Effect of Powder Feed Rate of Plasma-Sprayed Fe–Cr–Mo–B–C Coatings on Microstructure, Tribology and Corrosion in 3.5% NaCl Solution

Many process parameters of air plasma spray, such as deposition power, speed, and working distance, significantly influence the resulting microstructural, mechanical, tribological, and corrosion properties of the deposited coatings. Among these parameters, the feed rate plays a vital role in the microstructural evolution and achieved properties. Thus, this study investigates the effect of feed rates (10, 20, and 45 g/min) on the microstructure, tribological, and corrosion of Fe–Cr–Mo–B–C composite coatings fabricated using air plasma spray. The results showed that as the powder feed rate was reduced, the thickness, porosity, and amorphous fraction of the coating decreased due to a higher temperature and velocity of the particles at the point of impact. The lower feed rate increased the formed crystalline phases (Fe, Cr, Mo)₂₃C₆ and (Fe, Cr, Mo)₇C_6, which were embedded in the amorphous matrix. As a result, the hardness increased from about 0.72 GPa (at 45 g/min feed rate) to about 1.0 GPa (at 20 g/min feed rate) and 1.3 GPa (at 10 g/min feed rate). Similarly, the elastic modulus increased from about 40 GPa at 45 g/min feed rate to about 52 and 68 GPa at 20 and 10 g/min feed rates, respectively. Consequently, the sliding wear resistance was improved by up to 27 and 50% at a reduced feeding rate of 20 and 10 g/min, respectively. The principal wear mechanisms were adhesion and oxidation. The electrochemical corrosion resistance in a 3.5% NaCl solution was significantly increased at the lowest feed rate.