Mechanical properties of silicon carbide-reinforced alumina nanocomposites: Machining-induced surface residual stress and crack healing behavior

The strengthening and toughening mechanisms of 5 vol% 0.2 μm SiC particles reinforced alumina nanocomposites were investigated, especially with respect to the effect of machining-induced residual stress and thermal treatment. The machining and subsequent annealing behavior of an Al₂O ₃-SiC nanocomposite was compared to that of single phase Al₂O ₃. The machining-induced residual line force was determined by measuring the extent of elastic bending in thin disk specimens, and the surface roughness was evaluated by profilometry. The results showed that when the two materials were subjected to the same grinding conditions, they developed compressive residual stresses (σ_res) and surface roughness values of similar magnitude. The maximum thickness (d_max) of the residual stress layers was estimated to be $\approx 10mm$ for the alumina, and $\approx 12mm$ for the nanocomposite. For different machining treatments, a direct linear correlation was observed between the residual force and the surface roughness. Annealing of the machined samples produced complete relaxation of residual stresses in the single phase alumina, whereas only partial stress relaxation occurred for the nanocomposites.; In addition, the indentation crack healing and strength behavior of an Al₂O₃-SiC nanocomposite (Al₂O₃ + 5 vol% 0.2 μm SiC particles) has been studied as a function of crack size and the annealing environment. Results showed that annealing treatments can significantly increase the indentation strength. The annealing atmosphere was found to have a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon resulted in a strength increase of 50%, whereas annealing in air yielded a 3-fold improvement in the indentation strength. SEM observation showed that healing of indentation cracks occurred in both environments, with the greater degree of healing taking place during annealing in air.