Characterization of tribological behaviour of graphitic aluminum matrix composites, grey cast iron, and aluminum silicon alloys

In recent years a number of aluminum-silicon alloys and some graphitic aluminum matrix composites have been fabricated for potential tribological applications in the automotive industry, in particular for lightweight high efficiency internal combustion engines to replace conventional uses of cast iron.; This study provides a systematic investigation for wear mechanisms in dry sliding of the graphitic aluminum-matrix composites (A356 Al-10%SiC-4%Gr and A356 Al-5%Al₂O₃-3%Gr) developed for cylinder liner applications. Two eutectic Al-Si alloys (modified with rare earth elements) developed for wear resistant engine blocks were also studied. The tribological behavior of grey cast iron (ASTM A30), which is a traditional material for engine components, was also investigated as reference.; For graphitic aluminum matrix composites, a wear mapping approach has been adopted. Three main regimes: ultra mild, mild and severe wear regions were determined in the maps; additionally, a scuffing region was observed. In the ultra mild wear regime the wear resistance was primarily due to the hard particles supporting the load. It was shown that the onset of severe wear in graphitic composites occurred at considerably higher loads compared to A356 aluminum alloy and A356 Al-20% SiC composite. At the onset of severe wear, the surface temperatures and coefficient of friction of the graphitic composites was lower than that of A356 Al-20% SiC. At all testing conditions in the mild wear regime, a protective tribo-layer was formed, which by increasing the speed and load became more continuous, more compact, smoother, and harder. The tribo-layers were removed at the onset of severe wear.; An experimental wear map of grey cast iron was constructed; it consisted of three wear regimes: ultra mild, mild and severe wear. In the ultra mild regime a compacted fine iron oxide powder formed on the contact. The onset of severe wear was started with local material transfer to the steel counterface, and continued by increasing the roughness of the counterface. In the mild wear regime the oxide layers on the contact surface controlled the wear. Because of the microstructure of grey cast iron, large size debris formed in the mild regimes, which could promote a transition from mild to severe wear at loading conditions close to the upper limit of the mild regime.; The effects of size and morphology of hard phases in eutectic Al-Si alloys on their scuffing and wear resistance was investigated using etching techniques. Scratch tests and sliding wear tests were performed on the Al-Si samples etched to different times. At low loads, the wear rates and scuffing resistance of Al-Si alloys were a strong function of the etching time, surface roughness before etching, and the morphology of the hard phases. In the etched condition, fracture of the hard phases from the roots of the protruded portions of particles caused scuffing; while fracture of the trailing edges of the hard phases controlled the wear rate of the Al-Si alloys.