Pressed ceramics onto zirconia: Adhesion, strength, toughness and residual stress

Objectives: The aim of this research was to investigate the intrinsic mechanical properties before and after bonding to zirconia of four commercially available pressed veneering porcelain materials. Methods: Biaxial strength, indentation fracture toughness and interfacial fracture toughness were determined. In addition bilayer plates were subjected to three cooling regimes to investigate whether this influenced the resultant residual stresses. Thermocouples were inserted onto the surface and at the porcelain-zirconia interface to enable the temperatures to be recorded during cooling. X-ray diffraction was also used to determine the crystalline phases present within the pressed porcelains. Results: The mechanical properties are summarised below for the strength, toughness and adhesion to zirconia for the four materials. Two materials Noritake and Vita had considerable leucite present whereas the E,max and Weiland materials were primarily amorphous. Conclusions: It was found that the residual stresses scaled with the temperature difference measured between the surface and interface thermocouples. Recommendations for a slower final cooling cycle to reduce the magnitude of residual stresses are suggested. Summary of fracture toughness (Kc) adhesion (G) and flexural strength results (SD - Standard Deviation) Fracture toughness MPa.m1/2(SD) Adhesion, Strain energy release rate, G(J/m2)(SD), Biaxial flexural strength Bilayer Monolayer MPa (SD) Noritake CZR 0.78(0.12) 26.7(2.1)122.6(6.5) 54.0(16.0) Vita PM9 0.72(0.09) 21.3(2.7)132.8(11.4)89.2(8.2) Wieland PXzr 0.61(0.07) 17.2(2.6)103.7(12.5)66.9(11.0) EmaxZirPress 0.6 (0.06) 17.1(3.2) 93.2(4.4) 89.6(16.2)