An Investigation Of Impact Wear Behaviors Of ZM5 Magnesium Alloy And Its Micro-arc Oxidation Coatings

As the lightest structural metallic materials, magnesium alloys have been used broadly in the automobile, aerospace and household appliances industries, at the same time, kinds of friction and wear problems are also emerging. While up to now, the research on the wear mechanism of magnesium alloys has been focused on sliding wear, few publications on the wear mechanism under normal impact are found in literatures. Nowadays, there have been automotive parts and components made of magnesium alloys, such as engine case and gearbox. Therefore, simulating the vibration impact phenomenon which appears when the engines work, the damage mechanism of magnesium alloys under impact condition has been investigated in this thesis, and the factors influencing the wear performances have been discussed. So the study has both practical significance and academic value.The impact wear tests of ZM5 magnesium alloy and its micro-arc oxidation (MAO) ceramic coatings have been performed by a specially designed ball-on-flat impact tester. The influences of test load and impact number on impact wear behaviors were investigated, and the wear depth, wear area and surface pattern were measured and analyzed by means of laser confocal scanning microscope (LCSM), X-ray diffraction (XRD), scanning electrical microscope (SEM), energy dispersive spectroscope (EDS) and Surface Profiler. Main conclusions were obtained as follows:1.The process of impact wear was a compound process of wear and fatigue. The damage of magnesium alloy mainly puts up the characters of fatigue, and the wear depth and area increased nonlinear with the increase of impact number. During the earlier stage of impact wear, plastic deformation and accumulation were the main form of damages, and the wear depth increased rapidly. With the proceeding of impact process, the phenomenon of adhesive wear appeared between the center and edge of impact wear scar. During the later stage, because of the work-hardening of surface layer, cracks initiated at the surface and subsurface, then delamination occurred due to the propagation and intersection of cracks. The thickness of delaminated layer measured by LCSM was about 0.7μm.2.The MAO ceramic coating was composed of three layers, from outer to inner, they are loose layer, compact layer and interface layer. For the earlier stage, a large number of asperities in the loose layer were damaged by the impact force, and then the surface materials were either flaked off as wear debris or transformed to the counterparts. With the proceeding of test, the contact stress between the specimens decreased, the wear debris layer formed, and the resistance of plastic deformation increased, all these factors resulted in the decrease of wear depth and area. In terms of the whole impact wear process of ceramic coatings, the wear mechanism was charactered by the alternation of wear and fatigue, namely the impact wear was such a cyclic process: formation of wear debris, formation of wear debris layer, fatigue delamination of wear debris layer, formation of new wear debris.3.Contrast the experiment results of magnesium alloys with that of MAO ceramic coatings, we could found that under two kinds of impact loads, the wear depth and area of MAO ceramic coatings were both larger than that of magnesium alloy. The results indicated that different from the sliding wear, the existence of in situ produced MAO ceramic coatings didn't alleviate the wear of magnesium alloy.