The firing procedure influences properties of a zirconia core ceramic

High-strength ceramics for dental restoration are used as an understructure (core) that subsequently is covered by veneering ceramic. The veneering process involves a firing procedure at high temperatures at least once, usually two to five times. The aim of this study was to investigate whether these firing procedures affect the mechanical properties of a zirconia ceramic.

Thirty-three specimens of an industrially sintered yttria-stabilized zirconia ceramic (DC Zircon, DCS Dental AG, Allschwil, Switzerland) were cut into bars (1.2mmx4mmx20mm). One set of specimens (n=13) remained untreated (controls). Another set of specimens (n=10) was heat-treated once, corresponding to the first step of the veneering process. The third set of specimens (n=10) was heat-treated five times to mimic the full veneering process. Flexural strength, microhardness, dimensions and surface roughness were measured. The fracture patterns were assessed by light microscopy.

The untreated specimens showed a statistically significant higher flexural strength (20%) and microhardness (9%) than both of the test groups (p< or =0.001). No significant differences were found for fracture patterns, dimensions or surface roughness.

The heat treatment associated with the veneering procedure on a zirconia core material reduced the flexural strength of the core after the first firing. Subsequent firings were not detrimental to the properties measured.