Simulating porcelain firing effect on the structure, corrosion and mechanical properties of Co–Cr–Mo dental alloy fabricated by soft milling

Abstract

This study aims at evaluating the effect of simulating porcelain firing on the microstructure, corrosion behavior and mechanical properties of a Co–Cr–Mo alloy fabricated by Metal Soft Milling (MSM). Two groups of Co-28Cr-5Mo specimens (25 × 20 × 3 mm) were prepared by MSM: The as-sintered (AS) specimens and the post-fired (PF) specimens that were subjected to 5 simulating porcelain firing cycles without applying the ceramic mass onto their surface. Phase identification by X-ray Diffraction (XRD), microstructure examination by optical microscopy and Scanning Electron Microscopy combined with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), corrosion testing by cyclic polarization and chronoamperometry in simulated body fluid (SBF), the latter test accompanied by Cr3+ and Cr6+ detection in the electrolyte through the 1.5-diphenylcarbazide (DPC) method and UV/visible spectrophotometry, and mechanical testing by micro-/nano-indentation were conducted to evaluate the effect of the post-firing cycles on the properties of Co–Cr–Mo. The results were statistically analyzed by the t test (p < 0.05: statistically significant). All specimens had a mixed γ-fcc and ε-hcp cobalt-based microstructure with a dispersion of pores filled with SiO2 and a fine M23C6 intergranular presence. PF led to an increase in the ε-Co content and slight grain coarsening. Both AS and PF alloys showed high resistance to general and localized corrosion, whereas neither Cr6+ nor Cr3+ were detected during the passivity-breakdown stage. PF improved the mechanical properties of the AS-alloy, especially the indentation modulus and true hardness (statistically significant differences: p = 0.0009 and 0.006, respectively). MSM and MSM/simulating-porcelain firing have been proven trustworthy fabrication methods of Co–Cr–Mo substrates for metal-ceramic prostheses. Moreover, the post-firing cycles improve the mechanical behavior of Co–Cr–Mo, which is vital under the dynamically changing loads in the oral cavity, whereas they do not degrade the corrosion performance.

CoxCrFeNiTi High-Entropy Alloys Prepared via Mechanical Alloying and Spark Plasma Sintering for Magnetron Sputtering Coatings

“…The indentation hardness (HIT) of the HEA coatings was approximately 2.8 to 2.9 times higher than that of the SS substrate, indicating a greater resistance to irreversible The instrumented nanoindentation measurements that are presented in Table 4 revealed superior mechanical characteristics for both HEA coatings compared to the SS substrate. The indentation hardness (H IT ) of the HEA coatings was approximately 2.8 to 2.9 times higher than that of the SS substrate, indicating a greater resistance to irreversible (plastic) deformation as well as reversible (elastic) deformation [33]. The CoCrFeNiTi equiatomic HEA coatings exhibited higher H IT values than the Co 0.5 CrFeNiTi HEA coatings, suggesting a better wear resistance.…”

“…The H IT /E IT ratio, also referred to as true hardness [33], is higher for both HEA coatings (0.0244-0.0279) compared to the SS substrate (0.0200). A higher true hardness in the HEA coatings signifies an improved resistance to plastic deformation and enhanced wear resistance [33,35].…”

“…The indentation modulus (E IT ), which provides a reasonably accurate approximation of the Young's modulus [34], was about 2 to 2.4 times higher for the HEA coatings compared to that of the SS substrate. Higher E IT values for the HEA coatings indicate a better resistance to reversible deformation [33] when compared to the SS substrate.…”

“…A higher true hardness in the HEA coatings signifies an improved resistance to plastic deformation and enhanced wear resistance [33,35]. However, the low values of the H IT /E IT ratio, which can be assimilated to an "elasticity index" [33], suggest that there is a tendency toward permanent deformation in both the HEA coatings and the SS substrate.…”

Enhanced Anti-Corrosion Performance of Co-Cr-Mo Alloy in Molten Al by Prior Oxidation Treatment

“…In recent decades, various techniques such as X-ray photoelectron spectroscopy (XPS) have been used to characterize the oxide films on both Co-based alloys and Fe-based alloys [12,13,[17][18][19][20][21][22][23][24]. In contrast to the single-layered oxide film in Fe-29Cr-6Mo alloy [17], a duplex oxide layer consisting of an outer CoO-rich layer and an inner Cr 2 O 3 -rich layer was observed in Co-Cr-Mo alloy subjected to oxidation at 750 • C for 4 h. With prolonged oxidation, CoO was gradually replaced by Cr 2 O 3 , resulting in a single-layered oxide film dominantly composed of Cr 2 O 3 [13]. Tunthawiroon et al [5] characterized the oxide films on Co-Cr-Mo-xSi alloys exposed to high-temperature oxidation, and the results indicated the key role of Si in stabilizing the Cr 2 O 3 -dominated film.…”