Fatigue and Fracture Resistance of Nanostructured Composite System: Chrome Carbide - Hard Oxide Aluminum - Aluminum

摘要

In this paper Al-Al_2O_3-CrC nanostructured composite coating was presented, fatigue and fracture of the composite coating were investigated by nanoindentation, and in situ experiments performed in a scanning electron microscope to permit examination of freshly exposed surfaces. Crystallographic and morphological texture was characterized and the fracture resistance measured using fracture-mechanics. A CrC layer may improve the fracture resistance of an oxide aluminum layer. A CrC layer produced by pyrolitic deposition effectively heals the pores and defects of an oxide aluminum layer. It results in high load rating of the coating. Experiments revealed that in all cases, detection of an acoustic signal corresponded to an appearance of circular cracks seen on the surface; in a very few cases, examination of the surface after detection of a signal revealed the presence of two ring cracks. The degree of toughening associated with crack healing is determined by the number of healed defects and the effectiveness of the individual healing. Macroscopically, a crack path in the oxide aluminum appears to be straight, propagating along pores and internal voids. However, microscopically, a crack path exhibits a high degree of intergranular fracture. Because cracks generally deflect at small angles in the oxide aluminum layer, a crack path moves through pores and internal voids that usually concentrate internal stresses. The crack path is primarily intergranular at all velocities.