Zirconia in dentistry - understanding the effects of yttria-content, surface processing and heat-treatment on surface and mechanical properties

Zirconia materials with 3-5 mol% of yttria are widely used in dentistry for implants, abutments, crowns and fixed dental prostheses depending on the required aesthetics and mechanical reliability. Nevertheless, in-depth analysis of the underlying mechanisms affecting the surface and mechanical properties of dental zirconia is lacking to date. The aim of this study was therefore to understand the effects of yttria-content, surface processing and heat-treatment on surface and mechanical properties. Five different zirconia materials varying in yttria-content with 3 different surface processing (as-sintered, polished, polished and heat-treatment) were therefore systematically investigated. Of these materials, morphological properties such as grain sizes and surface roughness were characterized, crystal phase composition of monoclinic, tetragonal, or cubic phase assessed, biaxial flexural strength and resistance to scratching evaluated. The study highlights that surface and mechanical properties of zirconia are determined by the yttria-content, surface processing and resulting crystallographic composition, presence of grain boundaries and grain sizes on the surface, as well as surface roughness in general. By polishing and heat-treatment of the zirconia specimens, it was possible to eliminate macro-structure and generate surfaces with distinct grain sizes in the mid nano-range while increasing strength significantly. The polymorphic behavior of zirconia is highly complex and not yet fully understood, mainly owned to interrelations between crude material properties and production processes that ultimately affect end-product material characteristics.