Preparation and mechanical properties of Al2O3 reinforced by submicrometerCo particles

Al2O3/Co composites were fabricated by vacuum hot-pressing a mixture of alp ha-Al2O3 powder and a fine cobalt powder. Submicron-sized cobalt particles were uniformly dispersed into the Al2O3 matrix, and the dispersed type was a more inter-/intragranular one with increases of cobalt content up to 40 w t % Co addition. The growth of cobalt particles occurred with increasing co balt content. At 50 wt % Co addition, however, the growth as well as coales cence of cobalt particles occurred. The phases formed in the Al2O3/Co compo sites were f-Co(fcc), h-Co(hcp), alpha-Al2O3, and a small amount of graphit e. Significant improvements in bending strength (from 341 to 771 MPa) and f racture toughness (from 3.7 to 6.7 MPa . m(1/2)) of the Al2O3/40 wt % Co(23 vol % Co) composite compared to monolithic Al2O3 were achieved by dispersi ng submicron-sized Co particles into the Al2O3 matrix. The improvement in b ending strength was attributed to the compressive thermal residual stress i n the matrix Al2O3 induced by the mismatch of the coefficients of thermal e xpansion (CTE) between the matrix Al2O3 grains and cobalt particles during cooling from hot-pressing temperature. The fracture tough ness of the compo site was enhanced by crack bridging, crack deflection, and compressive ther mal residual stress. (C) 1998 Kluwer Academic Publishers.