Pressed ceramics onto zirconia. Part 2: indentation fracture and influence of cooling rate on residual stresses.

OBJECTIVES: The aim of this study was to evaluate the fracture toughness and surface residual stresses present in various pressable ceramics to zirconia resulting from cooling induced temperature gradients.
MATERIALS AND METHODS: Indentation fracture toughness was used to evaluate the residual stress present in various pressable ceramics (Noritake CZR Press, Vita PM9, Wieland PressXzr and IPS e.max ZirPress) to zirconia when subjected to different cooling regimen. The cooling responses of two ceramics were evaluated by thermocouples embedded in the surface of the porcelains and at the porcelain-zirconia interface.
RESULTS: The effective K(c) results obtained by indentation tests confirmed the presence of surface residual compressive stress for all-ceramic systems subjected to different cooling procedures. The residual stresses were quantified from the change in the radial crack size and the values compared for each ceramic before pressing, pressed ceramic only and pressed ceramic veneered on zirconia, from fast to slow cooling rates. A significant level of residual stress was found in the materials before pressing. Slow cooling significantly reduced the formation of residual stress for all pressed ceramics. From data produced by the thermocouples it was found that 'slow cooling' generated the least temperature difference between inner and outer surfaces of porcelain. A direct relationship was found for the cooling induced temperature difference between the surfaces, and interface thermocouples, and magnitude of the surface residual stresses. SIGNIFICANCE: Leucite containing porcelains have higher intrinsic fracture toughness, and for all porcelains fast cooling generated significant residual stress within the veneering porcelain. To reduce development of residual stress, slow cool is recommended on the last heating cycle (e.g. glazing cycle).