Infiltration of porous alumina bodies with solution precursors: Strengthening via compositional grading, grain size control, and transformation toughening

Alumina powder compacts, partially densified with a low-temperature heat treatment and then cut into bars, were infiltrated with liquid precursors that decomposed to either mullite (3Al(2)O(3) . 2SiO(2)), fully stabilized zirconia (cubic Zr(8Y)O-2), or partially stabilized zirconia (tetragonal Zr(4Y)O-2), The specimens were repeatedly infiltrated and pyrolyzed to achieve a higher concentration of the precursor near the surface, The infiltrated bodies were then densified at 1500 degrees C/2 h. Residual stresses developed during cooling from the densification temperature because of the higher concentration of the second phase near the surface and their differential thermal expansion relative to the matrix material, At least 10 bars of each two-phase material were fractured in four-point bending to determine the effect of the second phase on strength, The alumina bars without a second phase had a larger grain size (approximate to 7 mu m) and a mean strength of 253 MPa. The intruded phases significantly reduced the Al2O3 grain size to approximate to 1 mu m, Despite their higher concentration near the surface and apparent surface tensile stress, both of the Zr(Y)O-2 phases increased the mean strength to 413 MPa (c-Zr(8Y)O-2) and 582 MPa (t-Zr(4Y)O-2, an apparent toughening agent), The mullite second phase produced a high mean strength of 588 MPa, apparently due to its concentration gradient creating a compressive surface stress.