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Aln) Coatings Through Suspension Plasma Spray (SPS) Technology coatings Article Synthesis of Cubic Aluminum Nitride (AlN) Coatings through Suspension Plasma Spray (SPS) Technology Faranak Barandehfard, James Aluha and François Gitzhofer * Department of Chemical & Biotechnological Engineering, Université de Sherbrooke, Sherbrooke, QC J1K 2R1, Canada; [email protected] (F.B.); [email protected] (J.A.) * Correspondence: [email protected]; Tel.: +1-819-578-7937 Abstract: Thermal spraying of aluminum nitride (AlN) is a challenging issue because it decomposes at a high temperature. In this work, the use of suspension plasma spray (SPS) technology is proposed for the in situ synthesis and deposition of cubic-structured AlN coatings on metallic substrates. The effects of the nitriding agent, the suspension liquid carrier, the substrate materials and the standoff distance during deposition by SPS were investigated. The plasma-synthesized coatings were analyzed by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). The results show higher AlN content in the coatings deposited on a carbon steel substrate (~82%) when compared to titanium substrate (~30%) or molybdenum (~15%). Melamine mixed with pure aluminum powder produced AlN-richer coatings of up to 82% when compared to urea mixed with the Al (~25% AlN). Hexadecane was a relatively better liquid carrier than the oxygen-rich liquid carriers such as ethanol or ethylene glycol. When the materials were exposed to a molten ◦ aluminum–magnesium alloy at 850 C for 2 h, the corrosion resistance of the AlN-coated carbon steel substrate showed improved performance in comparison to the uncoated substrate. Citation: Barandehfard, F.; Aluha, J.; Gitzhofer, F. Synthesis of Cubic Keywords: cubic aluminum nitride; suspension plasma spray (SPS); coatings; melamine; urea Aluminum Nitride (AlN) Coatings through Suspension Plasma Spray (SPS) Technology. Coatings 2021, 11, 500. https://doi.org/10.3390/ 1. Introduction coatings11050500 1.1. Background Aluminum nitride (AlN) is a promising material that can find diverse applications Academic Editor: Robert B. Heimann in the aluminum industry [1], for example, in crucibles for handling corrosive chemicals Received: 11 March 2021 and reaction vessels [2]. In addition, AlN has become the key component of semiconductor Accepted: 15 April 2021 equipment, which is appropriate in piezoelectric and electronic applications [3]. Some Published: 23 April 2021 of these amazing properties of AlN are summarized in Table1, as high thermal conduc- tivity [4], low thermal expansion coefficient [5], and a large band gap, which is a major Publisher’s Note: MDPI stays neutral factor that determines a solid’s electrical conductivity [6], and this makes AlN an electrical with regard to jurisdictional claims in insulator. Other characteristics include AlN’s chemical and physical stability at fairly high published maps and institutional affil- temperature regions, high hardness [7], and high resistance to molten metals, wear and iations. corrosion [5]. Table 1. Selected AlN properties. Properties Values Copyright: © 2021 by the authors. Thermal expansion coefficient α = 4.3 × 10−6 K−1 Licensee MDPI, Basel, Switzerland. Thermal conductivity k = 319 W·m−1·K−1 This article is an open access article Crystalline structure Hexagonal or cubic distributed under the terms and Hardness 1400 Hv conditions of the Creative Commons Electrical resistivity 1013 W·cm Attribution (CC BY) license (https:// Band gap 6.2 eV creativecommons.org/licenses/by/ 4.0/). Coatings 2021, 11, 500. https://doi.org/10.3390/coatings11050500 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 500 2 of 21 AlN exists in several crystal structures with different properties: hexagonal (wurtzite) crystal structure and two types of cubic structures (rock salt and zinc-blende). The cubic AlN is a metastable phase at ambient conditions, which renders its synthesis a difficult process [8]. In comparison to the hexagonal crystal structure, the rock salt cubic AlN by having a higher symmetry in its crystal structure shows greater thermal conductivity (250–600 W·m−1·K−1), electrical resistivity (1016 W·cm) and hardness (4079–5098 Hv) [9]. 