Microstructure and mechanical properties of a porous ceramic composite with needle-like mullite and zirconia

Porous mullite ceramics have good properties for high-temperature applications, but porosity gives place to ceramics with low mechanical strength, which restricts the service life in their potential applications. Therefore, performing modifications at the microscale to increase the mechanical strength has become a current challenge to expand its application fields. This work describes the properties of a porous mullite-zirconia composite produced by ceramic processing, using industrial kaolin and stabilized zirconia as raw materials. The growth of mullite needle-like grains to reinforce the ceramic was promoted by the addition of a molybdenum oxide precursor. The effect of zirconia on the composite was analyzed through an experimental multi-technique approach and considering a pure mullite sample, identically processed, as a reference. The novel composite has a porosity of about 50%, and presents a homogeneous microstructure, with interlocked mullite needle-like grains and dispersed rounded zirconia grains. This morphology restricts the mullite tendency to shrink during sintering, giving the material a higher stiffness. In particular, the presence of zirconia in the composite improves both the flexural strength and the apparent Young modulus of the material (about 20% and up to 600%, respectively). These results encourage further investigations to establish this composite for different technological applications.