A Factorial Analysis of Material Composition and Operating Parameters on Tribological Properties in Graphite-Plugged Bronze Bushings

Self-lubricating bronze bushings with inserts of solid graphite lubricant are considered an important part of a mechanical system when environmental limitations or design issues make normal liquid lubrication impossible. This research presents a comprehensive experimental study on frictional behavior under 243 structurally defined test conditions, investigating the effects of the interaction between the graphite ratio (10%, 20%, 30%), diameter of the graphite plug (8, 10, 12 mm), normal applied load (50, 100, 150 kg), sliding speed (250, 500, 750 rpm), and the sliding time (10, 20, 30 minutes) on the coefficient of friction (COF), wear loss, and friction temperature. A study using a full factorial experimental design (3⁵), analysis of variance (ANOVA), and correlation analysis revealed graphite content as the most significant physical factor. Increasing graphite content to 30% from 10% reduced the average coefficient of friction by 29% and wear loss by 58%. Significantly, the effects of graphite plug diameter on tribological performance are statistically insignificant (p > 0.05), which offers a high degree of design freedom without affecting the performance. Applied normal load is counterintuitive, with a 54% decrease in COF and a 59% decrease in wear loss between the 50–150 kg range, indicating antagonistic interaction of transfer film improvement and contact stress amplification. Sliding speed has become the most adverse parameter, and the wear loss grows 357% as the speed rises between 250 and 750 rpm, which is mostly due to the thermal deterioration of the lubricating ability of graphite. Correlation analysis shows that there are strong correlations between normal load and COF (r = -0.719), between sliding speed and wear loss (r = 0.658), and between graphite percentage and wear loss (r = -0.471). Optimal tribological performance is observed at 30% graphite content, maximum practicable load (150 kg), and minimum practicable sliding speed (250 rpm), which lead to a COF of 0.150–0.170 and wear loss of less than 0.100 mg. The research offers quantitative design considerations to the engineers who are adopting graphite-plugged bronze bushings in the challenging applications.