Synthesizing AlN Coatings Using Suspension Plasma Spraying: Effect of Promotional Additives and Aluminum Powder Particle Size

Aluminum nitride (AlN) coatings have been considered for corrosion-resistant materials applicable to the aluminum (Al) industry where the AlN comes in direct contact with molten Al above 933 K. AlN coatings were synthesized by suspension plasma spray (SPS) technology using Al powder mixed with melamine suspended in hexadecane. The use of fine Al (1-5 µm) particles did not yield more than 10% AlN in the coatings. Mixing the Al powder with promotional additives such as B, BN, Mo, Y 2 O 3 , AlN, or Al 4 C 3 solves the fine particle agglomeration and stimulates the formation of AlN in the coatings which enhances their corrosion resistance. The optimum amount of AlN promoter was 0.22 wt.% of the total suspension mass, producing up to 72% AlN in the coating as determined by Rietveld quantitative analysis (RQA) using x-ray diffraction (XRD). Another way to improve the AlN formation in the coating by post-deposition nitridation and also solve fine Al particle agglomeration is to use a wide particle size distribution of Al, with the optimum ratio being 3:1, that is, (1-5 µm):(17-35 µm). XRD analysis indicated that the coating exhibited up to 80% AlN. The coatings Vickers hardness is related to their AlN content reaching 1644 Hv (80% AlN). The coatings were tested for corrosion resistance by direct contact with molten Al-5 wt.%Mg alloy at 1123 K and found to be stable. Ab initio Born–Oppenheimer molecular dynamics (BOMD) simulation predicted these experimental results. Indeed at 1200 K, molten Al and AlN exhibit weak van der Waals interactions. The AlN (s) -Al (l) interfacial energy was calculated to be 18.2 kJ mol −1 for hexagonal AlN phase and 56.4 kJ mol −1 for cubic AlN, which means that it lies within the physisorption regime, and therefore, no reaction occurs between Al (l) and AlN (s) which confirms non-wetting application in the Al industry.