Influence of residual thermal stresses on the edge chipping resistance of PFM and veneered zirconia structures: Experimental and FEA study

Objective: Chipping fractures of the veneering porcelain are frequently reported for veneered all-ceramic crowns. In the present study, the edge chipping test is used to measure the toughness and the edge chipping resistance of veneered zirconia and porcelain-fused-to-metal (PFM). The aim is to describe an edge chipping method developed with the use of a universal testing machine and to verify the accuracy of this method to determine the influence of residual thermal stresses on the chipping fracture resistance of veneering porcelain. A finite element analysis (FEA) was used to study the residual stress profiles within the veneering porcelain. Methods: Veneered zirconia and PFM bar specimens were subjected to either a fast or a slow cooling protocol. The chipping resistances were measured using the edge chipping method. The load was applied in two different directions, in which the Vickers indenter was placed in the veneering porcelain either parallel or perpendicular to the veneer/framework interface. The mean edge chipping resistance (R_eA) and fracture toughness (K_C) values were analysed. R_eA was calculated by dividing the critical force to cause the chip by the edge distance. K_C was given by a fracture analysis that correlates the critical chipping load (F_C) regarding edge distance (d) and material toughness via K_C = F_C/(βd^1.5). Results: The R_eA revealed similar values (p > 0.005) of chipping resistance for loads applied in the parallel direction regardless of framework material and cooling protocol. For loads applied in the perpendicular direction to the veneer/framework interface, the most chip resistant materials were slow cooled veneered zirconia (251.0 N/mm) and the PFM fast cooled (190.1 N/mm). K_C values are similar to that for monolithic porcelain (0.9 MPa.√m), with slightly higher values (1.2 MPa.√m) for thermally stressed PFM fast cooled and veneered zirconia slow cooled groups. Significance: The developed and reported edge chipping method allows for the precise alignment of the indenter in any predetermined distance from the edge. The edge chipping method could be useful in determining the different states of residual thermal stresses on the veneering porcelain.