The strength of Al2O3/SiC nanocomposites after grinding and annealing

Alumina matrix nanocomposites containing about 5 vol.% SiC of < 100 nm mean particle size show a substantial increase in strength after machining and annealing. The final strength is controlled by the annealing process and achieves the same level after a coarse machining using 150-grit diamond as is achieved using a complex lapping and polishing sequence. In all cases the final anneal leads to an increase in strength. The nanocomposite retains a significantly higher residual surface-compression stress level than an equivalent grain size alumina after machining. A remnant stress, of about 20% of the initial level, is retained even after 10 h annealing at 1250 degrees C. Hertzian indentation and measurements of Rayleigh wave velocity show that the nanocomposite surfaces contain defects of smaller size and density than are found in equivalent aluminas. Annealing appears to result in healing of existing surface defects thus increasing the nanocomposite strength while leaving its toughness unchanged. The crack-healing mechanism is associated with a chemical process on the nanocomposite surface which has been tentatively identified as an oxidation leading to amorphous mullite formation. (C) 1998 Acta Metallurgica Inc.