Effect of slow-cooling protocol on biaxial flexural strengths of bilayered porcelain-ceria-stabilized zirconia/alumina nanocomposite (Ce-TZP/A) disks

Objective

The present study investigated the biaxial flexural strengths of bilayered ceria-stabilized zirconia/alumina nanocomposite (Ce-TZP/A) disks with various layering porcelains veneered using a slow-cooling protocol.

Methods

Five porcelain materials (VITA VM9, Cercon Ceram Kiss, and Vintage ZR with experimental coefficient of thermal expansions; CTEs of 8.45, 9.04, and 9.61 ppm/°C) were veneered on Ce-TZP/A disks and slow-cooled after firing to fabricate bilayered specimens (core-to-porcelain thickness: 0.8 mm/1.5 mm). Biaxial flexural strengths of the specimens with the porcelain layer in tension were tested based on the piston-on-three-ball method (ISO 6872:2008). The data were statistically analyzed using Weibull distribution and Fisher’s exact test.

Results

Tensile stresses were observed in the entire porcelain layer while compressive stress at the surface of the Ce-TZP/A layer shifted to tensile stress at the interface between the materials. The cases of small CTE mismatches between the materials showed high Weibull characteristic strengths at the internal and external surfaces of the specimens, except the VM9 group (CTE: 9.0–9.2 ppm/°C). The maximum tensile stress was observed on the surface of the porcelain layer, where cracks originated and continuously propagated into the Ce-TZP layer. The Ce-TZP/A fractured into two pieces for large CTE mismatches between the materials, resulting in significantly lower flexural strengths than those fracturing into three pieces for small CTE mismatches.

Significance

Flexural strengths and fracture behaviors of bilayered porcelain-Ce-TZP/A disks were influenced by the CTE mismatches, and a small CTE mismatch between the materials was preferred when using a slow-cooling protocol.