Quantitative phase analysis of BaAl2O4-AlN based composite coating deposited by a novel explosive-spray technique

Abstract

Using a novel explosive spray coating technique, barium aluminate (BaAl2O4)-based composite coatings are deposited on mild steel substrates by burning the pyrotechnic mixture comprised of barium nitrate [Ba(NO3)2] and Al powders. With 2g of Ba(NO3)2, the Al is mixed with different quantities (1, 1.5, 2, and 2.5g) to prepare various feedstocks for spraying at variable stand-off distances (SOD) of 60, 80, and 100 mm. The Rietveld refinement technique is employed to estimate the amount of phases retained in the coatings. The thickness, porosity, adhesion strength, crystalline phase, microstructure, and hardness of the as-sprayed coatings are estimated by using optical microscopy, image analysis, scratch tests, X-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), and Vickers micro-hardness tests, respectively. Along with some secondary phases, majorly BaAl2O4 and aluminum nitride (AlN) are formed in the coatings. At 60 mm of SOD, with 1g of Al content, the BaAl2O4 phase is majorly formed. The released energy from the incomplete exothermic reaction is lower, and the deposition is quick considering a short travel path. However, the harder AlN phase is formed predominantly as the Al content is raised to 2g at relatively higher SODs. In such a condition, the released energy from the chemical reaction is high, and sufficient time is available for further reactions to take place among the deposition species. Among all, the coating obtained from 2g of Al and 80 mm of SOD is found to be the hardest, with minimum defects due to the highest formation of the AlN phase. At any SOD with 2.5g of Al content, a very low quantity of hard phases is formed due to negligible chemical reactions between the feedstock materials. The steel substrates are mainly strain-hardened due to the effects of explosion energy. The depth of the hardened zone reduces with an increase in Al content at all SODs. © 2026 Elsevier B.V.