Microstructural, mechanical, and tribological characterization of Hybrid Si₃N₄–BN nanoparticle-reinforced AA5052 surface composites fabricated by multi-pass friction stir processing

This study investigated the improved surface properties of a hybrid material containing silicon nitride (Si₃N₄) and boron nitride (BN) nanoparticles introduced via a three-stage friction stir processing (FSP) method. Unlike the traditional blind hole powder addition method, which results in particle aggregation, the longitudinal groove method achieves a more uniform distribution of reinforcements. Three Si₃N₄:BN mixture ratios (70:30, 60:40, and 50:50) were prepared, and their effects on microstructural changes, mechanical response, and tribological properties were systematically analyzed. Compared to the untreated alloy, the 60:40 composition provided the most balanced properties, with a 42% increase in microhardness (121.1 HV), a 58% reduction in wear rate (1.99 × 10⁻⁴mm³/N⁴m), and a low coefficient of friction of 0.29. The 70:30 formulation exhibited the highest tensile strength (131.96 MPa), an 18% increase. Microstructural observations confirmed the formation of refined equiaxed grains (approximately 5.2 μm) and uniformly dispersed nanoparticles due to dynamic recrystallization and Zener pinning. Phase analysis using X-ray diffraction (XRD) and EDS confirmed stable α-Al, Si₃N₄, and h-BN phases, with no intermetallic compounds. These results demonstrate that multi-pass FSP with hybrid ceramic reinforcement can effectively improve the surface integrity of aluminum alloys and lay the foundation for future research into corrosion resistance, high-temperature stability, and fatigue performance in aerospace, marine, and automotive applications.

Keywords:
AA5052 alloy; Friction stir processing; Silicon nitride; Boron nitride; Tribological properties