Effect of high-energy ball-milling on the characteristics of Fe3Al-based HVOF coatings containing boride and nitride phases

In the present work we investigate the effect of boron nitride (BN) addition to Fe3Al on the characteristics of ball-milled powders, and we study the influence of heat-treatment and subsequent milling of the powder mixtures on the microstructure and properties of coatings prepared by the high velocity oxy-fuel (HVOF) deposition process. The tribo-mechanical properties of the coatings were evaluated using indentation and pin-on-disc tribometry. Ball-milling of the Fe3Al powder with BN and heat-treatment of the mixture lead to the formation of microstructures that contain BN, Fe2B and AlN components. Further milling refines such microstructures and disperses the ceramic phases homogeneously within the metallic matrix. Thermal spraying of these powders results in hard coatings with microstructures similar to that of the powders. Compared to coatings obtained from a pure Fe3Al milled powder, the coating hardness increases by ~40% (from 5.6 GPa to 7.8 GPa), and the wear rate of 9×10−6 mm3/N m decreases to 5×10−7 mm3/N m when 30 mol% of BN is added. The wear rate of the HVOF coatings further decreases to 2×10−7 mm3/N m when the same powder is annealed and re-milled at high energy for 10 h. The degradation mechanism appeared to be dominated by abrasive and fatigue wear.