Relationship between mechanical properties and microstructure of yttria stabilized zirconia ceramics densified by Spark Plasma Sintering

Yttria stabilized zirconia ceramics are well known in the field of thermal barrier coatings due to their high ability to thermally insulate hot metallic parts of turbo-engines. The present work is focused on the mechanical properties and the fracture behavior of such materials shaped by Spark Plasma Sintering.1 Two types of powders have been used: i) the first one is a commercial powder made of micrometric spheroidal agglomerates of nano-crystallites (Tosoh), and ii) the second one is a home-made Sol-Gel powder 2 consisting of dense agglomerates of micrometric non-spherical crystallites. Both powders were shaped by Spark Plasma Sintering. The microstructures and mechanical properties of the various samples were characterized for different porosity rates and composition. The adjustment of several SPS processing parameters such as the temperature and the applied pressure allowed to obtain ceramics with a porosity in the range from 0 to 50 % vol. with oriented microstructure. Please click Additional Files below to see the full abstract

Relationship between mechanical properties and microstructure of yttria stabilized zirconia ceramics densified by spark plasma sintering

Porous ceramics are widely used for many applications such as filters, insulators, electrodes for SOFC, membranes or bone scaffolds, with porosity in the typically range of 20–50%vol. The functionality of those materials comes at the expense of the degradation of their mechanical properties which are highly impacted by the rate, distribution, shape and size of the porosity. Among them tetragonal stabilized zirconia (TSZ) is one of the most industrially used; it is sometime called: “the ceramic steel” since in its dense state it exhibits the highest toughness for ceramics. It is known that the porosity has a huge impact on the thermo-mechanical properties of refractory ceramics as Yittria Stabilized Zirconia (YSZ). This study aims to capitalize the mechanical properties as a function of porosity to provide future applications and ensure the behavior in service of thermal barrier coating. In the present paper, the correlation between the microstructure and the mechanical properties such as Young modulus, hardness and strength of YSZ ceramics obtained by Spark Plasma Sintering (SPS) was investigated. Two types of YSZ powder, a nanometric one from Tosoh and a micrometric one obtained by sol-gel route were studied to prepare homogeneous mesoporous or oriented macroporous microstructure by partial sintering. SPS parameters have been determined and optimized to manage the porosity rate. Furthermore, a bimodal microstructure can be obtained, by mixing both powders, allowing the formation of linking bridges between the microporous zone and the nanopowder during sintering. The macroporous ceramics have lower Young modulus, hardness and strength than mesoporous ones. These characteristics are discussed in the paper taking into account the differences between microstructure and contacts between particles with various form factors. Thus, it is clearly evidenced that the morphology of raw powders and the level of porosity are key parameters to optimize the mechanical properties of such porous material

Single-step sintering of zirconia ceramics using hydroxide precursors and Spark Plasma Sintering below 400 °C

The densification of zirconia at very low temperatures (<400 °C) requires enhanced synergy between thermodynamics and kinetics. This work demonstrates an efficient single step approach combining amorphous hydrated zirconia and Spark Plasma Sintering at 350 °C and 600 MPa. The resulting zirconia ceramics exhibit a cohesive nanostructure with small average grain sizes (20 nm) and a predominantly monoclinic structural polymorph confirmed by both X-ray scattering analyses and High Resolution Transmission Electronic Microscopy. Remarkable Vickers hardness of 3.8 GPa for high level of porosity (30%) is explained by the density and homogeneous distribution of grains boundaries and meso/microporosities