrothermal treatment result in a unique oriented aggregated nanostructure of TiO 2 . Only TiO 2 with this nanostructure can be cold sprayed.

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.

Cold-spraying has been developed as a high-quality metallic coating process. Recently, it became possible to fabricate a brittle ceramic coating using a particular nano-structured powder. In order to improve and control the process, the bonding mechanism should be understood. In this study, the bonding mechanism between cold-sprayed TiO 2 particle and stainless steel substrate was investigated by cross-section observation and quantitative adhesion strength measurement for the individual particles. The adhesion strength was measured by the combination using of scanning probe microscope (SPM) imaging and nano scratch test. The result clearly revealed a correlation in adhesion strength with splat diameter. The smaller splat diameter exhibited higher adhesion strength, though it is generally difficult to penetrate the cold-sprayed small particles through a shock wave formed in front of the substrate. The cross-section microstructure of the splats prepared by focused ion beam (FIB) showed the particular deposition behavior of the TiO 2 particle. The collided particle didn’t deform the substrate surface and deformed the particle itself. The bottom center of the splat is densified by the deformation and the other parts maintain the porous structure or broke up. Hence the higher adhesion strength of smaller splat was caused by this typical TiO 2 deposition behavior.

We have shown the fabrication of ceramics titanium dioxide (TiO 2 ) coating via cold spray process. Cold spraying of any hard ceramics coating is considered to be difficult, including TiO 2 . However, we could obtain a coating with thickness greater than 100 µm. The adhesion mechanism of this cold sprayed TiO 2 coating needs to be clarified. The effects of process gas parameters and substrate materials on coating's adhesion strength were studied. It was found that adhesion strength did not vary significantly with the change of process gas parameters, indicating that mechanical embedment is not the sole factor of coating¡¦s adhesion. The study of changing substrate materials reveals the degradation of adhesion strength due to surface oxide layer and that TiO 2 shows better adhesion on bare metal surface. Hence, by altering the surface chemistry, the adhesion of TiO 2 is varied, indicating the possibility of physical or chemical bonding between cold sprayed TiO 2 and substrate.

Cold spraying has been developed as a high-quality coating process. However, the deposition materials were limited as metallic materials. In this paper, titanium dioxide (TiO 2 ) ceramic particles are deposited on several substrate materials and formed thick coatings, making this approach suitable for a wide range of photocatalytic applications. In order to understand the adhesion mechanism of solid ceramic particles, the structures of feedstock particles are carefully observed with a scanning electron microscope (SEM) and a high-resolution transmission electron microscope (TEM). We find a porous structure which is agglomerated with nano-scaled primary particles. It is assumed that the break down phenomenon occurs due to the cold spray process and influences the adhesion of ceramic particles. The primary particles are bonded within a single crystal. This particular structure is the main factor to make adhesion between the particle and the substrate. SEM and TEM analysis clearly reveals adhesion mechanisms related to the impact of spraying ceramic particles toward the substrate.

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.

This study investigates the deposition behavior of cold sprayed copper particles on flat surfaces. In the experiments, a Laval-barrel nozzle was used to spray water atomized spherical copper particles with a mean diameter of 5 µm onto mirror-polished stainless steel. The particles were similar in morphology regardless of spraying conditions with an average bonding strength of 60 MPa as determined by nano scratch (shear) testing. An amorphous-like layer at the particle-substrate interface indicates that the deformation of the particles initially destroys their surface oxide, revealing an active fresh surface that facilitates metallic bonding. At higher spray velocities, metal jetting is observed at the periphery of flattened particles and its relationship with deposition efficiency is statistically analyzed and put forth as a potential method for controlling the cold spray process.

Molybdenum disulfide (MoS 2 ) films are widely used to improve friction performance, but they are difficult to fabricate using conventional thermal spray processes due to thermal decomposition of the feedstock powder. In this study, Cu-MoS 2 composite coatings are fabricated by cold spraying using mechanically milled powders containing different concentrations of MoS 2 . Investigators found that increasing the concentration of MoS 2 in the powder improved some coating properties while degrading others. Through testing it was determined that the ideal concentration of MoS 2 is 5wt%. Increasing the milling time of the powder mixture also provided benefits in terms of hardness and wear resistance.

To improve the deposition efficiency of copper particles, especially fine particle, in cold spray process onto metallic substrate, optimization of nozzle shape and dimension was performed by numerical simulation. Maximum velocity of the particle reached up to 685 m/s under the optimum conditions by using self-designed nozzle based on the simulation results. In the spraying of copper particle onto steel substrate, lamellar-like unique micro-structure was observed near the interface region of the steel substrate. Correspondingly, hardness increase in this region of steel substrate was recognized. Work hardening was induced in the steel substrate due to the higher velocity of copper particles. Furthermore, to reduce the bow shock effect on the substrate surface region in cold spray process, special shaped nozzle was newly developed. While nominal particle velocity decreased in the special nozzle, deposition efficiency, Vickers hardness and adhesion strength increased significantly especially in the case of fine particle and higher pressure levels of the working gas. Numerical simulation showed that pressure level on the substrate surface decreased effectively in the special nozzle. In the observation of a cold sprayed individual particle onto mirror polished substrate, extended metal jet was recognized at particle’s periphery when the particle was sprayed by the special nozzle. The results indicate that the decrease of particles velocity by bow shock was suppressed effectively in the special nozzle as compared with conventional nozzle.

Titanium dioxide (TiO 2 ) is a promising material for photocatalyst coating. However, it was difficult to fabricate TiO 2 coatings which have excellent photocatalyst property by thermal spray processes. Because anatase phase of TiO 2 transforms into rutile phase under high temperature i.e. the photocatalyst property of TiO 2 declines by heating. In this study, TiO 2 photocatalyst coatings were fabricated by cold spraying. Agglomerated TiO 2 powder with 100% anatase phase was injected into nitrogen or helium gas stream and deposit onto steel substrate. It was possible to fabricate TiO 2 coatings with anatase phase and dense microstructure. The deposition efficiency was increased with gas temperature. The photocatalytic property of the coatings was evaluated by NOx elimination test. From the results, it became clear that cold sprayed TiO 2 coatings had excellent photocatalyst property.

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.