Reactive plasma spraying is a promising technology for the in situ formation of aluminum nitride (AlN) coatings. Recently, it became possible to fabricate cubic-AlN-(c-AlN) based coatings through reactive plasma spraying of Al powder in an ambient atmosphere. However, it was difficult to fabricate a coating with high AlN content and suitable thickness due to the coalescence of the Al particles. In this study, the influence of using AlN additive (h-AlN) to increase the AlN content of the coating and improve the reaction process was investigated. The simple mixing of Al and AlN powders was not suitable for fabricating AlN coatings through reactive plasma spraying. However, it was possible to prepare a homogenously mixed, agglomerated and dispersed Al/AlN mixture (which enabled in-flight interaction between the powder and the surrounding plasma) by wet-mixing in a planetary mill. Increasing the AlN content in the mixture prevented coalescence and increased the nitride content gradually. Using 30 to 40 wt% AlN was sufficient to fabricate a thick (more than 200 µm) AlN coating with high hardness (approximately 1000 Hv). The AlN additive prevented the coalescence of Al metal and enhanced post-deposition nitriding through N₂ plasma irradiation by allowing the nitriding species in the plasma to impinge on a larger Al surface area. Using AlN as a feedstock additive was found to be a suitable method for fabricating AlN coatings by reactive plasma spraying. Moreover, the fabricated coatings consist of hexagonal (h-AlN), c-AlN (rock-salt and zinc-blend phases) and certain oxides: aluminum oxynitride (Al₅O₆N), cubic sphalerite Al₂₃O₂₇N₅ (ALON) and Al₂O₃. The zinc-blend c-AlN and ALON phases were attributed to the transformation of the h-AlN feedstock during the reactive plasma spraying. Thus, the zinc-blend c-AlN and ALON phases were not included in the feedstock and were not formed through nitriding of the Al.

The surface of most metals is covered with thin native oxide films. It has generally been believed that to achieve bonding, the oxide covering the surface of metallic particles or metal substrates must be broken and removed by adiabatic shear instability (ASI), whether induced at the particle–substrate interface or at the particle–particle interface. The aim of the present research is to investigate the correlation between the remaining oxide amorphous layer and substrate-deformation with the adhesion strength of cold-sprayed TiO₂ coatings towards the bonding mechanism involved. Relevant experiments were executed using stainless steel (SUS 304), subjected to various annealing temperatures and cold-sprayed with TiO₂ powder. The results indicate an increasing trend of coating adhesion strength with increasing annealed substrate temperature. The influence of substrate plastic deformation and atomic intermixing at the remaining amorphous oxide layer is discussed as the factors contributing to the increasing adhesion strength of cold-sprayed TiO₂ coatings.

The cold spraying of ceramic materials is widely acknowledged as a difficult process because it necessitates the feedstock powder particles experiencing a plastic deformation for deposition on a substrate. The problem arises due to the brittle properties of ceramic powder feedstock such as titanium dioxide (TiO₂), combined with a lack of understanding of the bonding mechanisms. In this study, TiO₂ coatings were deposited onto copper and aluminum substrates and the adhesion strength was evaluated to investigate the bonding mechanism. The influence of substrate hardness and remaining surface oxide layer was investigated by annealing the substrates with various temperatures. The results showed that the adhesion strength of the coatings on the aluminum substrate was higher than the copper substrate. Furthermore, the adhesion strength was decreased with increasing the annealing temperature on both substrate materials. These results indicate that a softer aluminum substrate was advantageous for adhesion. Annealing led to thermal softening the substrate; however, the thickness of the surface oxide layer was increased. Therefore, bonding occurred between the cold-sprayed TiO₂ particle and newly deform substrate surface, which yielded the higher adhesion strength. The main bonding mechanism is metallurgical, similarly to the cold-sprayed metallic coatings.

The structure and crystallinity of a segregation on a borophosphosilicate glass (BPSG) film have been clarified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), transmission electron diffraction (TED), and electron energy loss spectroscopy (EELS). The segregation arises on the BPSG film surface after annealing in nitrogen ambient. Its size is below 0.5 μm. The segregation is identified as crystalline BPO4 from the results of TED and EELS. In this letter, TED pattern of crystalline BPO4 is indicated in detail. The growth mechanism of crystalline BPO4 is proposed on the basis of a series of the results.

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