In a recent article published in Scientific Reports, researchers evaluated the tribological parameters of graphite and boron carbide-infused aluminum hybrid composites synthesized using the stir casting technique.
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Background
Aluminum has gained significant attention from the scientific community in recent years due to its various beneficial properties, such as ease of composite fabrication imbued with balanced properties like enhanced compressive strength and wear resistance.
In hybrid composites, multiple reinforcements are integrated into the base metal matrix to achieve a combination of enhanced properties from both the reinforcements and the base metal. This integration leads to improved hardness and strength compared to the original alloy.
The stir casting technique is one of the most commonly used and economical composite fabrication methods. Different reinforcing additives, such as boron carbide, can increase the structural strength of fabricated aluminum metal matrix composites (AMMCs). Similarly, the incorporation of solid lubricants, such as graphite and silicon, results in a lower friction coefficient, which reduces the overall wear effect.
Many researchers have investigated the tribological and mechanical properties of hybrid aluminum matrix composites due to their growing applications in various fields. However, sufficient tribological studies specifically addressing aluminum-based boron carbide and graphite hybrid composites as a combination have not been performed.
The Study
In this work, researchers fabricated Al6063 (an aluminum alloy) reinforced with graphite and boron carbide using the stir-casting method. They then performed microstructural analysis, tribological tests, and wear analysis on the synthesized samples.
The objective of the study was to propose graphite as a crucial self-lubricating reinforcement for hybrid composite samples, aiming for a minimal wear rate for sliding pairs with fewer mechanical surface defects. A series of samples—S1, S2, S3, S4, and S5—were fabricated with varying compositions to assess the impact of boron carbide in conjunction with graphite.
S1 contained 100 wt% aluminum/pure aluminum; S2 contained 96 wt% aluminum and 4 wt% boron carbide; S3 contained 94 wt% aluminum, 4 wt% boron carbide, and 2 wt% graphite; S4 contained 92.5 wt% aluminum, 4 wt% boron carbide, and 3.5 wt% graphite; and S5 contained 91 wt% aluminum, 4 wt% boron carbide, and 5 wt% graphite.
Researchers demonstrated the combined effect on the wear rate of the cast hybrid composites using an EN 31 steel disc. Potassium fluorotitanate halide salt was used in powder form to ensure good wettability of the aluminum with the reinforcement.
The obtained sample morphology was studied using scanning electron imaging, energy-dispersive X-ray analysis (EDAX), and X-ray diffraction (XRD). Additionally, the experimental and theoretical densities of the S1-S5 samples were investigated and compared. A comparison study was conducted using Taguchi analysis, considering load, material, and sliding speed as factors.
Study Significance
The reinforcement homogeneity in different composite casts was verified through XRD and EDAX analysis, which was attributed to the use of potassium fluorocitrate. The fabricated hybrid composites displayed self-lubricating characteristics and reduced porosity compared to their theoretical density.
Additionally, wear tests revealed significant improvements in the overall surface morphology of the composite pin at specific sliding velocities and loads. The wear rate was 0.0351 × 10⁻³ mm³/m, 0.0286 × 10⁻³ mm³/m, 0.0187 × 10⁻³ mm³/m, 0.0140 × 10⁻³ mm³/m, and 0.0169 × 10⁻³ mm³/m for pure aluminum, aluminum-boron carbide composite, aluminum-boron carbide-graphite 2 %, aluminum-boron carbide-graphite 3.5 %, and aluminum-boron carbide-graphite 5 %, respectively.
Moreover, the average friction coefficients varied from 0.31 to 0.35 for aluminum-boron carbide-graphite (5 %), 0.32 to 0.36 for aluminum-boron carbide-graphite (3.5 %), 0.32 to 0.38 for aluminum-boron carbide-graphite (2 %), 0.42 to 0.5 for aluminum-boron carbide composite, and 0.45 to 0.53 for pure aluminum.
A maximum wear rate of 0.351 × 10⁻⁴ mm³/m was found for the pure aluminum sample against EN31 steel disc with 0.53 as friction coefficient, while the least wear rate of 0.140 × 10⁻⁴ mm³/m was observed for the composite having graphite 3.5 % weight with 0.36 friction coefficient. Scanning electron microscopy images of the worn surface clearly revealed crack formation and delamination on the pin face for the pure aluminum sample.
In contrast, the hybrid composites of aluminum-boron carbide-graphite (2% to 5%) exhibited minimal crack propagation and delamination, along with smoother tracks due to the formation of a protective tribological film from graphite.
The study also demonstrated the contributions of sliding speed, load, and pin type on the wear rate and friction coefficient, resulting in an F-value of 44.57 with R² of 0.895 for the wear rate model and an F-value of 54.2 with R² of 0.934 for the friction coefficient model.
The findings indicate that the wear rate significantly decreases when boron carbide and graphite are used as reinforcements in the aluminum matrix, validating graphite's role as a crucial self-lubricating reinforcement for hybrid composite samples.
Journal Reference
Thakur, A. et al. (2024). Evaluation of tribological parameters for boron carbide and graphite infused aluminum hybrid composite fabricated by stir casting technique. Scientific Reports. DOI: 10.1038/s41598-024-73877-9, https://www.nature.com/articles/s41598-024-73877-9
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