The bond strength of dental porcelain to cobalt-chromium alloys fabricated by casting, milling and by selective laser melting: a comparative analysis

Abstract

This study was set up to explore the effects of the applied routes for fabricating cobalt-chromium (Co-Cr) dental frameworks on the metal-ceramic bond strength. Three groups (n = 12/group) of Co-Cr specimens (25×3×0.5) mm were fabricated by casting, milling, and by selective laser melting (SLM), and then airborne-particle abraded (110 μm Al2O3 particles). Dental porcelain was applied (8×3×1.1) mm onto the Co-Cr substrates, and the metal-ceramic bond strength was assessed by a three-point bend test according to the ISO 9693-1:2012 standard. Failure modes were determined using stereomicroscopy. Representative specimens from each group were used to assess the element distribution across the metal-ceramic interface, together with an inspection of its microstructure using a scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS). Average profile roughness (Ra) values were obtained for each group of metal substrates. The metal-ceramic bond strength data and Ra data were statistically analyzed (one-way ANOVA, Tukey's HSD test, a = 0.05). The mean surface roughness was affected by the fabrication technology (p < 0.01), SLM Co-Cr demonstrating a significantly higher Ra value (1.78±0.20 μm), than cast and milled Co-Cr (1.21±0.07 μm and 1.05±0.11 μm respectively). However, the metal processing applied did not affect the metal-ceramic bond (p = 0.104). Differently processed metal-ceramic specimens showed a comparable distribution of elements on the interface, together with good wetting between the alloy and the porcelain. The porcelain bond strengths to cast, milled and SLM Co-Cr alloys are well above the minimum ISO 9693-1:2012 recommended value of 25 MPa for metal-ceramic systems, thus allowing the clinical application of SLM Co-Cr in porcelain-fused-to-metal prostheses.

Indirect restorative systems—A narrative review

“…In fact, three main factors control metal‐ceramic bonding, including mechanical interlocking at the interface between porcelain and the metal framework, interatomic bonding across the oxide‐porcelain interface along with the type and magnitude of residual stress present in the veneering ceramic, which might be reduced by using feldspathic ceramics with a compatible CTE 14 . Several other factors can be associated with veneering ceramic susceptibility to fracture, including feldspathic ceramic layer thickness, framework material and processing method, low strength and toughness of the porcelain, inadequate ceramic firing, and defects caused during the fabrication process and adjustments 14,32,39 …”

“…14 Several other factors can be associated with veneering ceramic susceptibility to fracture, including feldspathic ceramic layer thickness, framework material and processing method, low strength and toughness of the porcelain, inadequate ceramic firing, and defects caused during the fabrication process and adjustments. 14,32,39 The metallic framework manufacturing method can significantly affect metal-ceramic interface bond, which might also influence technical complication rates, with the conventional lost wax casting and selective laser melting (SLM) 3D-printing methods outperforming CAD/CAM method for Co-Cr frameworks [30][31][32] ; Such new technologies for metal framework fabrication seem to provide acceptable clinical levels of marginal misfit for tooth and implant supported single crowns and FDPs, comparable to conventional casting (all lower than clinically acceptable levels, 120 μm), although CAD/CAM method has shown inferior marginal adaptation relative to other methods. 31,40,41 Irrespective of metal alloy type and processing method, metal frameworks prevent light transmission and make it a challenge to achieve an acceptable optical effect, especially for tooth supported anterior reconstructions.…”