Edge chipping resistance of veneering ceramics for metal and zirconia

Objective: porcelain fused to metal (PFM) is the gold standard for fixed prosthodontic restorations. The aim of this study is to verify the edge chipping resistance of veneering porcelain applied to metal and zirconia.

Method: Porcelains for metal (VM13 - VITA Zahnfabrik Co, Germany) and zirconia (VM9 - VITA) were studies. Monolithic bar-shaped specimens (5x32x2.5mm, n=4) were prepared for each material, according to manufacturer's recommendations. The edge chipping tests were carried out using a Vickers indenter mounted in a universal testing machine, with a constant loading rate 0.1mm/min. The load was applied in different distances between the upper longitudinal edge and the indenter (d_a). Each specimen was chipped approximately twenty times, with five different d_a values, from 0.1 mm to 0.5 mm. Average force and standard deviation were calculated for each distance. The relationship between mean critical force (F_C) and edge distance (d_a) was plotted. Edge toughness (T_e) was defined as the slope of the linear portion on these graphs. Finite element analysis was utilized to analyze the maximum principal stresses (σ₁) and distribution; the materials properties were considered homogeneous, linear-elastic and isotropic. Microestrutural analysis was carried out using Scanning Electron Microscopy (SEM).

Results: The results, on Figure 1 show higher values of edge toughness for VM13 (T_E = 209.9N/mm) than for VM9 (T_E = 130.5 N/mm). For FEM analysis the highest peak stress was found at d_a =0.1 mm (163.7 MPa), while for larger d_a the stress peak remained steady (~70.5 MPa). SEM analysis revealed twice more leucite content for PFM (VM13=18.1%±2; VM9=9.45%±1).

Table 1 – σ₁ peak values for FEM results.

Conclusion: VM13 showed higher edge toughness and more leucite content than VM9; this may explain the lower fracture rate of PFM relative to porcelain-vennered zirconia.