Corrosion behavior and tribological-microstructural characterization of Al/ZrO2-Gr hybrid composite fabricated via Friction Stir Processing

The present study investigates the microstructural evolution, tribological properties, and corrosion behavior of hybrid metal matrix composites reinforced by a combination of graphite (Gr) and zirconia (ZrO2) particles fabricated by Friction Stir Processing (FSP). To do so, microstructural characterization was carried out by optical microscopy (OM) and electron backscatter diffraction (EBSD) analysis. In addition, pin-on-disk, microhardness measurement, potentiodynamic polarization, and electrochemical impedance tests were conducted to evaluate the tribological and corrosion behavior of the samples. Results showed that employing four passes of FSP led to significant grain refinement in stir zones because of dynamic recrystallization (DRX), particle-stimulated nucleation (PSN), and Zener pinning effect. Furthermore, the EBSD results illustrated that the average grain size area has significantly decreased from ∼115 µm2 in the only FSP-treated specimen to ∼1.2 µm2 in the hybrid composite. Also, the presence of the secondary phase particles caused an increase in the fraction of the high-angle grain boundaries (HAGBs) compared to the only FSP-treated specimen. The results of the pin-on-disk test revealed that the wear resistance improved in the particle-reinforced composites and, on the other hand, decreased in the FSP-treated sample compared to the base metal (BM). Furthermore, the lubricant effect of the Gr particles contributed to a pronounced reduction in the coefficient of friction (COF) and weight loss in the Gr-containing composites such that the COF of the hybrid composite and Gr-reinforced mono-composite dramatically decreased by ∼0.52 and ∼0.37, respectively. The corrosion behavior of the FSP-treated sample demonstrated better resistance compared to the BM. In addition, the particle-reinforced composites exhibited better corrosion resistance than the FSP-treated sample and BM, especially the Al/Gr mono-composite which had the minimum corrosion rate and the most positive pitting potential (Epit), making it the most pitting-resistant sample in comparison to other specimens.