Wear and seizure of aluminium - silicon alloys in dry sliding against steel

Abstract

Aluminium-Silicon cast alloys have gained popularity as compared to iron-based materials as a candidate for internal combustion engine components such as cylinder liners, pistons and cylinder heads. This is primarily because of the high strength to weight ratio of the aluminium alloys. The components of the internal combustion engine during service are subjected to different kinds of wear namely adhesion, abrasion, scuffing (or seizure), corrosive wear and fatigue wear. Wear studies of these alloys are related to the effects of silicon content on wear resistance and the mechanisms of wear of these alloys. Available work on seizure of aluminium-silicon alloy is limited. This thesis studies the wear and seizure of binary aluminium-silicon alloys as well as those of alloys with copper, magnesium and nickel additions in uni-directional and reciprocating sliding modes. Tests in uni-directional sliding mode are conducted at room temperature and 0.8 m/s sliding speed. Speed in reciprocating sliding mode is varied in range 0.6 - 1.8 m/s. The counterface used was EN 24 steel. To elucidate the mechanisms of wear and of seizure of a range of hypoeutectic and hypereutectic alloys additional uniaxial compression and single pass ball scribing experiments were done. The compression tests were carried out at temperatures in a range of 25 °C to 400 °C and strain rate in a range of 0 .1 to 100 /s. The ball scribing experiments were done in a temperature range of 25 °C to 400 °C. Surface and sub-surface deformation and fracture of the slid specimens were studied in scanning electron microscope. Wear of binary Al-Si alloys proceeds in three stages namely mild wear, severe wear and seizure as a function of normal load. In the mild wear regime there forms m iron rich compacted layer on the pin surface which protects the alloy against wear. The protective layer is destabilised due to sub-surface plastic flow giving rise to severe wear as the load is increased. Further increase in the norma! load leads to seizure. Bulk shear appears to be the mechanism for seizure of Al-Si alloys. Frictional force and shear resistance of the material are the two controlling factors which together determine the onset of seizure. The resistance to shear depends on hardness and it decreases with subsurface depth. The shear stress which is directly proportional to the frictional force becomes equal to the resistance to shear at a certain sub-surface depth. Large scale shear is initiated in a plane situated at this depth and parallel to the sliding plane when the above condition is met. This heralds the onset of seizure. Changing the mode of sliding from uni-directional to reciprocating as well as increasing the average reciprocating speed affects adversely the wear and seizure resistances of the binary Al-Si alloys. Alloying additions such as copper and magnesium to the base alloy improve both these properties.