Effect of Al content in the pyrotechnic mixture on the properties of the BaAl2O4-based composite coating deposited on steel by a novel explosive spray technique

A low explosive material such as pyrotechnic mixture made of Al and Ba(NO3)2 powders, commonly known as the flash powder, is used to deposit BaAl2O4-based composite coatings on a flat mild steel surface by using a novel explosive coating setup developed in the laboratory. To achieve the high tribo-mechanical properties of the deposited coating, a suitable mixing ratio of Al to Ba(NO3)2 is to be found. In this work, the Al content of the flash powder is varied from 1 to 2.5 gm for 2 gm of Ba(NO3)2 to study the effect of Al content on the properties of the deposited BaAl2O4-based coatings. The fabricated coatings are characterized in terms of thickness, crystalline phase, microstructure, surface roughness, hardness, and wear resistance by using an optical microscope, X-ray diffractometer (XRD), Field emission scanning electron microscope (FESEM), stylus profilometer, Vickers micro-hardness tester, and linear reciprocating wear tester respectively. Up to 2 gm of Al addition, the hardness of the coatings is found to be enhanced remarkably. However, with further addition of Al, it is found to be decreased significantly. With 2 g of Al content, the major portion of Al powder is utilized well in the chemical reaction to produce mainly hard BaAl2O4 and AlN phases. The highest amount of energy is released from such a composition of feedstock. The presence of pure Ba and Al in the coatings from 1 g to 1.5 g of Al, respectively, have helped to enhance the wear resistance of those by reducing the coefficient of friction values. However, the high wear resistance obtained for the coating obtained from 2 g of Al content is due to the major presence of hard phases and low porosity in it. On the other hand, the high porosity and the major presence of partially melted pure Al particles make the coating from 2.5 g of Al less wear resistant. From a 1:1 mixing ratio of Ba(NO3)2 to Al powders in the flash powder, a hard and monolithic BaAl2O4-based composite coating with minimum defects can be obtained.