Abstract
Objective
We examined the surface characteristics and corrosion properties of selective laser melted (SLM) cobalt–chromium (Co–Cr) dental alloys before and after porcelain-fused-to-metal (PFM) firing.
Methods
Samples were manufactured utilizing SLM techniques and control specimens were fabricated using traditional casting methods. The microstructure and surface composition were examined using metallographic microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Corrosion properties were evaluated using electrochemical impedance spectroscopy. Student’s t -test was used to evaluate differences in numerical results of electrochemical corrosion tests between SLM and cast specimens before or after PFM firing. The results of electrochemical corrosion tests of the SLM and cast samples before and after firing were analyzed using one-way ANOVA.
Results
Although PFM firing altered the microstructure of the SLM specimens, they still exhibited a compact and homogeneous structure, and XPS analysis indicated that there were no significant differences in the surface composition of the specimens after firing. In artificial saliva at pH 5, the R p value of the SLM specimens was 6.21 MΩ cm −2 before firing and 2.84 MΩ cm −2 after firing, suggesting there was no significant difference in electrochemical corrosion properties ( P > 0.05). In artificial saliva at pH 2.5, the R p value of the SLM group was 4.80 MΩ cm −2 before firing and 2.88 MΩ cm −2 after firing, again indicating no significant difference in electrochemical corrosion properties ( P > 0.05). At pH 2.5, there was a significant difference in corrosion behavior between the cast and SLM groups, with the R p value of the cast group being 0.78 MΩ cm −2 vs. 2.88 MΩ cm −2 for the SLM group.
Significance
The improved post-firing corrosion resistance of SLM specimens provides further support for their use in prosthodontic applications, as the oral environment may become temporarily acidic following meals.
1
Introduction
Selective laser melting (SLM) is a widely used production process in which three-dimensional objects are created by selectively melting regions of a powder layer using a laser heat source . The melted areas are specified using a CAD file, and the solid object is progressively formed by melting layers on top of one another. Compared with traditional casting techniques, SLM reduces the probability of operator error and minimizes casting defects, providing greater production precision . SLM products exhibit higher density and improved corrosion and surface properties . Because of these advantages, the technique has recently been applied to denture manufacture and has attracted the attention of dentists. Traditional casting methods have been used to manufacture Co–Cr restorations for many years , but SLM has only recently been employed to fabricate Co–Cr products . For the sake of clinical safety, it is important to investigate the properties of Co–Cr products fabricated using SLM techniques.
Dental alloys used in fixed prostheses typically have a porcelain veneer fired onto the base metal for esthetic reasons. This type of metal–ceramic restoration is known as a porcelain-fused-to-metal (PFM) restoration. The facial surfaces of PFM restorations are veneered while the lingual and occlusal surfaces and sub-gingival margins are exposed. The porcelain firing process requires high-temperature treatment consisting of four operations between 950 and 1010 °C. The heating process inevitably changes the microstructure of the dental alloys, which may in turn affect their corrosion behavior.
Materials for dental applications have unique requirements including non-toxicity, biocompatibility, and mechanical strength . The biocompatibility of a material is influenced by the corrosion behavior, as corrosion may induce the release of toxic ions. It is therefore important to study the influence of the porcelain firing process on the corrosion behavior of the final product. Relatively few researchers have examined the effect of the PFM firing process on the surface characteristics and corrosion properties of alloys such as Ni–Cr . Other studies have evaluated the corrosion properties of Co–Cr alloys in artificial saliva , and Qiu et al. described the effects of the PFM firing process on the surface and corrosion properties of these alloys. However, the influence of the PFM firing process on the surface and corrosion properties of Co–Cr components fabricated using SLM has not been assessed. The pH within the oral cavity can become substantially lower after meals. The resulting ion exchange may affect the surface properties of the alloy and potentially modify the corrosion behavior .
We evaluated the effects of a simulated PFM firing process on the surface characteristics and corrosion properties of Co–Cr samples fabricated using SLM. The properties were compared to those of cast control specimens in artificial saliva at pH 2.5 and 5.
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