Factors Controlling the Sintering of Ceramic Particulate Composites: II, Coated Inclusion Particles

Abstract

Composite powders were synthesized by coating coarse ZrO2 inclusion particles with a cladding of fine‐grained, crystalline ZnO powder using a chemical precipitation technique. Three different inclusion sizes (1, 3, and 14 μm) were used by selecting the size of the starting ZrO2 powder, and the volume fraction of the inclusions was controlled by the amount of ZnO precipitated. The powders were compacted by uniaxial pressing in a die and then sintered at a constant heating rate of 4°C/min to 1500°C. The sintering kinetics were almost independent of the inclusion volume fraction, and of the inclusion size, for inclusion contents up to ∼40 vol%. Furthermore, composites containing up to ∼40 vol% inclusions were sintered to almost full density under the same conditions used for the unreinforced matrix. This is considerably better than the densities obtained for conventionally mixed powders, where a modest inclusion content (< ∼ 10 vol%) has been observed to cause a severe reduction in the sintered density of the composite matrix. The kinetic data and microstructural observations are a further indication that the main factors which oppose the free sintering of ceramic particulate composites are processing‐related; these factors are (i) inclusion‐inclusion interactions which constrain the matrix and (ii) the packing of the matrix phase in regions immediately surrounding the inclusions.