Indentations Cracks: Useful for Estimating Residual Incompatibility Stresses in Metal-Ceramics?

Introduction: Metal-ceramic prostheses may exhibit cracking, chipping or fracture due to residual thermal incompatibility stresses. Indentation-cracks have been used as an indirect method of analyzing residual stresses. Objective: To determine the effect of thermal contraction differences on the magnitude and distribution of residual stress in four metal-ceramic systems. Methods: Metal-ceramic specimens (n=6) and ceramic controls (n=6) were fabricated (40 mm x 1.5 mm x 4 mm) and polished through 1 µm abrasive. Specimen materials included: low expansion alloy (LA); low expansion ceramic (LC); high expansion alloy (HA); high expansion ceramic (HC). Vicker's microindentations (9.8 N load) were produced (n=50/specimen) within the 1 mm thick veneer at five distances (200, 300, 400, 600, 800 µm) from the ceramic surface. Indentations were placed at ~600 µm intervals to preclude interacting stress fields. Images were recorded via a calibrated image capture system (OmniMet®, Buehler LTD). Cracks parallel (C_ll) and perpendicular (C_┴) to the interface were measured. Residual stresses were calculated from the (C_┴) values (Kese and Rowcliffe 2003). Results: ANOVA indicated a significant effect of metal and ceramic on residual stress (p≤0.0001). Distance influenced (C_┴) stresses (p=0.019), but not (C_ll) stress (p=0.8231). At 200 µm from the metal-ceramic interface the (C_┴) residual stresses (MPa) were: LA/HC 10.5±7 (Δα=-0.6); HA/HC -1.8±11 (Δα=1.5); LA/LC -6.5±19 (Δα=1.9); HA/LC 0.7±10 (Δα=3.9). There was an interaction between Δα and distance for(C_┴) stresses (p=0.0003). Conclusion: The indentation crack method yielded relatively low stress levels, considering the thermal contraction differences among the metal-ceramic systems used. Capturing crack images for measurement within 10 s of indentation may lead to underestimation in the magnitude of stress. Further analysis of the difference between cracks lengths captured immediately and then again after 24 h is indicated to assess the influence of slow crack growth on calculated residual stresses. Supported by NIH/NIDCR Grant DE06672-24.