1.2. Synthesis Methods of AlN Some of the conventional synthesis methods of AlN powder involve the carbothermal reduction–nitridation of alumina [10], direct nitridation of Al powder [11], non-transferred arc plasma method [12], chemical routes [13], microwave-assisted urea route [14], pulsed laser ablation [15] and transferred type arc plasma [16]. Tables2 and3 summarize the different methods of producing AlN coatings and powder, respectively. Table 2. Summary of different methods used in producing AlN coatings. Method Precursors Coating Substrate Reference RF plasma spray Al powder + N2 stainless steel, quartz plate [17] DC plasma spray Al powder + NH4Cl mild steel (SS400) [18] Atmospheric plasma spraying (APS) Al powder + N2 + H2 soft steel (SS400) [8] Magnetron sputtering Al-disc + Ar + N2 stainless steel (SS) 304L [19] LT reactive magnetron sputtering Al powder + Ar + N2 crystal Si, amorphous SiN [20] Reactive DC magnetron sputtering Al powder + Ar + N2 crystal Si, amorphous SiN [21] Reactive plasma spray Al2O3 + N2 mild steel (SS400) [22] RF magnetron sputtering AlN + Ar glass, mica, and silicon [23] Atomic layer deposition Al(C4H9)3 + N2H5Cl + C12H27Al silicon [24] Wet-thermal treatment (WTT) NH3 + N2 + AlCl3 carbon fiber [25] Pulsed laser deposition AlN + N2 Si (100) [26] Magnetron sputtering system Al + N2 silicon nitride [27] RF = Radio frequency; DC = Direct current; LT = Low temperature. Table 3. Summary of different methods used for producing AlN powder. Method Precursors Reference Carbothermal reduction Al2O3 + N2 + C/CH4 [28] CVD AlCl3 + NH3 + N2 [29] ◦ Reaction 500–1000 C for 5 h Al2S3 + NH3 [30] Sol-gel C9H21O3Al + dextrose [31] Mechanochemical Al2O3 powder + N2 [32] Mechanochemical Al powder + Melamine [33] Transferred Arc-plasma Al powder + N2 + NH3 [16] Microwave plasma Al powder + NH4Cl + N2 + H2 [34] Solvent thermal synthesis AlCl3 + NaN3 [35] Low-temperature synthesis Al + NH4Cl [36] Reactive plasma spray Al + AlN powders [37] CVD = Chemical vapor deposition; LT = Low temperature. Thermal plasma technology is a well-adapted technology for synthesizing crystalline nano materials because of the fast reaction rates achieved due to the high plasma tempera- tures [5], its flexibility in the choice of various feedstock materials [38], its high conversion rates and energy efficiency [10] and its rapid quenching [39]. Furthermore, plasma spray technology simplifies the inflight reaction of the precursors and the in situ formation of the coatings in a wide range of thickness. Due to the high reaction rate, a dense coating is produced. Among all the thermal spray methods, plasma spraying is extensively used to produce coatings due to its high enthalpy, with the molten or semi molten particles being propelled to adhere and solidify on the substrate [40]. However, sometimes it is not necessary to melt the particles fully, because it is possible for particles in a semi molten state to be deposited as a coating. Moreover, a thermal plasma process can allow for a wide Coatings 2021, 11, 500 3 of 21 range of coating thicknesses, stretching from nm to mm [41]. In order for the deposited coating to stick to the substrate, the injected particles must melt or be in semi-molten state when they pass through the hot core of the plasma. The high melting point of AlN (2200 ◦C) as well as its decomposition at high temperatures does not facilitate the deposition of AlN particles injected through the plasma [41]. Besides ammonia [42], nitrogen gas is usually used as a nitriding agent [32], but recently urea [43], and melamine to produce Si-C-N [44], or AlN ceramics [45] have been proposed for nitriding reactions. Melamine (C3H6N6), an organic compound containing about 67 wt% nitrogen is a trimer of cyanamide, with a 1,3,5-triazine skeleton, that re- leases nitrogen at high temperatures [46], and has been used to produce AlN powder by mechanochemical reaction with Al [47]. Direct injection of nitriding powders such as urea or melamine with Al into plasma, unlike N2 gas, is challenging, particularly when there are different precursor components with various particle sizes and densities [41]. To alleviate the injection problems, suspension liquids have been employed as carrier media of transporting even nano particles into the high temperature zones of plasma, with the added benefits of providing chemical reactions within the suspension liquid. Indeed, powder precursors are dispersed in the liquid media and when injected into plasma, the liquid phase evaporates, while the solid particles melt and vaporize to form clusters. Suspension plasma spray (SPS) was invented to create a homogenous precursor to facilitate direct injection of small particles into plasma by dispersing them in a liquid carrier [48]. In order to produce a durable coating that can withstand thermal shocks, both the coating and the substrate should ideally have linear thermal expansion coefficient that are similar. Substrate materials should have higher melting points and lower thermal expan- sion coefficients than Al [49]. An interlayer coating can be used to adapt the expansion coefficient difference [50]. 1.3. Non-Wetting Properties In many material processing techniques, for example, in casting processes, molten metals are ordinarily in direct contact with ceramic refractories. Therefore, the properties of the
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