Microstructure and phase stability of three dental cobalt chromium alloys used for metal-ceramic restorations during thermal processing

Abstract

In dentistry, porcelain-fused-to-metal (PFM) dental restorations are one of the most predominant material choices for restoring missing or damaged tooth. Even with the rise in popularity of all-ceramic dental restorations, which give the impression of superior aesthetics, the transition from high noble alloys to the much more economical base metal alloys, allow the latter to remain a formidable competitor. Moreover, metallic structures exhibit superior fracture resistance properties (Pruitt and Chakravartula 2011) compared to all-ceramic based systems, despite the innovations and properties associated with yttria stabilized zirconia (Piconi and Maccauro 1999), which has relatively high fracture toughness due to the tetragonal to monoclinic phase toughening mechanism. Unfortunately, perhaps due to the widely accepted belief of the high toughness of base metal alloys such as Co-Cr, which is the most popular PFM alloy used today, knowledge of the microstructural and possible phase transformations of PFM restorations is poorly appreciated. This is especially critical since Co-Cr alloys have a fcc-hcp phase transformation (Okamoto 2003, Turrubiates-Estrada, Salinas-Rodriguez et al. 2011) under the conventional porcelain sintering conditions, which can potentially alter the properties of the alloy. Furthermore, the consequence of these developments, particularly when utilized in a bi-layered porcelain-metal system, has not been explored. The study aimed to explore the microstructure and phase stability of Co-Cr alloys used in PFM systems and determine the associated influence on the thermal, mechanical, and adhesive properties of the bi-layered material. The investigation utilized a wide range of analytical tools and methods: Electron backscatter diffraction (EBSD), energy dispersive spectrometry (EDS), x-ray diffraction (XRD), nano-indentation and four-point bend strain energy release rate test to determine adhesion. Phase stability and heat transfer modelling of the thermal history under conventional sintering procedures for cast Co-Cr dental alloy were analysed for two separate alloy thicknesses (0.5 mm and 1 mm). Obtained results indicated that the bulk phase stability was influenced by both the metal thickness and porcelain firing temperature with the thicker specimens exhibiting more substantial face-centered cubic (fcc) to hexagonal close-packed (hcp) transformation. A fined grain interfacial layer was also found to develop after undergoing porcelain firings with the depth of the layer increasing with more substantial fcc-hcp transformation. Both the bulk and interfacial layer exhibited independent transformation mechanisms with the bulk of the alloy displaying a “military” martensitic based transformation component while the interfacial layer was driven by a “civilian” diffusion based component. Subsequent analysis of the relationship between fine grained (powder metallurgy (PM) and CAD/CAM [CC]) and coarse grained (cast) microstructure as it relates to the phase stability of Co-Cr alloys were conducted. Fined grained microstructure was found to have superior fcc phase stability at both the bulk and interface and dramatically decrease the depth of the interfacial layer. Cr ion diffusion rates towards the surface was observed to be higher for Co-Cr alloys with finer grained microstructures, especially when manufactured using powder metallurgy due to the innate internal porosity network. However, this Cr concentration appears to have no effect on the oxidation of the alloy. Mechanical characterisation using nano-indentation test of the bulk and interfacial layer observed significant differences (p<.05) in hardness properties for all three alloys. Four-point bend strain energy release rate (Gc) results to quantify adhesive strength of the porcelain to the various Co-Cr alloys exhibited values of 92.15 J/m2 for CC, 62.24 J/m2 for PM and 42.83 J/m2 for cast. A clear relationship of higher adhesive strength associated with low interfacial hardness was observed because of the improved ductility and lower yield stress. That is, the total strain energy release rate Gc increased in order to compensate for the additional energy consumed from plastic flow. The phase stability of Co-Cr alloys has been shown in this study to be an important factor in determining the resultant mechanical and adhesive properties. Microstructure and conventional processing conditions have been shown to have a considerable effect on the phase stability on both the bulk and interface of Co-Cr alloys as well as the adhesion between the alloy and porcelain.