CRACK HEALING AND STRESS-RELAXATION IN AL2O3-SIC NANOCOMPOSITES

The crack-healing behavior of Al2O3 and Al2O3-SiC nanocomposite was st udied using Vickers indentations to generate precracks, After annealin g in argon for 2 h at 1300 degrees C, radial cracks in the nanocomposi te healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip, In contrast, radial cracks in alumi na grew when exposed to the same annealing treatment, The different re sponses are attributed to the fracture mode and toughening mechanism i n each material: In the nanocomposite, the cracks close as the residua l stresses surrounding the indentations relax, Radial cracks open and grow in Al2O3 because microstructural toughening is diminished during heating to the annealing temperature, An implication is that strength- limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this ''nanocom posite''' system.