The role of tribo-layers on the sliding wear behavior of graphitic aluminum matrix composites

This study provides a systematic investigation of the role played by the tribo-layers that form on the contact surfaces during the sliding wear of graphitic cast aluminum matrix composites. The graphitic composites include A356 Al-10% SiC-4% Gr and A356 Al-5% Al2O3-3% Gr that are being developed for cylinder liner applications in cast aluminum engine blocks. Three main wear regimes, namely, ultra-mild, mild and severe wear were determined. At nearly all sliding speeds and loads in the mild wear regime a protective tribo-layer was formed. By increasing the speed and load the tribo-layer covered a larger proportion of the contact surface and became more compact and smoother. The hardness of the tribo-layers increased with the applied load and speed and reached values as high as 800kg/mm(2). The tribo-layers were removed by extrusion process at the onset of severe wear. The topmost part of the tribo-layer consisted of iron-rich layers. The rest of the tribo-layer consisted of fractured SiC and Al3Ni particles and thin graphite films, which were elongated over long distances in the direction of sliding, thus, reducing shear stresses transmitted to the subsurface regions. It was shown that because of the thicker and more stable tribo-layers on the contact surfaces of graphitic composites, than that of non-graphitic composites and the A356 Al alloy, the graphitic composites displayed a transition from mild-to-severe wear at load and sliding speed combinations, which were considerably higher than those of the A356 aluminum alloy and the non-graphitic A356 Al-20% SiC composite. A negative effect of the hard constituents in the tribo-layers was the scuffing damage that they inflicted on the counterface. (C) 2001 Elsevier Science B.V. All rights reserved.