The need for effective electrical insulation coupled with good thermal conductivity in power electronics has led to an exploration of suitable solutions for components like Insulated-Gate Bipolar Transistors (IGBTs). Considering its material properties, AlN emerges as a promising candidate for this application due to its high thermal conductivity, good electrical insulation and ample dielectric strength. However, aluminium nitride (AlN) has a low deposition efficiency when applied by atmospheric plasma spraying (APS). In contrast to AlN, alumina has a very good deposition efficiency during thermal spraying. Feedstock development was conducted to enhance the coating deposition for AlN. Therefore, a parameter study was carried out with AlN feedstock material to form a protective alumina shell around the AlN particles. Subsequently, the heat-treated powder was applied on an aluminium substrate by APS. X-ray diffraction (XRD) analysis displayed that, the heat-treated feedstock material contained AlN and α-Al 2 O 3 phases. It was observed from scanning electron microscopy (SEM) analysis that the AlN particles formed an oxide shell which led to an enhanced deposition efficiency with a high amount of AlN in the coating. The coatings were also investigated by XRD and SEM to prove the presence of AlN and alumina. For the first time, oxide shelled AlN was successfully applied by thermal spraying with sufficient coating deposition and enhanced AlN-content in the coating.

In this work, a fine copper layer was deposited on an aluminum nitride substrate via cold spraying. Parametric modeling was used to develop a parameter selection map that helped guide preliminary experiments to validate the model for different substrate surfaces, nozzle geometries, gas dynamics, standoff distances, and deposition angles. Further modifications were then made to include failure prediction based on particle impact, leading to the deposition of well-adhered cold-sprayed copper on a ceramic surface.

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.

Aluminum nitride (AlN) ceramics is characterized with its high thermal conductivity and chemical stability. However, it was impossible to fabricate AlN coatings by conventional thermal spray processes directly from AlN feedstock powder due to thermal decomposition of AlN during spraying. In the last decade we were apple to fabricate the AlN coatings through the reactive plasma spraying process (RPS) in the atmospheric ambient. This study describes the way to fabricate high thermal conductivity plasma sprayed AlN coatings. The thermal conductivity of the AlN coatings was investigated by laser flash method. The as sprayed coating had very low thermal conductivity (2.43 W/m.K), compared to the AlN value. It is attributed to presences of high oxide content (Al 5 O 6 N, γ-Al 2 O 3 and α-Al 2 O 3 phases), low density (2.32 g/cm 3 ) and high porosity in the plasma sprayed coating (about 22%). Besides that, although the N 2 gas flow improved the nitride content, the thermal conductivity decreased gradually. It is related to the further increase of the coating porosity and decreasing its density with the N 2 gas. The influence of the process parameters on the thermal conductivity was investigated and to fabricate high thermal conductivity AlN coating adjusting the oxide content, the coating porosity and microstructure are the main factors. Very high thermal conductivity (about 95 W/m.K) atmospheric plasma sprayed AlN coating was fabricated. The coating consists of mainly AlN phase (more than 95% AlN), very small amount of oxide phases, low porosity (about 3%) with a sintered microstructure (nicked-shape sintered particles).

This work assesses the challenges of preparing dense technical ceramic substrates for thermal spraying and evaluates the capabilities of laser ablation in comparison with sandblasting. Sintered Si3N4 and AlN substrates were prepared by both methods and surface roughness was measured before and after treatment. Alumina coatings were deposited by suspension-HVOF and atmospheric plasma spraying, and coating cross-sections were analyzed by optical microscopy and SEM. Sandblasting had little or no effect on surface roughness and cracks were observed in coating cross-sections at the near-surface region of the substrate. Laser ablation, on the other hand, significantly increased surface roughness for both ceramics, producing hole patterns that are shown to vary with laser power and pulse timing. In the case of plasma spraying, the best coatings were achieved when the holes in the substrate were less than 100 µm in depth. With suspension sprayed coatings, the best results were obtained on substrates with deeper (> 100 µm) holes.

Vacuum kinetic spray (VKS) technology was employed to fabricate AlN films. In order to investigate the relationship between the deposition behavior with change of the size and microstructure of feedstock, AlN powders were pre-treated through ball-milling and heat-treatment. It is observed that the particle size decreased and defect density increased after the ball-milling. After the heat-treatment of the ball-milled powder, on the other hand, the particle size was not changed significantly, while the defect density was reduced by recovery. The coatings of ball-milled and heat-treated powder were obviously thicker than that of only ball-milled powder. It is inferred that, during the heat-treatment, although the defects were reduced by recovery, the dislocations were aligned, through which the cracks were able to propagate more easily. By the combined effects of ball-milling and heat-treatment, more fragmentations with new surfaces of the particles were generated during VKS, which would improve the deposition efficiency. Therefore, in VKS process, the deposition behavior is shown to be affected by not only particle size, but also the defect density and microstructure of the feedstock powder.

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.

A novel process to produce dense, well adherent aluminium coatings on ceramic materials is cold gas spraying (CGS). The mechanical, physical and chemical processes leading to the bonding of cold-sprayed coatings on ceramic substrates have only been described rudimentarily. A survey of the literature on adhesion mechanisms of cold spray coatings is given, where influences on bond strength are discussed and parameters identified. Using the example of coating Al 2 O 3 and AlN substrates with pure aluminium via cold gas spraying, a process and substrate parameter variation is presented. A significant raise in tensile coating strength was seen at elevated substrate temperatures and after subsequent annealing. Tensile strength also depended on chemical composition and roughness of the substrate. The results allow the discussion of the bonding mechanisms of cold spray aluminium on ceramic substrates as a function of deposition and annealing parameters.

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.

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.

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.